Late Ordovician Faunal Distribution and Ecospace - DocsLib

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Nothing shocks me. I'm a scientist.

Dr. H.W. Jones, Jr.

List of publications

I Frisk, Å.M. & Ormö, J. 2007: Facies distribution of post- impact sediments in the Ordovician Lockne and Tvären impact craters: In- dications for unique impact-generated environments. Meteoritics & Planetary Science 42 (11), 1971-1984. II Frisk, Å.M. & Ebbestad, J.O.R. 2007: Paragastropods, Tergomya, and Gastropoda (Mollusca) from the Upper Ordovican Dalby Lime- stone, Sweden. GFF 129 (2), 83-99. III Eriksson, M.E. & Frisk, Å.M. (submitted to Geological Magazine) Marine astrobleme palaeoecology in the early Late Ordovician of Sweden. IV Frisk, Å.M. & Harper D.A.T. (manuscript to be submitted to Pa- laeogeography, Palaeoclimatology, Palaeoecology) Late Ordovician brachiopod distribution and ecospace partitioning in the Tvären crater system, Sweden. V Frisk, Å.M. & Harper D.A.T. (manuscript to be submitted to Le- thaia) Palaeoenvironmental aspects of Late Ordovician (Sandbian) Sericoidea shell concentrations in an impact crater, Tvären, Sweden. VI Frisk Å.M. & Holmer L.E. (manuscript to be submitted to Acta Palaeontologica Polonica) Diversity and distribution of post-impact Linguliform and Craniiform brachiopod colonizers in Upper Ordovi- cian marine impact craters. VII Ebbestad, J.O.R., Högström, A.S. & Frisk. Å.M. (manuscript to be submitted to Journal of systematic palaeontology) Gastropods from the Upper Ordovician (Viru-Harju) of the Fågelsång area, Scania, southern Sweden.

Additionally, the following papers were written during the course of my PhD studies at Uppsala University, but are not included in this thesis:

I Frisk, Å.M. & Ebbestad, J.O.R. 2008: Trilobite bio- and ecostrati- graphy of the Tremadocian Djupvik and Köpingsklint formations (A. serratus trilobite Zone) on southern Öland, Sweden. GFF 130, 153- 160.

II Egenhoff, S., Cassle, C., Maletz, J., Frisk, Å.M., Ebbestad, J.O.R., & Stübner, K. (accepted in Sedimentary Geology). Sedimentology and sequence stratigraphy of the most pronounced Early Ordovician sea-level fall on Baltica - the Bjørkåsholmen Formation in Norway and Sweden.

Reprinting and publication is made with authorization from the copy- right holders.

Paper I © Meteoritical Society Paper II © GFF, Geologiska Föreningen Paper III © by the authors Paper IV © by the authors Paper V © by the authors Paper VI © by the authors Paper VII © by the authors

Statement of authorship Paper I: Å Frisk performed the field studies; J. Ormö performed the laboratory analysis of the drill cores. Both contributed to the interpretations and the writing. Paper II: Å. Frisk and J.O.R. Ebbestad sampled at Fjäcka and studied museum collections. Å. Frisk did the field work in Lockne. Both contributed to the writing. Paper III: Å. Frisk did sampling, lab work, and picked the majority of the material. M. Eriksson picked all the material supplied from Å. Wallin and did the identification of the material. Both contributed to the writing. Paper IV: Å. Frisk sampled and studied museum collections, and wrote the majority of the manuscript. Both contributed to the identification of the material and analysis of it. Paper V: Å. Frisk sampled the material, and wrote the majority of the manuscript. Both contributed to identification of the material and analysis of it. Paper VI: Å. Frisk sampled the material, did the lab work and picked the material. Å.Frisk & L.E. Holmer both contributed to the identification of the material. Å Frisk wrote the majority of the manuscript. Paper: VII: Å. Frisk assisted in data collecting and writing.

Contents

1. Introduction...... 9 The importance of being cratered...... 9 The Great Ordovician Biodiversification Event...... 11 Origin and structure of marine impact craters...... 12 Ordovician deposition patterns at the time of the impacts ...... 13 Research history and geological setting of the impacted areas ...... 15 Reconstructing intervals of survival and recovery following catastrophic events...... 19 2. Faunal recovery in marine impact craters...... 21 Palaeoenvironmental distribution patterns in the craters...... 21 Distribution of univalved molluscs in the craters and the contemporaneous Sularp Shale (Scania) ...... 22 Tvären as a local biodiversity hotspot...... 23 Polychaete colonization in the Tvären Crater ...... 26 Colonization of linguliform and craniiform brachiopods in the craters ...26 3. Future perspectives of unpublished data...... 29 4. Svensk sammanfattning ...... 30 Betydelsen av meteoritnedslag...... 30 Den ordoviciska uppblomstringen...... 31 Meteoritnedslag i Sverige...... 31 Forskningens syfte...... 32 Livet återvänder och koloniserar meteoritkratrarna ...... 33 5. Acknowledgements...... 35 6. References...... 37

1. Introduction

The importance of being cratered Catastrophic changes on Earth clearly influence the diversity and ecological construction of the biosphere both at local and global levels. Bolide impacts, as any other catastrophic event, generate distinctive patterns of biological destruction and recovery (Cockell & Blaustein, 2002), but few disturbances of an ecosystem are larger than those caused by impacts. Impacts are therefore highly significant in understanding the ecological effects of a catastrophic event. The debate on the catastrophic effects of impacts was first initiated by Alvarez et al. (1980), who put forward an impact event as the cause of the extinction at the Cretaceous-Tertiary (K/T) boundary. Later identification of the Chicxulub impact structure (Mexico) as the impact site gave further support to the theory (Hildebrand et al., 1991; Sharpton et al., 1992). Since then many important contributions have forwarded the importance of catastrophic events, but French (2004) argued especially for the importance of being cratered. During the Phanerozoic, the aftermath of events such as massive impacts are in general associated with major extinctions, truly devastating effects on the ecosystems, and thus they are alleged to be largely negative. Impacts, however, can serve as constructive events and produce wide-ranging environments enclosing new ecological niches for a diverse biota to occupy, besides initiating the bloom of disaster taxa (Cockell & Bland, 2005; Smelror & Dypvik, 2006; Schmitz et al., 2008). The process of impact cratering generates enormously high temperatures causing the substrate to be sterilised and devoid of life. If a crater forms a range of habitats within the recently formed structure, accessible for immigration, are introduced in the impacted area, bringing about a distinct post- impact succession of earliest colonizers and different phases of successive colonizers (Cockell et al., 2003). The colonization phases of benthic faunas in the post-impact sediments of the crater create a range of ecological niches. Acting as a restricted area the crater can be used as a foundation for analys- ing the tie between faunal characteristics and given environments. A majority of all known marine impacts are still covered by the sea and are thus largely inaccessible for detailed investigations. Marine craters make up approximately one fifth of all known craters on Earth (Dypvik & Jansa, 2003; Dypvik et al., 2004). The Ordovician of Baltoscandia is unique in that at least four well preserved marine craters with a good record of post-impact sediments are preserved (Lockne, Tvären, Granby, and Kärdla) (Fig.1).

9

Figure 1. Distribution of Ordovician impact craters in Baltoscandia.

