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.

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, which deserved generic separation from Rhomaleosaurus sensu stricto (Smith and Dyke, 2008). A. megacephalus is closely related to Eurycleidus and some authors have regarded A. megacephalus as a distinct species of Eurycleidus. The original holotype specimen of A. megacephalus was housed in the …


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 …


Under construction


Under construction


Under construction


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 …

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