Archaeothyris florensis (Reisz 1972) Late Pennsylvanian, ~306 mya, ~25 cm snout-to-vent length, was derived from a sister to, Varanops and phylogenetically preceded Ophiacodon and Haptodus.
Distinct from Varonops, the skull of Archaeothyris was relatively larger and narrower posteriorly. The postorbital region was shorter and the whole region angled ventrally toward a smaller occiput (back of skull). The lateral temporal fenestra was reduced here and further reduced in Ophiacodon. The reduction occured because the squamosal extended below the fenestra to contact the jugal. The bones above the orbits were raised slightly, further increasing the downward tilt at the back of the skull. The maxilla was convex ventrally, concave below the orbit and descended posteriorly. This combination produced a posterior component to the pull of the jaw muscles, which probably meant Archaeothyris was pulling with its canine teeth. The mandible was deeper and without a mandibular fenestra.
The ribs were much longer on Archaeothyris, creating a deeper torso.
The scapula was taller and slender. The clavicles were much longer medially. The fore and hind limbs were nearly the same size relative to all their elements.