Origin and early diversification of Plesiosauria and the influence of the end-Triassic extinction event: insights from the first unequivocal Triassic plesiosaurs
Final Report Abstract
Together with ichthyosaurs, plesiosaurs are the quintessential marine reptiles of the Age of Dinosaurs. Plesiosaurs belong to the clade Sauropterygia together with the exclusively Triassic placodonts, nothosaurs, and pistosaurs. Plesiosaurs are unique in that the forelimbs and hind limbs are of equal shape and were used in a four-winged underwater flight. The plesiosaur body is short and broad, and the primitive forms as well as the plesiosaurs proper have a very long neck with a small head. Plesiosaur origins are poorly understood because of the paucity of Late Triassic marine reptile localities and the lack of well-preserved intermediate forms between pistosaurs and plesiosaurs. Hitherto, plesiosaurs were exclusively known from the Jurassic and Cretaceous, with the oldest representatives from the earliest Jurassic of England. These are generally small-bodied forms, not exceeding 2 m in length. This project focusses on the first Triassic plesiosaur skeleton, a new find including skull remains and teeth, from the Rhaetian (latest Triassic) of Bonenburg, eastern Westphalia, Germany. We named a new taxon, Rhaeticosaurus mertensi, based on this skeleton and showed it through phylogenetic analysis to be a basal pliosaur deeply nested within plesiosaurs. Small body size (ca. 2.5 m total length) is partially due to the juvenile status of the individual, but clearly Rhaeticosaurus is a small-bodied taxon. Rhaeticosaurus shows all features of later plesiosaurs but differs in the long and slender humerus and femur with relatively little expanded distal ends. However, a large plesiosaur humerus from the same locality indicates that some latest Triassic plesiosaurs were large-bodied. Plesiosaurs thus suffered a decrease in body size across the Triassic-Jurassic boundary. Our research also included the first detailed comparative study of plesiosaur bone histology. We found that plesiosaur bone histology is as unique as several features of plesiosaurian osteology and functional morphology. Plesiosaur cortical bone histology is characterized by a peculiar kind of radial fibrolamellar bone in juveniles to early adults. Later in ontogeny this tissue is remodeled into dense Haversian bone and secondary trabecular bone, resulting in an apparent ontogenetic mass decrease of bone. A low number of growth cycles indicates that plesiosaur growth was extremely fast: juveniles reached over 60% of final cortical thickness by the end of the first year and skeletal maturity in less than five years. Histomorphometry indicates cortical apposition rates and inferred resting metabolic rates in the range of extant birds. Plesiosaurs thus must have been endotherms that gave birth to few but very large offspring for which they provided parental care. Together with the cruising capabilities in the pelagic environment provided by endothermy, this may explain plesiosaurian survival of the end-Triassic extinction event. Other aspects of the project concern that hallmark of plesiosaurs, the elongated and highly segmented neck. We found that, counter-intuitively, the plesiosaur neck was rather stiff, unlike that of other long- necked sauropterygians. The plesiosaur neck presumably served in visual and hydrodynamic camouflage for preying on schooling fish and squid. We also discovered a unique anatomical feature in neck of all plesiosaurs, intersegmental artery foramina, and extended our research on the vertebral column. This extension is a reconstruction of the evolution of the amniote intervertebral joint, again using histological evidence, showing that the intervertebral disk arose repeatedly in amniote evolution, as did synovial intervertebral joints. In conclusion, the project resulted in a deeper understanding of many aspects of plesiosaur evolution and paleobiology. The project resulted in a total of 11 peer-reviewed publications. The published results, especially on Rhaeticosaurus, received numerous reports in the public media.
Publications
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(2016). Die paläontologische Grabung in der Rhät-Lias-Tongrube der Fa. Lücking bei Warburg-Bonenburg (Kr. Höxter) im Frühjahr 2015. Geologie und Paläontologie in Westfalen 88, 11-37
Sander, P. M., Wintrich, T., Schwermann, A. H. & Kindlimann, R.
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(2017). A Triassic plesiosaurian skeleton and bone histology inform on evolution of a unique body plan. Sciences Advances 3, e1701144, 1-11
Wintrich, T., Hayashi, S., Houssaye, A., Nakajima, Y. & Sander, P. M.
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(2017). Foramina in plesiosaur cervical centra indicate a specialized vascular system. Fossil Record 20, 279-290
Wintrich, T., Scaal, M. & Sander, P. M.
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(2018). Quantitative histological models suggest endothermy in plesiosaurs. PeerJ 6, e4955
Fleischle, C., Wintrich, T. & Sander, P. M.
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(2019). Neck mobility in the Jurassic plesiosaur Cryptoclidus eurymerus: finite element analysis as a new approach to understanding the cervical skeleton in fossil vertebrates. PeerJ 7, e7658
Wintrich, T., Jonas, R., Wilke, H.-J. & Sander, P. M.
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(2019). Ontogeny of polycotylid long bone microanatomy and histology. Integrative Organismal Biology 1, oby007-oby007
O'Keefe, F. R. O., Sander, P. M., Wintrich, T. & Werning, S.