Of these, the Lockne crater in Jämtland is of particular interest since it is well exposed and gives excellent possibilities for a field-based study of faunal successions from the post-impact deposits (Dalby Limestone) in and around the crater as well as from existing drill cores. The Tvären crater in the Stockholm Archipelago (77 km SSW of Stockholm) is entirely under water, but the post-impact fauna of the Dalby Limestone can be studied both from a drill core and erratic boulders. Lockne and Tvären are nearly contemporaneous and therefore offer an excellent opportunity to compare the aftermath of the individual impacts. The specific aims of this thesis project were to analyse and compare the ecologic complexity of the pre- and post-impact fauna within and outside of the craters looking at the ecological and environmental parameters controlling the establishment of the new communities. Additional aims included analyses of the associated facies controlling post-impact deposition in the craters across an environmental gradient from the shallow to deep water. The post-impact sediments and fauna in the crater also provide important new

10 data for the understanding of the distribution of Ordovician marine biotas in relation to environmental conditions, such as depth, water energy, and turbu- lence that are represented within the structures. The Upper Ordovician biota contained mainly trilobites, brachiopods, conodonts, molluscs, crinoids, ostracodes, cystoids, scolecodonts, and chitinozoans. Drill cores, erratic boulders, field data, and museum collections were used and analysed to track changes in the abundance and diversity of the fauna throughout the post- impact colonization of the craters. Analyses of the functional morphology, diversity and specialization of the groups in the craters, especially in relation to the biota of the type locality of the Dalby Limestone, provide a useful approach to investigating the communities.

The Great Ordovician Biodiversification Event The Great Ordovician Biodiversification Event (GOBE) encompasses a period of 25 million years with a marked increase in the marine biodiversity (Webby et al., 2004). Owing to this short time span GOBE has been re- garded as the most rapid diversity boost of marine life in the history of the Earth (Servais et al., 2009) (Fig. 2). During GOBE, in the Middle to Late Ordovician, a vast increase of diversity at the order, family, genus, and species levels of both zooplankton and suspension feeding organisms occurred (Harper, 2006; Servais et al., 2009). The diversification of skeletal organisms was most important, especially the brachiopods, bryozoans, cephalo- pods, conodonts, corals, crinoids, graptolites, ostracodes, stromatoporoids, and trilobites (Harper, 2006). The Cambrian explosion, as seen in Fossil- Lagerstätten, such as the Burgess Shale (Canada), Sirius Passet (Greenland), and Chengjiang (China) witnessed the origin of new body plans and skele- talization accompanied by the extinction of the soft-bodied Ediacara fauna and the arrival of the Bilateralia. Whereas few new higher taxa emerged during the Ordovician biodiversification it provided a striking increase in diversity and disparity (Harper, 2006), noticeably in the readily preserved shelly fossil record (Servais et al., 2009). The factors behind the biodiversification are widely discussed (Servais et al., 2009). Numerous causual biological and environmental factors have been considered such as distinctive palaeogeographic patterns (Crame & Owen, 2002; Cocks and Torsvik, 2002, 2006), increased presence of phytoplankton (Vecoli et al., 2005; Lehnert et al., 2007; Servais et al., 2008), the highest sea levels in the Palaeozoic (Hallam, 1992; Barnes, 2004a), amplified nutrient supply owing to prominent volcanic activity (Bergström et al., 2004; Barnes, 2004b), and it has been considered that GOBE is rooted in the Cambrian explosion (Droser & Finnegan, 2003). Schmitz et al., (2008) linked a large asteroid breakup and increased frequency of impacts that was supposed to have accelerated the biodiversification process during this time of the GOBE.

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Figure 2. Family diversity curve of marine invertebrates during the Phanerozoic, including the Big five mass extinctions, Sepkoski’s three evolutionary faunas, the Ecological Evolutionary Units from Boucot (modified by Sheehan, 1996). C1–2 = Cambrian; P1–4 =Paleozoic; M1–3 = Modern. Geological periods, from left to right: C = Cambrian; O = Ordovician; S = Silurian; D = Devonian; C = Carboniferous; P = Permian; T = Triassic; J = Jurassic; C = Cretaceous; T = Tertiary. Modified from Harper, 2006.

Origin and structure of marine impact craters Earth´s surface is covered by roughly 70 % of water but only 27 out of 174 proven impact craters are of marine origin (Dypvik & Jansa, 2003; Dypvik et al., 2004). A small number of the marine craters are still submerged and avoided destruction (Dypvik and Jansa, 2003). However, the impact energy is reduced by deep water columns, and for this reason several marine craters are poorly preserved. The shallow platforms and continental margins therefore in all probability contain better preserved craters. If a crater experiences rapid infill and sediment burial it is more likely to be preserved.

12 The development of an impact crater has three main stages: contact and compression, excavation, and modification (Melosh, 1989). Distinct physical impact features control and modify every stage, although stages can grade into each other. A greater penetration depth can be required for a marine impact than a terrestrial one because water has lower density than the target rock. Accordingly, the nature of the target rock is of enormous significance throughout the contact and compression stage as it will influence the morphology of the crater. As the impactor hits, the target becomes substantially compressed immediately at contact. The kinetic energy of the impactor is transferred into the target and fluidization, fracturing, and acceleration of the material occurs as a shock wave distributes. The second stage is characterized by the crater opening up by the excavation flow, followed by the formation of an ejecta curtain that travels outward, and finally the formation of an ejecta blanket close to the crater. The shape of the crater during the end of the excavation stage is categorized by its shape as bowl-like (Melosh, 1989) or concentric sombrero-hat profile (Quaide & Oberbeck, 1968; Schultz et al., 1981; Gault & Sonett, 1982). In Lockne the impact caused transportation of material from the seabed. Enormous quantities of resurging seawater subsequently entered the recently formed crater, which at this stage, was empty and hot. As the resurge came in and eroded the crater, deep channels formed into its margin (von Dalwigk & Ormö, 2001). The resurge also ripped up the seabed and the immediate crater resulting in vast volumes of rocks, mixing large amounts of the eroded material with ejecta. Sedimentary structures in this material demonstrate that water participated throughout deposition. De- pending on the size of the crater and the amount of material involved, grav- ity will cause the crater to collapse. In the beginning of the modification stage debris sliding occurs, as well as late isostatic rebound and erosion (Ormö & Lindström, 2000).

Ordovician deposition patterns at the time of the impacts For the duration of the early and middle Palaeozoic the Baltoscandian epi- continental palaeobasin developed on the tectonically stable Baltoscandian shield. Sedimentation rates were slow and steady. Baltica underwent consid- erable climatic changes as it moved from southern high to tropical latitudes (Torsvik et al., 1996; Cocks & Torsvik, 2005). From the Middle to the Late Ordovician, sedimentation changed from temperate climate carbonate sedimentation to warm-water carbonate sedimentation (Nestor & Einasto, 1997). For the Ordovician a differentiation of three massive facies zones in this basin is prominent, determined by fauna and lithofacies (Männil, 1966; Jaa- nusson, 1976, 1995). The zones follow the division of Jaanusson (1976), although revision by Ainsaar et al. (2004) introduced the names Estonian Shelf, Scandinavian Basin, and Livonian Basin.

13 The Dalby Limestone was widely distributed in the Upper Ordovician Baltoscandian basin, in the part corresponding to present day Sweden (e.g. Västergötland, Östergötland, Dalarna, Jämtland, Bothnian Bay and sub- surface on Gotland, Gotska Sandön, Tvären Bay, and northern Öland). The unit is normally 15-20 m thick, composed of bedded or slightly nodular cal- carenites. However, under atypical conditions the thickness of the unit may become significantly greater, reaching as much as 88 m (Lindström et al., 1994, 1996, 2005). Volcanism occurred in the western margins of Baltica in the Upper Ordovician, manifested by the presence of numerous beds of bentonite, the Kinnekulle K-bentonite (Bergström et al., 1995), and is present in several Dalby Limestone successions. The original definition of the Dalby Limestone by Jaanusson (1960, 1982) used the top of the thickest K- bentonite bed in a complex of 7 K-bentonite beds (Jaanusson, 1960, 1982b; Holmer, 1989; Ebbestad & Högström, 2007) to designate the uppermost boundary of the unit.

