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The Fossil Flora of Shetland and Surrounding Areas

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The Fossil Flora of Shetland and Surrounding Areas THE FOSSIL FLORA OF SHETLAND AND SURROUNDING AREAS. VOLUME 1. A thesis submitted in candidature for the degree of Doctor of philosophy (2 Volumes). Ian Perry. Department of Botany. University of Bristol. MEMORANDUM. The work described in this thesis was carried out in the Department of Botany, University of Bristol. It is the original work of the author, except where otherwise stated, and no part of it has been submitted for any other degree at this or any other University. 30:8:89. SYNOPSIS. The fossil flora of Shetland, Orkney, northern Scotland, Norway and Greenland of the Middle Devonian (Givetian age) was investigated. Extensive collections were made from new and established localities resulting in the re-description of Svalbardia scotica sp. emend., Thursophyton milleri sp. emend. and Dawsonites roskiliensis sp. emend. 'Other fossil localities are recorded. The plants described here are preserved as; compressions/impressions, permineralised and fusainised material. Although the high thermal maturity of the deposits has rendered much of the material unidentifiable, some localities yielded well preserved morphology and anatomy. This allowed a detailed correlation between the morphological and anatomical details. The flora consisted of two major elements represented by Thursophyton milleri, a zosterophyll and Svalbardia scotica a progymno sperm. Anatomical preservation in pyrite and limonite was found and this represents the first of its kind from the study area. A polishing technique recently developed by Paul Kennrick was applied to investigate the anatomy. This provided high resolution images of anatomy comparable with that obtained by the Scanning Electron Microscope (SEM). An attempt was made to place the floral assemblage in Banks' (1980) stratigraphic zones, as well as interpreting the evolutionary significance of the plants found. ACKNOWLEDGEMENTS. I would like to thank Dr. Keith Allen for his supervision and encouragement during the course of this work. In addition I am indepted to; Dr. John Marshall for his comments and advice, Dr. Natascha Heintz of the University of Oslo for the loan of specimens from the Hoeg collection, Dr Olstein Jansen of the University of Bergen for the loan of type specimens from the Nathorst collection and the curators of the British Museum for their permission to inspect the Lang collection. Furthermore I appreciate the help and support of Diane Anstey while preparing this manuscript. CONTENTS. (Volume 1: text) CHAPTER 1. o. Introduction. 0. Early vascular plants a historical review. 1. Floral classification of early plants. 3. Devonian plant stratigraphy. 11. Devonian plant localities. 17. Middle Devonian plants of the Orcadian Basin. 21. CHAPTER 2. 26. Geology of the Orcadian Basin and the surrounding areas. 26. Shetland geology. 27. The Old Red Sandstone of Orkney. 38. Geology of Caithness. 41. Geology of Cromarty and the Moray Firth. 42. Geology of eastern Greenland. 44. Geology of western Norway. 47. Palaeogeography of the Middle Devonian. 49. CHAPTER 3. 53. Techniques. 53. CHAPTER 4. 66. Major plant bearing localites. 66. The South East Shetland basin. 66. The Walls Basin. 69. The Melby basin. 71. The minor plant bearing localities. 73. CHAPTER 5. Description of plants. 82. Thursophyton milleri. 82. Trimerophyton roskiliensis. 129. Svalbardia scotica. 138. CHAPTER 6. 177. Discussion. 177. Palaeoecology. 177. The Svalbardia/Archaeopteris complex. 184. Evolution of the leaf and Svalbardia. 187. Evolution and Thursophyton milleri. 188. Biostratigraphy. 190. Taphonomy. 192. REFERENCES. 195. TEXT FIGURES. Facing Page. Fig. 1. 17. Middle Devonian plant localities of the world. Fig. 2. 26. Devonian sediments of north east Scotland, Orkney and Shetland. Fig. 3. 27. Devonian sediments of Shetland. Fig. 4. 28. Devonian outcrop and locality map for south east Shetland. Fig. 5. 31. Devonian outcrop and locality map for west Shetland. Fig. 6. 33. Devonian outcrop and locality map for Foula. Fig. 7. 36. Devonian outcrop and locality map for Fair Isle. Fig. 8. 38. Devonian outcrop and locality map of Orkney. Fig. 9. 41. Devonian outcrop and locality maps for Caithness and Cromarty. Fig. 10. 45. Devonian sediments of east Greenland. Fig. 11. 45. Devonian sediments of east Greenland, cont. Fig. 12. 48. Devonian sediments of western Norway. Fig. 13. 53. Palaeogeography of the Middle Devonian. Fig. 14. 117. Reconstruction of a Metaxylem element from Thursophyton milleri. Fig. 15. 119. Reconstruction of Thursophyton Milleri. Fig. 16. 119. Anatomy of Thursophyton Milleri during branching. Fig. 17. 129. Comparison between the anatomy of Thursophyton milleri and related species. Fig. 18. 