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

蛇颈龙和鱼龙是恐龙时代最典型的海洋爬行动物。蛇颈龙与三叠纪的牙齿龙、无龙和气龙一起属于蜥脚类。蛇颈龙的独特之处在于它们的前肢和后肢形状相同，并能在水下进行四翼飞行。蛇颈龙的身体又短又宽，原始的蛇颈龙和真正的蛇颈龙都有一个很长的脖子和一个小脑袋。由于缺乏晚三叠世海洋爬行动物的位置，以及缺乏保存完好的介于pistosaur和蛇颈龙之间的中间形式，蛇颈龙的起源知之甚少。迄今为止，人们只知道蛇颈龙来自侏罗纪和白垩纪，最古老的代表来自英国最早的侏罗纪。它们通常体型小，长度不超过2米。这个项目的重点是第一具三叠纪蛇颈龙骨架，这是一个新发现，包括头骨遗骸和牙齿，来自德国东威斯特伐利亚的雷提亚人，以及来自德国（也是新发现）和英国雷提亚人的大量分离的假定蛇颈龙遗骸。早期蛇颈龙的颈椎骨通常有一个复杂的v形神经中枢缝合线，这可能与在水下飞行时颈部的僵硬有关，同时允许继续生长。有了三叠纪蛇颈龙起源的新证据，没有人反对将雷亚人孤立的骨头和牙齿归为蛇颈龙。这些标本告诉我们这个群体的早期多样化，因为它们的体型和形状差异很大，一些骨头记录了长度超过5米的动物！这表明蛇颈龙对三叠纪末灭绝事件的反应是缩小体型，而不是减少多样性。具体而言，拟建项目将对三叠纪蛇颈龙新（也是唯一的）骨骼进行描述，分析其系统发育位置，描述分离的雷蒂亚蛇颈龙骨骼，分析其在蜥脚类动物中的系统发育位置，研究早期蛇颈龙牙釉质微结构，分析v型神经中心缝合背景下蛇颈龙基底颈的生物力学，分析三叠纪蛇颈龙之间的形态和大小差异。将其与早侏罗世的记录进行比较通过与蛇颈龙的近亲——Pistosauridae科和Bobosaurus的比较，我们将对蛇颈龙的起源、三叠纪末灭绝事件对它们进化的影响以及它们独特的运动方式的进化有更深入的了解。

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.