Proboscidea
PROBOSCIDEA (animals "with a proboscis"), the scientific name of the group of mammals represented at the present day only by the two species of elephant. Although here regarded as a sub-order of UNGULATA (q.v.), the group is sometimes accorded the rank of an order by itself. 1 The existing elephants are widely sundered from all other living mammals, and for a long time palaeontology afforded but little clue as to their ancestry. Discoveries made during the first few years of the zoth century in the Lower Tertiary deposits of the Fayum district of Egypt have, however, brought to light the existence of several kinds of primitive proboscideans which serve to link the group with other ungulates, and likewise apparently indicate affinity with the Sirenia.
The following are some of the leading characteristics of existing elephants. The combined upper lips and nose are produced into a long muscular, flexible and prehensile proboscis, or trunk, with the nostrils at its tip. The teeth consist of a pair of large upper permanently growing incisors or tusks; and a set of cheekteeth having their crowns composed of a series of tall transverse vertical plates gradually increasing in number from the first to the last of the series; and only portions of two of these teeth being in use at any one time. There are no clavicles; and the limbs are stout, with their component segments placed nearly in a vertical line, and the upper segment, especially in the hindlimb, the longest; the radius and ulna are distinct, the latter articulating extensively with the carpus; the fibula and tibia also distinct; the astragalus very flat on both surfaces; and both front and hind feet short, broad and massive, with five toes (though the outer pair may be more or less rudimentary), all encased in a common integument, though with distinct, broad, short hoofs; third digit the largest. Two anterior venae cavae entering the right auricle. Stomach simple. A capacious caecum. Testes permanently abdominal. Uterus bicornuate. Placenta deciduate and zonary. Teats two, pectoral.
In order to understand the peculiar nature of the dentition, it is necessary to discuss to some extent those of the immediate ancestors _ u . of the true elephants, such as the mastodons (see MASTODON). As regards the incisors, or tusks, which 1 Cuvier's order Pachydermata (Gr. raxbs, thick and tepua, skin), containing the elephants, hippopotami, rhinoceros, swine, tapirs, hyraxes, etc., is now abandoned, its members now forming the orders Proboscidea and Hyracoidea and the sub-order Parissodactyla. A few Artiodactyla are also included.
project largely out of the mouth, and are of an elongated conical form and generally curved, these are composed mainly of solid dentine, the fine elastic quality and large mass of which renders it invaluable as " ivory " for commerce and the arts. A peculiarity of the dentine of the Proboscidea is that it shows, in transverse fractures or sections, fine lines proceeding in the arc of a circle from the centre t FIG. i. Longitudinal Sections of the Crowns of Molar Teeth of various Proboscideans, showing stages in the gradual modification from the simple to the complex form. The dentine is indicated by transverse lines, the cement by a dotted surface, and the enamel is black.
I, Mastodon americanus; III, Elephas afrtcanus; II, Elepkas (Stegodon) insignis; IV, Elephas primigenius.
to the circumference in opposite directions, and forming by their decussations curvilinear lozenges, as in the " engine-turning " of the case of a watch. The enamel-covering in existing species is confined to the extreme apex, and very soon wears off, but in some extinct species it forms persistent longitudinal bands of limited breadth. The tusks have small milk-predecessors, shed at an early age.
As regards the cheek-teeth, these are composed in the mastodons of a variable number of enamel-covered transverse ridges, often divided into inner and outer columns, which may partially alternate, and complicated by smaller additional columns; but in the unworn tooth they stand out freely on the surface of the crown, with deep valleys between (fig. i, I). In the elephants the ridges are increased in number, and consequently become narrower from before backwards, while they are greatly extended in vertical height. In order to give solidity to what would otherwise be a comb-like tooth, the whole structure is enveloped and united in a large mass of cement, which completely fills the valleys, and gives a general smooth appearance to the unworn tooth; but as the wear consequent upon the masticating process proceeds, the alternate layers of tissue of different hardness cement, dentine and enamel which are disclosed upon the surface form a fine and efficient grinding instrument. The intermediate stages between the molar of a modern elephant and that of a mastodon are so fully known that it is not possible to draw a definite line between the two types of toothstructure (see fig. I, II, III, IV).
