Ambedus pusillus, new genus, new species, a small diadectid (Tetrapoda: Diadectomorpha) from the lower permian of Ohio, with a consideration of diadectomorph phylogeny

Ambedus pusillus, new genus, new species, is a small diadectid from the Lower Permian Greene Formation of Ohio that is distinguishable from all other members of the group by the presence of conical anterior teeth of the maxilla and dentary, high maxillary and mandibular tooth counts, a lesser degree of molarization of the cheek teeth, a shallow dentary, lack of a labial parapet, and a relatively shallow alveolar shelf that suggests a more shallow tooth implantation than that of other diadectids. The recognition of A. pusillus as a member of Diadectidae is supported by the presence of a heterodont dentition that possesses transversely expanded, molariform cheek teeth bearing: 1) a central cusp flanked by labial and lingual shoulders; and 2) wear facets developing on the lingual and labial shoulders of the maxillary and dentary cheek teeth, respectively. Although lacking a deep lower jaw, which is characteristic of herbivores and found in other diadectids, the presence of procumbent anterior teeth, molariform cheek teeth, and wear facets similar to those of other diadectids suggest that Ambedus, like all other diadectids, may have been herbivorous. Phylogenetic analysis of Diadectomorpha confirms its status as a natural group, and it supports both the monophyly of Diadectidae and the sister-group relationship of Diadectidae to Tseajaia. The recognition of Limnoscelis as the sister-taxon to Tseajaia + Diadectidae is also supported. Within Diadectidae, Ambedus is the sister-taxon to all other diadectids, with Orobates, Desmatodon, and Diasparactus + Diadectes representing progressively more derived taxa in a series of nested clades. With a greater species-richness and wider geographic range relative to other diadectomorph taxa, it is hypothesized that the evolution of herbivory in Diadectidae led to an evolutionary radiation of species with in the group.