The earliest and most primitive members of the group of mammal-like reptiles belong to the order Pelycosauria. The most primitive members of this group, such as Varanosaurus and Mycterosaurus are small, rather slender animals, with elon gated pointed heads. They had no visible neck, the shoulder girdle being placed behind the skull. The body was long and the tail even longer. Their skulls differ from those of the Captorhino morpha most obviously in that the dermal roof is no longer com plete, but is perforated by a large lateral vacuity which is bounded by the jugal, postorbital and squamosal bones. This open ing serves to give room for the thickening of the masticatory muscles, which necessarily occurs when they are shortened so as to close the mouth.
Another important difference is that the supraoccipital bone be comes so widened that, with the overlying interparietal and tabu lar, it forms a plate on the hinder surface of the skull, which reduces the post-temporal fossae to very small proportions.
The only important changes in the post cranial skeleton are that the neural arches become light and narrow, the neural spines high, and the articulation faces of the zygapophyses are obliquely placed.
From such animals a series of short evolutionary lines arose, which led to the development of some extraordinary forms in which the neural spines from the head to the root of the tail become immensely elongated, and, in some cases, provided with lateral processes like the yard-arm of a ship. These animals, Dimetrodon and Naosaurus, must have been of grotesque ap pearance, one with a huge head with great piercing teeth, the other with a very small head with crushing dentition, each with a crest, as high as its own length from head to the root of the tail. Animals so specialized naturally had only a short range in time, they only occur in Lower Permian rocks, but the latter ranged from Czechoslovakia to Texas.
More conservative members of the group gave rise to a number of reptilian orders whose remains have been found in the Middle and Upper Permian rocks of Europe and South Africa, and in the Trias of South Africa, Asia and North America. The most important of these orders, the Theriodontia, included the ancestors of the mammals, and its members exhibit a series of stages which seem to bridge the structural gap between a Pelycosaur and a mammal very completely.
Some of these changes are illustrated by a comparison of the skulls of Scymnognathus and Cynognathus.
The skull of Scymnognathus, a Gorgonopsid, differs from that of a primitive Pelycosaur in that in it the whole head is flattened, the temporal vacuity, instead of facing laterally, is directed up ward and is greatly increased in size ; this change implies that the muscles which close the mouth had changed, an originally rather simple mass splitting up into pterygoidal, temporal and masseter muscles. In order to give room for this powerful devel
opment the side of the roof of the skull, formed by the jugal post orbital and squamosal, is bowed out, with the result that the quadrate and quadrato-jugal, being fixed in position by their articulation with the lower jaw, become detached from the side of the head and remain inserted in a depression on the front face of the squamosal, within the temporal vacuity. At the same time they are somewhat reduced in size.
The enlarged masticatory muscles require a more extensive area of attachment on the lower jaw, to provide which the upper and hinder end of the dentary becomes free and grows upward. At the same time, the hinder half of the jaw, composed of the surangular, angular, articular and prearticular bones, become con verted into a thin sheet by a lateral compression, and the lower border of the angular is notched, a special lamina of the bone being reflected over the outer surface of its posterior part. To this reflected lamina the lower edge of the tympanic membrane seems to have been attached.
The palate, though it still has the posterior nares placed far forward, is advanced because it is very much vaulted, owing to the downgrowth of the maxillae on each side of it. Posteriorly, the pterygoids, with a parasphenoid held between them, form a nar row girder which connects the basisphenoid with the anterior part of the palate.
The stapes still articulates with the quadrate. The brain case is incompletely ossified in front of the point of exit of the fifth cranial nerve. The branches of that nerve pass on each side of the rod-like epipterygoid, and the cerebral hemispheres are en closed in a single ossification homologous with the sphenethenoid of a frog.
Cynognathus has advanced beyond Scymnognathus in that the face has become deeper and more rounded, and the nostrils larger. The temporal vacuity has enlarged so that it is bounded above by the parietal, and this bone is drawn up into a deep sagittal crest which allows of longer temporal muscles. The quadrate and quadrato-jugal have become greatly reduced in size, but retain their position and function.