133. Reconstruction of Trimerophyton roskiliensis. Fig. 19. 164. Reconstruction of Svalbardia scotica. Fig. 20. 164. Reconstruction of the anatomy of Svalbardia scotica. Fig. 21. 164. Anatomy of some of the progymnosperms. Fig. 22. 164. Diagramatic reconstruction of secondary tracheids in Svalbardia scotica. Fig. 23. 190. Evolutionary relationships of Svalbardia and Thursophyton. TABLES Facing Page Table. 1.1 14. Devonian assemblage zones. Table. 1. 91. Metaxylem element dimensions of Thursophyton milleri from Skersund. Table. 2. 93. Metaxylem element dimensions of Thursophyton milleri from Sandwall. Table. 3. 99. Metaxylem element dimensions of Thursophyton milleri from Foula. Table. 4. 105. Metaxylem element dimensions of Thursophyton milleri from Norway. Table. 5. 108. Metaxylem element dimensions of Thursophyton milleri from Greenland. Table. 6. 108. Comparisons of element dimensions for Thursophyton milleri. Table. 7. 129. Comparison of xylem cell wall structure of Thursophyton with other species. Table. 8. 154. Comparison of measurements for Svalbardia scotica. CHAPTER 1. INTRODUCTION. During the past three decades there has generally been an increase in interest in the plants of Devonian age. This has been paralleled in the Orcadian basin by an increase in the study of the palynology of the area and in particular that of the Shetland basin, but not of the plants found in association with them. The Late Silurian and Early'Devonian periods probably represent the age of greatest vascular plant evolution and radiation in establishing the beginnings of land flora. None of these early plants survive to the present day but they gave rise to several diverse lines which evolved in the Middle Devonian and saw the appearance of all the major groups of Pteridophytes; and by the Late Devonian the first platyspermic and radiaspermic seeds. The investigation of Middle Devonian plants is thus an exciting and rewarding area of palaeobotanical study. The flora from the Orcadian basin has not been comprehensively studied since the work of Lang (1925, 1926.) and recent research into the palynology of the area has suggested that numerous plant localites are present, particularly within the Shetland Basin. The collections of John Marshall in particular have pointed to this area as being one worth further study. Other areas such as Western Norway and Eastern Greenland were thought to be worthy of inclusion within the study area because of the geographical proximity of the land masses during the Devonian and the similarity of the plant flora. Western Norway has long been known to have productive plant localites. While eastern Greenland has 0 recently been show to contain a limited plant flora. EARLY VASCULAR PLANTS A HISTORICAL REVIEW One of the earliest palaeobotanical contributions was the description of "fossil bamboo shoots" by the Chinese scholar Shen Kuo, published in 1086 (Li,1981). This may be the beginnings of descriptive palaeobotany and even now'the science still relies heavily upon observation and description. Today the discipline has extended to include theoretical approaches such as modelling, statistical analyses (Alvin et al , 1982; Cichan and Taylor, 1982; Hill, 1980; Spicer, 1981.), studies of reproductive biology (Dilcher, 1979), and analyses of fossil plant communities (Scott, 1979). J. W. Dawson 1859: Dawson published a description together with a reconstruction of a new genus Psilophyton, which at the time was regarded with much scepticism by his contemparies (Solms-Lombach, 1891) and despite his extensive pioneering collecting and work (1862, 1870, 1871, 1884) he had little impact on evolutionary thought at the time. Kidston and Lang 1917: The prevailing attitude of comparision between the fossil record and extant plants persisted until the description of a beautifully preserved silicified flora from the Lower Devonian Chert beds at Rhynie Scotland (Kidston and Lang, 1917, 1920a, b, 1921a, b) which allowed the accurate description of the plants clearly 1 far more primative than any similar extant plant. This together with the publication of a Lower and Middle Devonian flora from Western Norway (Nathorst, 1913, Halle, 1916) and the discovery of sclariform tracheids in a spiny axis of Psilophyton ornatum Dawson (Halle, 1916) supported Dawsons claims that these plants were indeed vascular plants of great simplicity. From these early works a series of floral classifications emerged, notably the concept of a psilophyte flora eirected by Lang (1917) which included Rhynia and Psilophyton under the term Psilophytales. Lang suggested that these plants were "characterised by the sporangia being borne at the ends of certain branches of the stem without any relation to leaves or leaf like organs." The name was derived from the genus Psilophyton and was used to suggest a resemblence between plants of the class and the
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