The pelycosaurs (from Greek "bowl"/"pelvis" and σαῦρος sauros "lizard") are an informal grouping (previously considered an order) composed of basal or primitive Late Paleozoic synapsid amniotes. Some species were quite large and could grow up to 3 meters 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.
Evolutionary history 
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 late Permian. 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 possibly functioned as a thermoregulatory device and/or for 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.
Unlike lepidosaurian reptiles, pelycosaurs lacked epidermal scales. Fossil evidence from some ophiacodonts shows that parts of the skin was covered in rows of osteoderms, presumably overlain by horny scutes. The belly was covered in rectangular scutes, looking like those present in crocodiles. Parts of the skin not covered in scutes could have had naked, glandular skin like that found in some mammals. Dermal scutes are also found in a diverse number of extant mammals with conservative body types, such as in the tails of some rodents, sengis, moonrats, the opossums and other marsupials, and as regular dermal armour with underlying bone in the armadillo.
Pelycosauria is a paraphyletic taxon because it excludes the therapsids. For that reason the term is sometimes avoided by proponents of a strict cladistic approach. Eupelycosauria is used to designate the clade that includes most Pelycosaurs along with the Therapsida and the Mammals. In contrast to "Pelycosaurs", this is a monophyletic group. Caseasauria refers to a pelycosaur side-branch or clade that did not leave any descendants.
The pelycosaurs appear to have been a group of synapsids that had direct ancestral links with the mammalia, having differentiated teeth and a developing hard palate.
In traditional classification, the order Pelycosauria is paraphyletic--that is, it is a grouping of animals that does not contain all descendants of a common ancestor, as is often required by a different system of naming organisms, phylogenetic nomenclature. In traditional taxonomy, Therapsida was separated from Pelycosauria in its own biological order, and mammals were separated from both as their own class, though such usage has not been continued by a majority of scientists since the 1990s. In phylogenetic nomenclature, the "Pelycosauria" is not used, since it does not constitute a clade (a group of organisms descended from one common ancestor and including all the descendants of that ancestor), as the group excludes the therapsids. Instead, it represents a paraphyletic "grade" of basal synapsids leading up to the clade Therapsida. The following classification was presented by Benton in 2004.
- Order Pelycosauria*
- Order Therapsida
See also 
- Botha-Brink, J. and Modesto, S.P. (2007). "A mixed-age classed ‘pelycosaur’ aggregation from South Africa: earliest evidence of parental care in amniotes?" Proceedings of the Royal Society B, 274(1627): 2829–2834. doi:10.1098/rspb.2007.0803
- Carroll, R.L. (1969). "Problems of the origin of reptiles." Biological Reviews, 44: 393-432.
- Benton, Michael J. (2004). Vertebrate palaeontology (3rd ed. ed.). Oxford: Blackwell Science. ISBN 978-0-632-05637-8.
- Reisz, R. R., 1986, Handbuch der Paläoherpetologie – Encyclopedia of Paleoherpetology, Part 17A Pelycosauria Verlag Dr. Friedrich Pfeil, ISBN 3-89937-032-5 .
- Romer, AS & Price L.I (1940), Review of the Pelycosauria. Geol. Soc. Amer. Spec. Papers 28: 1-538.
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