Silurian Vertebrates of Gotland (Sweden) and the Baltic Basin

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Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1544

Silurian vertebrates of Gotland (Sweden) and the Baltic Basin

OSKAR BREMER

ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 ISBN 978-91-513-0039-9 UPPSALA urn:nbn:se:uu:diva-328234 2017 Dissertation presented at Uppsala University to be publicly examined in Ekmansalen, EBC, Norbyvägen 14, Uppsala, Friday, 6 October 2017 at 13:00 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Associate Professor Héctor Botella (University of Valencia, Cavanilles Institute of Biodiversity and Evolutionary Biology, Valencia, Spain).

Abstract Bremer, O. 2017. Silurian vertebrates of Gotland (Sweden) and the Baltic Basin. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1544. 61 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-0039-9.

During the Silurian, the Swedish island Gotland was positioned close to the equator and covered by a shallow sea called the Baltic Basin. The sedimentary rocks (predominantly carbonates) comprising most of the island today were initially formed in this warm sea, and the relatively complete succession of rocks often contains fossil fragments and scales from early vertebrates, including heterostracans, anaspids, thelodonts, osteostracans, acanthodians, and a stem-osteichthyan. Fossils of early vertebrates become increasingly more common in younger Silurian rocks, but are mostly represented by fragmentary remains and rarer occurrences of articulated jawless vertebrates (agnathans). However, the record of articulated specimens and jawed vertebrates (gnathostomes) are more numerous in rocks of the following Devonian Period. Isolated peaks of agnathan diversity during the Silurian and disarticulated remains of gnathostomes from this period hint at a cryptic evolutionary history. A micropaleontological approach with broader sampling may provide a better understanding of early vertebrate distribution patterns and hopefully give some insights into this history. The objective of this study was to build upon previous sampling on Gotland and to use established frameworks for disarticulated remains with the aim of making comparisons with similar studies performed in the East Baltic. However, difficulties locating the collections from these previous works necessitated a different focus. Undescribed museum collections and newly sampled material enabled some taxonomical revisions and greatly improved the understanding of vertebrate distribution in the youngest part of the Gotland sequence. It also indicated that this interval may represent the early stages of the diversification of gnathostomes that become increasingly dominant toward the end of the Silurian. Furthermore, the description of samples from partly coeval sections in Poland enabled some preliminary comparisons outside of Gotland, and presented a striking example of restricted environmental occurrences for a thelodont taxon. This is encouraging for future sampling and investigations on Gotland. Together with the establishment of a facies-framework comparable to that developed in the East Baltic and correlations to other areas, this may prove fruitful for an increased understanding of early vertebrate distribution and evolution during the Silurian.

Keywords: early vertebrates, vertebrate microremains, scale taxonomy, early vertebrate distribution, environmental preferences, Silurian, Baltic Basin, Gotland, Sweden

Oskar Bremer, Department of Organismal Biology, Evolution and Developmental Biology, Norbyv 18 A, Uppsala University, SE-75236 Uppsala, Sweden.

ISSN 1651-6214 ISBN 978-91-513-0039-9 urn:nbn:se:uu:diva-328234 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-328234) List of Papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals.

I Bremer, O., Blom, H. (2015) An updated stratigraphic and environmental framework for the distribution of Silurian vertebrates on Gotland. Estonian Journal of Earth Sciences, 64(1):13–18. doi: 10.3176/earth.2015.03 II Jarochowska, E., Bremer, O., Heidlas, D., Pröpster, S., Vanden- broucke, T. R. A., Munnecke, A. (2016) End-Wenlock terminal Mulde carbon isotope excursion in Gotland, Sweden: Integra- tion of stratigraphy and taphonomy for correlations across restricted facies and specialized faunas. Palaeogeography, Palae- oclimatology, Palaeoecology, 457:304–322. III Bremer, O., Jarochowska, E., Märss, T., Blom, H. Vertebrate remains and conodont biostratigraphy in the Ludlow Burgsvik Formation of Gotland, Sweden. Manuscript. IV Bremer, O., Jarochowska, E., Märss, T. Vertebrate dermal remains and conodont distribution in the upper Silurian Hamra and Sundre formations of Gotland, Sweden. Manuscript. V Bremer, O., Niedźwiedzki, G., Blom, H., Dec, M., Kozłowski, W. (2017) Vertebrate microremains from the upper Silurian Winnica Formation of the Holy Cross Mountains, Poland. Ge- ological Magazine:1–19. doi:10.1017/S0016756817000681

In paper I, OB gathered the data, created the figures and wrote the bulk of the text with contributions from HB. In paper II, OB made the identifications, wrote the text and created the figures for the vertebrate section, and made small contributions to the discussion and conclusions. The data for paper III and IV was mainly collected by EJ and OB. EJ and OB created figures and co-authored the bulk of the text for paper III with contribution from HB and input from TM, and paper IV with input from TM. Material for Paper V was

3 collected and prepared by MD and WK. The data, text and figures in Paper V were prepared by OB and GN with input from HB and WK.

The following papers were also written during the doctoral studies but are not included in the thesis. VI Jarochowska, E., Viira, V., Einasto, R., Nawrot, R., Bremer, O., Männik, P., Munnecke, A. (2017) Conodonts in Silurian hyper- saline environments: Specialized and unexpectedly diverse. Ge- ology, 45(1):3–6. VII Jerve, A., Bremer, O., Sanchez, S., Ahlberg, P. E. Morphology and histology of acanthodian fin spines from the late Silurian Ramsåsa E locality, Skåne, Sweden. Resubmitted after minor revisions to Palaeontologia Electronica.

Disclaimer: The papers presented in this work are for the purpose of public examination as a doctoral thesis only. The description of a new taxon presented in paper IV is therefore not valid following ICZN article 8.2.

Front cover: The view from the top of a “faros reef” on southernmost Gotland.

4 Contents

1. Introduction ...... 7 1.1. Gotland and the Baltic Basin ...... 9 1.2. Evolutionary history and interrelationships of early vertebrates ...... 12 1.2.1. Heterostracans ...... 15 1.2.2. Anaspids ...... 16 1.2.3. Thelodonts ...... 17 1.2.4. Osteostracans ...... 18 1.2.5. Acanthodians ...... 18 1.2.6. Andreolepis hedei ...... 19 2. Climatic and environmental influences on vertebrate distributions ...... 21 2.1. Previous studies ...... 23 2.2. A different approach ...... 25 3. Silurian vertebrate distribution on Gotland ...... 27 4. Gotland in a broader context ...... 35 5. Conclusions and future directions ...... 41 6. Svensk sammanfattning ...... 43 7. Acknowledgements ...... 48 8. References ...... 50

1. Introduction

Vertebrates are an extremely diverse and widespread group of organisms that today inhabit a wide range of environments from deep oceans to high mountains, and occur from pole to pole. All vertebrates (including ourselves) can, like many other metazoans, trace their origin back to the so called Cambrian explosion as they make their first appearance in the fossil record in lower and middle Cambrian rocks from ca. 543–510 Million years ago (Ma) (see Marshall, 2006; Friedman and Sallan, 2012). A number of soft-bodied animals preserved in Lagerstätten, such as the lower Cambrian Chenjiang and middle Cambrian Burgess Shale, have been interpreted as either stem or crown group vertebrates, or at least related to them (Chen et al., 1999; Shu et al., 1999; Shu et al., 2003; Holland and Chen, 2001; Mallatt and Chen, 2003; Morris and Caron, 2014). However, interpreting the features preserved in these fossils is problematic and the affinities of many of them are uncertain (Donoghue and Purnell, 2009; Sansom et al., 2010b). Of these, Haikouichthys seems most likely to be at least a total group vertebrate, since several specimens seem to preserve diagnostic vertebrate characters, such as paired sensory capsules and vertebrae (Shu et al., 2003; Sansom et al., 2011; Janvier, 2015). Today, the only jawless vertebrates (agnathans) around are the hagfishes and lampreys that, based on both molecular and developmental data (Heimberg et al., 2010; Ota et al., 2011; Oisi et al., 2013a, 2013b), most likely form a clade (the cyclostomes) and therefore constitute the sister-group of the vastly outnumbering jawed vertebrates (gnathostomes). During the early parts of the Paleozoic, however, the now extinct armored agnathans (collectively termed “ostracoderms”) were the most prominent component of the vertebrate faunas (Friedman and Sallan, 2012; Sansom et al., 2015). This evidently changed during the Devonian Period, often called the “Age of Fishes”, as gnathostomes increased in abundance and diversified to fill a wide variety of trophic roles (see Friedman and Sallan, 2012). This has been coupled to both the acquisition of jaws and the shift from benthic to nektonic lifestyles (Klug et al., 2010; Anderson et al., 2011; Friedman and Sallan, 2012). However, fragmentary remains and scales of gnathostome affinities from older rocks (Friedman and Sallan, 2012; Sansom et al., 2015), and subsequently the min- imum divergence time between jawless and jawed stem-gnathostomes based on fossil data (Brazeau and Friedman, 2015), has turned the attention of researchers to Silurian and even older rocks to investigate the origin and early evolution of gnathostomes. This has, however, proven difficult because the

7 body fossil record of them is not as good as in the Devonian (Friedman and Sallan, 2012) and there seems to be a facies bias in the early record of fragmentary remains (Sansom et al., 2015). To get a better understanding of the environmental preferences and lifestyles of these early vertebrates may therefore be important for understanding their early evolution.

Figure 1. Map of the Silurian world (A) and detail of the Baltic Basin (B), both adapted from Eriksson and Calner (2005) and based on Baarli et al. (2003). C: extent of the erosional remnant of late Ludlow sediments from the Baltic Basin (see Paper V for figure references).

8 1.1. Gotland and the Baltic Basin Gotland is an island in the Baltic Sea off the east coast of the Swedish main- land. During the Silurian Period (ca. 444-419 Ma), Gotland and most of southern Sweden was situated close to the equator (Fig. 1A) and covered by a warm epicontinental sea referred to as the Baltic Basin (Fig. 1B) (Poprawa et al., 1999; Cocks and Torsvik, 2002). Today, Gotland is entirely built up by sedimentary rocks that originated in this sea and forms part of an erosive remnant (Fig. 1C) truncated by cratonward erosional limits to the north and northeast, as well as by the Teisseyre-Tornquist Line toward the southwest (Martinsson, 1958; Flodén, 1980; Poprawa et al., 1999). The warm waters of the Baltic Basin accommodated reefs that subsequently formed the majority of the rock formations that build up the island today (Fig. 2) (Calner et al., 2004a; Eriksson and Calner, 2005; Erlström et al., 2009). The reefs were also home to a plethora of marine organisms, the fossils of which can be found all over the island today. The succession of sedimentary rocks (Fig. 3) can be divided into a number of depositional sequences separated by minor discontinuities that relate to carbonate platform generations (Calner et al., 2004b). The strata show a prominent transition along the strike, generally passing from alternations of open marine, argillaceous shelf limestones and marls in the southwest into contemporaneous, shallow water carbonates to the northeast (Eriksson and Calner, 2005). The Gotland sequence has a collective thickness of 500-750 m and represents approximately 10 Million years of time, from latest Llandovery to late Ludlow (Jeppsson et al., 2006; Erlström et al., 2009; Kaljo et al., 2015). The strata have been divided into a series of groups and formations (Figs. 2 and 3) that are mainly built up by carbonates, but siliciclastics also form parts of the succession (Erlström et al., 2009). The reader is referred to the unpublished licentiate thesis (Bremer, 2016) prepared for this project, and references therein, for an exhaustive review of the Gotland stratigraphy, geology, and depositional environments, as well as for clarifications of the stratigraphical nomenclature used in Figure 2 and 3, as well as in later sections of this thesis.

Figure 2. Geological map of Gotland with the geographical extent of the formations and their lithologies. Based on Eriksson and Calner (2005) and data from the Swe- dish Geological Survey (SGU).

Figure 3. Stratigraphical column of Gotland (left) and simplified distribution of lithologies through the entire Gotland section and general differences between south- western and northeastern areas. Modified from Erlström et al. (2009) with additions from Samtleben et al. (1996).

The sedimentary rocks of Gotland have experienced little late diagenetic al- teration and tectonic displacement (Calner et al., 2004a), and consequently the fossils display excellent states of preservation (Jeppsson, 1983; Munnecke et

11 al., 1999; 2000). This has attracted the interest of scientists since the 18th century (Erlström et al., 2009) and the first fossil vertebrates were reported in the middle of the 19th century (Von Volborth, 1861). This was followed by infre- quent reports of vertebrate remains, but more detailed description of vertebrates were performed by Gross (1968a, 1968b) who also described the first remains of Andreolepis hedei Gross, 1968a. The most extensive works on Si- lurian vertebrate remains from Gotland were performed by Fredholm (1988a, 1988b, 1989, 1990). A short review of the history of research on Gotland and its vertebrates can be found in the unpublished licentiate thesis (Bremer, 2016), and a more exhaustive review of the history of research on Gotland can be found in Manten (1971). All of the previous reports, including the ones mentioned above, are reviewed further in section 2.

1.2. Evolutionary history and interrelationships of early vertebrates The “ostracoderms” effectively form a grade on the stem leading up to gnathostomes and are generally set apart from the living cyclostomes by having dermal hard tissues (Janvier, 2015), although their interrelationships are still unclear (see Fig. 4). The earliest records of potential dermal vertebrate remains composed of layered hard tissues (including a dentine-like tissue) come from the late Cambrian to Middle Ordovician deposits of Euramerica and Aus- tralia (Repetski, 1978; Smith et al., 1996; Young et al., 1996; Clark et al., 1999). These remains are mostly attributed to Anatolepis and even though they share similarities with a younger vertebrate group called arandaspids (Friedman and Sallan, 2012), their affinity remains unclear (Janvier, 2015). Arandaspids form a potentially paraphyletic assemblage near the root of the gnathostome stem that are viewed by some as related to heterostracans (Janvier, 1996; Sansom et al., 2001; Friedman and Sallan, 2012). Together with the similar astraspids (Janvier, 2015), they make up the oldest articulated specimens of definite “ostracoderms” that come from a handful of middle to late Ordovician rocks in Australia (Young, 2009), North America (Sansom et al., 2001), and Bolivia (Gagnier and Blieck, 1986). Fragmentary material that share similarities to these has also been found in other parts of the world (Wang and Zhu, 1997; Blieck and Turner, 2003; Karatajūtė-Talimaa and Smith, 2004; Sansom et al., 2009).

Figure 4. Roughly time-calibrated phylogenetic tree and fossil record of extinct and extant vertebrates. Phylogeny mainly based on Sansom et al. (2010a) after Janvier (2015), but excluding conodonts and with some collapsed nodes to reflect the uncertain interrelationships discussed in the text. Ranges of groups are based on Sansom et al. (2015), Janvier (2015), and references discussed in the text. Individual records outside the tree represent reports of isolated remains of potentially “ostracoderm” (Anatolepis) and gnathostome (Skiichthys, mongolepids and sinacanthids) taxa. Note that the single branches do not necessarily denote monophyly (e.g., acanthodians and placoderms). Time-line based on Cohen et al. (2013). See Figure 5 for individual figure references.

