Anatomy of <i>Colymbosaurus megadeirus</i> (Reptilia, Plesiosauria) from the Kimmeridge Clay Formation of the U.K., and high diversity among Late Jurassic plesiosauroids

Abstract

<div><p>ABSTRACT</p><p>We describe the syntype postcranial skeletons of the cryptoclidid plesiosauroid <i>Colymbosaurus megadeirus</i>. <i>C. megadeirus</i> is the type species of <i>Colymbosaurus</i>, and one other valid species, <i>C. svalbardensis</i>, is known. Although it is in widespread usage, <i>C. trochanterius</i> is a not valid species, although the presence of an anteroposteriorly oriented ridge bisecting the distal surfaces of the holotype humerus allows its referral to <i>Colymbosaurus</i> sp. Some specimens previously referred to <i>C. trochanterius</i>, including the holotype of ‘<i>Plesiosaurus</i>’ <i>manselii</i>, lack this feature, or show other differences from our hypodigm, suggesting that they are taxonomically distinct from <i>Colymbosaurus</i>, and countering previous suggestions that the Kimmeridge Clay Formation represents a depauperate plesiosauroid fauna. Substantial differences between the atlas-axis complexes and cervical vertebrae of <i>Kimmerosaurus langhami</i>, ‘<i>Plesiosaurus</i>’ <i>manselii</i>, <i>C. megadeirus</i>, and specimens referred here to cf. <i>Spitrasaurus</i> indicate that at least four plesiosauroid taxa were present in the Kimmeridge Clay Formation. This proposition is also supported by coracoid morphotypes, and demonstrates that Late Jurassic plesiosauroids were taxonomically diverse. Most Late Jurassic–Early Cretaceous cryptoclidids belong to a monophyletic Colymbosaurinae, but <i>Kimmerosaurus</i> and its sister taxon <i>Tatenectes</i> originated independently from Middle Jurassic cryptoclidids.</p><p>SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP</p></div