As regards the mode of succession, that of modern elephants is very peculiar. During the complete lifetime of the animal there are but six cheek-teeth, which it will be convenient to allude to as molars, on each side of each jaw, with occasionally a rudimentary one in front, completing the typical number of seven. The last three represent the molars of ordinary mammals, those in front are milk-molars, which are never replaced by permanent successors, the whole series gradually moving forwards in the jaw, and the teeth becoming worn away and their remnants cast out in front, while development of others proceeds behind. The individual teeth are so large, and the processes of growth and destruction by wear take place so slowly, that not more than one, or portions of two, teeth are ever in place and in use on each side of each jaw at one time, and the whole series of changes coincides with the usual duration of the animal's life. On the other hand, the earlier representations of the proboscidean series referred to below have the whole of the cheek-teeth in place and use at one time, and the milk-molars vertically displaced by premoiars in the ordinary fashion. Among mastodons transitional forms occur in the mode of succession as well as in structure, many species showing a vertical displacement of one or more of the milk-molars, and the same has been observed in one extinct species of true elephant (Elephas planifrons) as regards some of these teeth.
Most proboscideans are animals of large dimensions, and some are the most colossal of land mammals. The head is of great proportionate size; and, as the brain-case increases but little in bulk Character- during growth, while the exterior wall of the skull is isiks. required to be of great superficial extent to support the trunk and the ponderous tusks, and to afford space for the attachment of muscles of sufficient size and strength to (Flower's Osteology of Mammalia.')
FIG. 2. Section of the Skull of the African Elephant (Elephas africanus) taken to the left of the middle line, and including the vomer (Vo) and the mesethmoid (ME).
an, Anterior, pn, Posterior nasal aperture. OrV nat. size.)
wield the skull thus heavily weighted, an extraordinary development of air-cells takes place in the cancellous tissue of nearly all the bones of the cranium. These cells are not only formed in the walls of the cranium proper, but are also largely developed in the nasal bones and upper part of the premaxillae and maxillae, the bones forming the palate and the basi-cranial axis, and even extend into the interior of the ossified mesethmoid and vomer. Where two originally distinct bones come into contact, the cells pass freely from one to the other, and almost all the sutures become obliterated in old animals. The intercellular lamellae in the great mass which surrounds the brain-cavity superiorly and laterally mostly radiate from the inner to the outer table, but in the other bones their direction is more irregular. Like the similar but less developed aircells in the skulls of many other mammals, they all communicate with the nasal passages, and they are entirely secondary to the original growth of the bones, their development having scarcely commenced in the new-born animal, and gradually enlarge as the growth of the creature proceeds. The nasal bones are very short, and the anterior nasal aperture situated high in the face. The zygomatic arch is slender and straight, the jugal bone being small, and forming only the middle part of the arch, the anterior part of which (unlike that of true Ungulates) is formed only by the maxilla. The maxillo-turbinals are rudimentary, the elongated proboscis supplying their place functionally in warming and clearing from dust the inspired air.
The neck is very short. The limbs, as already mentioned, are long and stout, and remarkable for the great length of the upper segment (especially the femur) as compared with the lower segment, as represented by the foot. It is owing to this and the vertical position of the femur that the knee-joint in the hind-leg is placed much lower, and is more conspicuous externally than in most quadrupedal mammals; and this having been erroneously compared with the hock-joint or ankle of the 'more ordinary ungulates, gave rise to the popular fallacy that the joints of the elephant's leg bend in a contrary direction to that of other mammals. There is no round ligament in the hip-joint, or third trochanter to the femur. The radius and ulna are distinct, though fixed in a crossed or prone position; and the fibula also is quite separated from the tibia. The feet are short and broad, the carpal and tarsal bones being very square, with flattened surfaces for articulation ; the astragalus especially differs from that of the more typical ungulates in its flatness, in the absence of distinct pulley-like articular surface at either extremity, and in having no articular facet for the cuboid. The fibula articulates with the calcaneum, as in the artiodactyle sub-order of Ungulata. Of the five toes present on each foot, the middle one is somewhat the largest, while the lateral ones are the smallest, and generally lack (especially in the hind-foot) the complete number of phalanges. The terminal phalanges are all small, irregular in form, and late in ossification. The whole are encased in a common integument, with a flat, subcircular, truncated sole, the only external indication of the toes being the broad oval nails or hoofs arranged in a semicircle around the front edge of the sole. The hind foot is smaller and narrower than the front. The liver is small and simple, and there is no gall-bladder. In form the brain resembles that of the lower orders of mammals in that the cerebellum is entirely behind and uncovered by the cerebrum, but the hemispheres of the latter are richly convoluted.
