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).
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.