1911 Encyclopædia Britannica/Tylopoda

TYLOPODA (Gr. for boss-footed, in reference to the cushion-like pads forming the soles of the feet), the scientific name of the section of ruminating artiodactyle ungulate mammals (see Artiodactyla) now represented by the Old World camels (see Camel) and the South American Llamas (see Llama).

Characters.—In the skull there is a sagittal crest; the tympanic bulla is filled with cancellous tissue; the condyle of the lower jaw is rounded; and the premaxillae, or anterior bones of the upper jaw, have the full number of incisor teeth in the young state, the outermost of these being persistent through life as an isolated tooth. The tusk-like canines are present in both jaws, those of the lower jaw being differentiated from the long, horizontal and spatulate incisors; in form they are sub-erect and pointed. The crowns of the molars belong to the crescentic or selenodont type, and are tall-crowned or hypsodont; but one or more of the anterior premolars is usually detached from the series, and of simple pointed form. The hinder part of the body is much contracted, and the femur long and vertically placed, so that the knee-joint is lower in position, and the thigh altogether more detached from the abdomen than in most mammals. The limbs are long, but with only two digits (the third and fourth) developed on each, no traces of any of the others being present. The trapezoid and magnum of the carpus, and the cuboid and navicular of the tarsus are distinct. The two cannon-bones of each limb are confluent for the greater part of their length, though separated for a considerable distance at the lower end. Their lower articular surfaces, instead of being pulley-like, with deep ridges and grooves, as in other Artiodactyla, are simple, rounded and smooth. The first phalanges are expanded at their lower ends, and the wide, depressed middle phalanges embedded in a broad cutaneous pad, forming the sole of the foot, on which the animal rests in walking instead of on the hoofs. The terminal phalanges are small and nodular, not flattened on their inner or opposed surfaces, and not completely encased in hoofs, but bearing nails on their upper surface only. The neck is long and curved, and its vertebrae are remarkable for the position of the canal for the transmission of the vertebral artery, which does not perforate the transverse process, but passes obliquely through the anterior part of the pedicle of the arch. There are no horns or antlers. Though these animals ruminate, the stomach differs considerably in the details of its construction from that of the Pecora. The interior of the rumen or paunch has no tags or villi on its surface, and there is no distinct psalterium or many plies. Both first and second compartments are remarkable for the presence of a number of pouches or cells in their walls, with muscular partitions, and a sphincter-like arrangement of their orifices, by which they can be shut off from the rest of the cavity, and into which the fluid portion only of the contents of the stomach is allowed to enter. The placenta is diffuse, not cotyledonary. Finally, the Tylopoda differ not only from other ungulates, but from all other mammals, in the fact that the red corpuscles of the blood, instead of being circular in outline, are oval as in the inferior vertebrate classes.

Camels.—Of the two existing generic representatives of the Camelidae (as the family in which they are both included is named), the Old World camels (Camelus) are characterized by their great bodily size, and the presence of one or two fleshy humps, which diminish or increase in size according to the physical condition of the animals themselves. There is a total of 34 teeth, arranged as i. 1/3, c. 1/1, p. 3/2, m. 3/3. Of these the first upper premolar is a simple tooth placed close behind the premaxilla and separated by a long gap from the two other teeth of the same series; while the lower incisors, of which the outermost is the largest, are directed partially forwards. The skull is elongated, with an overhanging occiput, complete bony rims to the orbits, and the premaxillae separated from the arched and rather long nasals. The vertebrae are C. 7. D. 12. L. 7. S. 4 and Ca. 13 to 15. The ears are short and rounded; the toes of the broad feet very imperfectly separated; the tail is well developed, with a terminal tuft; and the straight hair is not woolly.

