|Petrolacosaurus kansensis (Lane 1945, Reisz 1977) Late Carboniferous ~302 mya, ~40cm in length, is the oldest known diapsid reptile. Eudibamus, a more primitive diapsid is known from more recent strata. Derived from a sister to Spinoaequalis, Petrolacosaurus phylogenetically preceded Araeoscelis and Acerosodontosaurus at the base of the Diapsida.
Distinct from Spinoaequalis, the skull of Petrolacosaurus had a shorter rostrum and a larger naris and orbit. The teeth were smaller and sharper. The upper temporal fenestra was posterior to the orbit, not above it.
The cervicals were elongated and increased their number to seven. The torso was relatively short and the tail was attenuated distally. With such proportions, Petrolacosaurus resembled a little sprawling sauropod.
The pectoral girdle was robust and the forelimbs were long. The manus and pes were subequal in size. Metacarpal 1 was reduced such that metacarpals 2 and 3 were aligned with m1.1.
The ilium was relatively small. The puboischium was large, forming a ventral plate. The hind limbs were longer than the forelimbs. While the forearm elements remained relatively straight, the tibia and fibula were bowed with a large space between them. The intermedium fused to the tibiale, creating an astragalus. All four metatarsals were aligned. Metatarsal 1 was longer, or less reduced.
The family tree of the Enaliosauria is here. The complete reptile family tree is here.
It has gone previously unrecognized that sphenodontids, such as Sphenodon, and other lepidosaurs, such as Pteranodon and Hyperodapedon with their diapsid skulls, are convergent with Petrolacosaurus according to the present cladistic analysis. These taxa are not related to Petrolacosaurus and its various diapsid descendants except that all are reptiles.