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Unit 390: Synapsida |
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The Vertebrates |
Overview |
Synapsid CladogramREPTILOMORPHA |--+--ANAPSIDA | `--EUREPTILIA Synapsida |--Caseasauria | |--Eothyrididae | `--Caseidae `--Eupelycosauria |--Varanopseidae `--+--Ophiacodontidae `--+--Edaphosauridae `--Sphenacodontia |--Sphenacodontidae `--THERAPSIDA |
Contents390.000 Overview |
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.
'Pulses' of synapsid evolution. "Note: Taxa illustrated are
designed to give a flavour of the range of types of
mammal and other vertebrates in each time period, and are
not meant to be a comprehensive listing. Body mass ranges
are approximate estimates based on overall body size. |
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Each of these pulses is a major evolutionary radiation of 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.
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A skeleton of a large carnivorous pelycosaur, Ctenospondylus casei, length 3 meters (photo from Fossilnet Gallery) |
The "pelycosaurs" are the Permo- Carboniferous synapsids. The Pelycosauria were among the very first groups of amniotes to evolve, early in the Late Carboniferous (Pennsylvanian) period. By the end of that period all 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. 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, and dinosaurs and birds.
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.
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Pelycosaur and Therapsid evolution, showing the main types. Mammals evolve from the ictidosaurs (top left). Illustration by Lois M. Darling, from Colbert (1969). |
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
epoch, 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
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