Pachycostasaurus Cruickshank et al. 1996
Classification
Sauropterygia > Eosauropterygia > Eusauropterygia > Pistosauroidea > Plesiosauria >
P. dawni Young, 1958 (type species)
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.
Distribution
Middle Jurassic (Callovian). Peterborough Member, Oxford Clay Formation (Jason Subzone), the King's Dyke Clay Pit, Wittlesey, Cambridgeshire, England.
Images
Pachycostasaurus teeth, scale bar = 5mm (from Cruickshank et al. 1996)
Pachycostasaurus skeleton in preparation (Dawn, 1997)
Discussion
Etymology: Pachus = thick, costa = rib, sauros = lizard. Pachycostasaurus is approx. 3.1 meters long. The rib cage and vertebrae of this recently published genus (Cruickshank et al. 1996) exhibit bone thickening, a condition termed pachyostosis. This heavy ossification is unusual in plesiosaurs (another exception may include Kronosaurus), although it is a common character in basal sauropterygians, especially the pachypleurosaurs. Pachycostasaurus probably used the extra ballast provided by its heavy bones to traverse the sea bottom searching for prey, or to hide in ambush. As a pliosaur, Pachycostasaurus is also rather unique in it's delicately constructed skull and short mandibular symphysis (the area where the left and right dentary rami meet at the anterior of the mandible). A detailed description of Pachycostasaurus is pending.