Pelycosauria

"Pelycosaurs" Temporal range: Pennsylvanian - Capitanian, 308–260.4 Ma Descendant taxon Therapsida survives to present. PreЄ Є O S D C P T J K Pg N
Mounted skeleton of Dimetrodon mileri, Harvard Museum of Natural History
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Reptiliomorpha
Clade:	Amniota
Clade:	Synapsida
Informal group:	†Pelycosauria

The pelycosaurs (from Greek πέλυξ pelyx "bowl" or "axe" and σαῦρος sauros "lizard") are an informal grouping (previously considered an order) composed of basal or primitive Late Paleozoic synapsids, sometimes erroneously referred to as "mammal-like reptiles". They consist of all synapsids except for the therapsids and their descendants. Some species were quite large and could grow up to 3 metres (10 ft) or more, although most species were much smaller. Because more advanced groups of synapsids evolved directly from 'pelycosaurs', the term had fallen out of favor among scientists by the 21st century, and is only used informally, if at all, in the modern scientific literature.

The pelycosaurs appeared during the Late Carboniferous and reached their acme in the early part of the Permian Period, remaining the dominant land animals for some 40 million years. A few continued into the Capitanian. They were succeeded by the therapsids.

At least two pelycosaur clades independently evolved a tall sail, consisting of elongated vertebral spines: the edaphosaurids and the sphenacodontids. In life, this would have been covered by skin, and likely functioned as a thermoregulatory device or as a mating display. Pelycosaur fossils have been found mainly in Europe and North America, although some small, late-surviving forms are known from Russia and South Africa.