Llamas.—Although the name llama properly applies only to one of the domesticated breeds, zoologically it is taken to include all the South American representatives of the Camelidae, which form the genus Lama. In this sense, llamas are characterized as follows. The dentition in the adult is i. 1/3, c. 1/1, p. 2/2, m. 3/3; total 32. In the upper jaw there is a compressed, sharp-pointed, tusk-like incisor near the hind edge of the premaxilla, followed in the male at least by a moderate-sized, pointed, curved canine in the anterior part of the maxilla. The isolated canine-like premolar which follows in the camels is not present. The teeth of the cheek-series which are in contact with each other consist of two small premolars (the first almost rudimentary) and three broad molars, constructed generally like those of Camelus. In the lower jaw the three incisors are long, spatulate and horizontal, with the outer one the smallest. Next to the latter is a curved, sub-erect canine, followed after an interval by an isolated minute and often deciduous simple conical premolar; then a contiguous series of one premolar and three molars, which differ from those of recent species of Camelus in having a small accessory column at the anterior outer edge. The skull generally resembles that of Camelus, the relatively larger brain-cavity and orbits and less developed cranial ridges being due to its smaller size. The nasal bones are shorter and broader, and are joined by the premaxillae. Vertebrae: C. 7, D. 12, L. 7, S. 4, Ca. 15 to 20. Ears rather long and pointed. No hump. Feet narrow, the toes being more separated than in the camels, and each with a distinct plantar pad. Tail short. Hairy covering long and woolly. Size smaller and general form lighter than in the camels. Llamas are now confined to the western and southernmost parts of South America, though fossil remains have been found in the caves of Brazil, and in the pampas of the Argentine Republic. (See also Alpaca; Guanaco; Llama and Vicugna.)

Fossil History.—As regards the past history of the group, remains of fossil species of Camelus have been obtained from the superficial deposits of various parts of Russia, Rumania, and Siberia, and others from the Lower Pliocene of northern India; the molar teeth of these latter presenting the additional column referred to above as distinguishing those of the llamas from those of modern camels. In addition to these Dr M. Schlosser has described remains of a large camel-like animal from China, with apparently generalized affinities, for which the name of Paracamelus is proposed. Mme Pavlow, of Moscow, has brought to notice a fossil camel-skull of great interest, which was collected in the district Alexandrie, of the government of Kherson, Russia. Unfortunately, the precise age of the formation from which it was obtained is unknown, but it is considered probable that it dates from the later Tertiary. Although it has the deciduous dentition, Mme Pavlow considers herself justified in referring the Kherson skull to the genus Procamelus previously known only from the Lower Pliocene or Upper Miocene strata of North America, and differing from modern camels, among other features, by the retention of a fuller, series of premolar teeth. Part of the cannon-bone of a camel from another district in Russia is provisionally assigned to the same species. Possibly this Russian camel (Procamelus khersonensis), as it is called, may form the connecting link between the typical Procamelus of North America and the fossil camel (Camelus sivalensis) of the Siwalik Hills of India. Be this as it may, the identification of a North American type of camel from the Tertiary strata of eastern Europe forms another connecting link between the extinct faunas of the northern half of the Old World and North America, and thus tends to show that the claim of America to be the exclusive birthplace of many Old World types may have to be reconsidered.

Remains of camels (C. thomasi) have also been found in the Pleistocene strata of Oran and Ouen Seguen, in Algeria; and certain remains frorn the Isle of Samos have been assigned to the same genus, although the reference requires confirmation. The Algerian Pleistocene camel was doubtless the direct ancestor of the living African species, which it serves to connect with the extinct C. sivalensis.