Figure 3. Stratigraphic chart showing parts of the Late Ordovician and the chrono- stratigraphical and biostratigraphical age of the Dalby Limestone of the Tvären crater (based on the Tvären-2 drill core and erratic boulders of the Ringsö island). L and T indicate the time of the impacts in Lockne and Tvären respectively. Figure modified from Frisk & Ormö (2007) and Ebbestad & Högström (2007) and references therein.

14 As for the base of the formation, it is lithologically transitional to the underlying Furudal Limestone (Jaanusson & Martna, 1948; Hadding, 1958). The stratotype locality of the Dalby Limestone is situated at the classic Fjäcka locality, Siljan District (Jaanusson, 1960, 1963; see Ebbestad & Högström, 2007 for summary). The Dalby Limestone was defined topostratigraphically by Jaanusson (1982, p. 22), by the first occurrence of the Dalby Limestone fauna. Hence its recognition is somewhat unreliable in the field and is in great need of a modern lithological definition. Stratigraphically the Dalby Limestone belongs to the Upper Ordovician Sandbian Stage, ranging from the regional Kukruse Stage to the Idavare Substage (Fig. 3). In the type section at Fjäcka in the Siljan Region, the Dalby Limestone corresponds to a number of very short-ranged chitinozoan zones, specifically the middle to upper Laufeldo- chitina stentor Zone, the Armoricochitina granulifera, Lagenochitina? dal- byensis, Belonechitina hirsuta, and Spinachitina cervicornis chitinozoan zones (Laufeld, 1967; Nõlvak & Grahn, 1993; Nõlvak et al., 1999). The stratotype locality holds the reference section for the Amorphognathus tvae- renensis conodont Zone with three subzones (Bergström, 1971, 2007). The Dalby Limestone is fairly rich in both macro- and microfossil re- mains. Trilobites, rhynchonelliform brachiopods, ostracodes, and cystoids are the most abundant macrofossils, while linguliform brachiopods, conodonts, and chitinozoans make up the majority of the microfossils. The nearly contemporaneous Sularp Shale in Scania, southern Sweden, contains a fauna comparable with that of the Dalby Limestone (Lindström, 1953; personal communication).

Research history and geological setting of the impacted areas Previous studies of the Lockne crater (Fig. 4), located in the Östersund Re- gion, began in 1900 when Wiman reported the occurrence of very coarse clastic deposits from the Ordovician. Thorslund (1940) continued with the mapping of Lockne and surrounding areas and assigned the clastic deposits described by Wiman (1900) to Ordovician shore deposits. However, in 1987, Simon concluded that the deposits had been subjected to some kind of strong crushing force, but offered no explanation. This problem was addressed by Wickman (1988) who proposed that the only possible processes that could explain these structures were the immense forces released by a meteorite impact. This suggestion was further investigated when a drill core from the Tvären crater was recovered (Lindström et al., 1994), containing similar clastic deposits to those found in the Lockne crater (Lindström & Sturkell,

15 1992). The identification of Planar Deformation Features (shocked quartz) (Therriault & Lindström, 1995) and high contents of Iridium in the impact deposits (Sturkell 1998a) confirmed the impact theory. Detailed studies on the Lockne impact crater were conducted in the 90´s by Sturkell (1998b) and Ormö (1998) in co-operation with Maurits Lindström (Stockholm Univer- sity) and have been followed by further work (Lindström et al., 2005; Ormö et al., 2007; and references therein). Faunal investigations, on the Dalby Limestone inside the crater, before this study were limited to the descriptive work of Thorslund (1940) on mainly ostracodes and trilobites, and by Grahn (1997) and Grahn et al. (1996) on chitinozoans. Sturkell et al. (2000) dated an ejecta layer 45 km outside the crater centre by chitinozoans and conodonts. Given that the areas were suggested to be cratered by meteorites in the 90’s, no palaeontological work prior to this date is placed in the context of impacts. Lockne is a well-preserved marine impact crater in Jämtland County and at present easily accessible in the field. At the time of the impact the Lockne site was placed in deeper water than Tvären, presumably a deeper shelf (>200 m). Depths have been suggested to be between 500-700 m (Lindström et al., 2005; Shuvalov et al., 2005; Ormö et al., 2007). The impact took place during the period of deposition of the Dalby Limestone, which therefore represents the youngest pre-impact sediments. The target rocks are made up of a crystalline granitic basement overlain by roughly 30 m of Cambrian shale and 50 m of Ordovician limestones. A 7.5 km wide inner crater formed in the basement, bordered by a 3 km wide zone of ejected crystalline basement representing the brim (Lindström et al., 2005). Openings in the crater brim channeled the resurging water (von Dalwigk & Ormö, 2001; Lindström et al., 2005), this is exemplified by the Tandsbyn gully (Fig.4). The inner crater contains breccia (Tandsbyn breccia) composed of fractured and brecciated basement assembled with crystalline ejecta. The first sediments to enter and partially fill the crater were coarse clastic resurge deposits (Lockne breccia), with a gradual transition into sandstones and silt- stones (Loftarstone) that diminished when all the resurge material had settled. Deposition was then replaced entirely by sediments of the Dalby Lime- stone, comprising the earliest post-impact secular sediments, as well with a gradual transition from the underlying deposits. Post-impact Dalby Lime- stone can be found overlying the resurge deposits with a gradual transition or lying directly on the impact breccia. Deposits of the Örå Shale are found on top the Dalby Limestone at a number of localities within the crater.

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Figure 4. Geological map of the Lockne crater, showing the location of the two drill cores through the structure. Map modified from Lindström et al. (1996) and Frisk & Ormö (2007).

During research excursions to the island of Ringsö in the Stockholm archipelago lead by B. Asklund and A.H. Westergård (Thorslund, 1940) fos- siliferous erratic boulders were collected and subsequent studied by Thors- lund (1940). Their provenance to the Dalby Limestone was affirmed by Thorslund (1940), Strachan (1959), and Bergström (1962), the studies con- cluding that they originated from submerged bedrock transported south- eastwards by ice. At Tvären bay the otherwise Precambrian crystalline rocks

17 contain a partially sediment-filled circular 2 km depression (Fig. 5). Flodén et al. (1986) carried out geophysical investigations in the bay that directed Wickman (1988) to interpret the structure as an impact crater.

Figure 5. Geological map of the Tvären crater and the position of the two drill cores made in 1991 through the structure. The fan of glacial erratics of the Dalby Lime- stone is marked by triangles. Map modified from Lindström et al. (1994) and Frisk & Ormö (2007).

