Pelycosaur
Template:Short description Template:Paraphyletic group
Pelycosaur (Template:IPAc-en Template:Respell)<ref name="Co78">Template:Cite journal</ref> is an older term for basal or primitive Late Paleozoic synapsids, excluding the therapsids and their descendants. Previously, the term mammal-like reptile was used,<ref>Carroll, R.L. 1988. Vertebrate Paleontology and Evolution. WH Freeman and Company, New York ISBN 0-7167-1822-7</ref> and Pelycosauria was considered an order, but this is now thought to be incorrect and outdated.
Because it excludes the advanced synapsid group Therapsida, the term is paraphyletic and contrary to modern formal naming practice.<ref name="Hennig 81">Template:Cite book</ref><ref name="Dilkes1996">Template:Cite journal</ref> Thus the name pelycosaurs, similar to the term mammal-like reptiles, fell out of favor among scientists by the 21st century, and is only used informally, if at all, in the modern scientific literature.<ref name=brink&modesto2007>Template:Cite journal</ref><ref name="La10" /> The terms stem mammals, protomammals, and basal or primitive synapsids are instead used where needed.
Etymology
The modern word was created from Greek [[wikt:πέλυξ|Template:Lang]] meaning 'basin' and [[wikt:σαῦρος|Template:Lang]] meaning 'lizard'.<ref>Template:Cite book</ref> The term pelycosaur has been fairly well abandoned by paleontologists because it no longer matches the features that distinguish a clade.<ref name="La10">Template:Cite book</ref>
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 Mammalia. In contrast to "pelycosaurs", Eupelycosauria is a proper monophyletic group. Caseasauria is a pelycosaur side-branch, or clade, that did not leave any descendants.Template:Citation needed
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.<ref name="DiLa21">Template:Cite journal</ref> They were succeeded by the therapsids.
Description
Some species were quite large, growing to a length of Template:Convert or more, although most species were much smaller. Well-known pelycosaurs include the genera Dimetrodon, Sphenacodon, Edaphosaurus, and Ophiacodon.<ref name=Cowen2013>Template:Cite book</ref>
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.Template:Disputed inline Fossil evidence from some varanopids shows that parts of the skin were covered in rows of osteoderms, presumably overlain by horny scutes.<ref name=brink&modesto2007/> The belly was covered in rectangular scutes, looking like those present in crocodiles.<ref name="caroll1969">Template:Cite journal</ref> 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<ref>Template:Cite book</ref> or as a mating display.
Taxonomy
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.<ref name="BrFr18">Template:Cite journal</ref>
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.<ref name=RomerPrice1940>Template:Cite book</ref>
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,<ref name="La10" /> but several paleontologists nevertheless continue using this word.<ref name="Amson&Laurin11">Template:Cite journal</ref><ref name="BrFr18" />
The following classification was presented by Benton in 2004.<ref name=Benton2004>Template:Cite book</ref>
- Order Pelycosauria*
- Suborder Caseasauria
- Family Eothyrididae
- Family Caseidae
- Suborder Eupelycosauria
- Family Varanopidae
- Family Ophiacodontidae
- Family Edaphosauridae
- Infraorder Sphenacodontia
- Family Sphenacodontidae
- Suborder Caseasauria
- Order Therapsida