Diplocaulus magnicornis (Cope 1882) Early Permian, skull width up to 33 cm, is a member of the lepospondyl clade Nectridea, derived from a sister to Keraterpeton and Scincosaurus.
All prior references (see figure below) indicate that the trailing edge of the "horn" was the tabular, but here it is identified as the supratemporal based on the positions of these bones in ancestral taxa.
Distinct from Keraterpeton 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.
The ontogeny (growth to maturity) of Diplocaulus is well known (see below).
Diplocaulis minimus (Detuit 1988, Germain 2010; Late Permian; MNHN ARG-501; below) is a small diplocaulid with asymmetrial horns that is more closely related to Diploceraspis.
Diploceraspis burkei (Romer 1952; Late Carboniferous to Early Permain)