The stratigraphy and structure were further revealed by two drill-cores in the bay (Lindström et al., 1994). Tvären-1 represents a short drilling into the crystalline basement. Tvären-2 on the contrary generated a 140 m sequence of impactites, resurge sediments and post-impact Dalby Limestone (Lindström & Sturkell 1992; Lindström et al., 1994; Ormö, 1994). An analogous succession to that in the Lockne crater was encountered, beginning with a crystalline breccia succeeded by 60 m of fining-upward resurge deposits, and 80 m of secular sediments of carbonate mudstone representing the Dalby Lime- stone (Lindström et al., 1994; Ormö et al., 2007). Ordovician carbonates on non-lithified sands of Early to earliest Middle Cambrian age made up the sedimentary target sequence at the time of the impact, covering a crystalline basement. The time of the impact event is established by means of conodonts and chitinozoans to late Kukruse Stage (Lindström et al., 1994; Ormö, 1994; Grahn et al., 1996). Prior to the impact, the youngest sediments belong to the

18 Dalby Limestone and deposition of Dalby sediments continued after the impact event and subsequent resurge sediments. However, the pre-impact Dalby Limestone in the crater is only accessible in the resurge deposits. Faunal investigations from the Tvären crater are essentially the works of Thorslund (1940) on mainly trilobites and ostracodes, Bergström (1962) and Ormö (1994) on conodonts. In addition Strachan (1959) worked on the graptolites from the Tvären boulders, Grahn et al. (1996) studied chitinozoans and Wallin & Hagenfeldt (1996) studied acritarchs from the Tvären-2 core. A sea depth of 100-150 m, presumably closer to 100 m (Ormö et al., 2007; Frisk & Ormö, 2007), has been suggested at the time of the impact. Through- out the modification state the crater has undergone subaerial erosion in addition to glacial erosion (Thorslund 1940; Lindström et al., 1994). The post- impact sequence of the Dalby Limestone biostratigraphically belongs to the Laufeldochitina stentor chitinozoan Zone (Lindström et al., 1994; Grahn et al., 1996) and the Prioniodus variabilis conodont Subzone of the Amorphog- nathus tvaerensis conodont Zone (Lindström et al., 1994; Ormö, 1994). The chitinozoan Conochitina tigrina was recorded in the uppermost beds of the core, corresponding to the topmost Kukruse Stage (Grahn et al., 1996).

Reconstructing intervals of survival and recovery following catastrophic events Large global catastrophic events are often coupled to mass extinctions. While events such as bolide impacts do not always generate a large-scale aftermath, they can still provide valuable information as to how life responds to the intense stress associated with mass extinctions. The well established five major mass extinctions (Raup & Sepkoski, 1982) have all been well studied although only moderate attention has been given to the biotic recovery from each event. Regarding the aftermath of marine meteorite impacts and the colonization and palaeoecology of different environments in a crater, no corresponding studies have been presented than the initial study by Lind- ström et al. (1994). Following the devastating event, the Lockne and Tvären impacts resulted in a sterile and novel sea floor with altered substratum and topography, configured by means of the deep crater depression and the shallower rim, and hence a variety of ecospace. This provides a good basis for studying the regional diversity patterns and ecological response of taxa following the crisis of extraterrestrial input. Even though no actual mass extinction event has been related to the Lockne and Tvären impacts, many of the terms and definitions relating to extinctions can be used for the localised catastrophes. The majority of mass extinctions are followed by a survival interval, distinguished by an irregular period of little (or simply absent) diversification

19 (Erwin, 1998), although a fast diversification is seen in the following recovery interval. Throughout the catastrophic event several different types of fauna can be recognized also including the pre-extinction interval; pre- extinction-taxa and extinction taxa (Hallam & Wignall 1997). The survival interval is represented by two groups of taxa; disaster taxa (Kauffman & Harries, 1996; Harries et al., 1996; Roland & Bottjer, 2001) and opportunistic taxa (Schubert & Bottjer, 1995; Kauffman & Harries, 1996; Hallam & Wignall, 1997; Harries et al., 1996). Finally, a phase with recovery taxa can be recognized. Pre-extinction taxa are defined as assemblages below the extinction event. In the survival interval only a small number of taxa prevail in the communities, typified by broad ecological ranges and between-habitat segrega- tion (Harries et al., 1996). Early on in this stage the vacant ecospace is colonized by disaster taxa. They are opportunistic generalists that adapt to high- stressed environmental conditions, inhabiting empty guilds, and are evident as short lived and fairly large populations, or blooms, driving the boundaries of their geographical range in the initial survival interval (Kauffman & Har- ries, 1996; Harries et al., 1996). During this interval the disaster taxa will in the end be pushed into peripheral settings and subsequently return to low abundances (Hallam & Wignall, 1997; Bottjer, 2001). As the area is repopu- lated opportunists and additional survivor taxa replace the disaster taxa, with a dynamic increase and distribution in the stressed ecospace (Kauffman & Harries, 1996; Harries et al., 1996). The recovery is defined in different ways but a general view is that it represents a new and stable ecosystem. Recovery is considered as the point in time when conditions return to those prevalent prior of the extinction, illustrated by numerical recovery of taxa (Sole et al., 2002; Sahney & Benton, 2008), or when an ecological equilib- rium nevertheless different to the pre-extinction taxa is accomplished.

20 2. Faunal recovery in marine impact craters

Palaeoenvironmental distribution patterns in the craters Prior to evaluating the individual fossil elements and their colonization of the craters it is necessary to recognize a greater division of the environments. To establish the distribution patterns of disparate litho- and biofacies in the Lockne and Tvären craters field data and drill-core were analysed, and aspects controlling the depositional environments were evaluated. Three central facies (rim facies, cephalopod facies, and argillaceous facies) of the post- impact Dalby Limestone were recognized at the Lockne crater, depending on sea-floor topography, location with respect to the crater, and local water currents (Frisk & Ormö, 2007) (Fig. 6). Each facies have characteristic faunal assemblages, corresponding facies are also present in the Tvären crater. Two drill cores from Lockne (Lockne-1 and Lockne-2) (Fig. 4) and one from Tvären (Tvären-2) (Fig. 5) were analysed for inorganic carbon (IC) and or- ganic carbon (Corg). Carbonate contents (IC values) was used to establish the cessation of the resurge deposition (Loftarstone) and the commencement of secular sedimentation (i.e., Dalby Limestone). Based on earlier lithological and palaeontological data (Lindström et al., 1994; Grahn et al., 1996) a visible boundary between the resurge deposits and the ‘normal’ Dalby Lime- stone in Tvären-2 has been established at ~149 m depth. Two studies (Frisk & Ormö 2007; Ormö et al., 2009) confirm that estimated boundary level. In general a nodular argillaceous facies dominates the deposition in the deeper and quiet regions of the crater floors and depressions, presenting low values of inorganic carbon. At the crater rim at Tvären (Fig. 7), consisting of crushed crystalline basement ejecta, a rim facies with a reef-like fauna was established, most certainly due to topographical highs, more oxygenated waters and substrate- derived nutrients. Occasionally, biocalcarenites with a distinctive fauna occur in the Tvären succession, manifested in the Tvären-2 drill core (Fig. 8), originating as detritus from the facies developed on the rim. They are found early in the succession above the lowermost part of the Dalby Limestone boundary at 149 m, suggesting that the development of a reef-like environment on the crystalline crater rim was initiated shortly after the resurged sediments settled (Lindström et al., 1994; Frisk & Ormö, 2007). The cal- carenitic turbidites are evident as peaks in IC and lows in Corg, in contrast to the remainder of the succession. These turbidites also show a close resem-

21 blance in biota (rhynchonelliformean brachiopods, bryozoans, and echinoderms) lithology, and Corg and IC values to glacial erratics boulders from the vicinity of the crater area. All of these data point to an origin from the crater rim. It is evident that comparable distribution patterns of the facies in Lockne and Tvären existed despite differences in depth of the impacted sea and individual crater size.

