A juvenile gomphodont cynodont specimen from the Cynognathus Assemblage Zone of South Africa: implications for the origin of gomphodont postcanine morphology

Abstract

The partial skull and lower jaws of a small gomphodont cynodont from the Cynognathus Assemblage Zone of South Africa has a well-preserved postcanine dentition distinctly different from that of contemporaneous adult Diademodon and Trirachodon. On the basis of its small size and great amount of tooth replacement it is interpreted to be a juvenile individual. The postcanines are compared with those of adults and juveniles of Diademodon and traversodontids and is seen to differ from them. Comparison with adults of Trirachodon shows some unique postcanine resemblances, such as well-developed anterior and posterior many-cusped cingula and three transverse cusps joined by a prominent ridge. Thus it is identified as a probable juvenile Trirachodon of uncertain species. Unlike in Trirachodon adults, tall central and internal cusps of the upper postcanines lie close together on the medial side of the crown, separated from the tall external cusp by a deep valley. In these features it shows a striking resemblance to the traversodontid Scalenodon angustifrons, but not to more primitive traversodontids. The lower postcanines superficially resemble those of traversodontids in that two cusps (central and internal) are very tall and the posterior basin is elongated, but, unlike in traversodontids, the external cusp is present, though relatively small. Evidence of tooth replacement occurs in the incisors, canines, and postcanines. At least two replacement waves of gomphodont teeth are indicated, as well as replacement of small, possibly sectorial teeth at the rear of the tooth row. Probable homology of (at least) the external and internal cusps in the three gomphodont families suggests that the common ancestor also possessed transversely-expanded crowns developed from an external sectorial position (homologous with the ancestral blade-like tooth) and a hypertrophied internal cingulum.National Science Foundation Research Grant BMS75-0115