13 Besides astraspid and arandaspid remains, rocks of Ordovician age have also produced scales of thelodonts and fragmentary remains of possibly heterostracan-, placoderm-, as well as acanthodian- (e.g., Skiichthys, Fig. 4) or even chondrichthyan-grade taxa (Sansom et al., 1996, 2001, 2012; Smith et al., 1997; Young, 1997; Donoghue et al., 2003; Friedman and Sallan, 2012). How- ever, the potential gnathostome affinities of these remain unresolved (see Friedman and Sallan, 2012; Janvier, 2015). Fossils of vertebrates are scarce also in lower Silurian rocks, but the oldest articulated thelodonts (Dineley and Metcalf, 1999) and the “naked” agnathan Jamoytius have been found in rocks from the middle of the Llandovery series (ca. 444–433 Ma) (Janvier, 1996; Sansom et al., 2010a). Anaspid remains have been described from upper Llandovery rocks of Scotland and the first articulated specimens of Lasanius and Birkenia come from end-Llandovery- aged rocks in the same area (Blom et al., 2002). Specimens of galeaspids and possible heterostracans occur in upper Llandovery strata of China (Zhao and Zhu, 2007) and Canada (Janvier, 1996; Soehn et al., 2001) respectively. Body fossils of definite heterostracans and osteostracans have been found in rocks from the succeeding Wenlock series (ca. 433-427 Ma) (Janvier and Blieck, 1993; Wilson and Caldwell, 1993; von Bitter et al., 2007). Gnathostomes are commonly represented by microremains from the Silurian Period: chondrichthyan-like scales (mongolepids, Fig. 4) and acanthodian-type spines (sinacanthids, Fig. 4) have been found in upper Llandovery rocks (Karatajūtė- Talimaa, 1995; Janvier, 1996; Sansom et al., 2005b; Zhao and Zhu, 2007; Zigaite et al., 2011), isolated placoderm plates and stem-osteichthyan fragments occur in faunas of Wenlock age in China (Zhao and Zhu, 2007; Qu et al., 2010) and in Ludlow sites elsewhere (Janvier, 1996; Friedman and Brazeau, 2010) including Gotland (Botella et al., 2007). The oldest articulated gnathostomes are represented by both placoderms and crown gnathostomes in rocks of latest Ludlow age (ca. 427-423 Ma) (Zhu et al., 1999; Zhu et al., 2009; Zhang et al., 2010). An enigmantic and often controversial group not mentioned above are the conodonts (Fig. 5A). Their affinity has been debated ever since the discovery of these isolated tooth-like elements (Fig. 5A) in the mid-1800s (Donoghue, 1998), with suggestions ranging from vertebrates to annelid worms (see Turner et al., 2010). The nature of the conodont elements became much clearer with the discovery of soft-bodied, eel-like animals that had a complex feeding apparatus composed of these tooth-like elements in its mouth-region (see Donoghue, 1998; Turner et al., 2010; Dzik, 2015). Conodont elements are phosphatic and composed of a solid basal body of dentine-like tissue, and a crown of lamellar, enamel-like tissue and so called white matter, but the interpretation of the composition of these tissues has var- ied (see Donoghue, 1998 for a thorough review). Conodonts were historically divided into the three groups protoconodonts, paraconodonts, and euconodonts that were initially proposed to form a grade in that order (Murdock et

14 al., 2013), but it has been shown that protoconodonts are instead related to chaetognaths (Szaniawski, 1982). However, it has also been demonstrated that euconodonts most likely derived from the paraconodonts (see Murdock et al., 2013 and references therein), which also led to the conclusion that their hard tissues had evolved convergently to those of vertebrates, rather than being ho- mologous (Murdock et al., 2013; Donoghue and Rucklin, 2016). Despite this, the vertebrate-like morphological features of whole-body specimens (head with large eyes, myomere-like structures along the body, and a caudal fin) have led some researchers to suggest that they are vertebrates (see Janvier, 2015), while others are more skeptical of these similarities and have ques- tioned their vertebrate, and even chordate, affinity (Turner et al., 2010). There- fore, paraconodonts and euconodonts could either be placed within the vertebrate clade in Figure 4, or on the stem leading up to them. Regardless of their affinity though, conodonts must have been an important component of past ecosystems, since they are so abundantly found in mainly shallow marine Paleozoic rocks. Furthermore, the works on conodonts from Gotland by Len- nart Jeppsson (1940–2015) and colleagues have resulted in a highly detailed biostratigraphical framework for the Gotland sequence (summarized in Jeppsson et al., 2006), which has been of great importance to this work. Out of the vertebrate groups mentioned above, the ones so far recognized on Gotland (Fig. 5B-C) are anaspids, thelodonts, heterostracans, osteostracans, acanthodians, and the stem osteichthyan (Botella et al., 2007) Andre- olepis hedei.

1.2.1. Heterostracans Heterostracans were a taxonomically large and diverse group of “ostracoderms” (Keating et al., 2015; Randle and Sansom, 2016) with a fossil record ranging from early Silurian times to the latter half of the Devonian (Janvier, 1996). They had oblong, fusiform bodies covered in scales and a head composed of two large shields (Fig. 5B), one dorsal and one ventral, a differing number of branchial plates in between, and an array of plates in a fan-like arrangement below the mouth (Janvier, 1996). The dermal armor of heterostracans consisted of a basal laminated layer and a middle honeycomb-like layer of trabecular bone, as well as a top bone layer with canals underneath individually grown tubercles or ridges (odontodes) composed of dentine with an enameloid cap (Janvier, 1996; Keating et al., 2015; Keating and Donoghue, 2016). Both the overall morphology and histology of heterostracans are similar to astraspids and arandaspids, and they are therefore often gathered (Fig. 4) in the pteraspidomorphs (Sansom et al., 2005a). However, heterostracans only had a single pair of common branchial openings while the other two had several (Janvier, 1996; Janvier, 2015). Only one taxon, Archegonaspis lindstroemi Kiær, 1932, has been found on Gotland, and only from a restricted interval of the sequence (Fredholm, 1988a,

15 1988b). It is mostly represented by fragmentary and disarticulated remains (Fig. 5B), but the dorsal and ventral head shields from the Lau channel described by Lindström (1895) are still the most complete remains of vertebrates ever found on Gotland.

Figure 5. Sketches of a conodont (A) and the early vertebrates represented on Got- land (B-G), as well as examples of microremains from respective group that form the basis for this work. B: heterostracan, C: anaspid, D: thelodont, E: osteostracan, F: acanthodian, G: Andreolepis hedei. The microremains are not representative of the sketched species, except for D (Phlebolepis elegans). A from Aldridge et al. (1993), B from Soehn and Wilson (1990), C from Blom et al. (2002), D from Ritchie (1968), E from Janvier (1996) based on Ritchie (1967), F from Romer (1964), and G modified from Chen et al. (2012).

1.2.2. Anaspids Anaspids (Fig. 5C) were a group of jawless vertebrates characterized by laterally compressed and slender bodies covered in mineralized scales, a fusiform head covered by small plates, a hypocercal tail, and tri-radiate spines behind a slanting row of external branchial openings (Janvier, 1996; Blom and Märss, 2010). The scales of anaspids (Fig. 5C) are composed of a basal laminated bone layer, an upper layer with more vascularized laminar bone, as well as a superficial layer of tubercles (Blom et al., 2002, 2003) composed of

16 spheritic mineralizations, possibly being derivatives of odontode dentine and enamel (Keating and Donoghue, 2016). As mentioned before, the earliest definite fossil anaspids show up in lower Silurian rocks, and their fossil record extends into the Lower Devonian (Blom and Märss, 2010). However, the relationship of anaspids to “naked” forms such as Jamoytius (placed near the basal node of the vertebrate tree, or associated with cyclostomes, or even nested within anaspids) mentioned earlier, their monophyletic status, as well as their relative position along the gnathostome stem compared to heterostracans (see Fig. 4) have been debated (Sansom et al., 2010a; see Blom and Märss, 2010; Blom, 2012; Keating and Donoghue, 2016). Both articulated and isolated material from the Northern Hemisphere (including the Baltic Basin) of birkeniid anaspids were studied in detail by Blom et al. (2002) and provided a taxonomical framework for disarticulated scales and platelets. This has been of great importance to this work because anaspids are only represented by such remains on Gotland. These occur sporadically through the Gotland sequence and are mainly concentrated to two stratigraphical levels (see section 2).

1.2.3. Thelodonts Thelodonts (Fig. 5D) were a cosmopolitan group of jawless fishes with a fossil record stretching from the Late Ordovician (Sansom et al., 1996; Märss and Karatajūtė-Talimaa, 2002) to Late Devonian times (Märss et al., 2007; Hairapetian et al., 2016). They have remained enigmatic since their discovery and their relation to the other vertebrate groups (Fig. 4) is unclear (see Keating et al., 2015). Most of them were small animals with a total body length of only a few centimeters, but some reached lengths of at least 60 cm (Märss et al., 2007). As summarized by Märss et al. (2007), the group displays a range of body proportions and morphologies, but three main body types have been recognized: 1) dorsoventrally flattened with wide head region that narrows behind the branchial structures into a slender body that ends in a generally large hypocercal tail; 2) fusiform head and trunk, i.e. more rounded in in cross section, with more laterally flattened and slender bodies with hypocercal tails; 3) fork-tailed, laterally compressed, and deep-bodied forms with multilobate tail fins. Many thelodonts probably also had pectoral fin folds, as well as dorsal and anal midline fins (see Märss et al., 2007). The entire body surface as well as the buccopharyngeal cavity of thelodonts were covered by individual, non-growing scales (Fig. 5D) with bases composed of laminated bone and crowns composed of dentine tissue capped with enameloid (Donoghue et al., 2006; Märss et al., 2007). Articulated specimens of thelodonts are rare (Märss et al., 2007), in fact, only 29 out of 147 described species are known from articulated material (Ferron and Botella, 2017). The

17 majority are described from so called scale-associations, where scales display- ing a range of morphologies can be ascribed with some certainty to a single taxon using a body-zonation framework that has been developed and refined for thelodonts (see Märss et al., 2007). This has proven crucial for any work about thelodonts on Gotland, since they are only represented by isolated scales.

1.2.4. Osteostracans Osteostracans are another large and diverse group of jawless fishes with a fossil record ranging from lower Silurian to Upper Devonian rocks (Janvier, 1996). They generally had bullet- to horseshoe-shaped head shields, paired pectoral fins, one or two dorsal fins, and a epicercal tail sometimes with a small lobe underneath (Fig. 5E) (Janvier, 1996). The body was covered by scales of different sizes that were probably formed by the fusion of smaller scales or by the addition of new odontodes (Keating et al., 2012; Qu et al., 2015). The dermal skeleton of osteostracans (Fig. 5E) consisted of a basal layer of laminated bone, a vascular middle layer of cellular bone, and a superficial layer (often of individual tubercles) composed of dentine with an enameloid cap (see Keating et al., 2015; Qu et al., 2015), although the detailed composition of the middle layer may be more complicated (Qu et al., 2015). Osteostracans are generally considered as the sister-group of gnathostomes (Sansom, 2009), (Fig. 4), largely based on features preserved by their exten- sively calcified or ossified endoskeleton of the head (otherwise only seen among galeaspids), as well as in their pectoral girdles and fins (Janvier, 1996; Janvier et al., 2004; Janvier, 2015). Another study of great importance to this work is the extensive review and revision of both articulated and isolated material of Silurian osteostracans from the East Baltic by Märss et al. (2014), which provided a framework for the material from Gotland.

1.2.5. Acanthodians Acanthodians are perhaps more “fish-like” in appearance than any of the groups described above (Fig. 5F) and they are often referred to as “spiny sharks” owing to their many fin spines, the two dorsal fins, the epicercal tail fin, and separate gill slits (Janvier, 1996). Their bodies were covered in growing (possibly polyodontode) scales (Fig. 5F) consisting of bony bases that could either be acellular or cellular, and a layered dentinous crown often with an enameloid cap (Janvier, 1996). Besides scales, the head and shoulder girdle were also covered in small bony platelets (tesserae), or sometimes by larger dermal bony elements (Denison, 1979; Janvier, 1996). The mouths of acanthodians were often equipped with tooth spirals, tooth whorls, or powerful jaw bones with large and firmly attached teeth (Janvier, 1996). Acanthodians were

18 classically considered a clade united in part by the presence of fin spines in front of all paired and mid-line fins (except the tail fin), as well as a series of intermediate spines in some forms (Denison, 1979; Janvier, 1996). However, paired fin spines have since been described in early chondrichthyans (Miller et al., 2003; Maisey et al., 2017) and a few fossil osteichthyans were also dis- covered to have median and paired fin spines (Zhu et al., 1999; Zhu et al., 2009). Together with cranial data, this has led to the rejection of a monophyletic Acanthodii (Brazeau, 2009; Davis et al., 2012). Instead, acanthodians are now considered as a paraphyletic assemblage within the chondrichthyan total group (Fig. 4) (Zhu et al., 2013; Giles et al., 2015; Burrow et al., 2016; Qiao et al., 2016). Definite remains from representatives of this paraphyletic assemblage have been found in lower Silurian to Permian rocks (Janvier, 1996; Davis et al., 2012). Only a single near complete acanthodian has been reported from strata older than the Devonian (Burrow and Rudkin, 2014). Hence, the Silurian acanthodians of the Baltic Basin are only known from scales and other disarticulated material such as tesserae, fin spines and jaws. The acanthodians on Gotland can generally be divided into three groups based on these disarticulated remains (mainly scales), the “nostolepid”, “gomphonchid”, and poracanthodid scale types. However, the details regarding the taxonomy of these remain problematic and are discussed in Papers III, IV, and V. Acanthodians make a late appearance in the Gotland sequence, and they generally see an increase in numbers toward late Silurian times (see section 2).

1.2.6. Andreolepis hedei The description of And. hedei by Gross (1968a) was based on scales and a single bone fragment, but other remains (parts of the shoulder girdle and tooth plates) were described later by Janvier (1978). The understanding of the squamation pattern in this fish was greatly improved in a study by Chen et al. (2012). They used geometric morphometrics on large amounts of isolated scales and comparative anatomy to identify body positions of the different kinds of Andreolepis scales. In the earlier studies, And. hedei was considered an actinopterygian (ray- finned fish) within crown group osteichthyans (bony fishes and tetrapods), but its phylogenetic placement shifted down onto the osteichthyan stem (Fig. 4) based on features in jaw bones with similar sculpture from the same localities (Botella et al., 2007). Investigation of these jaw bones has led to new insights into the evolution of tooth replacement among osteichthyans (Chen et al., 2016). The scales of And. hedei (Fig. 5G) consist of a base of cellular bone covered by several generations of odontodes (polyodontode) composed of dentine with enamel caps (Gross, 1968a; Qu et al., 2013), all being intercon- nected by a complicated system of vascular canals (Qu et al., 2016).

19 Andreolepis hedei and a related species have been found in sediments of similar age or slightly younger in other parts of the Baltic Basin (Vergoossen, 1999a; Märss, 2001), as well as in other regions in England and Russia (Märss, 2001), but so far it is only know from disarticulated material.

20 2. Climatic and environmental influences on vertebrate distributions

The Silurian Period follows the end-Ordovician Hirnantian glacial episode, which is associated with one of the “big five” mass extinctions in Earth history (Raup and Sepkoski, 1982). Furthermore, the Silurian is characterized by extremely large and more frequent isotopic shifts compared to any other part of the Phanerozoic, which indicates major changes in the global carbon cycle during this time (Calner, 2008; Munnecke et al., 2010). These were most likely the result of normal but abrupt changes in the global climate (Calner, 2008) and are associated with changes in the global sea-level (Munnecke et al., 2010). Friedman and Sallan (2012) pointed out that the generally scarce fossil record of vertebrates in the Ordovician makes it frustratingly difficult to eval- uate the impact of this end-Ordovician episode on vertebrates, and the climatic perturbations of the Silurian may obscure potential recovery patterns among them. However, looking at microremains, Blieck and Turner (2003) and Turner et al. (2004) identified some large-scale patterns among vertebrates during the Ordovician, including a faunal turnover towards the end of the period and an enigmatic gap in the fossil record between Late Ordovician and early Silurian times, which they named Talimaa’s Gap (Turner et al., 2004). Overall though, the vertebrates generally show comparably greater abundance and diversity during the Silurian (Qu et al., 2010). During investigations of conodont biostratigraphy and distribution on Got- land, it became evident that the conodont faunas were affected by a series of perturbations and extinctions (Jeppsson, 1984, 1987). These were associated with changes in rock facies connected to carbonate production, and the sedimentary changes were later confirmed on a global scale by Brunton et al. (1998). The discoveries led Jeppsson (1990) to present a model explaining these changes, or events, by the switchover between to two separate stable oceanic states and climatic episodes. This model was later refined and ex- panded upon (Jeppsson, 1997; Jeppsson and Aldridge, 2000, 2001) and the Silurian was divided into more than 20 alternating stable episodes and associated transitional events (Jeppsson, 1998). Following Lennart Jeppsson’s discoveries, isotopic studies on Gotland by Wenzel and Joachimski (1996) and Samtleben et al. (1996) demonstrated that the shift between stable oceanic states were coupled with significant and parallel stable isotope excursions in δ13C and δ18O isotopes. Similar patterns were observed both on Gotland and

21 globally in other works (Talent et al., 1993; Samtleben et al., 2000; Bickert et al., 1997; Saltzman, 2001; Cramer and Saltzman, 2005, 2007) and the discoveries were accompanied by alternative models to explain them (Wenzel and Joachimski, 1996; Samtleben et al., 1996; Bickert et al., 1997; Munnecke et al., 2003; Cramer and Saltzman, 2005; Cramer et al., 2005). A total of eight events have been described on Gotland: Ireviken, Ansarve, Boge, Valleviken, Mulde, Linde, Lau, and Klev events (Jeppsson, 1993, 1998, 2005b; Aldridge et al., 1993; Jeppsson et al., 1995; Jeppsson and Aldridge, 2000). Out of these, the Ireviken Event, Mulde Event, and Lau Event have been recognized globally (e.g., Märss, 1992; Jeppsson, 1993, 1998; Märss et al., 1998; Jeppsson and Aldridge, 2000; Calner et al., 2004a) and had faunal, sedimentary, and long lasting isotopic effects (Eriksson and Calner, 2005). A thorough review of the models, isotopic shifts, and repercussions of the events was presented in my unpublished licentiate thesis (Bremer, 2016). Fredholm (1989) compared some of the stratigraphical distribution of vertebrates on Gotland to the emerging patterns of conodont distribution observed by Jeppsson and speculated that low numbers of thelodont scales could be associated with two of these episodes. One of the isotopic events, the Lau Event, was the focus of a study by Eriksson et al. (2009). They investigated what impact this had on the vertebrate fauna on Gotland and concluded that the vertebrates were affected by the event in a similar fashion to the conodonts with step-wise disappearances of taxa. The thelodont Paralogania martinssoni Gross, 1967 disappeared at the onset of the event, while A. lindstroemi, And. hedei, and the thelodonts Phlebolepis elegans Pander, 1856 and Thelodus carinatus Pander, 1856 go before the top of När Formation. They are joined by the acanthodian Nos- tolepis striata Pander, 1856, while another acanthodian Gomphonchus sandelensis Pander, 1856 was found in a sample also from lowermost Eke For- mation. Thelodus parvidens Agassiz, 1839 ranges through the När Formation and was recovered in a sample from lower Eke Formation but was lacking in three samples from the upper part. However, N. striata, G. sandelensis, and Th. parvidens re-appear in later strata of the Burgsvik Formation. The samples of uppermost När Formation (middle of Lau Event) see the first appearance of the thelodont Lanarkia horrida Traquair, 1898 but no more remains of it were reported above lower Eke Formation, while another thelodont, Paralo- gania ludlowiensis Gross, 1967 makes its first appearance in the top of Eke Formation and also occurs in younger strata (Eriksson et al., 2009). The post- event recovery was rapid according to Eriksson et al. (2009), with the return of many taxa as well as the addition of the acanthodian Poracanthodes porosus Brotzen, 1934, the anaspids Septentrionia mucronata Blom et al., 2002 and Tahulalepis elongituberculata Blom et al., 2002, and the thelodont Thelo- dus sculptilis Gross, 1967. The extinction of And. hedei was pinpointed before the top of Botvide Member (När Formation). However, fragmentary scales and potentially other remains of this taxon was found in a sample from

