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.
A friend of mine has asked me for a reconstruction of Rhomaleosaurus cramptoni to illustrate a book he is writing on prehistoric animals from England. R. cramptoni is one of these marine reptiles belonging to a group called plesiosaurs. It lived during the Early Jurassic in the shallow sea that was covering most of Western Europe at that time.
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.
Rhomaleosaurus and related taxa (the rhomaleosaurid family) were kind of oddballs among the plesiosaurs, as they combine features of the two morphotypes, i.e. they have relatively long necks and relatively large heads. A comprehensive phylogenetic analysis by Adam S.Smith and Gareth J. Dyke (2008) placed them firmly within the pliosaurs as sister group to the Leptocleidoidea and the Pliosauridae (pliosaur sensu stricto).
Rhomaelosaurus cramptoni, originally described in 1863 as a species of Plesiosaurus, was medium-size, measuring something like 7 meters in length. It is known from a complete skeleton of an adult specimen, which was unearthed in 1848 near Whitby, Yorkshire. Besides R. cramptoni, two other species are still tentatively recognized within the genus, R. zetlandicus and R. thorntoni, both from the Early Jurassic of England. The species R. propinquus is considered a junior synonym of R. cramptoni. “R. victor” (Early Jurassic of Germany) was transferred in 2010 to the new genus Meyerasaurus and “R. megacephalus” (Lower Jurassic, England) was found to be more closely related to Eurycleidus and Macroplata than to the other species of Rhomaleosaurus and will most likely be redescribed as a new genus in the near future. Rhomaelosaurids were a remarkably shorted lived family, being present only in the Early and perhaps in the Middle Jurassic.
Proportions for my reconstruction is based on the skeletal published by A. S. Smith (2008).
References:
Carte A, Bailey WH. 1863. “Description of a new species of Plesiosaurus, from the Lias, near Whitby, Yorkshire”. J R Dublin Soc 4:160–170.
O’Keefe FR. 2001. “A cladistic analysis and taxonomic revision of the Plesiosauria (Reptilia: Sauropterygia)”. Acta Zool Fenn 213: 1–63.
Smith AS, Dyke GJ. 2008. “The skull of the giant predatory pliosaur Rhomaleosaurus cramptoni: implications for plesiosaur phylogenetics” Naturwissenschaften. 95, pp 975-980.
Smith AS, Vincent P. 2010. "A new genus of pliosaur (Reptilia: Sauropterygia) from the Lower Jurassic of Holzmaden, Germany". Palaeontology 53 (5): 1049–1063.
Taylor MA. 1992. Functional anatomy of the head of the large aquatic predator Rhomaleosaurus zetlandicus (Plesiosauria: Reptilia) from the Toarcian (Lower Jurassic) of Yorkshire, England. Philos Trans R Soc Lond Ser B 335:247–280.
