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Encyclopedia Britannica - Main :: TUM-VAN |
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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 tipper jaw, have the full number of incisor teeth in the young state, the outer-most of these being persistent through life as an isolated tooth. The tusk-like canines are present in both jaws, those of the lower jawMr F. W. D. Brie (English Historical Review, 1906) vol. xxi. advances the theory that Tyler and Straw are one and the same person.being differentiated from the long, horizontal
series , and of simple pointed form. The hinder part of the body
pulley
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. I, c. I, p. , m. R. 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. c. if, p. , in. I; total 32. In the upper jaw there is a compressed, sharp
horizontal
recent
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
Kherson
Kherson
Remains of camels (C. thomasi) have also been found in the Pleistocene strata of Oran and Ouen Seguen, in Algeria; and certain remains from 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 fore-limb 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 Poebrotherium, in which a distinct increase in bodily size is notice-able, as also in the relative length of the two bones which unite in the higher types to form the cannon-hone. 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, end 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 hone; while the dentition begins to approximate to the camel typenotably 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 typeProcameluswhich is entitled to be regarded as a camel, and approximates in size to a small llama. Here themetacarpals 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 Pliauchensa 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 metatarsals coalesced into canon-bones late
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 re-mains 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 A griochoerus, 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 note-worthy 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). Homacodon 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 unfrequently classed among the ancestral pigs, but its cameline affinities are strongly emphasized by Professor Scott. (R. L.*) End of Article: TYLOPODA (Gr. for boss-footed, in reference to the cushion-like pads forming the soles of the feet) If you wish, you can link directly to this article.
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