Figure 6. The three different facies in the Lockne crater (outcrops at the Tandsbyn gully). a) Lithology of the rim facies. A light gray biocalcarenite containing crystalline clasts of the brecciated basement found only on the crater rim. b) Part of the brachiopod coquina bed found in the upper layers in the rim facies. c) The cephalopod facies, a thick-bedded calcilutite with a high abundance of nautiloid conchs. d) The argillaceous facies at the bottom of the Tandsbyn gully characterized by argillaceous limestone nodules set in a hard, slightly fractured mudstone. Modified from Frisk & Ormö (2007).

Distribution of univalved molluscs in the craters and the contemporaneous Sularp Shale (Scania) During the Ordovician the gastropod diversity showed a significant expan- sion in taxonomic richness (Frýda & Rohr, 2004) with taxa predominantly found in shallow-water habitats. Later in the Ordovician, gastropods spread to more offshore environments. The early Sandbian origination bioevent

22 (time slice 5a-b of Webby, 2004) increased the radiation of most gastropod groups (Frýda & Rohr, 2004), coinciding with the time of the impacts in the Baltoscandian sea. Two studies in the thesis (Frisk & Ebbestad, 2007; Ebbestad et al., manuscript) present the diversity and distribution of Swedish Ordovician ter- gomyan, paragastropod, and gastropod faunas during the Sandbian. The studies looked at the faunas in the Lockne and Tvären craters, as well as the type locality of the Dalby Limestone in Fjäcka, and the contemporaneous fauna of the Fågelsång area in Scania, southern Sweden. The diversity of univalved molluscs in the Dalby Limestone was shown to be much higher than previously presumed; two new species were also documented. The bel- lerophontoid Bucania erratica of the shallow rim facies of Tvären is one of the oldest species of the genus in Baltoscandia. It is also the only taxon that is present in the deeper water settings of the contemporaneous Sularp Shale in Scania. Some of the faunal elements here were likely transported from the shallower platform. The deeper water taxa are taxonomically comparable to forms described from the Barrandian area, Czech Republic, and the slightly older Elnes Formation in the Oslo Region, Norway. While a diverse fauna of paragastropods, tergomyans, and gastropods is present in the shallow water carbonate facies of Tvären and the type area of the Dalby Limestone (Frisk & Ebbestad, 2007), these are on the whole absent in the Lockne crater. Their occurrence has only been confirmed at one locality, which in all probability is related to the presence of a deeper sea in the area.

Tvären as a local biodiversity hotspot This study (Frisk & Harper, manuscript a) presents analysis of the rhynchonelliform brachiopod guild structure. The purpose is to describe patterns of distribution and ecospace utilisation of brachiopods from the recently colonized Tvären crater and contemporaneous successions (the Dalby Lime- stone at Fjäcka, the Siljan District and the Sularp Shale in Scania). A noticeable increase of marine life and communities occurred during the Ordovician and Great Ordovician Biodiversification Event. It was also characterized by specialization and decreasing niche width expressed by a rise in diversity and guild partitioning (Harper, 2006). Waisfeld et al. (2003) suggested that growth of guilds and hence the partition of megaguilds within communities assisted the diversification. This work show that single events like localized meteorite impacts can increase the biodiversity as communities were offered a greater range of different topographic environment due to the morphology of the impact crater. Analyses of taxonomic diversity and ecospace utilization in settings within the Baltoscandian Basin, developing at corresponding times albeit situated at varied palaeodepths, show different levels of - diversity among rhynchonelliformean brachiopods. Low levels of ecospace

23 utilization and diversity prevailed in the deeper water Sularp Shale as distinct to the rim facies of Tvären demonstrating opposite levels. Rhynchonel- liform brachiopods display an increase in -diversity and number of guilds following a pattern from deeper mudstone to shallower grainstones settings. The brachiopods on the crushed basements rocks of the rim constitute an assemblage of genera and morphotypes with specializations for a more reef- like environment together with moderate levels of ecospace utilisation, and also attaining the highest levels of taxonomic and ecologic diversity. In the more typical settings of the Dalby Limestone, also represented by the type locality, similar numbers of guilds are present as at the crater rim although the species diversity of brachiopods within the guilds is much lower and there are fewer specimens. This study shows that the establishment and diversification of brachiopod communities in the Tvären crater was driven above all by variations in depth, sediment and nutrient supply. Variation in guild structure among the localities indicates that the distribution of brachiopods is related to their dif- ferential response to the environmental evolution in the crater. An increase in -diversity, characterized by more packed communities, during the Ordo- vician radiation is linked to increased specialisation specified by division of guilds (Harper, 2006). Environments were exploited on the crater rim and its elevated crest forming an abundance of ecospace. Hence great biodiver sity was centered on the crater rim and ecosystems expanded primary as a result of the initial opportunity to exploit unused and non- populated areas. Varia- tions in depth, sediment and nutrient supply defined the habitats on the crater rim and caused a significant increase of brachiopod diversity within the guilds. As an event the Tvären impact was significant in serving as a local pump and reservoir for biodiversity, pushing the development of new com- munity types and narrowly-defined niches. Further more, it helped to drive both - and -diversity during a critical phase of the Great Ordovician Bio- diversification Event. A separate study on the rhynchonelliformean brachiopod communities within the crater depression was also conducted (Frisk & Harper, manuscript b). The first recovery fauna of rhynchonelliformean brachiopods inside the crater is represented by Sericoidea (Fig. 8). As they represent the only rhynchonelliformean brachiopods in the crater depression, the survival rate of the subphylum is considered low. Sericoidea entered the crater environment only after it had become less constrained through filling and consequent reduction of depth. The shell concentrations are related to shallower-water conditions and this may partially solve the dilemma of why they normally do not occur with the deeper-water Foliomena fauna. This study suggests that Upper Ordovician Sericoidea bearing associations associated with muddy and shaly substrates did not merely, favour and occupy deep water environments but employed an opportunistic strategy, constructing shell concentrations, in outer shelf depths

24 posed rim nfilling of the al., 1994) and pro ed from Lindström et from Lindström ed pper central depression deposits are not preserved. preserved. not are deposits depression central pper osion the crater rim and u and rim crater osion the sed on seismic profiling and drillings (modifi on seismic profiling and sed crater was probably completed but due to completed crater was probably er heights (Ormö et al., 2007). The key environments in the crater are situated on the crater rim and in the central depression. I depression. central the in rim and crater on the situated crater are in the environments key The 2007). et al., (Ormö heights Figure 7. Cross section of the a crater ba of the a crater 7. Cross section Figure

25 Polychaete colonization in the Tvären Crater This study Eriksson & Frisk (submitted) examines the different phases of immigration by polychaetes, represented by scolecodont jaws, and their environmental preferences. The post-impact sediments of Tvären preserve a notable palaeoecological succession with regards to the scolecodonts which firm one of the most abundant fossil groups throughout the drill-core (Fig. 8). Successive colonization of vacant ecospace by scolecodonts in the crater is discussed as they belong to the first non-planktonic groups to appear in the crater depression. The initial post-impact Dalby Limestone immigration phase is typified by a low diversity polychaete fauna, dominated by three species. The polychaete assemblages do not necessarily represent a crater floor colonization of vagrant benthos. Instead, the lowermost assemblage seems to be a transported thanatocoenosis, as indicated by its taxonomic correspondence to the rim facies fauna recovered from the Ringsön erratics. Scolecodont occurrence and abundance show a rough positive correlation with levels of increased bioturbation in the Tvären-2 drill core. The taxa from deepest part of the crater differ from the upper parts of the succession, as the crater become infilled with sediments and experience more shallow, higher energy, water settings. By the early Late Ordovician the diversity of jawed polychaete faunas had increased on a global scale and at least 27 polychaete genera are recorded at the time of the impact, in the Kukrusean (Hints & Eriksson, 2007). Scole- codont material from the Ordovician of Sweden is poorly studied and only one description of coeval material from the Sularp Shale (Schallreuter, 1983) has been presented. In general, the material from this study is characteristic of the Upper Ordovician of Baltoscandia, comparable to faunal descriptions from Poland and Estonia (see Kielan-Jaworowska, 1966 and references therein). Differences can be seen in the poorly represented ramphoprionid fauna and a high frequency of “Xanioprion” in some levels of the crater. No scolecodonts has been found inside the nearly contemporaneous Lockne crater; this might be related to shallower water settings in the Tvären area.

