AT the closing stages of the Age of Reptiles and the time of primitive, toothed birds, the mammals were slowly but surely evolving. During that time they must have been numerous but unobtrusive, for though we have seen that they came from their ancestral reptilian stock during the aridity of the Trias, it is not until much later, in the Jurassic period, and then only rarely, that we find their remains in any number. Examination of these early forms shows that many of them were marsupials—that is, animals that carry their young in their pouches—whose protected and isolated
descendants constitute the animal population peculiar to Australia to-day.
The great era of the mammals is the Tertiary, comprising the Eocene, Oligocene, Miocene, Pliocene, and Pleistocene, but as the last two of these are concerned essentially with Man, we may concentrate on the earlier times. However scarce or unenterprising the Cretaceous mammals may have been, the same cannot by any means be said of their successors in the Eocene, for here we have numerous large forms of grotesque appearance and herbivorous habits which strongly remind us of the dinosaurs. Here again the constructive powers of Nature ran riot and produced great horny skulls and other modifications which seem to be the death warrant of the creatures they adorn.
‘TERRIBLE-HORN’: A GIANT WITH ITS WEAPONS ON ITS FACE ONE of these mammals was called Dinoceras (‘terrible-horn ‘). It had a body larger than a rhinoceros and almost as big as an elephant, supported on pillar-like legs with five-toed stumpy feet. The large skull was very curiously ornamented with three pairs of horns. The first pair was on the nose, the second pair stood in front of the eyes, while the third was on top of the brain-case. The last two pairs were connected by bony ridges and formed a basin-shaped depression on top of the skull. In life these horns would be covered with skin something like those of a giraffe. The teeth were small and adapted for the succulent vegetation of the well-watered lowlands, but the males possessed long, dagger-like fangs which must have been for fighting and which were protected by flanges on the lower jaw.
Dinoceras comes from the Middle Eocene deposits found at Wyoming, and a very similar but larger form called Tino-ceras is known from the same locality. These and related animals are placed in a group called the Amblypoda, or ‘blunt-feet ‘and were hoofed, or ungulate. Although specialised and large, they had very small brains. They paralleled many of the dinosaurs, and, like them, soon became extinct. Brain is always more important than brawn, and they were inadaptive creatures.
A curiously-skulled mammal of the Oligocene times was the great, horned Arsinoithcrium of Egypt. This animal was in some ways related to the Amblypoda, but it had a larger
brain and teeth adapted to a less juicy diet, and had no great incisors. The skull had two pairs of horns; one small pair over the eyes, and an enormous pair developed from the nose bones in front of the eyes. Both these horn structures were really hollow horn-cores and were sheathed during life with true horn which probably increased their size. Arsinoitherium played the part in that ancient world that the rhinoceros plays now, and there is little doubt that it had the same sort of habits.
Other curious forms of somewhat later date were also rhinoceros mimics and were confined to South America. Large in size, about nine feet long, they had heavy bodies, feet modified for living on and travelling over the pampas, and heavy skulls with extraordinary ever-growing and powerful teeth adapted for cutting and grinding up the plant food. Toxodon is a typical example which is exhibited in the British Museum (Natural History).
Another interesting group, also Miocene and later in age, is the Chalicotheres, whose remains have been found in Africa, America, and Asia. They are related in some degree to the horses, and are quite well known in such forms as Moropus. This peculiar animal shared the features of the ground sloth and the horse. It had clawed feet, suitable for digging up such plants and roots as it fed upon; its head was like that of a horse and its front legs were distinctly longer than the hind ones.