In North America, apart from certain still older and more primitive mammals, with teeth of the tubercular type, the earliest known form which can definitely be included in the camel-series is Protylopus, of the Uinta or Upper Eocene. In this creature, which was not larger than a European hare, there was the full number of 44 teeth, which formed a regular series, without any long gaps, and with the canines but little taller than the incisors, while the hinder cheek-teeth, although of the crescentic type, were low-crowned. In both jaws the anterior front-teeth were of a cutting and compressed type. Unfortunately, the skull is incomplete, and the rest of the skeleton very imperfectly known; but sufficient of the former remains to show that the socket of the eye was open behind, and of the latter to indicate that in the hind-foot, at any rate, the upper bones of the two functional toes had not coalesced into a cannon-bone. The lateral hind-toes (that is to say the second and fifth of the typical series) had, however, become rudimentary; although it is probable that the corresponding digits of the forelimb were functional, so that this foot was four-toed. In old individuals the bones of the forearm (radius and ulna) became welded together about half-way down, although they remained free above. On the other hand it appears that the smaller bone of the leg (fibula) was welded to the larger one (tibia), and that its upper portion had disappeared. Nothing is known of the neck vertebrae. It is, of course, evident that there must have been an earlier form in which all the feet were four-toed, and the bones of the forearm and lower part of the leg separate.

A stage higher in the series, viz. in the Oligocene, we meet with Pöebrotherium, in which a distinct increase in bodily size is noticeable, as also in the relative length of the two bones which unite in the higher types to form the cannon-bone. Moreover, the crowns of the hinder cheek-teeth are taller, and more distinctly crescentic, both feet are two-toed, the ulna and radius are fused, and the fibula is represented only by its lower part. In the vertebrae of the neck the distinctive cameloid characters had already made their appearance. On the other hand, the skull was short and rabbit-like, showing none of the characteristic features of modern camels.

In the Lower Miocene occurs Protomeryx or Gomphotherium, in which there is a considerable increase in the matter of bodily size, the two metacarpal and metatarsal bones (or those which unite in the latter forms to constitute the cannon-bones) being double the length of the corresponding elements in Protylopus. These bones, although separate, have their adjacent surfaces more closely applied than is the case in the latter; while in this and the earlier genera the terminal toe-bones indicate that the foot was of the normal hoofed type. In the skull the socket of the eye is surrounded by bone; while the dentition begins to approximate to the camel type—notably by the circumstance that the lower canine is either separated by a gap from the outermost incisors, or that its crown assumes a backwardly curved shape. In Protolabis of the Middle Miocene, while no cannon-bone is formed, the first and second pairs of incisor teeth are retained, and the limbs and feet are short and disproportionately small. In the Upper Miocene we come to a distinct type—Procamelus—which is entitled to be regarded as a camel, and approximates in size to a small llama. Here the metacarpals and metatarsals have partially united to form cannon-bones, the skull has assumed the elongated form characteristic of modern camels, with the loss of the first and second pairs of upper incisors, and the development of gaps in front of and behind each of the next three teeth, that is to say, the third incisor, the canine and the first cheek-tooth. The approximately contemporaneous Pliauchenia makes another step by the loss of the second lower cheek-tooth. Both these genera have the toe-bones of the irregular nodular form distinctive of modern camels, so that we may safely infer that the feet themselves had assumed the cushion-type.

In one species of Procamelus the metacarpals and rnetatarsals coalesced into canon-bones late in life; but when we come to the Pleistocene Camelops such union took place at an early stage of existence, and was thoroughly complete. In the living members of the group it occurs before birth. The species of Camelops were probably fully as large as llamas, and some, at any rate, resembled these animals as regards the number of teeth, the incisors being reduced to one upper and three lower pairs, and the cheek-teeth to four or five in the upper and four in the lower jaw; the total number of teeth thus being 28 or 30 in place of the 44 of Pöebrotherium. The sole difference between Camelops and Llama seems to consist in certain structural details of the lower cheek-teeth. An allied extinct genus (Eschatius) is also distinguished by certain features in the dentition.

Apart from Procamelus the foregoing genera are exclusively North American. A lower jaw from the Pleistocene deposits of that continent has, however, been referred to the Old World Camelus.

In addition to the above there is an extraordinary North American Miocene giraffe-necked camel (Alticamelus), a creature of the size of a giraffe, with similarly elongated neck and limbs, and evidently adapted for browsing on trees. The feet and number of teeth were generally similar to those of Procamelus. Unlike the giraffe, the length of the limbs is due to the elongation of their upper segments, and that of the neck to the lengthening of only the hinder vertebrae.