Colonization of linguliform and craniiform brachiopods in the craters In Lockne and Tvären the impact events caused a high magnitude of stress on the environments in the associated areas. To understand the processes on how biota responds to catastrophes we looked at the colonization intervals of post-impact deposits holding linguliform and craniiform brachiopod fauna (Frisk & Holmer, manuscript). Material was sampled from three different environments in the Lockne crater

26 (Frisk & Ormö, 2007) and in Tvären sampling was conducted from the Tvären-2 drill core (Fig. 8) and erratic boulders. All material was compared to faunas from normal contemporaneous Dalby Limestone sections in the Siljan District, Sweden (Holmer, 1989). In the aftermath of the local disasters, the onset of faunal colonisation in the craters was established in the initial parts of post-impact Dalby Lime- stone deposition. These assemblages are typified by faunas dominated by a low number of species and generic diversity. Environment favouring of the post-impact faunas is observed and in general faunas experienced a rapid re- population and vacated newly formed ecospace in both regions. A noticeable increase in the abundance of Acanthambonia sp. nov. in the argillecous facies, acting as a disaster taxon, is prominent for a period in the survival interval. Most opportunistic taxa return to low level of abundance following the survival interval (Hallam & Wignall, 1997), as in this case. The study shows that a wide range of ecological niches and environments are inhabited by linguliform and craniiform brachiopods. This seems to depend on topography and consequently facies within the diversified impact craters. Com- pared to the monotonous environments distributed in contemporaneous sections an increased diversification of habitats is visible for post-impact recovery brachiopods faunas. Three species found in the post-impact intervals of the Tvären crater, have not been reported previously from the Dalby Limestone, the first record of Pseudocrania sp., Craniida indet sp. A., and Spinilingula radiomellosa.

27 r nces lme t transported ct sediments. . (submitted), and Frisk & Ho (submitted), and Frisk ssil elements in the post-impa in the ssil elements the papers within this thesis this within the papers al. (1996), Eriksson & Frisk al. (1996), based on unpublished data, courtesy of ÅMF. Numerous of ÅMF. appeara on unpublished data, based courtesy distribution of structures and fo Hagenfeldt (1996), Grahn et Hagenfeldt (1996), Grahn rmation see Lindström et al. (1994) and et al. Lindström rmation see n of the Tvären-2 drill core with Tvären-2 n of the Figure 8. Lithologic successio 8. Lithologic Figure Dalby Limestone of within the is conodonts The range (manuscript). Based on Lindström et al. (1994), Wallin & et al. Based on Lindström of(especially biota echinoderms ostracods) and drill-core in the occur through slumping/turbiditichence material and represen fauna from the crater rim. For detailed info

28 3. Future perspectives of unpublished data

By the means of geochemistry and micropalaeontology we are trying to de- velop a method to recognize the end of impact-related sedimentation in impact craters distinguished by the transition from resurge to secular sediments after the Lockne, Tvären and Chesapeake impacts (Ormö et al., 2009). On- going geochemical investigations by Jens Ormö and Andrew Hill, comple- mented with my micropaleontological studies from the Lockne and Tvären drill-cores, will investigate the circumstances of recognized abrupt geochemical shifts to see if they are as well reflected in the palaeontological record. If so, it is hoped this will increase our understanding how the local biota was affected by the sedimentary environment within the craters. In the Siljan District Elsa Warburg (1910) described successions from the Ordovician. In Nittsjö one section serves as a key locality for the Ordovician succession in Sweden and extends from the lower Ordovician (Lanna For- mation) to its youngest unit the Dalby Limestone. There are only few earlier sections through this interval in the Siljan District. This projects main focus has been to excavate and re-investigate the Nittsjö locality and it is incorpo- rated in a large study on the Ordovician of the Siljan district in co-operation with Jan Ove Ebbestad and Anette Högström. The section has been sampled for isotopic studies, fossils and lithological studies. Moreover, this section will be used for the discussion and reassessment of the Ordovician formation boundaries, especially the Dalby Limestone, based on Jaanussons (1960, 1982) division. The present study has produced a large compilation of data concerning conodonts and trilobites from Lockne and Tvären, in addition to Thorslund’s (1940) earlier investigations on trilobites from the areas. This information will be integrated with all the palaeontological data from other groups presented within the thesis to establish a more comprehensive representation of the colonization and palaeoecological constructions in the craters. The different studies in the thesis will be used for extensive multivariate data analysis.

29 4. Svensk sammanfattning

Betydelsen av meteoritnedslag Avhandlingen analyserar processerna som styrde det marina djurlivets åter- hämtning och diversifiering efter två meteoritnedslag i det ordoviciska havet. Katastrofala förändringar på jorden påverkar den biologiska mångfalden och den ekologiska uppbyggnaden av biosfären både i ett lokalt och globalt per- spektiv. Meteoritnedslag likväl som andra katastrofer genererar specifika mönster av biologisk förstörelse och återhämtning. Väldigt få katastrofer skapar sådan stor obalans i ekosystem som de orsakade av meteoriter. Mete- oritnedslag är därför väldigt betydelsefulla för förståelsen av de ekologiska effekter som uppstår efter katastrofala händelser. Debatten angående katastrofala effekter till följd av meteoritnedslag etablerades i och med att Alva- rez et al. (1980) introducerade idén att ett meteoritnedslag orsakade utdöen- det av exempelvis dinosaurier vid den nu berömda krita-tertiär gränsen. Yt- terligare stöd för idén framfördes då platsen för nedslaget fastställdes till Chicxulubstrukturen i Mexiko. Sedan dess har flera viktiga vetenskapliga bidrag framhävt betydelsen av att förstå katastrofala händelser och återhämt- ningen efteråt, särskilt vid meteoritnedslag. När man talar om konsekvenserna av katastrofala händelser i jordens historia syftar man ofta på utdöenden, vilket i sin tur generar förödande effekter på ekosystem. Dessa tolkas mestadels som negativa. Emellertid kan meteoritnedslag leda till konstruktiva händelser genom att skapa nya miljöer och därmed oanvända ekologiska utrymmen som de olika djurgrupperna kan kolonisera. Meteoritnedslagets förödelse kan även orsaka en uppblomstring av katastroftaxa (Cockell & Bland, 2005; Smelror & Dypvik, 2006; Schmitz et al., 2008). Själva processen som sker vid meteoritens nedslag genererar enormt höga temperaturer, som i sin tur resulterar i en steril miljö utan liv. Om nedslaget bildar en krater kommer den att introducera en rad av habitat inom den nybildade strukturen tillgängliga för kolonisation. Efter nedslaget kommer det att uppstå en tydlig succession av de tidigaste kolonisatörerna följd av olika faser av dessa (Cockell et al., 2003). Eftersom kratern är en begränsad yta kan den användas som en unik undersökningslokal för att analysera sambandet mellan djurgruppernas egenskaper och de etablerade miljöerna. Meteoritkratrar som orsakats av nedslag på land är idag väl undersökta, däremot är effekterna av nedslag till havs mindre kända. Ett stort problem är

30 att majoriteten av de marina nedslagen fortfarande är täckta av hav och där- för svåra att studera. Jag har analyserat de nästan likåldriga ordoviciska kratrarna Lockne och Tvären i Sverige och de omedelbara och efterföljande fossilförande sediment som lagrats i dem. Undersökningarna utfördes på de avlagringar som bildades i kratrarna och den ekologiska respons som den koloniserande biotan visade på de katastrofala händelserna. Att förstå den ekologiska effekten och återhämtningen av biotan är viktigt för jämförelser med andra marina meteoritkratrar som exemplevis Chicxulub (Mexiko), Chesapeake Bay (USA) och Mjölnir (Norwegian Barents Sea).