22 Uddvide (upper Burgsvik Sandstone Member) described in Paper III, although a potential re-deposition of these scales cannot be excluded. On a larger scale, the event saw a shift from gnathostome-dominated faunas before the event to a thelodont-dominated fauna in the aftermath according to Eriksson et al. (2009). This faunal turnover among vertebrates had previously been identified by Märss (1992), which she named the And. hedei Event, but it seems to be represented in more detail in the Gotland succession (see Paper III). In later years, investigations in other parts of the Baltic Basin, where the sedimentary setting was less sensitive to changes in sea-level, have failed to identify the extinction patterns observed on Gotland (Jarochowska and Munnecke, 2016). Instead, they suggested that the patterns in the Gotland conodont fauna may partly be the result of shifting biofacies parallel with the changing sea-level and unconformity bias. Changes in facies has been suggested as an impeding factor for our understanding about early osteostracan evolution and distribution (Sansom, 2008; Märss et al., 2014). Following this, Sansom et al. (2015) investigated the impact of sea-level driven facies change on the fossil record of heterostracans, thelodonts, galeaspids, osteostracans, and gnathostomes. They concluded that the fossil record of galeaspids, osteostracans, and to some extent heterostracans are more affected by these changes compared to the less ecologically restricted thelodonts and gnathostomes, and the possible origination dates of the former extend much further back than their first appearances in the fossil record. Sansom et al. (2015) therefore suggested that a micropaleontological approach in addition to extended sampling in Upper Ordovician to lower Si- lurian rocks might help us understand the early evolution of these stem gnathostomes. This was also one of the main purposes of this PhD-project.

2.1. Previous studies Several studies of environmental preferences among early vertebrates have been done before. Märss and Einasto (1978) compared faunas from different facies belts of the Estonian and north Latvian deposits. They concluded that osteostracans and anaspids were most common in lagoonal and shoal belts, heterostracans mostly inhabited the open platform and the distal shelf, thelodonts commonly occurred in all facies belts, and acanthodians were most common in the shoal and open platform in the Silurian (Fig. 6). Furthermore, An- dreolepis and another stem osteichthyan, Lophosteus (Botella et al., 2007), mostly occurred in shoal and open shelf facies (Fig. 6). Differences among the widespread thelodonts were indicated in a comprehensive review of their occurrences by Turner (1999). She described thelodont communities of different ages and identified some large-scale distributional patterns, as well as differences in distributions pertaining to differing environments.

Figure 6. Average number of scales (left) in samples from different facies (top) of osteostracans (a), anaspids (b), heterostracans (c), thelodonts (d), acanthodians (e) and stem osteichthyans (f) as presented by Märss and Einasto (1978) in Wenlock (yellow), Ludlow (red), and Pridoli (blue) strata. Modified from Turner (1999) and based on Märss and Einasto (1978).

Märss and Einasto (1978) also noted that out of the two most widespread groups (thelodonts and acanthodians), thelodonts were more common than acanthodians during Wenlock and Ludlow, while the opposite was true in the Pridoli. The temporal and spatial distribution of Silurian vertebrates in Estonia and Latvia was studied in great detail by Märss (1986a), and in a later work she concluded that the early vertebrate assemblages are similar in a wide range of environments, from carbonate to siliciclastic deposits (Märss, 1989). Märss (1991) described early vertebrates as mainly living in the neritic belt but also in more offshore pelagic zones, thereby confirming previous observations.

24 Their presence among reefs was supported by vertebrate remains being found in sedimentary traps such as coral lacunae, but they appeared to not have inhabited “mature” bioherms or extreme environments rich in cyanobacteria. Some distributional patterns of vertebrates related to depositional environments were also observed by Fredholm (1988a, 1988b, 1989, 1990) on Got- land. The heterostracan A. lindstroemi was suggested as a nektonic animal that preferred environments with muddy bottom waters (Fredholm, 1989). Re- mains of anaspids and osteostracans were only found at certain stratigraphical levels and, similar to the East Baltic sections, they tended to co-occur according to Fredholm (1989). A similar pattern was observed in Paper IV, where anaspid and osteostracan remains are abundant in the Burgsvik Sandstone and Burgsvik Oolite members in the Sudret area, but missing entirely in the potentially coeval sediments of Burgen outlier. The fragmentary state of these remains, together with the potential near-shore nature of the concerned deposits led Fredholm (1989) to suggest that these were washed out from nearby rivers. Fredholm (1989) concluded that most thelodonts on Gotland were bottom-dwelling taxa in near-shore, muddy and more quiet environments, but a few species evidently preferred higher-energy, reef-associated areas. Spiny scales of the taxa that inhabited muddier waters were tentatively compared to studies done on shark scales (Reif, 1982, 1985), and Fredholm (1989) subsequently suggested that the spines served a protective purpose against ectopar- asites. A switch from an acanthodian-dominated fauna in the middle part of the Hemse Group to a thelodont-dominated fauna toward the top of it was observed by Fredholm (1988a, 1988b), who explained this with a shift from deep-water to shallow-water facies. The acanthodians on Gotland were viewed as less dependent on bottom conditions, but seemed to have preferred higher water energy (Fredholm, 1988a).

2.2. A different approach Besides looking at what kinds of rocks preserve what kinds of vertebrates, researchers have also looked at the fossils themselves to investigate the ecology and swimming capabilities of Paleozoic vertebrates. Studies with this approach has seen an increase during the last decade, which can complement the investigations discussed in the previous section. Botella and Fariña (2008) investigated the flow patterns around the head-shield of a heterostracan and concluded that its shape was important both for generating lift and increasing ma- neuverability when the animal was swimming. The importance of flow patterns, both over the entire fish and on a smaller scale, were also pointed out by Fletcher et al. (2014) in a review of hydrodynamic adaptations among fossil fishes. They discussed the possible lifestyles and the presence of drag-reduc- ing adaptations among some Paleozoic groups such as thelodonts, acanthodians, and anaspids.

25 In an extensive study by Ferron and Botella (2017), the connection between the lifestyles of modern sharks and the morphology of their scales developed by Reif (1982, 1985) was used to investigate the paleoecology of thelodonts. This connection had been implied before (e.g., Fredholm, 1989; Reif, 1978; Märss et al., 2007), but Ferron and Botella (2017) quantified this by creating a large database of shark scale-morphologies and using statistical methods (morphometrics) to couple them with the lifestyles of the sharks. This effectively created a statistical framework which could then be used to infer the ecology of thelodonts based on their squamation. After analyzing the squamation patterns in a large number of both articulated and scale-based taxa, they concluded that thelodonts display a wide range of lifestyles and habitat preferences, but four main ecological groups were recognized: demersal thelodonts on hard substrates, shoaling or schooling thelodonts, slow-swimming thelodonts in open water, and pelagic swimming specialist thelodonts. A potential fifth type, demersal thelodonts on soft substrate, was also identified. The vast majority of thelodonts seemed to be demersal taxa living on hard substrates, and Ferron and Botella (2017) suggested that the micromeric squamation gave thelodonts more flexible bodies which presented an advantage in restricted environments over the less mobile “ostracoderms” covered in large plates. A surprising number also indicated shoaling or schooling patterns, sug- gesting that grouping behaviors were important also for thelodonts (Ferron and Botella, 2017).

26 3. Silurian vertebrate distribution on Gotland

A historical account of previous works reporting vertebrate remains from the Silurian of Gotland was given in the licentiate thesis (Bremer, 2016). Most of these reports were also summarized and included in Paper I. Figure 7 includes all of this data, plus the vertebrate remains reported in Papers II, III, and IV, as well as unpublished material (detailed below). The description of new material in Papers II-IV necessitated some taxonomical revisions of previous reports by Fredholm (1988a, 1990), Nilsson (2005), and Eriksson et al. (2009). However, despite extensive efforts to locate these collections, their wherea- bouts remain unknown, thereby limiting any taxonomical revisions. The oldest vertebrate remains on Gotland were reported by Fredholm (1990) from the Lower Visby Formation (unit b). These are represented by one Loganellia sp. scale (Fredholm, 1990, fig. 4A) and one Thelodus sp. scale (Fredholm, 1990, fig. 6B) from the Rönnklint 1 and Brusviken 1 localities, respectively (see Appendix for map with localities). Before this, scales described as Thelodontida gen. et sp. indet had been reported from Högklint For- mation (topostratigraphical unit c, below Pterygotus beds in Figure 7) from Vattenfallet section by Ørvig (1979, p. 249, fig. 73), which Turner (1999) pu- tatively considered as a waterworn trilobatiform scale or as belonging to the squamation of Thelodus laevis Pander, 1856. Martinsson (1967) reported a possible thelodont fragment from Svarven 1 (Svarvarhuk) in sediments that are now considered to belong to the upper Tofta Formation (Jeppsson, 2005a; Paper I). Märss (1989) also reported loganellid scales in the upper Tofta For- mation. A sample collected from lower Hangvar Formation during this project at Lixarve 1 (G14-12OB) contained a single “gomphonchid” acanthodian scale (Fig. 8), which is the earliest record of acanthodians on Gotland.

27 Figure 7. Taxonomical ranges of vertebrate taxa through the stratigraphy of Gotland with the identified events to the right. Stratigraphical column and the event intervals based on Jeppsson et al. (2006). Gray lines indicate uncertain taxonomical designations. Distribution of taxa and references are given in the text. Figure 8. Gomphonchus sp. scale (PMU 23112), the oldest acanthodian remains found on Gotland from Lixarve 1 (G14-12OB) in lower Hangvar For- mation. Photographed at the PMU using a Nikon DS-Fi1 mounted on a Leica MZ95 stereomicro- scope and stacked image created with the software CombineZP (Hadley, 2005). Scale bar equals 0.5 mm.

Besides the sporadic occurrences listed above, few vertebrate remains have been found in the lower part of the Gotland stratigraphy despite extensive sampling (Fredholm, 1990). Vertebrate fragments remain scarce also in the lower Slite Group with only Loganellia grossi Fredholm 1990 appearing in broskogs formation (Rhipidium tenuistriatum beds) at Oivide 1, Sojvide 1, and Valdarve 1. The first definitive record of Th. laevis was reported by Fredholm (1990) from the “Slite Marl” (middle Slite Group) at Slitebrottet 2. Both these taxa co-occur from the bottom of eskelhem formation to the top of the länna formation. The thelodonts are joined by two anaspid fragments (Fredholm, 1990: fig. 7K, L) and two osteostracan fragments (Fredholm, 1990: fig. 8D, E) in an especially vertebrate-rich sample from the top of eskelhem formation (Pentamerus gothlandicus Layer) at Slitebrottet 1. The former represents the earliest known occurrence of anaspids on Gotland and was referred to as Birkeniida sp. D by Fredholm (1990), but later reassigned as Pterygolepis nitida Kiær, 1911 by Blom et al. (2002). The osteostracan fragments were referred to as Procephalaspis? by Fredholm (1990: fig. 8D-E), but the figured specimens are more reminiscent of Ateleaspis sp. cf. A. tessellata (Märss et al., 2014) described from the lower Loganellia grossi Vertebrate Zone Märss et al., 1995 in the Maasi Beds of Jaagarahu Regional Stage (RS), Saaremaa island, Estonia. Two more samples described from the P. gothlandicus Layer at Hide 1 in Fredholm (1990) collectively contained a similar fauna, but the anaspids were only described as Birkeniida sp. Another anaspid fragment was also described together with Log. grossi and Th. laevis in a sample from Hide Fiskeläge 1 in the overlying länna formation. Based on differences in scale morphologies and different stratigraphical occurrences in Estonia, Märss (1996) separated Loganellia einari Märss, 1996 from the Log. grossi scale set. She did, however, report both forms in the same sample from the Samsungs 1 locality on Gotland and explained their co-occurrence here by a stratigraphical overlap not present in the East Baltic. The full range of Log. einari in the Gotland stratigraphy is currently unknown and until the material collected by Fredholm is tracked down it is unclear whether some of the scales referred to Log. grossi in those samples are more similar to Log. einari or not.

29 A sample from the top of länna formation at Skenalden 2 contained the youngest scales of Log. grossi, as well as Th. laevis and two osteostracan fragments described as Oeselaspis sp. (Fredholm, 1990: fig. 8G). These fragmentary remains share similarities with both Oeselaspis and Saaremaaspis (Märss et al., 2014). The following Fröjel Formation seems depauperate in vertebrate remains, except for four scales of Th. laevis from the Svarvare Member at Svarvare 3, and one Thelodus sp.? scale from a level corresponding to Gannarve Member in the Närborrningen 1 drill core (Fredholm, 1990). Both Log. grossi and Log. einari disappear somewhere in this interval and do not return in the Gotland sequence (Fredholm, 1990). Only two scales of Th. laevis were recovered from a sample at Bara 1 in lowermost Halla Formation (Fredholm, 1990). This taxon is joined by the first appearance of Paralogania martinssoni on Gotland in a level corresponding to lower parts of the Mulde Brick Clay Member. These taxa co-occur through- out Halla Formation and into the lower part of the following Klinteberg For- mation (Fredholm, 1990; Paper II). The middle part of Halla Formation is rather poor in vertebrate remains with only Th. laevis and Par. martinssoni occurring in samples from Robbjäns kvarn 3, Värsände 1, Stora Vikare 1, and Mulde Tegelbruk 1 (Fredholm, 1990). One section, divided into the two localities Möllbos 1 and 2, in the same interval proved extremely productive and a series of samples from this section were described in an unpublished manuscript by Fredholm. The samples contained thousands of Th. laevis scales, but less than a hundred Par. martinssoni scales. These two taxa were also joined by a spiny variant of Th. laevis seemingly unique to this section, although some of the Th. laevis scales described from the Gothemshammar section in Paper II also had spine-like extensions. Fredholm (1992) speculated that the development of these spines in Th. laevis was a response to environmental pressures and in the unpublished manuscript she discussed whether the low abundance of Par. martinssoni was the result of competition. The samples from Möllbos also contained remains of Birkeniida sp. D and Birkeniida sp. (Fredholm, 1990: fig. 7J, M-N). The former was revised as Rhyncholepis butriangula Blom et al., 2002 in conjunction with the establishment of this taxon, but no successful comparison has been made for the latter. Blom et al. (2002) also reported Schidiosteus mustelensis Pander, 1856 from Möllbos 1. One sample from Möllbos 1 contained a single osteostracan fragment which Fredholm (1990) referred to as Tremataspis sp., which was later referred to Tremataspis schmidti Rohon, 1892 by Märss et al. (2014) based on figured specimens from other localities (see below). New samples from the upper part of Halla Formation in the Gothemsham- mar section on eastern Gotland was described in Paper II, and previous reports from the same area were reviewed and partly revised in the process. In summary, this section has produced remains of the thelodonts Th. laevis, Par. martinssoni, Oeselia mosaica Märss, 2005, the anaspids Pt. nitida, Rh. butri-

30 angula, Rhyncholepis parvula Blom et al., 2002, Rytidolepis quenstedtii Pan- der, 1856, and the osteostracans Saaremaaspis mickwitzi Rohon, 1892, Thy- estes verrucosus Eichwald, 1854, Oeselaspis pustulata Patten, 1931, Tr. schmidti, Tremataspis milleri Patten, 1931, Meelaidaspis gennadii Märss et al., 2014. One fragment figured as Osteostraci gen. et sp. indet. (Paper II: fig. 8Q) may be referable to M. gennadii based on similarities to a special fragment figured in Märss et al. (2014: fig. 18H). Fredholm (1990) presented several samples from the upper Halla Formation on western Gotland, but these only produced a few Th. laevis and Par. martinssoni scales. One sample in Paper II comes from the lowermost Klinteberg Formation, just above the boundary to Halla Formation (following the suggestion of Calner and Jeppsson, 2003), and contains Th. laevis (a few of these scales have spiny extensions), Par. martinssoni, O. mosaica, Pt. nitida, Rh. parvula, R. quenstedtii, S. mickwitzi, and T. verrucosus. Samples in Fredholm (1990) from lower Klinteberg Formation at Gothemshammar only contained Th. laevis and Par. martinssoni scales. The Hällinge 1, Godrings 1 and 3, and Rågåkre 1 localities in central parts of Gotland, where lower Klinteberg Formation crops out, only produced scales of these two thelodonts as well. One sample from Gröndalen 1, on the small island Lilla Karlsö just west of Gotland, contained Th. laevis scales and fragments of Tremataspis sp. (Fredholm, 1990: fig. 8L) referred to Tr. schmidti by Märss et al. (2014). However, the precise placement of this island within the Gotland stratigraphy has proven difficult (Frykman, 1989). Only three samples from the upper part of Klinteberg Formation (one from Lilla Snögrinde 2 and two from Smiss 1) produced Th. laevis and Thelo- dus sp. scales (Fredholm, 1990). Following Klinteberg Formation is a relatively long interval through the lower half of Hemse Group with scarce reports of vertebrate remains. The exception is the so called Phlebolepis ornata layer in the middle of petes formation where thelodonts Phlebolepis ornata Märss 1986b, Thelodus sp., and Par. martinssoni occur (Fredholm, 1990). Possibly just above this level, Fredholm (1990) presented samples containing scales of Thelodus carinatus together with Par. martinssoni. In the following Etelhem Formation, a sample from Sigvalde 2 contained the thelodonts Th. carinatus and Phlebolepis elegans, anaspid fragments of Birkeniida sp. D (Fredholm, 1990: fig. 12C & D) referred to Sch. mustelensis and R. quenstedtii respectively by Blom et al. (2002), as well as osteostracan remains described as definite Dartmuthia gem- mifera Patten, 1931 by Fredholm (1989, not figured) and others figured as Tremataspididae sp. in Fredholm (1990: fig. 12A-B). The latter fragments are difficult to place taxonomically, but the flat external surface interrupted by large pore-like structures is reminiscent of Tremataspis mammillata Patten, 1931 from largely coeval layers of the East Baltic (Märss et al., 2014). It should be mentioned that the relation of the old Hemse units (used to place these samples stratigraphically in previous works) to the new subdivisions of the Gotland stratigraphy used in Figure 7 is unclear in some places.

