Pelycosaur

The pelycosaurs (pronounced PEL-ih-ko-sawrz) were previously considered an order, but are now only an informal grouping composed of basal or primitive Late Paleozoic synapsids, sometimes called "mammal-like reptiles". They consist of all synapsids excluding the therapsids and their descendants.
Because more advanced synapsid group therapsida evolved directly from the pelycosaurs, the pelycosaurs are paraphyletic.
 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 term pelycosaur has been fairly well abandoned by paleontologists
because it no longer matches the features that distinguish a clade. The
 modern word was created from Greek
 πέλυξ pelyx 'wooden bowl' or 'axe' and σαῦρος sauros 'lizard'. The
intended etymology presents a problem, since pelycosaur as created
basically means 'basin lizard', but there is a ambiguity between several
 original Greek words in that pélix is 'wooden bowl' and pelíkē 'basin',
 whereas pélyx is 'ax' like pélekys 'double-edged ax'.
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 have direct ancestral links with the mammals, having
differentiated teeth and a developing hard palate. The pelycosaurs
appeared during the Late Carboniferous and reached their apex in the early part of the Permian, remaining the dominant land animals for some 40 million years. A few continued into the Capitanian. They were succeeded by the therapsids.

Some species were quite large, growing to a length of 3 metres (10 ft)
or more, although most species were much smaller. Well-known pelycosaurs
 include the genera Dimetrodon, Sphenacodon, Edaphosaurus, and Ophiacodon.
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 reptilian epidermal scales. Fossil evidence from some varanopids shows that parts of the skin were 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 might 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.
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.

In phylogenetic nomenclature, "Pelycosauria" is not used formally, since
 it does not constitute a group of all organisms descended from some
common ancestor (a clade),
 because the group specifically excludes the therapsids which are
descended from pelycosaurs. Instead, it represents a paraphyletic
"grade" of basal synapsids leading up to the clade Therapsida.
In
1940, the group was reviewed in detail, and every species known at the
time described, with many illustrated, in an important monograph by Alfred Sherwood Romer and Llewellyn Price.
In traditional classification, the order Pelycosauria is paraphyletic in that the therapsids
 (the "higher" synapsids) have emerged from them. That means
Pelycosauria is a grouping of animals that does not contain all
descendants of its common ancestor, as is often required by phylogenetic nomenclature. In evolutionary taxonomy, Therapsida is a separated order from Pelycosauria, and mammals
 (having evolved from therapsids) are separated from both as their own
class. This use has not been continued by a majority of scientists since
 the 1990s.
The following classification was presented by Benton in 2004.
• Order Pelycosauria\*
• Order Therapsida