Pelycosauria

Pelycosauria is an extinct order of synapsids, a group of mammals-like reptiles that lived during the late Paleozoic and early Mesozoic eras. They are considered to be the earliest known synapsids and are often referred to as the "stem-group" of mammals. Pelycosaurs were characterized by their large size, robust bodies, and a variety of adaptations for both terrestrial and aquatic environments. They had a single temporal fenestra, a large opening in the skull, which is a synapomorphy, or shared derived characteristic, of synapsids. Pelycosaurs also had a unique jaw structure, with a single tooth row and a large, flat tooth surface, which is thought to have been used for crushing and grinding food. The order Pelycosauria is divided into two suborders: Eothyropoda and Sphenacodontia. Eothyropoda includes the earliest known pelycosaurs, such as Hylonomus and Dimetrodon, while Sphenacodontia includes more derived forms, such as Sphenacodon and Dina. Pelycosaurs went extinct during the Permian-Triassic extinction event, which is thought to have been caused by a combination of environmental changes and volcanic activity. Despite their extinction, pelycosaurs played a significant role in the evolution of mammals, as they are considered to be the closest living relatives of mammals.