31 The thelodonts Phl. elegans and Th. carinatus are joined by Thelodus parvidens in some samples in the Etelhem Formation, and all three taxa range through the remainder of the Hemse Group with rare and sporadic occurrences of Par. martinssoni in the När Formation and Loganellia cuneata Gross, 1947 or/and Lanarkia horrida toward the top (Fredholm, 1988a; Eriksson et al., 2009). Previous reports of Log. cuneata were revised as Lan. horrida by Nilsson (2005), but both taxa may be present (discussed in Paper III). Besides the mention of a Tremataspis sp. fragment in an undescribed sample from Botvide 1 (0.59-0.52 m below the Hemse/Eke boundary) in Fredholm (1989), no more anaspid or osteostracan remains were reported from the rest of Hemse Group by Fredholm (1988a, 1988b). Nilsson (2005) reported three indeterminable osteostracan fragments referred to as Gen. et sp. indet. A from Botvide 1 and 11 fragments described as Gen. et sp. indet. B from Nyan 2. These share similarities with fragments of Procephalaspis oeselensis Robertson, 1939 described from the East Baltic (Märss et al., 2014), but their affinities remain uncertain. It is worth noting that scales described as Thelodonti sp. A (Nilsson, 2005, fig. 13c) occur in several samples from the Botvide Member in the collection of Lennart Jeppsson at Naturhistoriska Riksmuseet (NRM) in Stockholm (pers. obs. 2017). These display morphologies reminiscent of loganellid thelodonts, but with the addition of a pair of thornlets on either side of the crown on some scales. Thirty similar scales from older parts of the När Formation were presented by Fredholm (1988a: fig. 4E, F) and described as “Logania” martinssoni, but according to Fredholm (1988a) the material was not enough for sufficient comparisons to older representatives of Paralogania martinssoni on Gotland, nor from other regions. Other scales described as ?”Logania” martinssoni by Fredholm (1988a: fig. 8G) from the Gogs 1 locality (När For- mation) are similar to scales described in Paper III (Fig. 19F-G) and Paper IV (Fig. 3M). These were referred to the palmatilobate scales of Paralogania? described by Märss (2006) from the Tahula Beds of the Kuressaare Stage in the East Baltic. När Formation in the top of Hemse Group also sees an unprecedented number of gnathostomes on Gotland with the addition of the acanthodians Nos- tolepis striata and Gomphonchus sandelensis, as well as Andreolepis hedei. The heterostracan Archegonaspis lindstroemi also appears at this level (Fredholm, 1988a, 1988b; Eriksson et al., 2009). However, most of these taxa, along with some of the thelodonts, disappear toward the top of the formation (see section 5) and no other heterostracan remains are known on Gotland outside this interval. Microremains described by Spjeldnaes (1950) came from the lower Eke Formation at Lau Backar and contained one N. striata scale, nine G. sandelensis scales, a number of Th. parvidens scales, as well as an unknown number of scales similar to both Log. cuneata (Spjeldnaes, 1950; pl. 1, fig. 3) and Lan.

32 horrida (Spjeldnaes, 1950: pl. 1, fig. 4). Fredholm (1989) presented preliminary picking results of samples from 31 localities with Eke Formation strata. One sample from Lau Backar 1 and Burgen 5 of lower Eke Formation only contained one Thelodus sp. and one Loganellia? sp. scale respectively. Two samples from Botvide 1 in lowermost Eke Formation presented in Eriksson et al. (2009) collectively produced six G. sandelensis scales, one Th. parvidens, one possible Lan. horrida, as well as one Paralogania sp. and one Thelodus sp. scale. Only one sample from middle Eke Formation produced a single scale of Th. parvidens (Eriksson et al., 2009). The upper Eke Formation is more productive, with around one hundred Par. ludlowiensis and Paralogania sp. scales, but only a few Th. parvidens scales and two osteostracan fragments of unknown affinities (Fredholm, 1989; Eriksson et al., 2009). The publications of Fredholm (1988a, 1988b, 1990) discussed above only treated vertebrate contents of samples from the lowermost stratigraphy on Gotland and up to the top of the Hemse Group. She did, however, sample the remaining part of the Gotland sequence and described some preliminary results in her PhD-thesis (Fredholm, 1989). Eriksson et al. (2009) presented the contents of three samples from Burgsvik Formation and Blom et al. (2002) described anaspids from the Burgsvik Sandstone Member. These earlier reports are discussed in Paper III, and more material from both the Burgsvik Sandstone and Burgsvik Oolite members is described, as well as one new sample from Kapellet 1 in the Burgen outlier. The samples from the sandstone and oolite collectively contained the thelodonts Th. parvidens, Th. trilobatus, Par. ludlowiensis, Log. cuneata, Lan. horrida; anaspids Septentrionia mucronata, Liivilepis curvata Blom et al., 2002, Tahulalepis elongituberculata; osteostracan Tahulapsis ordinata Märss et al., 2014; the acanthodians N. striata, G. sandelensis, as well as novel appearances of poracanthodids; three fragmentary scales and potentially two larger fragments of And. hedei. Samples from Burgen outlier produced Th. parvidens scales, one Th. trilobatus scale at Närshamn 2, as well as remains of N. striata, G. sandelensis, and poracanthodids. The most conspicuous difference between Burgen outlier and the Burgsvik Sandstone and Oolite members is the complete lack of anaspids and osteostracan remains in the former (Paper III). No samples from the lowermost part of Hamra Formation were studied by Fredholm (1989). Eriksson et al. (2009) described one sample from Ängvards 5 and one from Skradarve 1. The former only contained scales of N. striata and G. sandelensis, and the latter contained the thelodonts Th. parvidens, Th. sculptilis, Thelodus sp., Par. ludlowiensis, Paralogania sp., the acanthodian Por. porosus scales, as well as several indeterminable osteostracan fragments. The remainder of the sample from Skradarve 1 in Lennart Jeppsson’s collection at NRM was described in Paper IV, together with additional samples from Hamra Formation. Scales identified as Th. parvidens, Log. cuneata, Par. ludlowiensis, palmatilobate scales, two indeterminable osteostracan frag-

33 ments, and scales identified as G. sandelensis were found in this sample. Pa- ralogania ludlowiensis was only found in two more samples, also from lower Hamra Formation. Besides Skradarve 1, only one more sample which came from the upper Hamra Formation contained palmatilobate scales. A number of fragments similar to Tah. ordinata were recovered from a sample from Ho- burgen Lighthouse. Nostolepis striata and Gomphonchus sandelensis were present in several samples, but poracanthodids were only found at Skradarve 1 (two scales) and Hoburgen Lighthouse (four scales). No scales or fragments of anaspids were recovered in the samples treated in Paper IV, but Blom et al. (2002) reported remains of Sep. mucronata, L. curvata, and Hoburgilepis papillata Blom et al., 2002 from Hamra Formation. Fredholm (1989) described four samples from the Sundre Formation. One sample from the lower part at Juves 4 produced three Th. parvidens scales. Two samples were described from Västerbackar 1 with one containing an ischnacanthiform tooth and a Th. parvidens scale, while the other produced scales of G. sandelensis, N. striata, and Th. parvidens. An additional sample from Västerbackar 1 presented in Paper IV displayed a similar composition, but with the addition of one typical Log. cuneata scale. The final sample described from Sundre Formation by Fredholm (1989) came from Holmhällar 1 and contained Th. parvidens, Th. sculptilis, N. striata, and G. sandelensis scales. The remaining scales of the samples from Västerbackar 1 and Holmhällar 1 presented in Fredholm (1989) are described in Paper IV, together with additional material from Sundre Formation. The content of these samples agreed with previous findings. A single sample in Fredholm (1989) came from submarine strata off the south coast of Gotland and contained scales of N. striata, G. sandelensis, as well as one “Loganellia sp.” scale.

34 4. Gotland in a broader context

The natural place to start when comparing the fossil vertebrate faunas of Got- land to other parts of the Baltic Basin is with the well-described sections in Estonia and Latvia. There have been several correlations between the Gotland and East Baltic sections (see Bassett et al., 1989), but one of the most detailed work was done by Jeppsson et al. (1994) using conodont faunas. However, the biostratigraphy of both Gotland and the East Baltic has been refined and revised since then (Jeppsson et al., 2006; Märss and Männik, 2013). The correlated sections in Figures 8 and 9 are based on comparisons of the local conodont biozones on Gotland and in the East Baltic, except Uduvere Beds which is based on isotope stratigraphy and vertebrate faunas (see Paper IV). It does not, however, take into account potential local eco-stratigraphical trends among conodont taxa (Kaljo et al., 2015) and should only be regarded as ten- tative. References for the occurrences in the East Baltic section discussed below, and presented in Figures 9 and 10, are presented in the figure captions. Fredholm (1988a, 1988b, 1990) made some comparisons of the vertebrate faunas of Gotland to those described from the East Baltic sections (and other areas) and some of these were further commented on by Turner (1999). The older parts of the sections were discussed in Fredholm (1990) and both areas seem fairly barren in Llandovery and lower Wenlock strata, with only rare Loganellia sp. and Thelodus sp. that are difficult to determine taxonomically. Eventually, scales identified as Loganellia grossi, Loganellia einari, and Thelodus laevis start to appear in both sequences (Fig. 9). These thelodonts are joined by one or two taxa of anaspids and osteostracans in the eskelhem formation and the Maasi Beds of Jaani RS in Estonia (Blom et al., 2002; Märss et al., 2014). The reports of Gomphonchus sp. in the Llandovery and G. sp. and Nostolepis sp. in the early Wenlock of the East Baltic were previously unmatched in the Gotland sequence (Fredholm, 1990). Fredholm (1988b) speculated that their absence on Gotland might result from a lack of suitable environments during these times. However, the discovery of a “gomphonchid” scale from lower Hangvar Formation (section 2) confirms their earlier presence on Gotland, although the reason for their scarcity in lower Silurian rocks is still unclear.

35 Figure 9. Correlation of taxonomical ranges for thelodonts and anaspids in the Gotland (from Fig. 7) and East Baltic sections. Gray lines indicate taxonomical uncertainties, and red taxa are not represented on Gotland. Gotland stratigraphy based on Jeppsson et al. (2006) and East Baltic stratigraphy based on Märss and Männik (2013). East Baltic taxonomical ranges from Märss (1986, 1992, 1996, 2005, 2006), Märss et al. (2007), and Blom et al. (2002). Figure 10. Correlation of taxonomical ranges for osteostracans, acanthodians, heterostracans, and Andreolepis in the Gotland (from Fig. 7) and East Baltic sections. Gray lines indicate taxonomical uncertainties, and red taxa are not represented on Gotland. Gotland stratigraphy based on Jeppsson et al. (2006) and East Baltic stratigraphy based on Märss and Männik (2013). Ranges for East Baltic taxa from Märss et al. (2014) and Märss (1986). The low number of vertebrate remains in the Wenlock strata sees a drastic change in the Halla Formation on Gotland (comparable to the Rootsiküla RS) as a range of anaspid and osteostracan taxa appear in the fossil record (Fredholm, 1990; Paper II). The high diversity of osteostracans in the Wenlock and the presence of phylogenetically disparate groups suggest that the early evolutionary history of this group is largely obscured by facies bias (Sansom, 2008). Following a relatively barren interval through upper Klinteberg Formation and the lower Hemse Group, there is a distinct level with the thelodont Phlebolepis ornata together with Paralogania martinssoni on Gotland (Fredholm, 1988b). Phlebolepis ornata also occurs in the East Baltic, but is restricted to an interval of only a few decimeters in the upper Sauvere Beds of Paadla RS (Märss, 1986a, b), possibly corresponding to the upper part of the Phl. ornata layer on Gotland (Fredholm, 1988b). A coeval placement of the Sauvere Beds is also supported by conodont data. This interval is comparably more diverse in the East Baltic and also contains anaspid and osteostracan remains. The following Himmiste Beds produce even more anaspid and osteostracan taxa, as well as the first records of Gomphonchus sandelensis and Nostolepis striata (Fig. 10). The (possibly) younger Etelhem Formation on Gotland is slightly more diverse than the Phl. ornata layer, with three thelodont, two anaspid, and two osteostracan taxa, but the stratigraphical positions of some samples in the historical divisions of units c and d in the Hemse Group is unclear. Large parts of the remaining Hemse Group on Gotland (När For- mation) have a potential time-equivalent in the Uduvere Beds of upper Paadla RS in the East Baltic (Paper IV). The faunas of the two regions are similar to the underlying strata, consisting mostly of thelodonts and acanthodians, but with the addition of heterostracans. Fredholm (1988b) suggested that at least some of the fragmentary Archegonaspis sp. remains from the East Baltic (Märss, 1977) might be conspecific with Archegonaspis lindstroemi on Got- land. Another addition to the faunas in both sections is Andreolepis hedei. Following the discovery of And. hedei scales in the Burgsvik Formation (as well as in younger sediments elsewhere), Märss (1992, 2001) concluded that And. hedei occurs in one older faunal association with Phl. elegans, and one younger faunal association with Paralogania ludlowiensis or Th. sculptilis (Märss, 2001). A few osteostracan remains have been reported, but no anaspids occur in this interval in any of the sections. The youngest parts of the Gotland sequence, the Hamra and Sundre formations, have historically been correlated with Kuressaare RS in East Baltic (e.g., Jeppsson et al., 1994). More recently however, it has been suggested that the topmost Sundre Formation only reaches levels equivalent to lowermost Kuressaare RS (Kaljo and Martma, 2006; Kaljo et al., 2015), which is also supported by conodonts (Märss and Männik, 2013). Therefore, the Eke, Burgsvik, Hamra, and Sundre formations may represent a hiatus between Paadla and Kuressaare stages in the East Baltic. The lower part of Kuressaare

