Australia: The Land Where Time Began
Therapsida - previously called mammal-like reptiles
The Therapsida are an order of the class Synapsida. They had powerful, complex jaws in which the teeth were differentiated into incisors, canines and molars. Their legs were more vertically situated beneath the body than those of pelycosaurs, the group they evolved from, and reptiles. They had a more advanced jaw apparatus and limbs than the pelycosaurs. In the Late Permian they are best known from South Africa and western Russia. The therapsids were the first reptiles to dominate, outside Australia amphibians were scarce. Originally thought of as reptiles with mammal-like characteristics they are now thought of, at least by some scientists, to be closer to mammals than reptiles.
They were severely depleted, but survived the mass extinction that occurred at the close of the Permian and underwent a limited radiation to become the dominant forms of the Early Triassic. They flourished in the Early and Middle Triassic but succumbed across most of Gondwana in the Late Triassic, when dinosaurs were becoming dominant across the rest of the world. The lineage from Therapsids to mammals was the only one to survive past the Early Cretaceous.
That was the case outside Australia. In Australia they lived on in Queensland for another 110 million years. Parts of a skull, that is estimated to have been about 40 cm long, was found in the collections of the Queensland Museum that had been unrecognised for 90 years. It was from rocks dated to 110 million years ago.
Again, Australia is different.
Therapsid ancestors arose in the Early Permian, in the form of Sphenacodontia, a pelycosaur. By the Middle Permian they had replaced the pelycosaurs to become the dominant land animals. By the Late Permian they were the most abundant and diverse herbivorous terrestrial vertebrates.
These were a mixed group of carnivorous and herbivorous therapsids. Titanosuchus from South Africa was a typical carnivore. While being dog-like it had a heavier skull and much shorter limbs. The incisors and canines were well developed, and the cheek teeth were like those of modern carnivorous mammals, with sharply ridged for shearing flesh and bones.
The herbivorous dinocephalians looked less like their modern counterparts than did the carnivorous ones. Moschops from South Africa was 5 m (17 ft) long and had heavy limbs and a massive rib cage but tiny hands and teeth. Like the pelycosaurs, the head was small compared with the rest of the body, and it had a massively powerful neck. An unusual feature of the skull was a very thick skull roof, 10 cm (4 inches) thick in some specimens. The only function for the heavily built head that scientists have been able to suggest is that it was used in head-butting as in some modern mammals such as sheep, goats, deer, etc., during fights between males. If this was the case the main force of the butt would have occurred on the thickened skull roof. It would then have been transmitted along the thickened bone to the massive neck and then to the heavily muscled shoulders. It may be the neck is so powerful to act as a shock absorber.
The dicynodonts (2-dog-toothed)
During the Late Permian and much of the Triassic the dominant herbivores were the dicynodonts. They were characterised by the loss of teeth. Some forms have only 2 canines while others have lost even the canines. There is evidence that their jaws were covered by a scaly beak as in birds and turtles.
Jaw motion of dicynodonts
Studies of the jaws of Endothiodon, a primitive dicynodont from South Africa, have shown that it and a related group of species, ranging from about 1 inch to about 3 m (10 ft), the lower jaw moved in a counterclockwise rotation, as viewed from the right side. The tongue and palate pulled plant material into the mouth. The lower jaw then slipped forward, moving up to the closed position while grasping the material with the beak, then dragged the lower jaw back while in contact with the upper jaw. This motion would have bitten through tough vegetation, cutting it into small enough pieces that could be swallowed. As with all other reptiles, Endothiodon was unable to chew its food as mammals do. They swallowed their food in large chunks, the food being broken down in the stomach.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|