Den ordoviciska uppblomstringen En av de största evolutionära händelserna i livets historia på jorden skedde under den geologiska tidsperioden ordovicium som varade för ungefär 489 till 443 miljoner år sedan. Den marina diversiteten genomgick en påtaglig ökning under en period av 25 miljoner år och har kommit att kallas The Gre- at Ordovician Biodiversification Event (GOBE), den stora ordoviciska uppblomstringen. På grund av den korta tidsrymd som den biologiska mångfal- den expanderade betraktas GOBE som den snabbaste ökningen av marint liv i jordens historia. Ökningen är synbar på familj-, släkt- och artnivå för både zooplankton och filtrerande organismer. En viktig diversifiering skedde speciellt hos organismer med någon typ av skelett, speciellt gällande brakiopoder, bryozoer, bläckfiskar, konodonter, koraller, krinoideer, graptoliter, ostrakoder, stromatoporider och trilobiter. Orsakerna bakom ordoviciska uppblomstringen har diskuterats och flertalet biologiska och miljöpåverkande faktorer har framförts. Exempelvis har paleogeografiska förändringar, ökade nivåer av det atmosfäriska syret, de högsta havsnivåerna under paleozoikum, samt väldigt stora näringstillgångar knutna till den ökade vulkaniska aktiviteten föreslagits. Likaså har ett stort asteroiduppbrott vilket ökade frekvensen av meteoritnedslag under denna tid kopplats till accelereringen av den biologiska mångfalden.

Meteoritnedslag i Sverige För 480 miljoner år sedan splittrades en stor himlakropp i asteroidbältet och till följd av detta utsattes jorden för ett långvarigt och förhöjt bombardemang av meteoriter (Schmitz et al. 2001). Flertalet meteoriter slog ned i det hav som delvis täckte vad som motsvarar dagens Sverige. Idag är området, som motsvaras av den dåtida kontinenten Baltica, väldigt unikt då där finns fyra dokumenterade nästan likåldriga marina kratrar med väl bevarade sediment avsatta efter nedslaget (Lockne, Tvären, Granby), likväl som den estniska kratern Kärdla. Av alla kända meteoritkratrar på jorden är en femtedel mari-

31 na (Dypvik & Jansa, 2003; Dypvik et al., 2004). Två av dessa marina nedslag var nästan samtida och det största av dem slog ned för ungefär 458 miljoner år sedan i Lockne, Jämtland. Havsbottnen i vilken meteoriten slog ned bestod av sediment från kambrium och ordovicium samt underliggande kris- tallint urberg. I samband med nedslaget slungades vatten och material från havsbottnen. Detta följdes av en gigantisk återsvallning av havsvatten, likt tsunamivågor, in i den nybildade, tomma och varma kratern. Återsvallet eroderade och slet loss stora volymer material från havsbottnen. I Lockne bildades en 7,5 km bred inre krater omgiven av en tre km bred yttre krater. Efter nedslaget och avsättningen av det tillhörande materialet fortsatte sedi- mentationen av dalbykalkstenen som hade påbörjades redan innan katastro- fen. En liknande process skedde vid bildandet av den något yngre två km breda Tvärenkratern, som numera är delvis bevarad i utkanten av Stock- holms skärgård. Vid bildandet av kratrarna genererades enormt höga temperaturer som bidrog till ett sterilt substrat utan liv. Dessa katastrofala händelser orsakade en lokal utplåning av områdenas marina fauna. Följaktligen kom detta enorma område att erbjuda nya miljöer för djurlivet som fortfarande levde i det om- givande havet, men inte var påverkade av nedslaget. En dramatisk föränd- ring av havsbottens topografi, formad av kratern, skiljde sig nämnvärt från den tidigare homogena havsbottnen. Kratern erbjöd varierande levnadsmil- jöer och nya habitat för faunan att immigrera och bidrog till bildandet av nya ekosystem.

Forskningens syfte Målet är att dokumentera den ekologiska mångfalden, kolonisationen och distributionen av det marina djurlivet i de två ordoviciska meteoritkratrarna Lockne och Tvären. Det genomförda projektet är en pionjärstudie, då det inte finns några omfattande jämförelsebara studier angående koloniseringen av de olika delarna i marina meteoritkratrar efter de katastrofala händelserna. Majoriteten av andra marina kratrar är mycket dåligt exponerade och har därmed en låg grad av tillgänglighet. Dessa kratrar valdes för att de i stort sett är likåldriga, är välbevarade, och de har mycket material att studera. Locknekratern har väl blottade och tillgängliga sektioner, likväl som flertalet borrkärnor. Från Tvärenkratern finns tillgången av en borrkärna och mycket flyttblock med härkomst från den upphöjda kraterkanten. Sedimentära bergarter och fossil i kratern kan även bidra till viktig ny information för förståel- sen av det marina djurlivets spridning under ordovicium i relation till olika levnadsförhållanden (djup, vattenenergi, turbulens m.m.), representerade i olika delar inom strukturen. Material har samlats och analyserats från kratrarna genom studier av borrkärnor, flertalet fältarbeten, och museisamlingar.