38 RS (Tahula Beds) displays a diverse thelodont fauna with several new taxa not found on Gotland, and an increasing amount of acanthodian remains in younger parts (Kaljo and Märss, 1991). In Paper V, vertebrates from sections in the Holy Cross Mountains, Poland, that have been correlated to Hamra and Sundre formations on Gotland were described as largely similar to those observed on Gotland (Paper IV). Märss (1997) reported vertebrate remains from drill-cores in Poland, but most of those faunas were dominated by acanthodians and are probably considerably younger than those on Gotland (Paper IV). The distribution of Silurian vertebrates and the succession of several distinct faunal assemblages in Lithuania was summarized in Karatajūtė-Talimaa and Brazauskas (1994). No detailed correlations have been performed, but some comparisons can be made. In general, Wenlock faunas display low di- versities with only Log. grossi and a few other indeterminable thelodonts (comparable to lower parts of the Gotland sequence). The following fauna from early Ludlow is similar, but sees the replacement of Loganellia with Pa- ralogania sp. In mid-Ludlow times, the fauna is slightly more diverse and contains Thelodus laevis, Th. parvidens, Par. martinssoni, Phlebolepis cf. elegans, Archegonaspis sp. as well as acanthodians, which is comparable to the upper parts of the Hemse Group on Gotland. The following interval, repre- senting late Ludlow time, is even more diverse with a fossil fauna composed of Th. parvidens, Th. sculptilis, Paralogania ludlowiensis, Archegonaspis sp., acanthodians, as well as a number of taxa that are probably younger than those on Gotland. The acanthodian succession was summarized by Valiukevičius (2006) and displays a low diversity only represented by “gomphonchid” and “nostolepid” forms through Wenlock and the first half of Ludlow, but several new taxa occur toward the end of Ludlow and early Pridoli. Vergoossen (1999a, 2002a, 2002b, 2003a, 2004) described vertebrate remains from several outcrops in the Öved Sandstone Formation in Scania, southern Sweden. The age of these strata has proven challenging to determine, with suggested ages ranging from late Ludlow or early Pridoli (Jeppsson and Laufeld, 1986; Siveter, 1989) to early Devonian (Mehlqvist et al., 2015). Ver- goossen used the vertebrate faunas to place the localities in the following relative, temporal order: sample SW 24 from Rasmåsa D was viewed as the oldest mainly because it contained Th. carinatus scales (Vergoossen, 2002c); a sample from Helvetesgraven was viewed as the second oldest based on the presence of And. hedei together with Th. sculptilis and Thelodus admirabilis Märss, 1982 (Vergoossen, 1999a); another sample from site D (Q 685) was viewed as slightly younger since it lacked any Th. carinatus scales and contained Th. sculptilis (Vergoossen, 2002c); samples from site E also contained Th. sculptilis, but were more diverse and shared poracanthodid acanthodians with potentially younger samples (Vergoossen, 2004); a similar age was proposed for samples from Klinta and Rinnebäcks bro that similarly only contained Th. sculptilis out of the zonal index fossils, but they produced fewer poracanthodids (Vergoossen, 2002b); the youngest Ramsåsa assemblages

39 were those from site C (Vergoossen, 2003a) and site H (Vergoossen, 2002a) because they were dominated by Th. admirabilis and Th. sculptilis respectively (Vergoossen, 2004). The faunas from Scania are peculiar, especially the occurrence of And. hedei together with Th. sculptilis and Th. admirabilis (Vergoossen, 1999a). This occurrence was not included in the review of Andreolepis by Märss (2001), but Vergoossen (1999a) suggested it as the second place, besides the central Urals, where And. hedei and Th. sculptilis co-occurred. Vergoossen (2002d) did not identify his Andreolepis scales as the younger species Andre- olepis petri Märss, 2001 and argued that it could be coeval with the Burgsvik Formation on Gotland (Vergoossen, 1999a). In the works mentioned in the previous paragraph, there were several attempts to correlate the Scanian faunas to those on Gotland. This was complicated by the attempted comparison of oldest Ramsåsa fauna in the SW 24 sample (Vergoossen, 2002c) to a loose slab described by Fredholm (1989) as coming from lower Eke Formation. This slab has been argued by Eriksson et al. (2009) to have been transported and misplaced, probably deriving from younger layers, which is corroborated here. Nevertheless, Vergoossen generally viewed the faunas of the Öved Sandstone Formation as potentially transitional between those described from the Udu- vere Beds (Paadla RS) and Tahula Beds (Kuressaare RS) in the East Baltic. Therefore, they may potentially represent the same hiatus as proposed for the youngest parts of the Gotland sequence. The compositions and richness of the Scanian samples are intriguing and could be an important source of infor- mation about the late Ludlow faunal shifts discussed in Papers III and IV, and in section 4. Sedimentary rocks of Silurian age in Ringerike, Norway, have produced both articulated specimens and microremains of vertebrates (Turner and Turner, 1974; Davies et al., 2005). As pointed out by Märss (1989), many of these are the same as, or similar to, the taxa in the Rootsiküla RS of the East Baltic and the Halla and Klinteberg formations on Gotland. Four species of articulated anaspids have been described from the Rudstangen locality, two of which (Pterygolepis nitida and Rhyncholepis parvula) have been described from Gotland (Blom et al., 2002; Paper II). The vertebrate microfossils have been reported from separate calcarenitic lenses or thin horizons (Turner and Turner, 1974) in clastic sediments interpreted as formed in a coastal plain environment (Davies et al., 2005). The latter authors viewed the calcarenitic lenses as stratigraphically uncontrolled and claimed that they contained vertebrate remains that indicated widely different ages. Vertebrate remains from other localities in Norway are briefly discussed in Turner (1999), but detailed comparisons cannot be made until this material is better described.

40 5. Conclusions and future directions

The initial aim of this project was to compare the well-sampled sequence of Gotland to other parts of the Baltic Basin in order to create a more complete picture for investigations of the distribution of Paleozoic vertebrates during the Silurian and how the depositional environment affected this. Important questions were: how much of the patterns that we see (e.g., those coupled to isotopic events) are real and how much is caused by sampling bias? Subse- quently, the sampling-strategy for this project was to complement the material from previous studies. Paper I reflects the early stages of this work as it reviewed and digitized the data from all previous studies and put the samples in an updated stratigraphical framework. A crude division of facies based on Erlström et al. (2009) was also implemented. The licentiate thesis represents the other component of this ground-laying work, namely to establish a detailed geological and sedimentological context for vertebrate samples through the entire Gotland sequence. The realization that the material from previous studies would not be avail- able for study necessitated a different approach. The focus of the project shifted to describing newly collected (Paper II) and undescribed material from museum collections (Papers III and IV). These studies confirmed previous finds and provided confidence for revisions of some previous reports. The new samples also produced taxa not reported from Gotland before, as well as novel appearances of familiar groups within the stratigraphy (i.e. earliest records of acanthodians, section 2). This provides incentive for further sampling and investigation of the Gotland sequence. However, these studies also made it clear that some taxonomical issues will remain unresolved or uncertain until the original material is found or until the same localities are re-sampled. The taxonomical framework established for thelodonts (Märss et al., 2007), anaspids (Blom et al., 2002), and osteostracans (Märss et al., 2014) proved extremely useful for the identification of isolated scales and fragmentary material in this work. Papers III-V made it clear that there is a dire need of a similar framework for acanthodians. Detailed investigations of the acanthodian scale-types (including histology) from different parts of the Baltic Basin in a stratigraphical and geographical context, as well as comparing them to the variations within the squamation of articulated specimens could potentially help elucidate this. This would also aid in evaluating the rise in both abun-

41 dance and diversity of acanthodians through the late Silurian evident on Got- land and other areas of the Baltic Basin, as well as making it possible to compare them to similar studies performed on the Lithuanian sections (Valiukevičius, 2006). Paper V described vertebrate microremains from an otherwise poorly studied area (with regards to Silurian vertebrates) on the opposite side of the Baltic Basin relative to Gotland. A striking example of environmental preferences for a single thelodont taxon was observed, which largely agrees with the distribution of this thelodont on Gotland. Sequence- and chemostratigraphy was used in previous studies to correlate the Polish sections to the Hamra and Sun- dre formations on Gotland. The fossil vertebrate fauna was similar to that observed on Gotland, but it was difficult to place it within the vertebrate biozo- nation established for the East Baltic. Following the conclusions presented above a few future directions can be suggested:  sampling for conodont- and isotope-stratigraphy to investigate the stratigraphical position of some problematic areas (e.g., Burgen outlier). This, together with sequence stratigraphical investigations, could also aid in correlating the sequence outside of Gotland  sampling for microfacies analysis and further sedimentological investigations may help resolve uncertainties regarding the depositional environment and mode of formation for less investigated areas of Gotland  development of a detailed facies-framework for Gotland compatible with the one developed for the East Baltic (see Märss and Einasto, 1978)  re-sampling to elucidate some of the taxonomical concerns flagged in this work (if the old collections of Doris Fredholm and Eva Nilsson are not recovered) Environmental preferences have been identified among Silurian vertebrates before. The results presented in this thesis encourage further, detailed investigations of vertebrate occurrences on Gotland. The suggestions outlined above may also enable better comparisons of the distribution on Gotland to the East Baltic and other parts of the Baltic Basin to give a more complete picture, as well as testing for potential effects of climatic perturbations or biases in the fossil record of Silurian vertebrates. This could complement studies such as the ones presented in section 4.2 to give a more complete picture of the ecology of early vertebrates during the Silurian, and perhaps help elucidate some of their cryptic history highlighted in previous works.

42 6. Svensk sammanfattning

Introduktion Idag är ryggradsdjuren en extremt framgångsrik djurgrupp som lever i alla möjliga miljöer från djupa hav till höga berg och från pol till pol. Likt de flesta djurgrupper kan ryggradsdjuren (inklusive oss själva) spåra sitt ursprung till- baka till den kambriska explosionen, då de första fossila lämningarna börjar dyka upp i ungefär 543–510 miljoner år gamla sedimentära bergarter. De enda käklösa ryggradsdjur som lever idag är ett fåtal arter pirålar och nejonögon, som tillsammans troligen bildar en monofyletisk grupp (rundmunnar) och ut- gör systergruppen och en parallell utvecklingslinje till alla käkförsedda ryggradsdjur. De käkförsedda ryggradsdjuren var vanliga redan under tidsperioden devon för 419–359 miljoner år sedan (mås), även om de vid denna tid fram- förallt utgjordes av olika typer av fiskar eftersom de ännu inte hade invaderat land. Innan devon, under ordovicium (485–444 mås) och silur (444–419 mås), dominerades dock fiskfaunorna av flera grupper av så kallade pansarrundmun- nar, det vill säga käklösa fiskar som oftast var täckta av hudbenplattor eller fjäll. Forskare har börjat intressera sig alltmer för perioderna silur och ordovicium för att undersöka de tidiga fiskarnas utveckling. Kroppsfossil av käk- försedda fiskar från ordovicium och silur är ovanliga och de flesta spår består av fragment eller isolerade fjäll vars förekomster tycks vara begränsade till specifika levnadsmiljöer som de tidiga fiskarna föredrog. En bättre förståelse för miljöpreferenser hos de tidiga ryggradsdjuren kan därför bidra till en ökad förståelse för deras tidiga evolution. Gotland och merparten av södra Sverige var under silur beläget nära ekva- torn och täckt av ett varmt och grunt inlandshav som kallas för den Baltiska bassängen. I detta hav avsattes sediment som sedermera bildade den berg- grund, framförallt kalksten (ofta i form av fossila revkroppar), som utgör Got- land idag. Det levde även en mängd olika djurarter bland dessa rev, inklusive både käklösa och tidiga käkförsedda fiskar, vars fossila lämningar man nu kan hitta över hela ön. Den lagrade berggrunden på Gotland har en sammanlagd tjocklek på 500-750 m och motsvarar ca 10 miljoner år. Lagren har delats in i flera formationer av olika åldrar som generellt även visar en tydlig gradient i avsättningsmiljön från grundare vatten i nordost till djupare i sydväst. De första fossilen av fiskar från Gotland rapporterades redan 1861, men den mest omfattande kartläggningen av dessa utfördes under slutet av 1900- talet av Doris Fredholm. Majoriteten av de fossil som har rapporterats utgörs

43 dock av isolerade fjäll eller fragment. Av de tidiga fiskgrupper som levde under silur har heterostracer (pansarpirålar), anaspider, thelodonter, osteostracer (pansarnejonögon), acanthoder (taggpansarhajar), samt Andreolepis hedei hittats på Gotland. De två sistnämnda är stam-broskfiskar respektive stam-ben- fiskar, medan övriga tillhör de s.k. pansarrundmunnarna som, namnet till trots, troligen alla är närmare besläktade med käkförsedda fiskar än de nu levande käklösa fiskarna. Tandlika fragment från ytterligare en utdöd djurgrupp, konodonter, är också vanligt förekommande. Det är dock oklart exakt var dessa djur hör hemma i livets träd, men de har varit extremt betydelsefulla för att etablera en detaljerad biostratigrafi på Gotland, vilket har varit till stor an- vändning i detta arbete. I det här arbetet sammanfattas tidigare rapporter i en uppdaterad kontext (artikel I) och en del av taxonomin revideras. Dessutom beskrivs nya fossil från olika delar av Gotlands stratigrafi (artikel II-IV), samt från samtida lager i centrala Polen (artikel V) som var beläget på andra sidan av den Baltiska bassängen.

Klimatet och miljöns påverkan på de tidiga fiskfaunorna Silurperioden efterföljer den Hirnantiska istiden som inträffade vid slutet av ordovicium och som är kopplad till ett av de fem stora massutdöendena i livets historia. Även silur verkade vara en instabil period, då ovanligt många isotop- förändringar av ovanligt stora magnituder har identifierats i berglager från silur. Dessa var troligtvis kopplade till klimatförändringar som i sin tur orsakade förändringar i havsnivåerna. På Gotland identifierades dessa förändringar, eller episoder, först bland konodonterna då de uppvisade periodvisa faunaför- ändringar, som senare även upptäcktes bland andra djurgrupper på Gotland och även globalt. En tidigare undersökning tittade närmare på en av dessa episoder bland fiskarna och drog slutsatsen att även fiskfaunorna påverkades. Förändringar i havsnivån ledde såklart även till förändringar i avsättnings- miljön i det grunda hav som täckte Gotland, vilket i sin tur påverkade fiskarnas livsmiljöer. En mängd studier har undersökt miljöpreferenser hos de tidiga fiskarna och det verkar som att thelodonter förekom i en mängd olika vatten- djup och marina miljöer, medan anaspider och osteostracer föredrog landnära och mer skyddade områden. Acanthoder levde både på djupt vatten och i strandnära miljöer mot slutet av silur, men innan dess verkar det som att de var betydligt mer restriktiva i sin utbredning. Heterostracer verkade vanligast i djupt vatten under silur, men blir vanligare i grundare vatten senare. Andra studier har visat att vår förståelse för de tidiga ryggradsdjurens evolution är begränsad mycket på grund av deras levnadsmiljöpreferenser. Syftet med det här projektet var därför att öka vår förståelse för de tidiga ryggradsdjurens utbredning i olika avsättningsmiljöer genom att inkorporera Gotlands fiskfau- nor i tidigare undersökningar, samt att jämföra dem med andra områden i den

44 Baltiska bassängen. Potentiellt kan detta komplettera ett växande fält där stat- istiska metoder används för att analysera de fossila fiskarnas hydrodynamiska egenskaper jämfört med djur som lever idag.

Tidiga fiskar på Gotland De äldsta delarna av Gotlands stratigrafi är relativt fattiga på fiskfossil och endast enstaka fjäll från thelodonter har hittats i Visby-, Högklint-, Tofta- och Hangvarformationerna, trots omfattande tidigare provtagning. Under detta projekt hittades dock ett acanthodfjäll i ett prov från nedre Hangvarformat- ionen, vilket är det äldsta av sitt slag på Gotland. Käkförsedda fiskar lyser annars med sin frånvaro på Gotland i ytterligare några miljoner år innan ett flertal plötsligt dyker upp på en och samma gång. Slitegruppen är också fattig på fiskfossil och förutom ett kort intervall med två thelodonter, en anaspid och en osteostrac, dröjer det till toppen till Halla- formationen innan fler fiskarter dyker upp på Gotland. Förutom åtminstone tre arter av thelodonter dyker minst fem olika anaspider och sex olika osteostracer upp. Flera av dessa beskrevs i artikel II, där även en del äldre fynd reviderades taxonomiskt. Fiskfossilen blir dock ovanliga nästan lika plötsligt igen vid basen av Klintebergformationen, och förblir ovanliga genom resten av Klintebergformationen och den nedre halvan av efterföljande Hemsegrup- pen. En distinkt nivå har dock identifierats i mitten av Hemsegruppen där en viss thelodontart, Phlebolepis ornata, förekommer endast i ett kort intervall tillsammans med ytterligare en thelodont innan den försvinner. Ganska snart därefter dyker dock den närbesläktade Phlebolepis elegans upp som får säll- skap av ytterligare två nya arter av thelodonter, samt ett par anaspider och osteostracer. De sistnämnda två grupperna blir dock kortvariga och lite senare får thelodonterna istället sällskap av de tidigare nämnda käkförsedda fiskarna (acanthoder och Andreolepis), samt den enda heterostracen som har hittats på Gotland. Två av fyra thelodonter och heterostracen förvinner vid toppen av Hemse- gruppen, medan de två andra thelodonterna bara försvinner tillfälligt i Eke- formationen och dyker upp igen i den efterföljande Burgsviksformationen. Acanthoderna försvinner också bara tillfälligt och även fossil av Andreolepis hedei (som förmodades ha dött ut under en av isotop-episoderna som nämndes tidigare) har hittats i Burgsviksformationen (artikel III). Det är dock oklart om de senare fragmenten har blivit omarbetade från äldre sediment eller inte. En ny thelodont dyker upp i toppen av Ekeformationen och är tillsammans med en osteostrac extremt vanlig i Burgsviksformationen (artikel III), framförallt i sandstenen och ooliten där det även förekommer fyra arter av anaspider. Dess- utom får de två acanthodgrupperna som hittills varit representerade på Gotland sällskap av ytterligare en variant.

