We have repeatedly seen that all aquatic animals have some or all the bones of the limbs shortened, and it is of interest to observe that the early plesiosaurs had longer forearm and foreleg bones than the later ones, just as we have seen was the case with the early ichthyosaurs. It would seem probable that all the early plesiosaurs had long necks, though some of the late ones in Cretaceous times had relatively short necks, shorter even than the known nothosaurs possessed.
The nothosaurs doubtless lived about the shores of the ancient seas, spending much of their time in the water, leaving it perhaps when hard pressed by their enemies, as do some modern reptiles, or to rear their young. The teeth of the nothosaurs are long and slender in front, shorter behind. The animals must therefore have been carnivorous in habit, feeding probably upon such fishes as they could catch, and the various invertebrates which live in shallow water. The structure of the jaws and their attachments are quite as in the plesiosaurs, proving that they could not have swallowed large objects; but the skull is broader and flatter than that of most plesiosaurs, indicating habits not unlike those of the modern alligators and crocodiles.
Some time we shall doubtless find remains of nothosaurs or nearly allied animals elsewhere than in Europe, but probably not from later deposits than the Triassic. So far as we now know, their geological range and geographical distribution were much restricted; they evidently wholly died out shortly after the plesiosaurs appeared.
CHAPTER VII
ANOMODONTIA
LYSTROSAURUS
Over a large area of South Africa, chiefly along the Orange River and its tributaries, there is an extensive series of deposits many hundreds of feet in thickness, usually called the Karoo beds, which, for more than fifty years, have been widely famous among scientific men for the many and remarkable vertebrate fossils which they have yielded. These deposits seem to represent the whole of the vast interval of time from the Carboniferous to the Jurassic, that is, the whole of the Permian and Triassic, though not many fossils have been found in the lowermost strata. Among the fossils of the lower strata are those of the strange creatures described in the following pages as Mesosaurus. From the deposits representing the Upper Permian and the Triassic the fossils that have been obtained are both abundant and diverse. Unfortunately, however, of the scores of forms that have been discovered few are known completely, and still fewer are known sufficiently well to enable us to picture the living animals.
From the Upper Permian Karoo rocks two orders of reptiles have been recognized, the Cotylosauria, represented by more specialized forms than those from the Lower Permian of North America; and the order or group called by Broom the Therapsida. While the forms of this latter group have certain definite structural relationships with each other, they show so great a diversity among themselves that, when they shall be better known, it will be found necessary perhaps to separate them into several distinct orders.
At least five groups of the Therapsida are now recognized by Broom, the Dromasauria, Dinocephalia, Anomodontia, Therocephalia, and Theriodontia. Of all these the members of the last-mentioned group have attracted the greatest interest among geologists and naturalists, because of their intimate relationships to the mammals—so intimate, indeed, that they seem almost to bridge over the interval between the two classes. From higher Karoo beds primitive representatives of the more crocodilian types have been discovered, forms which seem to be the beginning of that order described on later pages as the Parasuchia.
It would lead us too far astray to mention even, let alone describe, the many forms of reptiles that have been discovered in the Karoo beds; nor indeed is it possible for anyone who has not attentively studied their remains to get a very clear conception of many of them, so incompletely have they been made known.
Doubtless from among all these diverse forms there have been not a few which sought wider opportunities in the water, but, if so, we have as yet very little knowledge of them. One form only, so far as the writer is aware, has been credited with aquatic habits, a remarkable reptile belonging to the group originally called by Sir Richard Owen, the Anomodontia, a word meaning “lawless teeth,” and to the genus Lystrosaurus, also described by the same noted paleontologist. A restoration of the skeleton of Lystrosaurus has recently been published by Watson. This restoration the writer has reproduced in the present pages, though he has taken the liberty of making some minor changes, to accord better with what he believes must have been the position of the shoulder-blades and the hind legs. And he would also suggest that the tail in life did not turn down so much at its extremity as depicted by Watson.