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

INTRODUCTION Gomphodonts are a clade of derived non-mammalian cynodonts characterized by the possession of transversely expanded postcanine teeth with crown-to-crown occlusion. These molar-like postcanines, characterized as ‘gomphodont’ teeth, are generally considered to be specialized for processing a predominantly herbivorous diet (Hopson 1971). If the contentious Tritylodontidae are omitted (see discussions in: Hopson & Kitching 2001; Abdala & Ribeiro 2003), three family-level groups of gomphodont cynodonts are recognized: Diademodontidae, Trirachodontidae and Traversodontidae. In all three families, the expanded postcanines show a progressive decrease in crown wear from the first expanded tooth to the last, indicating that the teeth decrease in age from front to back with no indication of replacement within the row. However, this simple replacement sequence may not be sufficient to explain gomphodont tooth succession, for most taxa show evidence of more complex replacement patterns involving teeth of other morphologies. Furthermore, replacement patterns have been studied in growth series of very few species (e.g. Scalenodon angustifrons (Crompton 1955); Diademodon sp. (Hopson 1971)), so large portions of the full ontogenetic series are unknown for most taxa. Diademodontids appear to represent the most primitive condition of dental morphology and replacement pattern among gomphodont cynodonts (Fourie 1963; Hopson 1971; Osborn 1974). In Diademodon, the tooth rows end in a series of three or more teeth in which the crown morphology grades from the fully expanded type to a fully sectorial (bladelike) type. Diademodon also appears to be primitive among gomphodonts in having the anterior expanded teeth sequentially replaced by simpler, more pointed, non-occluding postcanines (Crompton 1963; Hopson 1971). In trirachodontids and traversodontids, the anterior series of simple teeth is not seen, but sectorial teeth often occur at the rear of the tooth row, though the intermediate morphologies of Diademodon do not occur; rather, sectorial crowns usually lie immediately behind fully expanded teeth (Crompton 1955). In more derived taxa of traversodontids, such as Massetognathus and Exaeretodon, posterior sectorial teeth are unknown and the entire postcanine series appears to consist only of fully expanded teeth. Disagreement also persists with regard to the homologies of the main postcanine cusps among the three gomphodont families and also between gomphodonts and their carnivorous ancestors with more sectorial postcanines. As noted by Abdala & Ribeiro (2003, p. 534), it appears that the external, sectorial, border of the postcanines in Diademodon is homologous with the sectorial postcanines of non-gomphodont cynodonts, with the expanded medial portion of the tooth originating by hypertrophy of the lingual cingulum of the ancestor, as