45 I de yngsta delarna av Gotlands stratigrafi försvinner många av de fiskar som var vanliga i Burgsvikformationen, men thelodonten Thelodus sculptilis, som ofta ses som ett viktigt indexfossil, blir allt vanligare (artikel IV).

Gotland i ett större perspektiv Det bästa stället att jämföra Gotlands fossila fiskfaunor med är de väl under- sökta och till stora delar jämngamla lagren i östra Baltikum (framförallt Est- land och Lettland), vilket även till viss del gjordes i de tidigare studierna på Gotland. Det verkar som att de östbaltiska sektionerna har ett flertal luckor (hiatus) på grund av avbrott i sedimentationen, luckor som Gotland möjligtvis kan fylla i. Dessutom kan avsättningsmiljöerna i Estland komplettera de som finns bevarade på Gotland. I stort stämmer de två geografiskt separerade sektionerna väl överens, med enstaka fossila fynd i de äldre delarna (framförallt thelodontfjäll och sällsynta acanthodfjäll) och en plötslig och hög taxonomisk mångfald i lager i östra Baltikum som är jämngamla med den fossilrika Hallaformationen på Gotland. Det distinkta, efterföljande lagret med Phl. ornata finns också representerat i den östbaltiska sektionen. De något yngre lagren på Gotland (toppen av Hem- segruppen) där acanthoder blir allt vanligare, samt där Andreolepis och heterostracen återfinns, verkar också ha en östbaltisk motsvarighet. Efter detta föl- jer dock ett hiatus som troligen motsvaras av Eke, Burgsvik-, Hamra- och Sundreformationerna på Gotland. Där Gotlands sektion tar slut verkar den öst- baltiska sektionen ta vid och flera nya arter dyker upp i dessa lager. Dessutom blir de käkförsedda fiskarna så småningom allt vanligare och får en allt större artrikedom. Sektioner i Litauen är inte riktigt lika väl korrelerade med Gotland, men ett storskaligt mönster som liknar det man ser både på Gotland och östra Baltikum kan observeras. De yngsta lagren på Gotland kan möjligtvis ha motsvarigheter både i Polen (artikel V) och i Skåne, vilket potentiellt kan ge ytterligare insik- ter i hur miljöberoende faunorna var och hur de förändrades under denna tid i och med den ökade betydelsen av käkförsedda fiskar. Det finns även likheter med sedimentära bergarter i Norge där fiskfaunor (däribland välbevarade anaspider) med liknande sammansättning som de på Gotland har återfunnits. Detaljerade jämförelser måste dock vänta tills de frag- mentariska och disartikulerade fiskfossilen från dessa sektioner har blivit bättre beskrivna och placerade i ett stabilt stratigrafiskt ramverk.

Slutsatser och framtida arbete Det ursprungliga syftet med det här arbetet var att bygga vidare på tidigare undersökningar om Gotlands fossila fiskfaunor, framförallt de som gjordes av Doris Fredholm under 80-talet. Tanken var att göra detaljerade undersök- ningar om de tidiga vertebraternas miljöpreferenser och hur detta påverkar vår förståelse för deras evolution. I artikel I sammanfattades merparten av tidigare

46 rapporter och de gamla proverna placerades i en uppdaterad stratigrafi. Den andra komponenten av detta grundläggande arbete, nämligen en detaljerad ge- nomgång av Gotlands geologi och stratigrafi, sammanfattades sedermera i en licentiatavhandling. Ungefär halvvägs in i projektet stod det dock klart att vi trots idoga försök inte skulle kunna hitta de prover som Doris Fredholm un- dersökte. Således ändrades projektets inriktning och artikel II-IV beskriver därför nya prover från Gotland, samt en del annat material från olika museer som fanns tillgängligt. De taxonomiska ramverk som har utvecklats för att inkludera även disarti- kulerat material från thelodonter, anaspider och osteostracer var till stor hjälp för undersökningarna i detta projekt. Det blev dock tydligt att ett motsvarande gediget och jämförbart ramverk saknas för acanthoder. Något liknande kom- mer troligtvis behövas innan ytterligare undersökningar om deras evolutionära mönster under silur kan genomföras tillfredställande, eftersom de mestadels är representerade av isolerade fjäll. Kanske kan detaljerade undersökningar av acanthodfjäll i ett kontrollerat geografiskt och stratigrafisk sammanhang, samt jämförelser med fjällvariation på artikulerade exemplar vara till hjälp. Den sista artikeln (artikel V) erbjöd en möjlighet att studera fiskfaunorna i en annars dåligt beskriven och till stora delar okänd del av den siluriska Bal- tiska bassängen. Dessa polska faunor liknade delvis de yngsta delarna på Got- land, vilka dessa sekvenser hade korrelerats med tidigare. Dessutom visade en specifik thelodontart en tydlig miljöpreferens då den enbart fanns i en av de jämngamla sekvenserna, nämligen i ett prov från väldigt grunt vatten som till stora delar liknar något äldre sediment på Gotland där samma art också är väldigt vanlig (artikel III). Det finns en del föreslagna riktningar som framtida undersökningar kan ta. Ytterligare provtagning för konodonter och isotopstratigrafi kan hjälpa till att tidsplacera vissa problematiska områden på Gotland. I samband med s.k. se- kvensstratigrafiska undersökningar kan detta dessutom underlätta korrele- ringar med områden utanför Gotland. Ytterligare sedimentologiska undersök- ningar på Gotland skulle också kunna förbättra vår förståelse för dess varie- rande och ibland svårtolkade avsättningsmiljöer. Ett etablerat facies-ramverk för Gotland som är kompatibelt med det som har utvecklats i tidigare arbeten om östra Baltikum skulle också underlätta framtida jämförelser. Sammantaget skulle detta kunna leda till en än mer komplett bild av de tidiga ryggradsdjurens utbredning under silur och förhoppningsvis leda till en ökad förståelse för de tidiga fiskarnas evolution.

47 7. Acknowledgements

First and foremost I want to thank my supervisors. Henning Blom for giving me the opportunity to do a PhD in the first place and for helping this project both scientifically and financially over the years. I want to thank Per Ahlberg for letting me be part of his team, it has been truly inspiring and the research being done around me never cease to amaze me. I will be forever grateful that Tiiu Märss agreed to supervise my work and guide me through the intricacies of fossil fish scales.

I also want to thank my opponent, Héctor Botella, and my committee for their participation in this process.

It is no overstatement when I say that this PhD would not have been possible without the help from Emilia Jarochowska, who so kindly invited me to be a part of her projects and patiently helped me during later works. Many thanks also goes out to collaborators in the department: Sophie Sanchez and Grzegorz Niedźwiedzki helped me venture outside Gotland, Helena Malmikumpu aided me with setting up a heavy liquid separation lab (and has helped me with all sorts of things). I also have to thank Daniel Snitting for helping me out with everything from computer issues to correcting typos. Miloš Bartol and Mi- chael Streng were always there to help me with the SEM and Jan Ove Eb- bestad always made me feel welcome to the museum collections and photog- raphy setup in Uppsala.

I need to thank Henrik Viberg, Rose-Marie Löfberg, Tove Broberg, Afsaneh Ahmadzadeh, and Karin Lundberg for their help with departmental and ad- ministrative matters over the years.

I want to thank Thomas Mörs at Naturhistoriska riksmuseet in Stockholm for kindly lending me material from the collections and for being so welcoming and helpful during my short visits to the museum. Linda Wickström helped me get access to the geographical data at SGU, which was crucial for the initial stages of this project. I need to thank the people in Lund for always being so welcoming and for letting me loan and work with material from there. And of course I need to thank Git Klintvik Ahlberg for her patient lab work.

48 I spent my first two years in Uppsala over at Geo, which really opened up my eyes to so many interesting aspects of paleontology. A special shout-out to Heda, Luka, Åsa, Ralf, Illiam, Linda, Aodhán, Oskar, Jenny, Emma, Haizhou, Steve, and all the other people over there who made my first couple of years in Uppsala really awesome.

After this, I moved over to EBC where I also had the pleasure of getting to know really great people. Thanks to Qingming, Lovisa, Maïtena, Susanne, Cecile, Donglei, Muhammed, Martin, Gizem, Jordi, Laura, Manolis, Erik, Va- leria, and all the other people in the department for the good times, the fikas, the crucial distractions, and the occasional recreational exercise (i.e. Table tennis). It is always risky to start naming people in case I forget someone, but you all know who you are, thank you!

I have to give a special thank you to Erik Elgh, who had a taste of the micros as he picked through some of my samples and found the scale from lower Hangvar!

And, of course, a big thank you to Anna. I would not have been able to do this without your help and support!

Last but not least I would like to thank my family and friends from back home and from afar for their belief in me. I wouldn’t have made it without you!

This project received funding from several sources. The Archimedes Foundation grant for the DoRa programme funded my research stay in Tallinn during the early stage of this project. Participation in conferences was made possible by grants from the Swe- dish IGCP committee, IGCP 591 student funding, Johanssons Inez travelling grant, and the research educational fund at the Department of Organismal Biology. Partici- pation in courses abroad was funded by Anna Maria Lundins stipendienämnd and visits to museums was sponsored by Liljewalchs travelling grant. The field work and collection of samples on Gotland during this PhD-project was granted by Helge Ax:son Johnsons stiftelse and Stiftelsen Lars Hiertas mine.

49 8. References

AGASSIZ, L. 1839. Fishes of the Old Red Sandstone. In: MURCHISSON, R. I. (ed.) The Silurian System. John Murray, London. ALDRIDGE, R. J., BRIGGS, D. E., SMITH, M. P., CLARKSON, E. & CLARK, N. 1993a. The anatomy of conodonts. Philosophical Transactions of the Royal Society B: Biological Sciences, 340, 405-421. ALDRIDGE, R. J., JEPPSSON, L. & DORNING, K. J. 1993b. Early Silurian oceanic episodes and events. Journal of the Geological Society, 150, 501-513. ANDERSON, P. S., FRIEDMAN, M., BRAZEAU, M. D. & RAYFIELD, E. J. 2011. Initial radiation of jaws demonstrated stability despite faunal and environmental change. Nature, 476, 206-209. BAARLI, B. G., JOHNSON, M. E. & ANTOSHKINA, A. I. 2003. Silurian stratigraphy and palaeogeography of Baltica. In: LANDING, E. & JOHNSON, M. E. (eds.) Silurian lands and Seas: Paleogeography outside of Laurentia. New York State Museum Bulletin. BASSETT, M. G., KALJO, D. & TELLER, L. 1989. The Baltic Region. In: HOLLAND, C. H. & BASSETT, M. G. (eds.) A global standard for the Silurian system. National Museum of Wales. BICKERT, T., PÄTZOLD, J., SAMTLEBEN, C. & MUNNECKE, A. 1997. Paleoenvironmental changes in the Silurian indicated by stable isotopes in brachiopod shells from Gotland, Sweden. Geochimica et cosmochimica Acta, 61, 2717-2730. BLIECK, A. & TURNER, S. 2003. Global Ordovician vertebrate biogeography. Palaeogeography, Palaeoclimatology, Palaeoecology, 195, 37-54. BLOM, H. 2012. New birkeniid anaspid from the Lower Devonian of Scotland and its phylogenetic implications. Palaeontology, 55, 641-652. BLOM, H. & MÄRSS, T. 2010. The interrelationships and evolutionary history of anaspids. In: ELLIOTT, D. K., MAISEY, J. G., XIAOBO, Y. & DESUI, M. (eds.) Morphology, Phylogeny and Paleobiogeography of Fossil Fishes. München: Verlag Dr. Friedrich Pfeil. BLOM, H., MÄRSS, T. & MILLER, C. G. 2002. Silurian and earliest Devonian birkeniid anaspids from the Northern Hemisphere. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 92, 263-323. BLOM, H., MÄRSS, T. & MILLER, C. G. 2003. A new birkeniid anaspid from the Upper Silurian of Skåne, south Sweden. GFF, 125, 57-61. BOTELLA, H., BLOM, H., DORKA, M., AHLBERG, P. E. & JANVIER, P. 2007. Jaws and teeth of the earliest bony fishes. Nature, 448, 583-586. BOTELLA, H. & FARIÑA, R. A. 2008. Flow pattern around the rigid cephalic shield of the Devonian agnathan Errivaspis waynensis (Pteraspidiformes: Heterostraci). Palaeontology, 51, 1141-1150. BRAZEAU, M. D. 2009. The braincase and jaws of a Devonian ‘acanthodian’ and modern gnathostome origins. Nature, 457, 305-308.

50 BRAZEAU, M. D. & FRIEDMAN, M. 2015. The origin and early phylogenetic history of jawed vertebrates. Nature, 520, 490-7. BREMER, O. 2016. Geology, stratigraphy, and fossil vertebrates of Gotland, Sweden: a review. Uppsala University. BROTZEN, F. 1934. Erster Nachweis von Unterdevon im Ostseegebiete durch Konglomeratgeschiebe mit Fischresten. II Teil (Paläontologie). Zeitschrift für Geschiebeforschung, 10, 1-65. BRUNTON, F. R., SMITH, L., DIXON, O. A., COPPER, P., NESTOR, H. & KERSHAW, S. 1998. Silurian reef episodes, changing seascapes, and paleobiogeography. In: LANDING, E. & JOHNSON, M. E. (eds.) Silurian Cycles: Linkages of Dynamic Stratigraphy with Atmospheric, Oceanic and Tectonic Changes. New York: New York State Museum Bulletin. BURROW, C. J. & RUDKIN, D. 2014. Oldest near-complete acanthodian: the first vertebrate from the Silurian Bertie Formation Konservat-Lagerstätte, Ontario. PloS one, 9, e104171. CALNER, M. 2008. Silurian global events–at the tipping point of climate change. In: E., A. M. T. (ed.) Mass extinctions. Springer-Verlag Berlin Heidelberg. CALNER, M. & JEPPSSON, L. 2003. Carbonate platform evolution and conodont stratigraphy during the middle Silurian Mulde Event, Gotland, Sweden. Geological Magazine, 140, 173-203. CALNER, M., JEPPSSON, L. & MUNNECKE, A. 2004a. The Silurian of Gotland— Part I: Review of the stratigraphic framework, event stratigraphy, and stable carbon and oxygen isotope development. Erlanger geologische Abhandlungen, Sonderband, 5, 113-131. CALNER, M., JEPPSSON, L. & MUNNECKE, A. 2004b. The Silurian of Gotland— part II: guide to the IGCP 503 field meeting 2004. Erlanger geologische Abhandlungen, Sonderband, 5, 133-51. CHEN, D., BLOM, H., SANCHEZ, S., TAFFOREAU, P. & AHLBERG, P. E. 2016. The stem osteichthyan Andreolepis and the origin of tooth replacement. Nature, 539, 237-241. CHEN, D., JANVIER, P., AHLBERG, P. E. & BLOM, H. 2012. Scale morphology and squamation of the Late Silurian osteichthyan Andreolepis from Gotland, Sweden. Historical Biology, 24, 411-423. CHEN, J.-Y., HUANG, D.-Y. & LI, C.-W. 1999. An early Cambrian craniate-like chordate. Nature, 402, 518-522. CLARK, D. L., SORENSON, J. K., LADD, A. N. & FREIHEIT, J. R. 1999. Probable microvertebrates, vertebrate-like fossils, and weird things from the Wisconsin Ordovician. Journal of Paleontology, 73, 1201-1209. COCKS, L. R. M. & TORSVIK, T. H. 2002. Earth geography from 500 to 400 million years ago: a faunal and palaeomagnetic review. Journal of the Geological Society, 159, 631-644. COHEN, K. M., FINNEY, S. C., GIBBARD, P. L. & FAN, J.-X. 2013; updated. The ICS International Chronostratigraphic Chart. Episodes 36, 199-204. CRAMER, B. D., JEPPSSON, L., MUNNECKE, A., SALTZMAN, R. & KERSHAW, S. A unified Silurian oceanographic model: past, present, and future. The dynamic Silurian Earth, subcommision on Silurian Stratigraphy, field meeting, 2005. CRAMER, B. D. & SALTZMAN, M. R. 2005. Sequestration of 12C in the deep ocean during the early Wenlock (Silurian) positive carbon isotope excursion. Palaeogeography, Palaeoclimatology, Palaeoecology, 219, 333-349. CRAMER, B. D. & SALTZMAN, M. R. 2007. Fluctuations in epeiric sea carbonate production during Silurian positive carbon isotope excursions: A review of

