318-300 million years ago


Pelycosaurs (the most primitive synapsids) first appear in the Upper Carboniferous Period (the Pennsylvanian).

The first pelycosaur, Archaeothyris, would have resembled a large iguana with short limbs. There is an earlier species Protoclepsydrops known only from a few bone fragments. Like early anapsid reptiles, Archaeothyris was a sprawling reptile and it was closely related to the first anapsid reptiles, the captorhinomorphs. Captorhinomorphs had a few derived characteristics compared to synapsids such as a reduced tabular bone; supratemporal and postorbital bones which don’t contact each other, a single coronoid bone in lower jaw as opposed to two in synapsids, and lacked the large medial centrale bone in the foot which synapsids possessed. These characteristics of the earliest known reptiles which are more advanced than the condition of pelycosaurs indicate that the pelycosaurs evolved from a slightly more primitive but as yet unknown group of anapsid reptiles. There is an early reptile, Promeriscus from the Upper Carboniferous, in which the supratemporal and postorbital bones contact each other as in pelycosaurs. Although Promeriscus lacks the temporal fenestra that gives the synapsids their name, there is an open suture (a line of weakness) in this same region. Many amphibians have this characteristic as well and it is thought that this open suture was a remnant of a movable hinge from the skull of crossopterygians fish (Kemp, 1982; Carroll, 1988).