Thalassiodracon hawkinsi

Apart from the name (which means “sea dragon”), there is a priori nothing particularly impressive about the plesiosaur Thalassiodracon. With a body length of less than 2 meters, it lies on the small side size wise in this highly successful group of mesozoic marine reptiles that roamed the seas for some 150 millions years between the Late Triassic and the end of the Cretaceous. What so special about Thalassiodracon is that it happens to be the plesiosaurs’ earliest representative known from complete remains, and thus possibly one of the most primitive. A thorough study of the remains of Thalassiodracon would therefore likely hint clues about the evolutionary history of the entire group. The monotypic genus Thalassiodracon (with a single species T. hawkinsi named after the discoverer of the first fossil of the animal, Thomas Hawkins) is known from several skeletons from the late Triassic- Early Jurassic boundary of Somerset, England.

Plesiosaurs were traditionally divided into two morphology-based clades: the long-necked small-headed Plesiosauroidea (the plesiosaurs sensu stricto) with forms such as Plesiosaurus and Elasmosaurus, and the short-necked large-headed Pliosauroidea (the pliosaurs), represented by such animals as Kronosaurus and Liopleurodon of WWD fame. Thalassiodracon has been previously classified either as a basal Plesiosauroidea (Druckenmiller and Russell (2008), Smith and Dyke (2008), Ketchum and Benson (2010)) or a basal Pliosauroidea (O’Keefe (2001)).

A reexamination of the skull of a specimen of Thalassiodracon using a modern 3D imaging technique known as X-ray microtomography, and conducted by Roger B. J. Benson, Karl T. Bates, Mark R. Johnson, and Philip J. Withers, has revealed previously overlooked details of the cranial anatomy that helped solving the exact systematic position of this animal. The study shows that Thalassiodracon, albeit its relatively small skull and long neck is a basal member of the Pliosaurid family (the most derived family within the Pliosauroidea), to which belong the huge cretaceous marine monsters, Liopleurodon and Kronosaurus. A consequence of this phylogenetic placement is that the other large-headed short-necked family, the Rhomaleosauridae (which was the subject of a previous post) would have developed their morphology quite independently from the Pliosauridae. Thalassiodracon also extends the Pliosaurid lineage to the Late Triassic (The title of earliest pliosaurid was previously held by Hauffiosaurus from the Early Jurassic). The study by Benson and co-workers has just been published in the May 2011 issue of the Journal of Vertebrate Paleontology.

References:

Benson, R.B. J. , K. T. Bates, M. R. Johnson, and P. J. Withers. 2011. Cranial Anatomy of Thalassiodracon hawkinsii (Reptilia, Plesiosauria) from the Early Jurassic of Somerset, United Kingdom. 31(3):562-574.

Druckenmiller, P. S., and A. P. Russell. 2008. A phylogeny of Plesiosauria (Sauropterygia) and its bearing on the systematic status of Leptocleidus Andrews, 1922. Zootaxa 1863:1–120.

Ketchum, H. F., and R. B. J. Benson. 2010. Global interrelationships of Plesiosauria (Reptilia, Sauropterygia) and the pivotal role of taxon sampling in determining the outcome of phylogenetic analyses. Biological Reviews 85:361–392.

O’Keefe, F. R. 2001. A cladistic analysis and taxonomic revision of the Plesiosauria (Reptilia: Sauropterygia). Acta Zoologica Fennica 213:1–63.

Smith, A. S., and G. J. Dyke. 2008. The skull of the giant predatory pliosaur Rhomaleosaurus cramptoni: implications for plesiosaur phylogenetics. Naturwissenschaften 95:975– 980.

Storrs, G. W., and M. A. Taylor. 1996. Cranial anatomy of a new plesiosaur genus from the lowermost Lias (Rhaetian/Hettangian) of Street, Somerset, England. Journal of Vertebrate Paleontology 16:403–420.