Cranial anatomy of Thalassiodracon hawkinsii (Reptilia, Plesiosauria) from the Early Jurassic of Somerset, United Kingdom

Benson, R. B. J. and Bates, K. and Johnson, M. R. and Withers, P. J. (2011) Cranial anatomy of Thalassiodracon hawkinsii (Reptilia, Plesiosauria) from the Early Jurassic of Somerset, United Kingdom. Journal of Vertebrate Paleontology, 31 (3). pp. 562-574. ISSN 0272-4634 DOI

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The taxonomy and systematics of the earliest plesiosaurians is poorly resolved. This limits our understanding of the diversification of one of the most successful clades of secondarily aquatic tetrapods. Here we provide a robust diagnosis of Thalassiodracon hawkinsii from the Pre-planorbis Beds (Triassic–Jurassic boundary interval) of the United Kingdom, and suggest that at least two other, previously unrecognized plesiosaurians are present in the same deposits. Computed tomography of an exceptionally preserved skull, and examination of previously undescribed (or briefly described) specimens yields new anatomical data. Thalassiodracon has a dorsomedian ridge on the premaxilla, a squamosal bulb, four premaxillary teeth, and a heterodont maxillary dentition. Several features of Thalassiodracon, including the squmosal bulb, broad anterior termination of the pterygoids, heterodont dentition, and single foramen in the lateral surface of the exoccipital, are plesiomorphic or represent pliosauroid synapomorphies. Among pliosauroids, Thalassiodracon shares a parietal that extends far anteriorly, a broad, interdigitating posterior termination of the premaxilla, and a short posteroventral process of the postorbital with Hauffiosaurus and pliosaurids. Thus, we suggest pliosaurid affinities for Thalassiodracon, in contrast to most recent phylogenetic studies. The early stratigraphic position of Thalassiodracon coincides with the earliest occurrence of Rhomaleosauridae (the sister taxon of Pliosauridae). The relatively long neck and small skull of Thalassiodracon indicate that the robust skeleton and macropredaceous habits of rhomaleosaurids and pliosaurids were derived independently.

Item Type: Article
Uncontrolled Keywords: 2011AREP; IA64;
Subjects: 04 - Palaeobiology
Divisions: 04 - Palaeobiology
Journal or Publication Title: Journal of Vertebrate Paleontology
Volume: 31
Page Range: pp. 562-574
Identification Number:
Depositing User: Sarah Humbert
Date Deposited: 09 Jan 2012 12:14
Last Modified: 23 Jul 2013 10:03

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