Silurian Vertebrates of Gotland (Sweden) and the Baltic Basin

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. © Oskar Bremer 2017 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 Reprints were made with permission from the respective publishers. Paper II: © 2016 Elsevier B.V. All rights reserved. Paper V: © Cambridge University Press 2017. 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 5 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

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