32 Livet återvänder och koloniserar meteoritkratrarna Den första studien visar att det fanns åtminstone tre olika nyckelmiljöer i kratrarna med skilda sedimentära avsättningar och fossil fauna. Meteoritned- slagen och bildandet av de två kratrarna förändrade den annars så homogena havsbottnen, där kraterns botten, som motsvarar den djupaste delen, står i kontrast mot den upphöjda och grundare kraterkanten. Genom fältarbete och analyser av flertalet borrkärnor från Lockne och Tvären har ett samband i utvecklingen av miljöerna hos de två olika kratrarna kunnat påvisas. Genom analyser av organiskt och oorganiskt kol har den övre gränsen för de avsätt- ningar som associerats med det material som avsatts av tsunami-liknande vattenmassor från impakten kunnat fastställas. Vidare har de flyttblock som associerats med kraterkanten kunde korreleras med nedrasat material som hittats i kraterns djupare delar. Mer ingående studier av de olika miljöer som initierades i kratrarna visar att på kraterkanten uppkom en väldigt unik miljö som kan liknas vid ett rev. Detta är i synnerhet påtagligt i Tvären. Efter meteoritnedslaget koloniserades den upphöjda kraterkanten av djurgrupper från de delarna av havet som meteoritnedslaget inte påverkade, främst brakiopoder, trilobiter, krinoideer och ostrakoder. Resultaten visar att speciellt brakiopoderna på kraterkanten har en helt annan diversitet och kvantitet jämfört med nivåer med likåldriga sediment som är opåverkade av meteoritnedslaget. I jämförelse med kraterns djupa delar är skillnaden i faunans mångfald och spridning väldigt stor. En- dast en art av brakiopoder koloniserade den djupa delen av kratern och inte förrän under ett väldigt sent stadium. Den höga diversiteten av brakiopoder i den revliknande miljön beror till synes på möjligheten av att kolonisera en steril miljö, som är en ekologiskt outnyttjat yta. Möjligheten att kolonisera kraterbotten fanns även, men på grund av den inledningsvis representerade en djupare havsmiljö var troligtvis inte möjligheterna att leva där lika stora då syrenivån var lägre. Dessutom var sedimentationshastigheten väldigt hög. En ytterligare möjlig orsak till denna diversifiering är skillnaderna i substrat, då kraterkanten bestod av ett hårt krossat urberg, medan kraterbottnen bestod av finkorniga mjuka sediment. En del av studien beskriver närmare två olika djurgruppers kolonisation av kratrarna. Tanduppsättningar av maskliknande djur (polychaeter) hittas genom hela lagerföljden i Tvären, men representeras av olika släkten och/eller arter. De koloniserade kratern i ett tidigt stadium och utgör en stor del av den bevarade fossila faunan. I svenska fossilförande bergarter är väl- digt lite beskrivet om dessa. Denna studie bidrar även till att bygga på pusslet om djurgruppen. Små brachiopoder har studerats från kratrarna i Tvären och Lockne. I och med kratrarnas morfologi har flera nya och skilda miljöer uppkommit, från kraterns djupare botten till dess upphöjda krater- kant. Dessa miljöer har bidragit till att kraterns delar har koloniserats av olika arter och mängder av brachiopoder.

33 Två delstudier presenterar mångfalden och distributionen av gastropod- liknande mollusker från de två kratrarna, typlokalen för dalbykalkstenen (Fjäcka, Siljan) och den likåldriga sularpskiffern (Skåne). Mångfalden i dalbykalkstenen visade sig vara mycket högre än tidigare påvisats och nya arter har beskrivits. En av de nya arterna, Bucania erratica, från den i kratern grundare belägna kraterkanten, är ett av de äldsta fynden av detta släkte i Baltoskandia. Denna art är den enda, från kratrarna, som hittats i sularpskiffern, skiffern bildades under djupa vattenförhållanden. Troligtvis har den transporterats från de grundare delarna. En mer artrik fauna av gastropodlik- nande mollusker levde i de grunda delarna av kratern i Tvären och även i typlokalen i Siljan. Medans denna fauna var helt frånvarande i Lockne som var belägen i en djupare del av det dåtida havet. Slutsatserna av studierna visar att de olika samhällen som utvecklades i kratrarna och de alltmer tätpackade miljöerna under ordovicium bidrog till att driva den biologiska diversiteten, speciellt under ett kritiskt skede i den stora ordoviciska uppblomstringen. Den biologiska diversiteten innefattar både alfa- (mångfalden av arter inom ett samhälle) och betadiversiteten (mångfalden av samhällen). Inom den begränsade ytan hos kratrarna erbjöds miljöer från grunda och revlika till över 200 m djupa och från väl syresatta till lågt syresatta. Diversiteten av kratermiljöer favoriserade koloniseringen av olika djurgrupper. Kraterkanterna, och då speciellt i Tvären, hade en viktig paleobiologisk effekt då de fungerade som en lokal pump och reservoar för biodiviersiteten. De olika djurgruppernas kolonisering av kratrarna drevs främst av variationer av djup, sedimentunderlag och näringstillgång.

34 5. Acknowledgements

First of all I would like to thank Uppsala University for funding my PhD. Years of fieldwork for numerous projects, courses, and meetings has been supported by personal funding from a range of sources: Leksand municipal grants (UU), Wallenberg Foundation (UU), Lars Hiertas Memorial Founda- tion, Letterstedska Föreningen, International Association of Sedimentolo- gists (IAS) postgraduate grant, Swedish royal Academy of Science (KVA) grants, Bjurzons travel grants (UU), Quantative Palaeontology Summer School (through Bologna University), NordForsk grants, Håkanssons travel grant (UU), Liljewalchs travel grant, Heino & Sigrid Jänes Foundation (UU), and Otterborg travel grant (UU). NIR (Network on Impact Research) assisted in several grants for meetings and courses through NordForsk. In addition funding from the Swedish Research Council (VR) to Lars Holmer and Jan Ove R. Ebbestad, and a shared grant with Lars Holmer from Magnus Bergwalls Stiftelse, is acknowledged.

All three of my supervisors are greatly acknowledged; each has contributed to my understanding of various aspects of palaeontology and geology and further supported me throughout my PhD-studies. Lars Holmer has always been positive regarding my diverse projects and has encouraged me to attend fieldtrips, conferences, and courses all over the world. Jan-Ove R. Ebbestad has introduced me to many aspects of the Ordovician, Early to Late, and has further spread great enthusiasm in the field as well as in the lab. Maurits Lindström has been an unlimited well of knowledge and has always listened and answered all of my questions regardless of their being minute or grand.

I have several co-authors outside Uppsala University to thank for their co- operation. Dave Harper (Geological Museum, Copenhagen) has from the first moment I met him as a master student supported my work on different Ordovician projects and been a great source of information and discussions. Mats Eriksson (Lund University), with whom I have had many productive scientific discussions, but moreover shared the same taste in music with. Jens Ormö (Centro de Astrobiologia, Spain) in the company of Maurits Lindström introduced me to the Lockne area and has explained the impact cratering aspects of the localities. Together with Erik Sturkell (University of Gothenburg) they introduced me to the world of impacts and instantly I felt part of this research group. I am very thankful for that.

35 I have many people to thank in my local habitat at the Department of Earth Sciences at Uppsala University. To all the PhD students I have known at Palaeobiology (a.k.a. Pallen) thank you for sharing many hours of laughter not forgetting blood, sweat and tears. Sofia, Cissi, and Sandra deserve extra warm thanks, as does Sebastian for his patient ears. I would like to thank all the PhD-students I have gotten to learn and spent countless hours with from LUVAL, Berg, and Geophysics, and also at Evolutionary Organismal Biol- ogy (EBC). Hanna Ridefelt and I shared many hours of work, and joy. You taught me many words of wisdom. Special thanks go to Anette Högström for having her door open for me, all the time, and helping me with scientific ideas, or just listening. Michael Streng, thank you for always helping me out, and everyone else, it is much appreciated. John Peel, thank you for correc- tions, numerous discussions and advice.

Moreover I would like to send recognition to my friends at other depart- ments of earth sciences in Sweden, especially to Barbro at the Evolutionary Organismal Biology, Uppsala University, who became a dear friend to me. Christian Mac Ørum Rasmussen (Copenhagen), he made me laugh in all regions of the world during conferences and excursions. Maria Liljeroth (Copenhagen), I look forward to more mojitos. NIR (Network on Impact Research) arranged excellent courses and introduced me to a lot of people. And to all the ‘palaeogirls’: superb fikas.

Hannes has spent quite a lot of weeks with me in the field (but so did I during his PhD), for the company, discussions and all his help I am truly grate- ful.

To my parents and close friends from Stockholm, and my extended family in Skellefteå, I appreciate you all truly being there for me. I cannot do anything else then send you all my love. However, an immense amount of it I send to Hannes.

36 6. References

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