Opisthotriton kayi

ATLAS: The cotyles of a partial atlas, AMNH 8481, are round in shape and are separated by a short intercotyle process. The posterior cotyle is round and is incompletely filled in with bone. The ventral side of the vertebra is marked by a depression, which is deeper on the anterior side and bounded by two ridges that meet along the midline. The atlas of Opisthotriton can be distinguished from that of Prodesmodon by its larger size, rounded cotyles, and more prominent intercotylar process. DORSAL VERTEBRAE: A dorsal vertebra of Opisthotriton kayi has been described in great detail by Estes (1964, pp. 82 - 83). TheJudith River Formation specimens are of the same size or slightly smaller than AMNH 8112 from the Lance Formation, but are otherwise morphologically indistinguishable. DENTARY: Unfortunately, specimen AMNH 8485 is fragmentary and without teeth. The tooth bases indicate that the teeth are pleurodont and extend close to the lingual edge of the tooth-bearing border. The anterior part of Meckel's fossa has been broken and the divergence between the dorsal and ventral border of the fossa is not so great as in Habrosaurus.