THE teeth in Paretasanrus, like those in the amphibia we have already mentioned, were not confined to the margins of the jaws as in mammals, but extended over the palate as well. The animal was large and unwieldy, sometimes so much as ten feet long, and is thought to have been a vegetarian. Its heavy and awkward limbs had the elbows and knees stuck out from the body, so it must have walked with a wide but slow and ungainly stride. The feet were adapted for digging like a mole, so probably this animal dug about for its vegetable food.
Pareiasaurus belongs to the most primitive reptilian order, and is at the base of the whole family tree that produced various other reptiles, the birds, mammals, and man, so that in a way, we may regard it as a reptilian Adam, the remote father of us all. Although it is known from South Africa, other animals closely similar to it have been found in deposits in northern Russia and Scotland, while smaller related forms of a later date are known from Scotland, Switzerland and the United States of America.
The other interesting Permian reptile we must mention comes from Texas in the United States, and is known as Dimetrodon. It belongs to a different order of reptiles from Pareiasaurus, although in general build and size they were somewhat alike. Dimetrodon was a flesh-eater, however, as its conical and compressed teeth indicate, and had an alligatorlike head and a body like a monitor.
The most remarkable feature in its make-up was the aston-
ishing length to which the neural or dorsal spines of the vertebra? grew. One spine was often a straight rod, three feet long, while sometimes this rod had cross-pieces like the yards on a mast. These neural spines extending along the body must have been covered by a thin web of skin during life, and the purpose of this awkward structure is not clear. It is possible that it was a sexual characteristic, and that only the males were thus adorned.
Living at this same time and till the Upper Trias x in South Africa, Russia, Scotland, and North America were many varied animals related to Dimetrodon but rather different in structure. They are all grouped under the names of Anomo-dontia or ‘irregular toothed,’ or Theromorpha, ‘beast shaped,’ and consist of forms varying in size from that of a little lizard to a large dog. Some were purely land-living, others were aquatic. It is noticeable that in nearly all groups of animals, some forms persist in returning to the ancestral habitat of the waters.
While there is considerable variation in the teeth of these animals, as is indicated by the name Anomodont given above, one group with a head like a dog had remarkably mammallike teeth. Instead of being constructed all on the same pattern as in most reptiles, the teeth of these creatures were differentiated into incisors, canines or eye-teeth, and the chewing teeth, pie-molars and molars. The only real difference, indeed, between these teeth and those of a mammal lay in the manner of their replacement, though it is possible that these reptiles had also only two sets of teeth during life. Like the mammals also, the skull had two condyles at the back for movement upon the neck. Many of the bones of the skeleton, too, were mammal-like.
We thus reach the conclusion that during the Trias age, when conditions were arid—always a stimulus to evolution— and the advantage was with the more adaptive and fast-moving types of animals, the mammals were evolved from the slower-moving, less efficient creatures. At a comparatively early stage in the development of the reptiles, therefore, the most significant feature of their history was accomplished.
This does not mean that the importance of the group dwindled in the immediately succeeding period, because, from the Trias1 onwards till the close of the Cretaceous ‘
some of the most remarkable creatures were evolved, perfected, and lost again to flourish no more.
In Triassic times these forms were already well indicated. Already many had given up the struggle for existence on land and had returned to, and adapted themselves with great success in, the seas.
One such group was the Plesiosaurs, whose first forms we know from the Trias, and which became dominant during the Jurassic and the Cretaceous periods. In one of the early accounts of them, they were aptly described as being like ‘a snake threaded through the shell of a turtle.’ In type they were small-headed animals with a very long neck, a flattened, barrel-shaped body and a long tail with a small fin at its end. In size they varied from a few feet to thirty feet in length. Their remains have been found in England, America, Germany, Australia, and Africa—indeed, all over the world.
The small head had the margins of the jaws lined with sharp, conical teeth fixed in deep sockets. The eyes were large and, again, a pineal eye was present on top of the head. The long and slender neck was composed of many vertebra;, but appears to have been almost inflexible; while the body was probably covered by smooth skin without any trace of armour.
For its swimming abilities, the animal did not depend on the sinuous movement of the fish or on the strength of the fins, but on the propulsion of its modified paddle-like limbs. Both the front and the hind limbs were of about equal size, and their construction bears ample testimony to the land-living habit of their ancestral form. All the ordinary limb and toe bones were present, though the joints of the toes had increased in number. The whole structure was strengthened by cartilage and covered with skin to form a powerful paddle.
While the soft structures of the body were protected above by the backbone and ribs, the flattened front or ventral surface was strengthened by the expanded plates of bone which supported the paddles, and by numerous rows of secondarily-developed abdominal ribs, the whole protective structure being not unlike the lower shell, or plastron, of a turtle.