Palaeos Vertebrates 390.000 Synapsida Overview

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Synapsida (=Theropsida)

Evolution

The Theropsids or "beast (mammal) faces" constitute an evolutionary lineage that developed a special opening, the synapsid arch, for attachment of jaw muscles, giving a superior bite and permitting adaptive radiation during the late Carboniferous. These basal forms evolved through the primitive pelycosaur stage, to the therapsids or mammal-like reptiles, and finally the mammals themselves.� Pelycosaur, therapsid, and mammal represent three evolutionary grades in a single progressive evolutionary axis. The therapsids, as forms transitional between basal amniote and mammal, can be thought of as occupying the same evolutionary space as the dinosaurs, which are transitional between reptiles and birds, do.

When we look at the history of the Theropsids we see that the lineage can be further understood in terms of evolutionary "pulses", each pulse corresponding to a major biota. The following diagram and associated text, from Janis & Damuth (1990), shows the major pulses (although two Permian ones have been left out!), and other tetrapods that were contemporary.

Each of these pulses is a major evolutionary radiationof the Theropsid lineage. And each follows the other sequentially.

The evolutionary systematic diagram to the right from Carroll (1988) shows how this works (note - time shown the opposite direction (bottom is oldest) to the preceding diagram). The branches of only the first two pulses mentioned here, the pelycosaurian and the Therapsid (the cynodonts and mammals are indicated but the numerous sub-branches are not shown). The Permo- carboniferous sees the flourishing adaptive radiation of the various pelycosaur groups (bottom left of the diagram). Then, as these decline due to changing environmental conditions (with only a few stragglers making it through to the middle Permian) the new adaptive radiation of the Therapsida takes off. This itself consists of a number of subgroups, one of which, the cynodonts, itself initiates a new adaptive radiation, from one line of which the mammals emerge. And so on.

Speaking personally, I find few things more awesome than contemplating this vast and majestic process of evolution, the ebb and flow of successive biotas through geological time. Creationists and others who cannot for ideological or religious reasons accept the fact of evolution miss out a great deal, and are left with a claustrophobic little universe in which nothing happens and nothing changes.

The Pelycosaurs

A skeleton of a large carnivorous pelycosaur, Ctenospondylus casei, length 3 meters (photo from Fossilnet Gallery)

The "pelycosaurs" are the stem group synapsids who inhabited the Permo-Carboniferous. The pelycosaurs were among the very first groups of amniotes to evolve, early in the Late Carboniferous (Pennsylvanian) Epoch. By the end of that epoch all of the major lines of pelycosaurs, except the caseasaurs 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 the age of mammals (Cenozoic). They were supplanted by their descendants, the Therapsids or "mammal-like reptiles", which had a short but glorious reign before being 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 quite early from the rest of the reptile line, including turtles, lizards and snakes (squamates), crocodiles, dinosaurs and birds.

The evolutionary development of the pelycosaurs are best known and most completely recorded in the fossil record from the Cisuralian and Guadalupian "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 generalized primitive features of early amniotes. 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 protorothyrid and captorhinid reptiles and diadectomorphs, only somewhat more slender. In many respects the skull of some of the early pelycosaurs was very close to the captorhinid skull, which is one reason for thinking that these reptiles had a protorothyrid / captorhinid ancestry. There is the tendency among cladist systematisers now to deny this, because the captorhinid skull has a few specialized 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 protorothyrids 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 up to 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. Thus, in the early morning the animal could stand with its sail oriented toward the sun. Like a solar heater, the sail would absorb heat and warm 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 could angle its sail away from the sun and into the wind, dissipating heat. The sail and its associated spines could also have served in sexual and intra-species display behavior, and possibly also for camouflage (e.g. if the animal was hiding among the bamboo-like calamite plants, or 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 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 dispatch 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 Period, to kill off many of the large stem tetrapods that kept the pelycosaurs insignificant, and these creatures were able to emerge as the dominant life-form on Earth during the Permian period, while the captorhinids remained small and relatively insignificant.� MAK, minor revisions ATW090307.

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