Preservation of Marine Reptile Soft Parts: Reconstructing the Life and Death of Ancient Leviathans
Abstract: Marine reptiles constitute a diverse group of secondarily aquatic tetrapods that have inhabited the world’s oceans for the greater part of the last 250 million years. While certain large-scale anatomical changes have been observed through progressive alterations of their skeletal morphology (e.g., the evolution of fin-like limbs), little is known about those changes that have occurred in the integument during their transition from landlubbers to leviathans.This thesis has three main objectives: (1) to obtain novel ultrastructural and chemical information from fossilized marine reptile skin using a combination of cutting-edge analytical tools (e.g., mass spectrometry and spectroscopy); (2) to infer aspects of evolution, paleobiology and paleoecology from the acquired data; and (3) to deduce how skin is retained in the fossil record. To accomplish these aims, four exceptionally preserved soft-tissue fossils representing two major marine reptile clades were used as exemplary animals: a sea turtle from the earliest Eocene (~54 Ma) Fur Formation of Denmark and three ichthyosaurs from the Early Jurassic (~183 Ma) Posidonia Shale of Germany.The fossil sea turtle comprises a partial but semi-articulated skeleton with adhering scutes and dark-colored organic matter that surrounds one of the hind limbs. Comparisons with previously reported Eocene turtles identify the Fur Formation specimen as a stem pan-cheloniid, and more specifically as a close relative of Eochelone. Furthermore, the fossil preserves a mosaic of integumental features not previously documented in any living or extinct turtle: a shell covered in scutes and limbs devoid of scales. Collectively, my findings suggest that the adaptive transition to neritic waters by the ancestral panchelonioids was more complex than hitherto appreciated.Ichthyosaur skeletons with associated body outlines, have been the focus of scientific studies for almost 200 years. The recent discovery of relict blubber and countershading in an exceptionally preserved Stenopterygius indicates that derived ichthyosaurs were warm-blooded and thus even more dolphin-like than previously thought. This new knowledge has been used to create the most scientifically accurate sculptural model of an ichthyosaur to date, and the process on how this was achieved is the focus of my third paper.Ichthyosaurs from the Jurassic Posidonia Shale exhibit a range of preservational modes. An isolated dorsal fin and tail fluke were examined and compared against a previously documented semi-complete individual to determine potential qualitative differences in preservation between the three specimens. Microscopic and molecular examination revealed that the fin fossils comprise mats of densely packed melanosomes sandwiched between layers enriched in calcium phosphate. I interpret this structural arrangement as representing incompletely preserved skin from both sides of the fins. When compared against specimens with higher structural fidelity (such as the semi-complete specimen), it is readily apparent that the fins had reached an advance stage of decay before entering the fossil record. I conclude that differences in preservational modes between Posidonia Shale soft-tissue ichthyosaurs largely are a function of when and how fast mineralization could outpace decomposition.
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