318-300 million years ago


Although pelycosaurs were the earliest “mammal-like reptiles”, there are only a few characteristics which had been modified from the ancestral condition of the anapsid reptiles. The most prominent are the synapsid opening and the presence of canines.

Archaeothyris measured about 50 cm and possessed the single temporal fenestra (with the postorbital and squamosal bones above and the jugal bone below) which would characterize the synapsids. It had a long snout and the region of the skull behind the eye socket was short. The canine teeth were slightly larger than the others. Archaeothyris was thoroughly reptilian with a brain about the average size of a reptile’s brain.

All known groups of pelycosaurs appeared before the end of the Carboniferous and they, not the anapsid or diapsid reptiles, are the dominant terrestrial vertebrates of the Permian. All three suborders of pelycosaurs evolved from forms similar to Archaeothyris.