In caverns and superficial deposits of South America occur remains of extinct species more or less closely related to modern llamas; but previous to the Upper Pliocene the group is unknown in South America, which it reached from the north.

All the foregoing genera are included in the sub-family Camelidae. Parallel to this is, however, the North American family Leptomerychidae (Hypertragulidae), as represented by Leptomeryx, Camelomeryx and Leptoreodon, which presents remarkable resemblances, especially in the type genus, to the Tragulina (see Chevrotain); camel-like features being, however, apparent in the two genera last mentioned. Generalized features are also displayed by the Oligocene Hypisodus, which in its short skull and large orbits presents a curious approximation to the African dik-dik antelopes of the genus Madoqua (see Antelope). Again, the remarkable horned North American Oligocene genus Protoceras, while displaying resemblances to Leptomeryx and Leptoreodon, presents also points of similarity to the Tragulina and Pecora (q.v.).

The North American genus Oreodon typifies a second family included by Professor W. B. Scott in the Tylopoda and generally known as the Oreodontidae. As Oreodon is, however, antedated by Merycoidodon, the latter name is properly entitled to stand, in which case the family should be called Agriochoeridae. It is not easy to point out the characters in which the family approximates to the Camelidae, and only its general characteristics can be indicated. The family ranges in North America from the Upper Eocene to the Lower Miocene, but Oreodon (or Merycoidodon), which is typified by an animal of the size of a sheep, is Oligocene. In the Oreodontinae or typical section of the family, which includes several genera nearly allied to Oreodon, the skull is shorter and higher than in the camels, with a swollen brain-case, a preorbital gland-pit, the condyle of the lower jaw transversely elongated, the tympanic bulla hollow, and the orbit surrounded by bone. The dentition comprises the typical 44 teeth, of which the molars are short-crowned, with four crescentic cusps on those of the upper jaw (selenodont type). The most characteristic dental feature is, however, the assumption of the form and function of a canine by the first lower premolar; the lower canine being incisor-like. The tail is very long; and the feet have five functional toes, with complete but short metacarpals or metatarsals. In the Miocene Agriochoerus, which typifies a second sub-family (Agriochoerinae), there is no gland-pit in the skull, of which the orbit is open behind; while the upper incisors are wanting in the adult and the terminal toe-bones are claw-like rather than of the hoofed type. The molars are less completely selenodont than in the type genus. It is noteworthy that a molar from the Tertiary of India has been referred to Agriochoerus, a determination which if correct probably indicates the occurrence of Oreodonts in the unknown Tertiary deposits of Central Asia. It may be added that in the Oreodontidae the vertebral artery pierces the transverse processes of the cervical vertebrae in the normal manner.

The earliest representatives of the Tylopoda according to Professor Scott is the Middle Eocene genus Homacodon, typifying the family Homacodontidae, which is regarded as the common ancestor of both Camelidae and Oreodontidae, with resemblances to the European Oligocene genus Dichobune (see Artiodactyla). Homaeodon was an animal of the size of a rabbit, with five toes (of which only five were functional to each foot) and 44 teeth, of which the molars are tuberculated (bunodont), with six columns on those of the upper jaw; the premolars being of a cutting type. It should be added that this generalized animal is not infrequently classed among the ancestral pigs, but its cameline affinities are strongly emphasized by Professor Scott.

Literature.—W. B. Scott, “On the Osteology of Poebrotherium,” Journal of Morphology (1891), vol. v.; “The Osteology of Protoceras” (1895), ibid., vol. xi.; J. L. Wortman, “On the Osteology of Agriochoerus,” Bull. Amer. Museum (1895), vol. vii.; “The Extinct Canielidae of North America” (1898), ibid., vol. x.; W. D. Matthew, “The Skull of Hypisodus” (1901), ibid., vol. xvi.  (R. L.*)