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|Mounted skeleton of Dimetrodon mileri, Harvard Museum of Natural History|
The pelycosaurs (pronounced PEL-ih-ko-sawrz) 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, growing to a length of 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 supplied etymology (from Greek πέλυξ pelyx 'wooden bowl' or 'axe' and σαῦρος sauros 'lizard') presents a problem, as follows: pelycosaur has the basically supplied meaning 'basin lizard', but there is an original etymological mismatch in that Greek pélix is 'wooden bowl' and pelíkē 'basin', whereas pélyx is 'ax' like pélekys 'double-edged ax'. This imperfect term pelycosaur has been fairly well abandoned by paleontologists because it no longer matches cladistic features.
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.
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.
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 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 in that the therapsids (the "higher" synapsids) have evolved 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 usage has not been continued by a majority of scientists since the 1990s. In phylogenetic nomenclature, "Pelycosauria" is not used formally, 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
- 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
- C. Richard Tracy et al.
- Carroll, R.L. (1969). "Problems of the origin of reptiles." Biological Reviews, 44: 393-432.
- Cowen (2013), pp. 91–92
- Benton, Michael J. (2004). Vertebrate palaeontology (3rd ed.). Oxford: Blackwell Science. ISBN 978-0-632-05637-8.
- Cowen, Richard (2013). History of Life. John Wiley & Sons. ISBN 978-1-11851-093-3
- 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.
- C. Richard Tracy, & J. Scott Turner & Raymond B. Huey (1986), A biophysical analysis of possible thermoregulatoryadaptations in pelycosaurs. pp. 195–205 In: Ecology and Biology of Mammal-Like Reptiles, P.D. MacLean, J.J. Roth, E.C. Roth, and N. Hotton (Eds.), Smithsonian Press, Washington, DC.
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