Spinoaequalis is an extinct genus of diapsid reptile. The 1 foot long creature, known from fossils found in Kansas was one of the first diapsids, along with Petrolacosaurus. It was also the first reptile to return to the water, evolving shortly after Hylonomus, the oldest confirmed reptile. Spinoaequalis was not fully aquatic, frequently returning to dry land. It probably swam using its laterally flattened, fanned tail. Spinoaequalis has been found along with preserved marine fish, suggesting it occasionally left fresh water streams for the sea.