Diplocaulus magnicornis (Cope 1882) Early Permian, skull width up to 33 cm, was originally considered a member of the Nectridea. Here, however, Diplocaulus was derived from a sister to the microsaur Tuditanus. Predecessor taxa with smaller "horns" include Diceratosaurus, Stegops and Eoserpeton (see below), but note that in none of these do the postparietal and the tabular expand along with the supratemporal.
All prior references (see figures) indicate that the leading edge of the "horn" was the squamosal, but here it is identified as the supratemporal. The confusion may have begun with Stegops, in which the supratemporal shifted posterior to the jugal and the squamosal was in the supratemporal position typically seen in nectrideans, such as Sauropleura. The situation is perhaps clarified with Eoserpeton, in which the bones have a more basal configuration.
Distinct from Tuditanus the skull of Diplocaulus had smaller orbits and narial openings further forward on the skull. The narial openings were on the leading edge, facing anteriorly. The premaxilla contacted the frontals, widely separating the nasals. The skull was flatter, with orbits on top of the skull and much wider due to the widening of the skull roof. The parietals, postparietals, tabululars and supratemporals expanded the most. The postorbital was tiny. The jaws were only half as wide as the back of the skull. The interpterygoid openings were larger and oval shaped. The marginal teeth were tiny. The vomer and palatine teeth formed a concentric single row of similarly-sized teeth lingual to the marginal teeth.
The entire body was flattened with elongated cervical and dorsal ribs contributing.. The tail apparently had no transverse processes, but the caudal vertebrae were robust and produced a tail longer than the presacral length.
Metacarpal 2 was the longest and most robust. The carpals and tarsals were poorly ossified. The femur included a long and sharp trochanter. The tibia was more robust than the fibula.