Petrolacosaurus was a small, 40-centimetre (16 in) long, reptile, and the earliest diapsid known. It lived during the late Carboniferous period. The strata where it was found in Kansas, USA, are of Pennsylvanian age, and are approximately 302 million years old. The prehistoric reptile's diet may have consisted mainly of small insects. Petrolacosaurus had distinctive canine-like secondary-sized teeth, a trait found primarily in therapsids, and later in mammals. Its fossils were found in Kansas, USA.