51 proposed paleoceanographic models. Palaeogeography, Palaeoclimatology, Palaeoecology, 245, 37-45. DAVIES, N. S., TURNER, P. & SANSOM, I. J. 2005. A revised stratigraphy for the Ringerike Group (Upper Silurian, Oslo Region). Norwegian Journal of Geology/Norsk Geologisk Forening, 85. DAVIS, S. P., FINARELLI, J. A. & COATES, M. I. 2012. Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes. Nature, 486, 247-50. DENISON, R. H. 1979. Acanthodii. Pt. 5, Stuttgart, Gustav Fischer Verlag. DINELEY, D. L. & METCALF, S. J. 1999. Fossil Fishes of Great Britain, Joint Nature Conservation Committee. DONOGHUE, P. C. & RUCKLIN, M. 2016. The ins and outs of the evolutionary origin of teeth. Evol Dev, 18, 19-30. DONOGHUE, P. C. J. 1998. Growth and patterning in the conodont skeleton. Philosophical Transactions of the Royal Society of London B: Biological Sciences, 353, 633-666. DONOGHUE, P. C. J. & PURNELL, M. A. 2009. Distinguishing heat from light in debate over controversial fossils. BioEssays, 31, 178-189. DONOGHUE, P. C. J., SANSOM, I. J. & DOWNS, J. P. 2006. Early evolution of vertebrate skeletal tissues and cellular interactions, and the canalization of skeletal development. J Exp Zool B Mol Dev Evol, 306, 278-94. DONOGHUE, P. C. J., SMITH, M. P. & SANSOM, I. J. 2003. The origin and early evolution of chordates: molecular clocks and the fossil record. In: DONOGHUE, P. C. & SMITH, M. P. (eds.) Telling the evolutionary time: molecular clocks and the fossil record. CRC Press Taylor & Francis Group. DZIK, J. 2015. Evolutionary roots of the conodonts with increased number of elements in the apparatus. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 106, 29-53. EICHWALD, E. V. 1854. Die Grauwackenschichten von Liv- und Esthland. Bull. Soc. Imp. Nat. Moscou, 27, 1-111. ERIKSSON, M. E. & CALNER, M. 2005. The dynamic Silurian earth, Sveriges geologiska undersökning. ERIKSSON, M. E., NILSSON, E. K. & JEPPSSON, L. 2009. Vertebrate extinctions and reorganizations during the Late Silurian Lau Event. Geology, 37, 739-742. ERLSTRÖM, M., PERSSON, L., SIVHED, U. & WICKSTRÖM, L. 2009. Beskrivning till regional berggrundskarta över Gotlands län. K 221. FERRON, H. G. & BOTELLA, H. 2017. Squamation and ecology of thelodonts. PLoS One, 12, e0172781. FLETCHER, T., ALTRINGHAM, J., PEAKALL, J., WIGNALL, P. & DORRELL, R. 2014. Hydrodynamics of fossil fishes. Proc Biol Sci, 281, 20140703. FLODÉN, T. 1980. Seismic stratigraphy and bedrock geology of the central Baltic. Almqvist & Wiksell. FREDHOLM, D. 1988a. Vertebrates in the Ludlovian Hemse Beds of Gotland, Sweden. GFF, 110, 157-179. FREDHOLM, D. 1988b. Vertebrate biostratigraphy of the Ludlovian Hemse Beds of Gotland, Sweden. GFF, 110, 237-253. FREDHOLM, D. 1989. Silurian vertebrates of Gotland, Sweden. . PhD thesis, Lund University. FREDHOLM, D. 1990. Agnathan vertebrates in the lower Silurian of Gotland, Sweden. GFF, 112, 61-80.

52 FREDHOLM, D. 1992. Possible environmental influences on the morphology of fossil vertebrate scales (abstract). In: MARK-KURIK, E. (ed.) Fossil Fishes as Living Animals. Tallinn: Academy of Sciences of Estonia. FRIEDMAN, M. & BRAZEAU, M. D. 2010. A reappraisal of the origin and basal radiation of the Osteichthyes. Journal of Vertebrate Paleontology, 30, 36-56. FRIEDMAN, M. & SALLAN, L. C. 2012. Five hundred million years of extinction and recovery: a phanerozoic survey of large-scale diversity patterns in fishes. Palaeontology, 55, 707-742. FRYKMAN, P. 1989. Carbonate ramp facies of the Klinteberg formation, Wenlock- Ludlow transition on Gotland, Sweden, Sveriges geologiska undersökning. GAGNIER, P.-Y. & BLIECK, A. R. 1986. First Ordovician vertebrate from South America. Geobios, 19, 629-634. GROSS, W. 1947. Die Agnathen und Acanthodier des obersilurischen Beyrichienkalks. Palaeontographica Abteilung A, 91-158. GROSS, W. 1967. Über Thelodontier-Schuppen. Palaeontographica Abteilung A, 1- 67. GROSS, W. 1968a. Fragliche Actinopterygier-schuppen aus dem Silur Gotlands. Lethaia, 1, 184-218. GROSS, W. 1968b. Die Agnathen-fauna der silurischen Halla-schichten Gotlands. GFF, 90, 369-400. HADLEY, A. 2005. CombineZ version 5.1. HAIRAPETIAN, V., ROELOFS, B. P. A., TRINAJSTIC, K. M. & TURNER, S. 2016. Famennian survivor turiniid thelodonts of North and East Gondwana. Geological Society, London, Special Publications, 423, 273-289. HEIMBERG, A. M., COWPER-SAL-LARI, R., SEMON, M., DONOGHUE, P. C. & PETERSON, K. J. 2010. microRNAs reveal the interrelationships of hagfish, lampreys, and gnathostomes and the nature of the ancestral vertebrate. Proc Natl Acad Sci U S A, 107, 19379-83. HOLLAND, N. D. & CHEN, J. 2001. Origin and early evolution of the vertebrates: New insights from advances in molecular biology, anatomy, and palaeontology. BioEssays, 23, 142-151. JANVIER, P. 1978. On the oldest known teleostome fish Andreolepis hedei Gross (Ludlow of Gotland), and the systematic position of the lophosteids. Eesti NSV Teaduste Akadeemia Toimetised, Geoloogia, 27, 88-95. JANVIER, P. 1996. Early vertebrates, Oxford University Press. JANVIER, P. 2015. Facts and fancies about early fossil chordates and vertebrates. Nature, 520, 483-9. JANVIER, P., ARSENAULT, M. & DESBIENS, S. 2004. Calcified cartilage in the paired fins of the osteostracan Escuminaspis laticeps (Traquair 1880), from the Late Devonian of Miguasha (Québec, Canada), with a consideration of the early evolution of the pectoral fin endoskeleton in vertebrates. Journal of Vertebrate Paleontology, 24, 773-779. JANVIER, P. & BLIECK, A. 1993. The Silurian-Devonian agnathan biostratigraphy of the Old Red continent. Palaeozoic Vertebrate Biostratigraphy and Biogeography. Belhaven Press, London, 67-86. JAROCHOWSKA, E. & MUNNECKE, A. 2016. Late Wenlock carbon isotope excursions and associated conodont fauna in the Podlasie Depression, eastern Poland: a not-so-big crisis? Geological Journal, 51, 683-703. JEPPSSON, L. 1983. Silurian conodont faunas from Gotland. Fossils and Strata, 15, 121-144.

53 JEPPSSON, L. 1984. Sudden appearances of Silurian conodont lineages: Provincialism or special biofacies? Geological Society of America Special Papers, 196, 103-112. JEPPSSON, L. 1987. Lithological and conodont distributional evidence for episodes of anomalous oceanic conditions during the Silurian, Department of Geology, University of Lund. JEPPSSON, L. 1990. An oceanic model for lithological and faunal changes tested on the Silurian record. Journal of the Geological Society, 147, 663-674. JEPPSSON, L. 1993. Silurian events: the theory and the conodonts. Proceedings of the Estonian Academy of Sciences, 42, 23-27. JEPPSSON, L. 1997. The anatomy of the mid-silurian Ireviken Event. In: BRETT, C. E. & BAIRD, G. C. (eds.) Paleontological events: Stratigraphic, ecological, and evolutionary implications. New York: Columbia University Press. JEPPSSON, L. 1998. Silurian oceanic events: summary of general characteristics. In: LANDING, E. & JOHNSON, M. E. (eds.) Silurian cycles: Linkages of dynamic stratigraphy with atmospheric, oceanic and tectonic changes. New York: New York State Museum Bulletin. JEPPSSON, L. 2005a. A revision of the Early Wenlock stratigraphy of Gotland. Sveriges Geologiska Undersökning Rapport, 20-21. JEPPSSON, L. 2005b. A second Wenlock secundo-secundo event. GFF, 127, 49. JEPPSSON, L., ALDRIDGE, R. & DORNING, K. 1995. Wenlock (Silurian) oceanic episodes and events. Journal of the Geological Society, 152, 487-498. JEPPSSON, L. & ALDRIDGE, R. J. 2000. Ludlow (late Silurian) oceanic episodes and events. Journal of the Geological Society, 157, 1137-1148. JEPPSSON, L. & ALDRIDGE, R. J. 2001. Discussion on Ludlow (late Silurian) oceanic episodes and events — reply. Journal of the Geological Society, London, 158, 732-733. JEPPSSON, L., ERIKSSON, M. E. & CALNER, M. 2006. A latest Llandovery to latest Ludlow high-resolution biostratigraphy based on the Silurian of Gotland— a summary. GFF, 128, 109-114. JEPPSSON, L. & LAUFELD, S. 1986. The late Silurian Öved-Ramsåsa Group in Skåne, southern Sweden. Sveriges Geologiska Undersökning: Avhandlingar och uppsatser, Ca. JEPPSSON, L., VIIRA, V. & MÄNNIK, P. 1994. Silurian conodont-based correlations between Gotland (Sweden) and Saaremaa (Estonia). Geological Magazine, 131, 201-218. KALJO, D., EINASTO, R., MARTMA, T., MÄRSS, T., NESTOR, V. & VIIRA, V. 2015. A bio-chemostratigraphical test of the synchroneity of biozones in the upper Silurian of Estonia and Latvia with some implications for practical stratigraphy. Estonian Journal of Earth Sciences, 64, 267. KALJO, D. & MÄRSS, T. 1991. Pattern of some Silurian bioevents. Historical Biology, 5, 145-152. KALJO, D. & MARTMA, T. 2006. Application of carbon isotope stratigraphy to dating the Baltic Silurian rocks. GFF, 128, 123-129. KARATAJŪTĖ-TALIMAA, V. 1995. The Mongolepidida: scale structure and systematic position. Geobios, 28, 35-37. KARATAJŪTĖ-TALIMAA, V. & BRAZAUSKAS, A. 1994. Distribution of vertebrates in the Silurian of Lithuania. Geologija, 17, 106-114. KARATAJŪTĖ-TALIMAA, V. & SMITH, M. M. 2004. Tesakoviaspis concentrica: microskeletal remains from the Ordovician and Lower Silurian of Siberia. In: ARRATIA, G., WILSON, M. V. H. & CLOUTIER, R. (eds.) Recent advances in the origin and early radiation of vertebrates. Munich: Verlag Dr. Friedrich Pfeil.

54 KEATING, J. N. & DONOGHUE, P. C. 2016. Histology and affinity of anaspids, and the early evolution of the vertebrate dermal skeleton. Proc Biol Sci, 283, 20152917. KEATING, J. N., MARQUART, C. L. & DONOGHUE, P. C. 2015. Histology of the heterostracan dermal skeleton: Insight into the origin of the vertebrate mineralised skeleton. J Morphol, 276, 657-80. KEATING, J. N., SANSOM, R. S. & PURNELL, M. A. 2012. A new osteostracan fauna from the Devonian of the Welsh Borderlands and observations on the taxonomy and growth of Osteostraci. Journal of Vertebrate Paleontology, 32, 1002-1017. KIÆR, J. 1911. A New Downtonian Fauna in the Sandstone Series of the Kristiania Area, a Preliminary Report, J. Dybwad. KIÆR, J. 1932. IV. Suborder Cyathaspida. In: HEINTZ, A. (ed.) The Downtonian and Devonian vertebrates of Spitsbergen. Skrifter om Svalbard og Ishavet. KLUG, C., KRÖGER, B., KIESSLING, W., MULLINS, G. L., SERVAIS, T., FRÝDA, J., KORN, D. & TURNER, S. 2010. The Devonian nekton revolution. Lethaia, 43, 465-477. LINDSTRÖM, G. 1895. On remains of a Cyathaspis from the Silurian strata of Gotland, Nordstedt. MAISEY, J. G., MILLER, R., PRADEL, A., DENTON, J. S. S., BRONSON, A. & JANVIER, P. 2017. Pectoral Morphology in Doliodus: Bridging the ‘Acanthodian’-Chondrichthyan Divide. American Museum Novitates, 3875, 1-15. MALLATT, J. & CHEN, J. Y. 2003. Fossil sister group of craniates: predicted and found. J Morphol, 258, 1-31. MANTEN, A. A. 1971. Silurian reefs of Gotland. Developments in sedimentology, 13, 539. MARSHALL, C. R. 2006. Explaining the Cambrian “Explosion” of Animals. Annual Review of Earth and Planetary Sciences, 34, 355-384. MÄRSS, T. 1977. The Upper Silurian cyathaspid Archegonaspis sp. A from the East Baltic. Eesti NSV. MÄRSS, T. 1982. Thelodus admirabilis n. sp. (Agnatha) from the Upper Silurian of the East Baltic. Eesti NSV Teaduste Akadeemia Toimetised, 31. Geoloogia, 3, 112-116. MÄRSS, T. 1986a. Silurian vertebrates of Estonia and west Latvia. Fossilia Baltica, 1, 104. MÄRSS, T. 1986b. Squamation of the thelodont agnathan Phlebolepis. Journal of Vertebrate Paleontology, 6, 1-11. MÄRSS, T. 1989. Vertebrates. In: HOLLAND, C. H. & BASSETT, M. G. (eds.) A global standard for the Silurian System. International Union of Geological Sciences. MÄRSS, T. 1991. Agnathans and fishes in shallow facies of the North-East Baltic Wenlock and Ludlow. Proceedings of the Estonian Academy of Sciences, 40, 89- 95. MÄRSS, T. 1992. Vertebrate history in the Late Silurian. Proceedings of the Estonian Academy of Sciences, Geology, 41, 204. MÄRSS, T. 1996. Loganellia (Thelodonti, Agnatha) from the Jaagarahu Stage, Wenlock, Estonia. Proceedings of the Estonian Academy of Sciences, Geology, 45, 189-202. MÄRSS, T. 1997. Vertebrates of the Pridoli and Silurian-Devonian boundary beds in Europe. Modern Geology, 21, 17-42.

55 MÄRSS, T. 2001. Andreolepis (Actinopterygii) in the upper Silurian of northern Eurasia. Proceedings of the Estonian Academy of Sciences, Geology, 50, 174- 189. MÄRSS, T. 2005. Thelodont Oeselia mosaica gen. et sp. nov. from the Wenlock and Ludlow of the East Baltic. Proceedings of the Estonian Academy of Sciences, Geology, 54, 181-190. MÄRSS, T. 2006. Thelodonts (Agnatha) from the basal beds of the Kuressaare Stage, Ludlow, Upper Silurian of Estonia. Proceedings of the Estonian Academy of Sciences, Geology, 55, 43-66. MÄRSS, T., AFANASSIEVA, O. & BLOM, H. 2014. Biodiversity of the Silurian osteostracans of the East Baltic. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 105, 73-148. MÄRSS, T., CALDWELL, M., GAGNIER, P., GOUJET, D., MÄNNIK, P., MARTMA, T. & WILSON, M. Distribution of Silurian and Lower Devonian vertebrate microremains and conodonts in the Baillie-Hamilton and Cornwallis Island sections, Canadian Arctic. Proceedings of the Estonian Academy of Sciences, Geology, 1998. Estonian Academy Publishers, 51-76. MÄRSS, T. & EINASTO, R. 1978. Distribution of vertebrates in deposits of various facies in the North Baltic Silurian. Eesti NSV Teaduste Akadeemia Toimetised Geoloogia, 27, 16-22. MÄRSS, T., FREDHOLM, D., TALIMAA, V., TURNER, S., JEPPSSON, L. & NOWLAN, G. 1995. Silurian vertebrate biozonal scheme. Geobios, 28, 369-372. MÄRSS, T. & KARATAJŪTĖ-TALIMAA, V. 2002. Ordovician and Lower Silurian thelodonts from Severnaya Zemlya Archipelago (Russia). Geodiversitas, 24, 381- 404. MÄRSS, T. & MÄNNIK, P. 2013. Revision of Silurian vertebrate biozones and their correlation with the conodont succession. Estonian Journal of Earth Sciences, 62, 181-204. MÄRSS, T., TURNER, S. & KARATAJŪTĖ-TALIMAA, V. 2007. Agnatha II- Thelodonti, Münich, Verlag Dr Friedrich Pfeil. MARTINSSON, A. 1958. The submarine morphology of thc Baltic Cambro-Silurian area. Bull. Geol. Inst. Univ. Upps. 38, 11, 35. MARTINSSON, A. 1967. The succession and correlation of ostracode faunas in the Silurian of Gotland. GFF, 89, 350-386. MEHLQVIST, K., STEEMANS, P. & VAJDA, V. 2015. First evidence of Devonian strata in Sweden — A palynological investigation of Övedskloster drillcores 1 and 2, Skåne, Sweden. Review of Palaeobotany and Palynology, 221, 144-159. MILLER, R. F., CLOUTIER, R. & TURNER, S. 2003. The oldest articulated chondrichthyan from the Early Devonian period. Nature, 425, 501-504. MORRIS, S. C. & CARON, J. B. 2014. A primitive fish from the Cambrian of North America. Nature, 512, 419-22. MUNNECKE, A., CALNER, M., HARPER, D. A. T. & SERVAIS, T. 2010. Ordovician and Silurian sea–water chemistry, sea level, and climate: A synopsis. Palaeogeography, Palaeoclimatology, Palaeoecology, 296, 389-413. MUNNECKE, A., SAMTLEBEN, C. & BICKERT, T. 2003. The Ireviken Event in the lower Silurian of Gotland, Sweden – relation to similar Palaeozoic and Proterozoic events. Palaeogeography, Palaeoclimatology, Palaeoecology, 195, 99-124. MUNNECKE, A., SAMTLEBEN, C., SERVAIS, T. & VACHARD, D. 1999. Sem- observation of calcareousmicro-and nannofossils incertae sedis from the Silurian of Gotland, Sweden: Preliminary results. Geobios, 32, 307-314.

