Spinoaequalis schultzei (deBraga and Reisz 1995) Late Carboniferous ~306 mya, ~30 cm in length, was originally considered a basal diapsid close to Petrolacosaurus and Araeoscelis, but with a deep, sculling tail. Here Spinoaequalis was derived from a sister to Milleropsis and Eudibamus and phylogenetically preceded the araeoscelids at the base of the Diapsida.
Distinct from Eudibamus, the skull was shorter, like that of Petrolacosaurus. A possible upper temporal fenestra appeared. The teeth were longer.
The neural spines of the tail were much taller. This created a sculling tail ideal for swimming.
The scapula and coracoid were not fused. The ulna and radius were straighter and closer together.
The tibia and fibula were straighter and closer together. Metatarsal 4 remained the widest proximally.