Diadectids come in many sizes, all bulky. Wiki considers them to be anamniotes (pre-reptiles), the first herbivores among tetrapods and the first tetrapods to attain large size. These are all debatable.
Ambedus pusillus (Kissel and Reisz 2004) is from the Early Permian of Ohio. It was considered the most primitive diadectid and one of the smallest. Like larger taxa, it had labiolingually broad blunt teeth with a central cusp over many of them. This genus is represented by MCZ 9436 (Fig. 1).
Kissel (2010) wrote: “Diagnosis: A small diadectid distinguishable from other members of the group by: 1) a shallow dentary; 2) relatively high maxillary and mandibular tooth count; 3) lack of a labial parapet of dentary; 4) anterior teeth of maxilla and dentary conical, in contrast to the incisiform anterior teeth of other diadectids; and 5) shallow alveolar shelf, which suggests a relatively shallow tooth implantation.”
Not a juvenile
Kissel (2010) wrote, “the remains described herein as A. pusillus possess none of the features that typify known juvenile individuals of previously described diadectid taxa. All elements are therefore thought to represent those of adult individuals.”
Short tooth roots
Kissel (2010) wrote, “The shallow alveolar shelf in Ambedus pusillus suggests that tooth implantation was not as deep as that in other diadectids. The shallow alveolar shelf of MCZ 9436 indicates that root length is less than crown height in Ambedus, as observed in the diadectomorphs Limnoscelis and Tseajaia.”
No dentine infolding
Kissel (2010) wrote, “In no specimen is it possible to determine if the marginal teeth of Ambedus exhibit infolding of the dentine, a feature present in all other diadectomorphs.”
No incisiform teeth
Kissel (2010) wrote, “The maxillary dentition of heretofore known diadectids consists of two incisiform teeth that are succeeded by a series of molariform cheek teeth. The maxillary dentition of Ambedus adheres to this general pattern, but the anteriormost teeth of MCZ 9436 are not incisiform.”
Unique tooth number
Kissel (2010) wrote, “MCZ 9438, a complete left dentary, possesses a complete tooth row, and a total of 22 teeth are present. Such a tooth count represents the greatest yet recorded for a diadectid, with the mandibular tooth counts of other diadectids including 14 to 18 for Diadectes, 15 for Diasparactus, 14 for Desmatodon and 17 for Orobates.”
Humerus not referred
Kissel (2010) also referred to MCZ 8667 an isolated humerus that was collected within the same vicinity as the maxillae and dentaries. Kissel wrote: “Because the humerus exhibits no features indicative of Diadectidae, it is not referred to Ambedus pusillus, and it is therefore not described herein.”
What we learn from the above is Ambedus is not very much like other diadectids. One wonders then, is it something else? Related taxa with a shallow dentary and more teeth include Solenodonsaurus and the chroniosuchids, neither of which had diadectid teeth. Milleretta is also similar (Fig. 1). Not sure about the tooth shapes there. Then again there’s a third, perhaps more likely possibility based on tooth shape and number. Ambedus may be the romeriid root taxon for all three of these clades with a nod toward the diadectidae based on tooth shape. If so, that humerus may come back “into play.” Notably the manus of Romeria priimus is very slender and very un-diadectid-like. Not sure what the rest of it looks like. We’ll see if the humerus data helps answer those questions. Currently it’s on loan.
Kissel R 2010. Morphology, Phylogeny, and Evolution of Diadectidae (Cotylosauria: Diadectomorpha). Toronto: University of Toronto Press. pp. 185. online pdf
Kissel RA and Reisz RR 2004. Ambedus pusillus, new genus, new species, a small diadectid (Tetrapoda: Diadectomorpha) from the Lower Permian of Ohio, with a consideration of diadectomorph phylogeny. Annals of Carnegie Museum 73:197-212.