56 MUNNECKE, A., SERVAIS, T. & VACHARD, D. 2000. A new family of calcareous microfossils from the Silurian of Gotland, Sweden. Palaeontology, 43, 1153- 1172. MURDOCK, D. J., DONG, X. P., REPETSKI, J. E., MARONE, F., STAMPANONI, M. & DONOGHUE, P. C. 2013. The origin of conodonts and of vertebrate mineralized skeletons. Nature, 502, 546-9. NILSSON, E. K. 2005. Extinctions and faunal turnovers of early vertebrates during the Late Silurian Lau Event, Gotland, Sweden. Master thesis, University of Lund. OISI, Y., OTA, K. G., FUJIMOTO, S. & KURATANI, S. 2013a. Development of the chondrocranium in hagfishes, with special reference to the early evolution of vertebrates. Zoolog Sci, 30, 944-61. OISI, Y., OTA, K. G., KURAKU, S., FUJIMOTO, S. & KURATANI, S. 2013b. Craniofacial development of hagfishes and the evolution of vertebrates. Nature, 493, 175-80. ØRVIG, T. 1979. Agnathan vertebrates. In: JAANUSSON, V., LAUFELD, S. & SKOGLUND, R. (eds.) Lower Wenlock faunal and floral dynamics - Vattenfallet section, Gotland. Sveriges Geologiska Undersökning: Rapporter och meddelanden, C 762. OTA, K. G., FUJIMOTO, S., OISI, Y. & KURATANI, S. 2011. Identification of vertebra-like elements and their possible differentiation from sclerotomes in the hagfish. Natur Communications, 2, 373. PANDER, C. H. 1856. Monographie der fossilen Fische des silurischen Systems des Russisch-Baltischen Gouvernements, Buchdr. der K. Akademie der wissenschaften. PATTEN, W. 1931. New ostracoderms from Oesel. Science, 73, 671-673. POPRAWA, P., ŠLIAUPA, S., STEPHENSON, R. & LAZAUSKIEN, J. 1999. Late Vendian–Early Palaeozoic tectonic evolution of the Baltic Basin: regional tectonic implications from subsidence analysis. Tectonophysics, 314, 219-239. QU, Q., BLOM, H., SANCHEZ, S. & AHLBERG, P. 2015. Three-dimensional virtual histology of Silurian osteostracan scales revealed by synchrotron radiation microtomography. J Morphol, 276, 873-88. QU, Q., SANCHEZ, S., BLOM, H., TAFFOREAU, P. & AHLBERG, P. E. 2013. Scales and tooth whorls of ancient fishes challenge distinction between external and oral 'teeth'. PLoS One, 8, e71890. QU, Q., SANCHEZ, S., ZHU, M., BLOM, H. & AHLBERG, P. E. 2016. The origin of novel features by changes in developmental mechanisms: ontogeny and three- dimensional microanatomy of polyodontode scales of two early osteichthyans. Biol Rev Camb Philos Soc, 92, 1189-1212. QU, Q., ZHU, M. & ZHAO, W.-J. 2010. Silurian atmospheric O2 changes and the early radiation of gnathostomes. Palaeoworld, 19, 146-159. RANDLE, E. & SANSOM, R. S. 2016. Exploring phylogenetic relationships of Pteraspidiformes heterostracans (stem-gnathostomes) using continuous and discrete characters. Journal of Systematic Palaeontology, 15, 583-599. RAUP, D. M. & SEPKOSKI, J. J. 1982. Mass extinctions in the marine fossil record. Science, 215, 1501-1503. REIF, W.-E. 1978. Types of morphogenesis of the dermal skeleton in fossil sharks. Paläontologische Zeitschrift, 52, 110-128. REIF, W.-E. 1982. Morphogenesis and function of the squamation in sharks. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen, 164, 172-183. REIF, W.-E. 1985. Squamation and ecology of sharks, Senckenbergische Naturforschende Gesellschaft.

57 REPETSKI, J. E. 1978. A fish from the Upper Cambrian of North America. Science, 200, 529-531. RITCHIE, A. 1967. Ateleaspis tessellata Traquair, a non-cornuate cephalaspid from the Upper Silurian of Scotland. Zoological Journal of the Linnean Society, 47, 69-81. RITCHIE, A. 1968. Phlebolepis elegans Pander, an Upper Silurian thelodont from Oesel, with remarks on the morphology of thelodonts. In: ØRVIG, T. (ed.) Current problems of lower vertebrate phylogeny: proceedings of the Fourth Nobel Symposium. Stockholm: Almqvist & Wiskell. ROBERTSON, G. M. 1939. An Upper Silurian vertebrate horizon, with description of a new species, Cephalaspis oeselensis. Transactions of the Kansas Academy of Science (1903-), 42, 357-363. ROHON, J. V. 1892. Die obersilurischen Fische von Oesel. I Teil. Thyestidae und Tremataspidae. Mém. Acad. Imp. Sci. St. Pétersbg., 38, 1-88. ROMER, A. S. 1964. The vertebrate body, Philadelphia, WB Saunders Company. SALTZMAN, M. R. 2001. Silurian δ13C stratigraphy: a view from North America. Geology, 29, 671-674. SAMTLEBEN, C., MUNNECKE, A. & BICKERT, T. 2000. Development of facies and C/O-isotopes in transects through the Ludlow of Gotland: evidence for global and local influences on a shallow-marine environment. Facies, 43, 1-38. SAMTLEBEN, C., MUNNECKE, A., BICKERT, T. & PÄTZOLD, J. 1996. The Silurian of Gotland (Sweden): facies interpretation based on stable isotopes in brachiopod shells. Geologische Rundschau, 85, 278-292. SANSOM, I. J., DAVIES, N. S., COATES, M. I., NICOLL, R. S. & RITCHIE, A. 2012. Chondrichthyan-like scales from the Middle Ordovician of Australia. Palaeontology, 55, 243-247. SANSOM, I. J., DONOGHUE, P. C. & ALBANESI, G. 2005a. Histology and affinity of the earliest armoured vertebrate. Biol Lett, 1, 446-9. SANSOM, I. J., MILLER, C. G., HEWARD, A., DAVIES, N. S., BOOTH, G. A., FORTEY, R. A. & PARIS, F. 2009. Ordovician Fish from the Arabian Peninsula. Palaeontology, 52, 337-342. SANSOM, I. J., SMITH, M. M. & SMITH, M. P. 1996. Scales of thelodont and shark- like fishes from the Ordovician of Colorado. Nature, 379, 628. SANSOM, I. J., SMITH, M. M. & SMITH, M. P. 2001. The Ordovician radiation of vertebrates. Major Events in Early Vertebrate Evolution, 156-171. SANSOM, I. J., WANG, N.-Z. & SMITH, M. 2005b. The histology and affinities of sinacanthid fishes: primitive gnathostomes from the Silurian of China. Zoological Journal of the Linnean Society, 144, 379-386. SANSOM, R. S. 2008. The origin and early evolution of the Osteostraci (Vertebrata): A phylogeny for the Thyestiida. Journal of Systematic Palaeontology, 6, 317-332. SANSOM, R. S. 2009. Phylogeny, classification and character polarity of the Osteostraci (Vertebrata). Journal of Systematic Palaeontology, 7, 95-115. SANSOM, R. S., FREEDMAN, K. I. M., GABBOTT, S. E., ALDRIDGE, R. J. & PURNELL, M. A. 2010a. Taphonomy and affinity of an enigmatic Silurian vertebrate, Jamoytius kerwoodi White. Palaeontology, 53, 1393-1409. SANSOM, R. S., GABBOTT, S. E. & PURNELL, M. A. 2010b. Non-random decay of chordate characters causes bias in fossil interpretation. Nature, 463, 797-800. SANSOM, R. S., GABBOTT, S. E. & PURNELL, M. A. 2011. Decay of vertebrate characters in hagfish and lamprey (Cyclostomata) and the implications for the vertebrate fossil record. Proc Biol Sci, 278, 1150-7.

58 SANSOM, R. S., RANDLE, E. & DONOGHUE, P. C. 2015. Discriminating signal from noise in the fossil record of early vertebrates reveals cryptic evolutionary history. Proc Biol Sci, 282, 20142245. SHU, D., MORRIS, S. C., HAN, J. & ZHANG, Z.-F. 2003. Head and backbone of the Early Cambrian vertebrate Haikouichthys. Nature, 421, 526. SHU, D. G., LUO, H. L., CONWAY MORRIS, S., ZHANG, X. L., HU, S. X., CHEN, L., HAN, J., ZHU, M., LI, Y. & CHEN, L. Z. 1999. Lower Cambrian vertebrates from south China. Nature, 402, 42-46. SIVETER, D. J. 1989. Ostracodes. In: HOLLAND, C. H. & BASSETT, M. G. (eds.) A Global Standard for the Silurian System. Cardiff: National Museum of Wales. SMITH, M. M. & SANSOM, I. J. 1997. Exoskeletal micro-remains of an Ordovician fish from the Harding Sandstone of Colorado. Palaeontology, 40, 645-658. SMITH, M. P., SANSOM, I. J. & REPETSKI, J. E. 1996. Histology of the first fish. Nature, 380, 702. SOEHN, K. L., MÄRSS, T., CALDWELL, M. W. & WILSON, M. V. H. 2001. New and biostratigraphically useful thelodonts from the Silurian of the Mackenzie Mountains, Northwest Territories, Canada. Journal of Vertebrate Paleontology, 21, 651-659. SOEHN, K. L. & WILSON, M. V. 1990. A complete, articulated heterostracan from Wenlockian (Silurian) beds of the Delorme Group, Mackenzie Mountains, Northwest Territories, Canada. Journal of Vertebrate Paleontology, 10, 405-419. SPJELDNAES, N. 1950. On some vertebrate fossils from Gotland with some comments on the stratigraphy, Almqvist & Wiksell. SZANIAWSKI, H. 1982. Chaetognath grasping spines recognized among Cambrian protoconodonts. Journal of Paleontology, 806-810. TALENT, J. A., MAWSON, R., ANDREW, A. S., HAMILTON, P. J. & WHITFORD, D. J. 1993. Middle Palaeozoic extinction events: faunal and isotopic data. Palaeogeography, Palaeoclimatology, Palaeoecology, 104, 139- 152. TRAQUAIR, R. H. 1898. Report on fossil fishes. Summary of Progress of the Geological Survey of the United Kingdom for 1897, 72-76. TURNER, P. & TURNER, S. 1974. Thelodonts from the upper Silurian of Ringerike, Norway. Norsk Geologisk Tidsskrift, 54, 183-192. TURNER, S. 1999. Early Silurian to Early Devonian thelodont assemblages and their possible ecological significance. In: BOUCOT, A. J. & LAWSON, J. D. (eds.) Paleocommunities: a case study from the Silurian and Lower Devonian. Cambridge University Press. TURNER, S., BLIECK, A. & NOWLAN, G. S. 2004. Vertebrates (agnathans and gnathostomes). In: WEBBY, B. D., PARIS, F., DROSER, M. L. & PERCIVAL, I. G. (eds.) The Great Ordovician Biodiversification Event. New York: Columbia University Press. TURNER, S., BURROW, C. J., SCHULTZE, H.-P., BLIECK, A., REIF, W.-E., REXROAD, C. B., BULTYNCK, P. & NOWLAN, G. S. 2010. False teeth: conodont-vertebrate phylogenetic relationships revisited. Geodiversitas, 32, 545- 594. VALIUKEVIČIUS, J. 2006. Event pattern in the development of Siluro-Devonian acanthodians of Lithuania. Neues Jahrbuch fur Geologie und Palaontologie- Monatshefte, 321-343. VERGOOSSEN, J. 1999a. Late Silurian fish microfossils from Helvetesgraven, Skåne (southern Sweden)(I). Geologie en Mijnbouw, 78, 267-280.

59 VERGOOSSEN, J. 2002a. Late Silurian fish microfossils from Ramsåsa, locality H, Scania, south Sweden, with some remarks on the body zonation scheme used in thelodont studies. Scripta Geologica, 123, 41-69. VERGOOSSEN, J. 2002b. Late Silurian fish microfossils from Klinta and Rinnebäcks Bro (Scania, south Sweden), with remarks on the morphology of Nostolepis striata trunk scales. Scripta Geologica, 123, 71-92. VERGOOSSEN, J. 2002c. Late Silurian fish microfossils from Ramsåsa (sites D and ‘south of church’), Skåne, south Sweden. Scripta Geologica, 123, 93-158. VERGOOSSEN, J. Observations on the correlation of microvertebrate faunas from the Upper Silurian Öved Sandstone Formation. In: SATKUNAS, J. & LAZAUSKIENE, J. J., eds. The Fifth Baltic Stratigraphic Conference, Basin Stratigraphy - Modern Methods and Problems. Vilnius, Lithuania: Extended Abstracts, 2002d Vilnius. Geological Survey of Lithuania, 222-224. VERGOOSSEN, J. 2003a. First record of fish microfossils from Ramsåsa, site C, Skåne, southern Sweden. Scripta Geologica, 126, 1-78. VERGOOSSEN, J. 2004. Fish microfossils from Ramsåsa, site E, Scania, southern Sweden (mid Palaeozoic). Scripta Geologica, 127, 1-70. VON BITTER, P. H., PURNELL, M. A., TETREAULT, D. K. & STOTT, C. A. 2007. Eramosa Lagerstätte—Exceptionally preserved soft-bodied biotas with shallow- marine shelly and bioturbating organisms (Silurian, Ontario, Canada). Geology, 35. VON VOLBORTH, A. 1861. Vorkommen von Conodonten in England und Schweden. Neues Jahrbuch für Mineralogie und Geologie, 2, 464-465. WANG, J. & ZHU, M. 1997. Discovery of Ordovician vertebrate fossil from Inner Mongolia China. Chinese science bulletin, 42, 1560-1563. WENZEL, B. & JOACHIMSKI, M. M. 1996. Carbon and oxygen isotopic composition of Silurian brachiopods (Gotland/Sweden): palaeoceanographic implications. Palaeogeography, palaeoclimatology, palaeoecology, 122, 143- 166. WILSON, M. V. & CALDWELL, M. W. 1993. New Silurian and Devonian fork- tailed 'thelodonts' are jawless vertebrates with stomachs and deep bodies. Nature, 361, 442-444. YOUNG, G. C. 1997. Ordovician microvertebrate remains from the Amadeus Basin, central Australia. Journal of Vertebrate Paleontology, 17, 1-25. YOUNG, G. C. 2009. An Ordovician vertebrate from western New South Wales, with comments on Cambro-Ordovician vertebrate distribution patterns. Alcheringa: An Australasian Journal of Palaeontology, 33, 79-89. YOUNG, G. C., KARATAJUTE-TALIMAA, V. N. & SMITH, M. M. 1996. A possible Late Cambrian vertebrate from Australia. Nature, 383, 810. ZHANG, G.-R., WANG, S.-T., WANG, J.-Q., WANG, N.-Z. & ZHU, M. 2010. A basal antiarch (placoderm fish) from the Silurian of Qujing, Yunnan, China. Palaeoworld, 19, 129-135. ZHAO, W.-J. & ZHU, M. 2007. Diversification and faunal shift of Siluro-Devonian vertebrates of China. Geological Journal, 42, 351-369. ZHU, M., YU, X. & JANVIER, P. 1999. A primitive fossil fish sheds light on the origin of bony fishes. Nature, 397, 607-610. ZHU, M., ZHAO, W., JIA, L., LU, J., QIAO, T. & QU, Q. 2009. The oldest articulated osteichthyan reveals mosaic gnathostome characters. Nature, 458, 469-474. ZIGAITE, Z., KARATAJUTE-TALIMAA, V. & BLIECK, A. 2011. Vertebrate microremains from the Lower Silurian of Siberia and Central Asia: palaeobiodiversity and palaeobiogeography. Journal of Micropalaeontology, 30, 97-106.

Appendix. Geological map of Gotland with the geographical extend of the formations and the localities and drill cores mentioned in the text. Based on Eriksson and Calner (2005) and data from the Swedish Geological Survey (SGU).

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