All rhomaleosaurids have a relatively large head with a moderately long neck composed of about 28 vertebrae. The group has a stratigraphical range from the Lower Jurassic to the Middle Jurassic. The Rhomaleosauridae is traditionally regarded as a pliosauroid family but some phylogenies place the clade outside of Pliosauroidea in a sister relationship or unresolved polytomy with Pliosauroidea and Plesiosauroidea.

Cast of the holotype of Rhomaleosaurus cramptoni. Taken in the Natural History Museum, London (photograph by Hector E. Rivera).

Rhomaleosaurid phylogeny

Cladogram of rhomaleosaurid pliosaurs (from Smith and Dyke, 2008)

Rhomaleosaurid genera


The genus Archaeonectrus was proposed by Novozhilov (1964) for ‘Plesiosaurus’ rostratus, a species named by Owen (1865). By modern standards, Owen’s (1865) original description is rather inadequate. One notable characteristic of Archaeonectrus rostratus is the relatively small size of the limbs relative to its body. It has been classified as a pliosauroid, sometimes within the …


The genus Atychodracon was erected by Smith (2015) to accommodate ‘Rhomaleosaurus’ megacephalus, because it is generically separarate from Rhomaleosaurus sensu stricto (Smith and Dyke, 2008). A. megacephalus is closely related to Eurycleidus and some authors have regarded A. megacephalus a distinct species of Eurycleidus. The holotype specimen of A. megacephalus was housed in the Bristol …


Avalonnectes is a small-bodied basal rhomaleosaurid. Avalonnectes was named by Benson, Evans and Druckenmiller (2012) for a partial skeleton including the rear part of the skull from the lowermost Jurassic of Street, Somerset, UK. The specimen (NHMUK PV OR 14550) was previously referred to Thalassiodracon hawkinsii and is one of many historical plesiosaur skeletons preserved …


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M. coccai (Gasparini, 1997) type material: MOZ 4386 V articulated skull and mandible, atlas-axis and first cervical vertebrae. From the upper part of the Los Molles Formation, Cuyo Group, Emileia giebeli and Emileia multiformis subzone.


M. victor was originally described and figured by Fraas (1910). Historically, M. victor has been regarded as a species of Rhomaleosaurus and is often associated in the literature under the defunct name ‘Thaumatosaurus’. Smith and Vincent (2010) identified M. victor as generically distinct from Rhomaleosaurus and erected the new name Meyerasaurus for it (Smith and Dyke, 2008). The …


Rhomaleosaurus is the largest known Lower Jurassic pliosaur and was the top predator in early Jurassic marine ecosystems. It has a reinforced skull to help resist torsion and a ferocious set of teeth, a combination of characters perfect for snatching and killing cephalopods, fish, and other marine reptiles. Historically, the genus Rhomaleosaurus has been interchangeable …


Thaumatodracon is a relatively large rhomaleosaurid from the Lower Jurassic (Sinemurian) of Lyme Bay – the coast between Lyme Regis and Charmouth – UK. The holotype specimen (NLMH 106.058) is an almost complete skull and cervical (neck) series. It has a 60 cm long skull, and based on comparison with other rhomaleosaurids I estimate its total …


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Several rhomaleosaurid genera are considered nomina dubia or are regarded as junior synonyms of other genera. The following names are therefore invalid :

  • ‘Thaumatosaurus’ = nominum dubium