Elephants are exclusively vegetable-feeders, living chiefly on leaves and young branches of forest trees and various kinds of herbage, or roots, which they gather and convey to their mouth by a very mobile proboscis, an organ which combines in a marvellous manner strength with dexterity of application, and is a necessary compensation for the shortness and inflexibility of the neck, as it is by this that many of the functions of the lips of other animals are performed. By its means elephants are enabled to drink without bending the head or limbs. The end of the trunk being dipped, for instance, into a stream or pool, a forcible inspiration fills the two capacious air-passages in its interior with water, which, on the tip of the trunk being turned upwards and inserted into the mouth, is ejected by a blowing action, and swallowed. Or if the animal wishes to refresh and cool its skin, it can throw the water in a copious stream over any part of its surface. Elephants can also throw dust and sand over their bodies by the same means and for the same purpose, and they have frequently been observed fanning themselves with boughs held in the trunk.
The following are the distinctive features of the genus Elephas, the type_of the family Elephantidae: Dentition: i. J, c. J, m. f =26. The incisors variable, but usually of very large size, especially in the male sex, directed somewhat outwards, and curved upwards, without enamel except on the apex before it is worn; preceded by small milk-incisors. The molars succeed each other by horizontal replacement from before backwards, never more than one or part of two being in use on each side of each jaw at the same time; each composed of numerous flattened enamel-covered plates or ridges of dentine, projecting from a common many-rooted base, surrounded and united together by cement. The number of plates increases from the anterior to the posterior molar in regular succession, varying in the different species, but the third and fourth (or the last milk-molar and the first true molar), and these only, have the same number of ridges, which always exceeds five. Skull of adult very high and globular. Lower jaw ending in front in a deflected, spout-like symphysis. Vertebrae: C. 7, D. 19-21, L. 3-4, S. 4, C. 26-33.
The two existing species of elephant are the Indian or Asiatic (Elephas maximus), and the African (E. africanus), the distinctive characteristics of which are given under ELEPHANT. See also MAMMOTH and MASTODON.
EXTINCT PROBOSCIDEA Elephas. The extinct representatives of the Proboscidea are of the greatest importance and interest, since they serve to connect the modern elephants with ungulates of more ordinary type. The MAMMOTH (Elephas primigenius) is treated in a separate article. Nearly allied is E. armeniacus of Asia Minor; but E. antiquus, of which the remains are abundant in many of the superficial formations of England and Europe generally, approximates in the structure of its molar teeth to the African elephant. It is represented in the Pleistocene of India by the closely allied or identical E. namadicus. Affinity with the African species is strongly marked in the case of the dwarf elephants of Malta (E. melUensis) and Cyprus (E. Cypriotes); and the gigantic E. meridionalis, of the " forest-bed " of the east coast of England and the Upper Pliocene of the Val D'Arno, has likewise molars showing the broad lozenges of enamelbordered dentine characteristic of the African type. These and other species indicate, however, that, so far as dental characters are concerned, generic separation of the African from the Asiatic elephant is impossible. In North America the mammoth occurs in the far north, E. columbi, more akin to E. antiquus chiefly in the Central United States, and E. imperator (allied to E. meridionalis) in the south. The oldest representatives of this group are E. hysudricus and E. planifrons of the Lower Pliocene of Northern India; the latter of which developed premolars vertically replacing the anterior teeth of the molar series.
From E. planifrons there is an almost complete transition to the ridge-toothed elephants, such as E. ganesa, E. insignis.
E. bombifrons and E. clifti, typically from the Lower Pliocene of India and Burma, but some of which extend eastwards to Java, Borneo, China and Japan. These constitute the group (or genus) Stegodon, and are characterized by the lowness of the crowns of the molar teeth, in which the tall plates of the more typical elephants are reduced to low ridges with more or less completely open valleys between them ; the number of ridges in each tooth is always much lower than in the corresponding teeth of the typical elephants. Premolars, vertically replacing the anterior molars, were often developed. These stegodont elephants appear to have been confined to India and the countries farther east, and exhibit an almost complete transition, so far as dental characters are concerned, to the mastodons of the same region.
