Pachycostasaurus is approximately 3.1 meters long. Its rib cage and vertebrae exhibit thickened bone (Cruickshank et al. 1996) a condition termed pachyostosis. This heavy ossification is unusual in plesiosaurs (another exception may include Kronosaurus), although it is common in basal sauropterygians, especially pachypleurosaurs. Pachycostasaurus probably used the extra ballast provided by its heavy bones to traverse the sea bed searching for prey. Pachycostasaurus has been classified as a pliosaurid and is notable for its its delicately constructed skull and relatively short mandibular symphysis.
Etymology: Pachus = thick, costa = rib, sauros = lizard.
Type material: PETMG R338, an almost complete skeleton including most of the skull and some of the mandible, almost complete vertebral column and associated ribs, left pelvis, right ilium, both humeri, three phalanges from a hindlimb.
From the Peterborough Member of the Oxford Clay Formation (Jason Subzone), King’s Dyke Clay Pit, Wittlesey, Cambridgeshire, England.