The Pelycosauria were among the very first groups of reptiles to evolve, early in the Late Carboniferous (Pennsylvanian) period. By the end of that period all the major lines of Pelycosaurs except the caesaurs had appeared. They remained the dominant life-form (the largest, fiercest, etc) for some 40-odd million years, which is about 3/5ths the length of time of the age of mammals. They were supplanted by their descendants, the Therapsids or "mammal-like reptiles", which had a short but glorious reign before being mostly decimated by the terminal Permian extinction events, an opportunity that allowed the Archosauria to take over.
In their position on the family tree of life, the Pelycosaurs are the earliest and most primitive members of the synapsids, the group that (in the old classification) leads to or (in the new classification) includes mammals. Thus the mammal-line split off from the rest of the reptile line (including turtles, lizards and snakes, crocodiles, and dinosaurs and birds) very early.
The evolutionary development of the pelycosaurs are best known and most completely recorded in the fossil record from the lower Permian "Red Bed" sediments of North America, particularly in Texas, Oklahoma, and New Mexico. Elsewhere the remains of pelycosaurs are fragmentary and scattered.
The pelycosaur skeleton shows the typical generalised primitive features of early reptiles. The skull has a full complement of bones (lacking only the intertemporal bone, which was lost during the transition from labyrinthodonts to early reptiles), there is a pineal opening (common in early tetrapods), there are vertebral intercentra, and the limbs are similar to the limbs of protorothyridid and captorhinomorph reptiles and diadectid amphibians, only somewhat more slender. In many respects the skull of some of the early pelycosaurs was very close to the captorhinomorph skull, which is one reason for thinking that these reptiles had a protorothyridid / captorhinomorph ancestry. There is the tendency among cladist systematisers now to deny this, because the captorhinomorph skull has a few specialised features not found in Pelycosaurs. I argue against this supposition, which is contradicted by the order of appearance in the fossil record, and in any case cladistics is not as infallible a methodology as some of its supporters would like to think. I would suggest instead that the Pelycosaurs were secondarily primitive, evolving from protorothyridids during the late Carboniferous period.
One distinctive feature of many pelycosaurs is a large sail along the back, formed by the great elongation of neural spines of the vertebrae, sometimes upto a meter in length in the larger species. This condition evolved independently at least three times (and quite possibly more) among pelycosaurs. The function of such spines would seem to be a thermoregulatory device for controlling body temperature. A membrane of skin would span the space between the spines, and this was richly equipped with blood vessels, warming the animal in the chilly morning and cooling it during the midday heat. How this would work would be that in the early morning the animal would stand with its sail oriented toward the sun. Like a solar heater, the sail would have absorbed heat and warmed the blood, which circulated through the body, raising the reptile's temperature so it could begin its daily hunt for food earlier than its non-sailback competitors. To prevent overheating after strenuous activity, it angled its sail away from the sun and into the wind, dissipating heat. The sail and its associated spines would also have served in sexual and intra-species display behaviour, and possibly also for camouflage (e.g. if the animal was hiding among the bamboo-like calamite plants (giant "horsetails"),
An interesting example of convergent evolution is seen between these pelycosaurs and the unrelated dinosaurs. Several genera of dinosaurs, such as Spinosaurus and Ouranosaurus, both from Gondwanaland during the middle Cretaceous period - developed similar sails on their backs, which may have served a similar purpose.
But the most distinguishing feature of the Pelycosaurs (and a character retained by their Therapsid descendants) were not their famous famous sail (which not all pelycosaurs had anyway) but the so-called synapsid skull, which features a single, large opening on the side of the skull (the temporal region) behind the orbit (eye socket). This special opening allowed the development of larger and longer jaw muscles, and hence stronger jaws that could be opened wider and closed forcefully, enabling the animal to despatch struggling or larger prey. It was this simple evolutionary adaptation that gave the pelycosaurs the edge in the struggle for survival. All that was needed was a prolonged period of drought, such as the sudden period of aridity during the Kasimovian epoch, to kill off many of the large amphibians that kept the Pelycosaurs insignificant, and these reptiles were able to emerge as the dominant life-form on Earth during the Permian period, while the captorhinomorphs remained small and relatively insignificant.
family Eothyrididae - small primitive insectivorous types, ancestors of the caesids - Sakmarian to Artinskian
family Caesidae - medium-sized to very large chubby-bodied upland herbivores. Include the largest known pelycosaurs. Continued after most of the other lines had died out. - Artinskian to Wordian
family Ophiacodontidae - small to large possibly aquatic or semi-aquatic fish-eaters - Moscovian to Artinskian
family Varanopseidea - unspecialised medium to large lizard-like types - Gzelian to Capitanian
family Edaphosauridae - fin-backed water-margin inhabiting herbivores - a large sail on the back - Gzelian to Artinskian
family Sphenacodontidae - the dominant predators of the early Permian - includes a number of sailback types - Kasimovian to Ufimian
|Books and Links|
There are three excellent websites, which makes it hard to give a single "best of the web" award here:
Introduction to the Pelycosaurs - this is the best intro to the group
Biology 356 - Major Features of Vertebrate Evolution - Permo-Carboniferous Synapsids - by Dr. Robert Reisz, University of Toronto - a bit more technical than the UCMP page - gives an excellent overview of each of the Pelycosaur families
SynapsidaPalaeos - incorporating Toby White's Vertebrate Notes and material from these pages - the best detailed description of the group. Very technical coverage.
The following sites give a briefer coverage:
Order Pelycosauria - brief intro - Glasgow University Zoology Museum
Synapsids The evolutionary tree for the Pelycosauria is on the Tree of Life web site
Alfred S. Romer and Llewellyn I. Price Review of the Pelycosauria : Geological Society of American Special Papers, No 28, ed. by Stephen J. Gould - classic monograph best book available
Robert R. Reisz, Pelycosauria, Encylopedia of Paleoherpetology, Part 17A, 1986, Gustav Fischer Verlag, Stuttgart and New York - a more recent coverage - more upto date, but less cool drawings of each species
cautionary note (please read before using this page as reference material!)