Petrolacosaurus lived during the carboniferous Period, about 300 million years ago in North America. It measured 1 foot (40 centimeters) in length. Petrolacosaurus is one of the oldest known diapsid reptiles. The diapsids were a group of reptiles characterized by two openings on the side of their skulls, to which groups of muscles were attached. These openings were located just behind the eyes. It was from this group of reptiles that dinosaurs were later to evolve.