Mastodon. The connexion between the stegodont elephants and the mastodons (see MASTODON) is formed by the Indian and Burmese Mastodon latidens and M. cautleyi. In fact the main distinction between these animals and the stegodont elephants is the smaller number of ridges in the third, fourth and fifth molars, which is usually four, and never exceeds five, whereas in the stegodonts it is at least six and the numbers are not the same in each of the three teeth. In the two species named the transverse ridges are more or less continuous. Many other species, such as the European M. aniernensis (see fig. 2 in art. MASTODON) and the Indian M. sivalensis, have, however, the ridges broken up into columns, or cones, more or less alternately arranged, and thus blocking the intermediate valleys. In these species, which are of Pliocene age, there are four ridges in molars 3, 4 and 5; but in the Pleistocene North American M . americanus (as well as in many other species) these are reduced to three in each of the aforesaid teeth. The lower jaw of the latter species frequently shows small tusks, which are, however, generally shed in mature age. Premolars, which vertically replace some of the anterior molars (milk-molars), are developed in many species, although not in M. americanus. Species of the genus are found over the greater part of the world, inclusive of Europe, Asia and North and South America; M. humboldli being the best known South American species. A single tooth referable to this or the next genus has been obtained from South Africa.
Tetrabelodon. The more primitive mastodons constitute the genus Tetrabelodon, and are characterized by the presence of a pair of short chisel-shaped tusks in the lower jaw, which is prolonged into a trough-like chin for their support; tusks being also present in the upper jaw. These animals were provided with a snout-like muzzle instead of a trunk (see MASTODON). Their birthplace was Africa; the Miocene European M . angustidens having been discovered in Egypt in strata overlying those from which were obtained the remains of the under-mentioned more primitive genera. Tetrabelodont mastodons were, however, by no means confided to the Miocene, Tetrabelodon longirostris occurring in the Lower Pliocene of Europe, and T. pandionis in that of India. Most of these four-tusked mastodons were smaller animals than modern elephants.
Palaeomastodon. No proboscidean earlier than Tetrabelodon occurs in Europe, but the group is represented in the Upper Eocene of Egypt by a smaller and more primitive type known as Palaeomastodon. This genus resembles Tetrabelodon in having four pairs of tusks, but differs in the less elephant-like skull, and the simpler character of the molar teeth, of which five pairs were in use at one time, whereas in Tetrabelodon and Mastodon there were never more than two pairs and a portion of a third in simultaneous wear.
Moeritherium. The earliest representative of the proboscidean stock at present known is Moeritherium, from the Middle Kocene of Egypt, which includes still smaller animals, whose relationship to Elephas would scarcely be realized were it not for the intermediate links. All six pairs of cheek-teeth (pm. 2-m. 3, fig. 3) were in use at once, and there was a comparatively full series of teeth in the front of the jaws; while the premolars were preceded by milk-molars in the normal manner. Very significant is the enlargement of the second pair of incisors in each jaw, thereby foreshadowing the tusks of Tetrabelodon. There was, however, no lengthening of the chin, so that the muzzle was (From the Geological Mosn:int. )
FIG. 3. Dentition of Moeritherium lyonsi. (\ nat. size.)
A, Upper teeth.
B, Front of snout, showing the tusk-like second incisors.
C, Left ramus of mandible from outer side.
probably of normal proportions. This animal was not larger than a tapir.
Dinolherium. The huge proboscidean from the Lower Pliocene and Middle Miocene strata of Europe and India, known as Dinolherium, indicates a type off the line of descent of the elephants. Upper tusks were apparently wanting, but the FIG. 4. Skull of Dinotherium giganteum (Lower Pliocene. Eppelsheim, Hesse-Darmstadt).
lower jaws carried a pair of large tusks bent downwards in a peculiar manner (fig. 4). The cheek-teeth formed five pairs, all in use at one time, and premolars vertically replacing milkmolars in the ordinary fashion. The ridge-formula of the permanent teeth of the cheek series was 2.2.3.1.2.
Barylhcrium and Pyrotherium. Very problematical are the affinities of Barytherium of the Egyptian Eocene and Pyrotherium of the Lower Tertiaries of Patagonia; although it is possible that they may both be offshoots from the primitive proboscidean stock. Pyrolherium had a pair of upwardly directed tusks in the lower jaw. The cheek-teeth are five in number and carry transverse ridges similar to those on the molars of Dinotherium, although there are only two to each tooth. If really related to the Proboscidea, Pyrotherium may be derived from the African ancestral stock of that group which reached South America by way of a former land-connexion between that continent and Africa. So far as can be determined, Barytherium approximates in many respects to Dinotherium, but in others seems to approach Uintatherium of the North American Tertiaries (see AMBLYPODA).
See C. W. Andrews, Descriptive Catalogue of the Tertiary Vertebrata of the Fayum, British Museum, 1906. (R. L.*)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)
