Acerosodontosaurus piveteaui (Currie 1980, Bickelmann, Müller and Reisz 2009; Late Permian). Acerosodontosaurus descended from a sister to Galesphyrus, was a sister to the BPI 3859 specimen attributed to Youngina (see below) and phylogenetically preceded the clade of Hovasaurus, Thadeosaurus and Tangasaurus.
The only more primitive skull belongs to Spinoaequalis. Distinct from Spinoaequalis, the maxilla rose to block lacrimal contact with the naris. The jugal extended further anteriorly to the anterior orbit. The mandible was deeper. The dorsal ribs were longer, enclosing a wider than tall torso.
The humerus had a greatly expanded distal end. This contributed to enlarging the space between the radius (which had a convex bow) and ulna. Metacarpal 4 was no longer than mc 3. The phalanges of digits 2-4 lined up parallel to the metacarpal base line. The digit length disparity was not as great. The pelvis was more robust with all three elements similar in size to one another.
Bickelmann et al. (2009) determined that a bone considered to be quadratojugal (in purple) was instead a portion of rib. Because of that reasssessment the authors considered Acerosodontosaurus a phylogenetic enigma in which the phylogenetic position was poorly understood. That is not the case when the rest of the anatomy also supports a basal diapsid nesting in the present study.