| Pelycosaurs | |
|---|---|
.jpg) | |
| Mounted skeleton ofDimetrodon milleri,Harvard Museum of Natural History | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Tetrapoda |
| Clade: | Reptiliomorpha |
| Clade: | Amniota |
| Clade: | Synapsida |
| Informal group: | †Pelycosauria Cope, 1878 |
| Groups included | |
| Cladistically included but traditionally excluded taxa | |
Pelycosaur (/ˈpɛlɪkəˌsɔːr/PEL-ih-kə-sor)[1] is an older term forbasal or primitive LatePaleozoicsynapsids, excluding thetherapsids and their descendants. Previously, the termmammal-like reptile had been used,[2] and Pelycosauria was considered anorder, but this is now thought to be incorrect and outdated.
Because it excludes the advanced synapsid groupTherapsida, the term isparaphyletic and contrary to modern formal naming practice.[3][4] Thus the namepelycosaurs, similar to the termmammal-like reptiles, had fallen out of favor among scientists by the 21st century, and is only used informally, if at all, in the modern scientific literature.[5][6] The termsstem mammals,protomammals, andbasal orprimitive synapsids are instead used where needed.
The modern word was created from Greekpélyx meaning 'basin' andsaûros meaning 'lizard'.[7] The termpelycosaur has been fairly well abandoned by paleontologists because it no longer matches the features that distinguish a clade.[6]
Pelycosauria is aparaphyletic taxon because it excludes thetherapsids. 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 propermonophyletic group.Caseasauria is a pelycosaur side-branch, or clade, that did not leave any descendants.[citation needed]
The pelycosaurs appear to have been a group of synapsids that have direct ancestral links with themammals, having differentiated teeth and a developing hard palate. The pelycosaurs appeared during theLateCarboniferous and reached their apex in theearly part of the Permian, remaining the dominant land animals for some 40 million years. A few continued into theCapitanian, but they experienced a sharp decline in diversity in the lateKungurian.[8] They were succeeded by thetherapsids.
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 thegeneraDimetrodon,Sphenacodon,Edaphosaurus, andOphiacodon.[9]
Pelycosaur fossils have been found mainly inEurope andNorth America, although some small, late-surviving forms are known fromRussia andSouth Africa.
Unlikelepidosaurian reptiles, pelycosaurs might have lacked reptilian epidermalscales.[disputed –discuss] Fossil evidence from somevaranopids shows that parts of the skin were covered in rows ofosteoderms, presumably overlain by hornyscutes.[5] The belly was covered in rectangular scutes, looking like those present incrocodiles.[10] Parts of the skinnot 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 somerodents,sengis,moonrats, theopossums, and othermarsupials, and as regular dermal armour with underlying bone in thearmadillo.
At least two pelycosaurclades independently evolved a tallsail, consisting of elongated vertebral spines: theedaphosaurids and thesphenacodontids. In life, this may have been covered by skin, and likely functioned as athermoregulatory device[11] or as amating display.
In phylogenetic nomenclature, "Pelycosauria" is not used formally, since it does not constitute a group ofall organisms descended from some common ancestor (aclade), 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.[12]
In 1940, the group was reviewed in detail, and every species known at the time described, with many illustrated, in an important monograph byAlfred Sherwood Romer andLlewellyn Price.[13]
In traditional classification, the order Pelycosauria isparaphyletic in that thetherapsids (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 byphylogenetic nomenclature. Inevolutionary taxonomy, Therapsida is a separated order from Pelycosauria, andmammals (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,[6] but several paleontologists nevertheless continue using this word.[14][12]
The following classification was presented by Benton in 2004.[15]
