Specimens reveal that Petrolacosaurus had a slightly elongated skull with two temporal fenestrae.
The upper temporal fenestra is located posteriorly to an enlarged orbit.
This is a distinctly diapsid character.

The largest teeth in the jaw were at the front of the snout, erupting from the premaxilla bone.
There were also a few larger-than average teeth further back on the maxilla, which were probably homologous with caniform teeth retained from the common ancestor between true sauropsid reptiles and synapsids (mammal ancestors).
On the dentary, there are marginal teeth, displaying a primitive type of shallow implantation.
The palatal arrangement bears close resemblance to Youngoides.
Specimens reveal that Petrolacosaurus had a slightly elongated skull with two temporal fenestrae. The upper temporal fenestra is located posteriorly to an enlarged orbit. This is a distinctly diapsid character.
The largest teeth in the jaw were at the front of the snout, erupting from the premaxilla bone. There were also a few larger-than average teeth further back on the maxilla, which were probably homologous with caniform teeth retained from the common ancestor between true sauropsid reptiles and synapsids (mammal ancestors). On the dentary, there are marginal teeth, displaying a primitive type of shallow implantation. The palatal arrangement bears close resemblance to Youngoides.
Petrolacosaurus