TYLOPODA, a section of ruminant cloven-hoofed mammals (see ARTIODACTYLA) including the single family Camelidae, of which the Old World camels (see CAMEL) and the South Ameri can llamas (see LLAMA) are the only living representatives. The outstanding distinctions of the Tylopoda are : (I) the retention of the lateral incisors, canines and usually the first premolars as small recurved laniary tusks; (2) reduction of the remaining pre molars and narrowness of the molars; (3) the peculiar form of tympanic bulla, folded in upon itself and filled with cancellous tissue; (4) elongate cervical vertebrae and peculiar course of the vertebral artery, perforating the inner side of the pedicle of the arch instead of the transverse process as in all other mammals (except Macrauchenia, q.v.) ; (5) carpal and tarsal bones remain separate, the trapezoid and magnum never co-ossified, nor the cuboid and navicular, while the trapezium is reduced to a small nodule in Miocene and disappears in later genera, only the ecto and mesocuneiform are consolidated; (6) fore and hind feet completely didactyl, the lateral pair of digits absent except for small vestigial splints or nodules in the early forms, the median pair long, slender, appressed in early stages, consolidated into a cannon-bone in all later Tertiary and modern Tylopoda; (7) the distal ends of metapodials remain separate and slightly divergent, the keels of their distal facets confined to the palmar surface and not extending over the dorsal surface as in the pecora; (8) phalanges flattened and widened in varying degree and the hoof correspondingly reduced and limited to the upper surface of the terminal phalanx, conformant with the development of a heavy cutaneous pad on the palmar surface, and a digitigrade rather than unguligrade gait. No horns or antlers are developed. The femur and humerus are relatively elongate and the thigh and upper fore-limb are more free from the flank than in other ru minants. Many features of the soft anatomy are peculiar to the group; the absence of a distinct psalterium and presence of pockets for storage of liquid in the stomach, the diffuse placenta and the oval blood corpuscles are the most remarkable.
The modern camels and llamas are the remnants of a group which played an important part among the Tertiary mammals of North America, and in the Pliocene and Pleistocene found its way to South America, Asia, eastern Europe and northern Africa. Fossil camels have been found in the Pleistocene of Alaska, in the Pliocene and Pleistocene of China, Siberia and Russia, the Pleistocene of Rumania and of Algeria, all of them related to the modern camels, but the Pliocene species from China and Russia are in a more primitive generic stage (Paracamelus). In South America the fossil Camelidae are related to the llama group, the older forms of late Pliocene and early Pleistocene more primitive in dentition (Palaeolama).
The ancestry of the camels is shown in North America by a very complete fossil record from Oligocene to Pleistocene, with more doubtful predecessors in the Eocene. The earliest stage is Proty lopus of the Upper Eocene, about the size of a jack-rabbit. It has none of the characteristics of the Tylopoda well developed but is said to show them in a rudi mentary stage. Xiphodon and other genera from the Upper Eocene of Europe have some claim to be associated with the Tylopoda.
Poebrotherium of the Ameri can Oligocene is the first of the undoubted ancestral line of the camels. It is about as large as a sheep and the long neck and limbs, peculiar course of the ver tebral artery, the characteristic form of the tympanic bulla and the structure of teeth and feet are unmistakably camelid. The dentition is complete, the ante rior teeth are all incisiform in some species, in others the ca nines are small, recurved, spaced tusks. The feet are two-toed, the lateral digits reduced to nodules, the median pair elongate and ap pressed but not co-ossified into a like those of modern Camelidae.