Oligocene of both hemispheres appears Protapirus, which ranges
well into the Miocene, and is essentially a tapir, having lost the third
lobe of the last lower molar, and being in process of acquiring
molar-like upper premolars, although none of these teeth have two
complete inner columns. Finally, Tapirus itself, in which the last
three upper premolars, makes its appearance in the Upper Miocene,
and continues till the present day. The characters of the genus
may be expressed as follows in a more detailed manner.
The dentition is i 33, c 11, p 43, m 33, total 42. Of the upper incisors the first and second are nearly equal, with short, broad crowns, the third is large and conical, considerably larger than the canine, which is separated from it by an interval. Lower incisors diminishing in size from the first to the third; the canine, which is in contact with the third incisor, large and conical, working against (and behind) the canine-like third upper incisor. In both jaws there is a long space between the canines and the commencement of the teeth of the cheek-series, which are all in contact. First upper premolar with a triangular crown narrow in front owing to the absence of the anterior inner column. The other upper premolars and molars all formed on the same plan and of nearly the same size, with four roots and quadrate crowns, rather wider transversely than from before backwards, each having four columns, connected by a pair of transverse ridges, anterior and posterior. The first lower premolar compressed in front; the others composed of a single pair of transverse crests, with a small anterior and posterior basal ridge. Skull elevated and compressed; with the orbit and temporal fossa widely continuous, there being no true post-orbital process from the frontal bone. Nasal apertures very large, and extending high on the face between the orbits; nasal bones short, elevated, triangular and pointed in front. Vertebrae: cervical, 7; dorsal, 18; lumbar, 5; sacral, 6; caudal about 12. Limbs short and stout. Fore-feet with four toes, having distinct hoofs: the first toe being absent, the third the longest, the second and fourth nearly equal, and the fifth the shortest and scarcely reaching the ground in the ordinary standing position. Hind-feet with the typical perissodactyle arrangement of three toes—the middle one being the largest, the two others nearly equal. Nose and upper lip elongated into a flexible, mobile snout or short proboscis, near the end of which the nostrils are situated. Eyes rather small. Ears of moderate size, ovate, erect. Tail very short. Skin thick and but scantily covered with hair. Tapirs are common to the Malay countries and tropical America; two species from the latter area differ from the rest in having a vertical bony partition to the nasal septum, and are hence subgenerically or generically separated as Tapirella (Elasmognathus) (see Tapir). Nearly related is the extinct family Lophiodontidae (inclusive of the American Helaletidae), in which both the upper and lower first premolar may be absent, while the upper molars present a more rhinoceros-like form, owing to the lateral compression and consequent lengthening of the outer columns, of which the hinder is bent somewhat inwards and is more or less concave externally, thus forming a more complete outer wall. In America the family is represented by Heptodon, of the Middle Eocene, which differs from the early members of the tapir-stock in having a long gap between the lower canine and first premolar; the dentition is complete, and the upper premolars are simple. The next stage is Helaletes, also of Middle Eocene age, in which the first lower premolar has disappeared, and the last two upper premolars have become molar-like. Finally, in the Oligocene Colodon the last three upper premolars are like the molars, and the first pair of lower incisors is lost. In Europe the group is represented by the long-known and tygical genus Lophiodon with three premolars in each jaw, of which the upper are simpler than the molars. The genus is especially characteristic of the Middle and Upper Eocene, and some of the species attained the size of a rhinoceros.
4. Rhinoceros Group.—The last section of the Perissodactyla is that of the Rhinocerotoidea, represented by the modern rhinoceroses and their extinct allies. In this group the incisors and canines are very variable in number and form; the lower canine being separated by only a short gap from the outer incisor (when present), but by a long one from the first premolar, which is in contact with the second. The second and third premolars, which are always present, are large and molar-like; the whole of these teeth being essentially of the lophodont type of Lophiodon, but the last upper molars assume a more or less triangular form, with an oblique outer wall, and there are certain complications in the structure of all these teeth in the more specialize types (fig. 2). The lower cheek-teeth have, unlike those of the Tapiroidea, crescentic ridges, which have not the loops at their extremities characteristic of the advanced Hippoidea; the last lower molar has no third lobe. The facial portion of the skull is generally shorter than the cranial; the orbit is freely open behind; and the premaxillae tend to be reduced and fuse with the nasals. Front toes, 3 or 4; hind toes, 3.
