The pelycosaurs were among the first mammal-like reptiles to evolve. They were the earliest and most primitive members of the synapsids (Therapsida), which were the precursors of mammals. The pelycosaurs were the dominant life form for many millions of years.

Many pelycosaurs had a very distinct and useful feature. Off of the backs of numerous pelycosaurs was a "sail", or fin like object. One possibility for this sail is thought to have been a device to regulate body temperature. The sail was rich in blood vessels. It would have been warmed by absorbing sunlight, or it could have given off heat. The pelycosaurs could have used their sails to warm themselves by facing the sail towards the sun, or to cool themselves facing it into the wind and away from the sun. The pelycosaurs may have also used their sails for sexual display and camouflage. It is important to note that not all pelycosaurs had a sail.

These are fossils of a type of pelycosaur, called edaphosaurus.

A feature that was very important to the pelycosaurs was the synapsid skull. It featured a large opening on the side of the skull, behind the eye socket. The opening gave the pelycosaurs, and their Therapsid descendents, an advantage over other animals because it allowed for the development of larger and longer jaw muscles. The development of these muscles allowed pelycosaurs to kill prey more effectively and efficiently, though not all were carnivores. Thus, in hard times, pelycosaurs, or their descendents with the synapsid skull, were more likely to survive due to their enhanced ability as killers. Most pelycosaurs were carnivores and had large, sharp teeth to aid them in killing and eating their prey. However, the pelycosaur's physical advantages where not enough to ensure their survival. The rein of the pelycosaurs ended during the Permian extinction.