Petrolacosaurus

Petrolacosaurus was a small, 40-centimeter (16 in) long, reptile, and the earliest diapsid known. It lived during the late Carboniferous period, the Joggins strata where it was found are dated to 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.

In popular culture
Petrolacosaurus was featured in the BBC television show Walking with Monsters. The animal is incorrectly used as an example of an ancestral amniote, or a "reptile" in the traditional Linnaean sense of the word. From an evolutionary point of view, this means that Petrolacosaurus was used to illustrate the first vertebrates which were already fully adapted to terrestrial life, but were still cold-blooded. It was portrayed as living alongside several species of giant arthropods, like giant mesothelae spiders, and Meganeura, a giant dragonfly, as well as anthracosaur amphibians like Proterogyrinus. In accordance with its use as a proto-amniote, Petrolacosaurus is portrayed as the common ancestor of both synapsids such as Dimetrodon, including mammals, and sauropsids, including modern reptiles and birds; it is also stated that its heart was the template for our own.

While Petrolacosaurus was indeed a relatively early amniote and was typically lizard-like, as were all the first amniotes, it was nevertheless too derived to be portrayed as the ancestor of all modern forms. In fact, it was already a diapsid, with two openings known as "temporal fenestrae" on each side of its skull to add attachment points for jaw muscles. Hence, it cannot have been the ancestor of any synapsids, which have only one such opening and diverged from the common amniote tree before the diapsids did.