The most primitive group is that of the American Hyracodontidae, represented in the Oligocene by Hyrachyus, Hyracodon and Triplopus. With the exception of the first lower premolar, the dentition is complete; the incisors being normal, but the canine rudimentary, and the last upper molar distinctly triangular. The upper molars have a crista and a crochet (fig. 2). The skull is high, with the facial and cranial portions approximately equal. There are only three front toes, and the limbs are long and adapted for running.
In the Amynodontidae, represented by the North American Middle Eocene Amynodon and Metamynodon, the premolars may be either 44 or 33, making the total number of teeth either 44 or 40. The incisors tend to become lateral, the canines are enlarged, and the last upper molar is sub-quadrangular. The upper molars have a crista but no crochet (fig. 2). As in the last family, the post-glenoid process of the skull is broad; the whole skull being depressed with a shortened facial portion. The fore-foot is five-toed and spreading; indicating that the members of the family were swamp-dwelling animals.
Finally, we have the family Rhinocerotidae, which includes the existing representatives of the group. In this family the dentition has undergone considerable reduction, and may be represented inclusive of all the variations, by the formula i 2 or 01 or 0 c 0 or 11 or 0 p 4 or 24 3 or 2 m 33. The first upper incisor, when present, has an antero-posteriorly elongated crown, but the second is small; when fully developed, the lower canine is a large forwardly directed tusk-like tooth with sharp cutting-edges, and biting against the first upper incisor. The third upper molar is triangular, and most of the teeth of the upper cheek-series may have both crochet and crista (fig. 2). The post-glenoid process is small, and the facial and cranial portions of the skull are approximately of equal length. Usually there are three, but occasionally four front toes; and the limb-bones are short.
A large number of representatives of the group are known from both the Old and the New World: specialization displaying itself in the later ones in the development of dermal horns over the nasal bones, either in laterally placed pairs as in some of the early forms, or in the median line, either single or double. In North America rhinoceroses became extinct before the close of the Pliocene period; but in the Old World, although their geographical distribution has become greatly restricted, at least five well-marked species survive. The group is unknown in South America.
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Fig. 2.—Grinding Surface of moderately worn Right Upper Second Molars of Rhinoceros. | ||||||||||||||
| A, Rhinoceros unicornis. | B, Rhinoceros sondaicus. | |||||||||||||
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As regards the dentition of the existing species, the cheek-series consists of the four premolars and three molars above and below, all in contact and closely resembling each other, except the first, which is much smaller than the rest and often deciduous; the others gradually increasing in size up to the penultimate. The upper molars present a characteristic pattern of crown, having a much-developed flat or more or less sinuous outer wall, and two transverse ridges running obliquely inwards and backwards from it, terminating internally in conical eminences or columns, and enclosing a deep valley between. The posterior valley is formed behind the posterior transverse ridge, and is bounded externally by a backward continuation of the outer wall and behind by the cingulum. The anterior valley is formed in the same manner, but is much smaller. The middle valley is often intersected by vertical “crista” and “crochet” plates protecting into it from the anterior surface of the posterior transverse ridge or from the wall, the development of which is a useful guide in discriminating species, especially those known only by teeth and bones. The depressions between the ridges are not filled up with cement. As stated above, the lower molars have the crown formed by a pair of crescents; the last having no third lobe.
The head is large, and the skull elongated, and elevated posteriorly into a transverse occipital crest. No post-orbital processes or any separation between orbits and temporal fossae. Nasal bones large and stout, co-ossified, and standing out freely above the premaxillae, from which they are separated by a deep and wide fissure; the latter small, generally not meeting in the middle line in front, often rudimentary. Tympanics small, not forming a bulla. Brain-cavity small for the size of the skull. Vertebrae: cervical, 7; dorsal 19–20; lumbar, 3; sacral, 4; caudal, about 22.
