Pelycosaur

Evolutionary history

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, but they experienced a sharp decline in diversity in the late Kungurian. They were succeeded by the therapsids.

Description

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 might have 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 may have been covered by skin, and likely functioned as a thermoregulatory device or as a mating display.

Taxonomy

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 recommended by a majority of systematists since the 1990s, but several paleontologists nevertheless continue using this word.

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. The following classification was presented by Benton in 2004.

• Order Pelycosauria*
  • Suborder Caseasauria
    • Family Eothyrididae
    • Family Caseidae
  • Suborder Eupelycosauria
    • Family Varanopidae
    • Family Ophiacodontidae
    • Family Edaphosauridae
    • Infraorder Sphenacodontia
      • Family Sphenacodontidae
• Order Therapsida