Origin and early diversification of Plesiosauria and the influence of the end-Triassic extinction event: insights from the first unequivocal Triassic plesiosaurs

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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 of eastern Westphalia, Germany, and on abundant isolated putative plesiosaur remains from Rhaetian bonebeds of Germany (also new finds) and England. The neck vertebrae of early plesiosaurs commonly show a complex v-shaped neurocentral suture which may be linked to a stiffening of the neck during underwater flight while allowing continued growth. With the new evidence of a Triassic origin of plesiosaurs, nothing argues agains assignment of the Rhaetian isolated bones and teeth to Plesiosauria. The specimens inform us about the early diversification of the group because of their great size and shape disparity, with some bones recording animals exceeding 5 m in length! This suggests that plesiosaurs reacted to the end-Triassic extinction event by size reduction but not by diversity reduction. Specifically, the proposed project will describe the new (and only) Triassic plesiosaur skeleton, analyze its phylogenetic position, describe the isolated Rhaetian plesiosaur bones, analyze their phylogenetic position among sauropterygians, study early plesiosaur tooth enamel microstructure, analyze the biomechanics of the basal plesiosaur neck in the context of the v-shaped neurocentral suture, and analyze morphological and size disparity among Triassic plesiosaurs, comparing it with the Early Jurassic record. The comparison with the closest plesiosaur relatives, Pistosauridae and Bobosaurus, will lead to a refined understanding of plesiosaur origins, the influence of the end-Triassic extinction event on their evolution, and the evolution of their unique mode of locomotion.