Echinerpeton and Archaeothyris are among the oldest known synapsids. They mark the mammal-side branch of amniotes, recognized by the single temporal opening behind the eye. UCMP frames Synapsida as a 300-million-year-old group with mammals as one surviving branch, while Archaeothyris is commonly treated as one of the oldest undisputed synapsids.

Picture in the foreground an Echinerpeton. It had a slightly more robust body and a subtle raised/back-ridge look. The Echinerpeton is an ancestor to the Dimetrodon so it had taller neural spines., but not a big Dimetrodon-style sail, yet.

In the midground, picture an animal on a log, that’s the Archaeothyris. It was smaller, lower, slimmer, and more agile. Kind of like a small, lizard-like early synapsid predator.

A common environment for these first synapsids was a swampy Carboniferous coal forest with wet ground, shallow water, giant lycopsid-like trunks, horsetail/clubmoss-style plants, and dense ferns. Giant dragonfly-like insects were around too. The environment of the synapsids was a “Coal Age forest” mood.

The synapsid breakthrough was not hearing yet. It was bite power. One skull opening behind the eye gave jaw muscles room to grow, and much later, the remodeling of that jaw system helped create the mammal ear.