# 1911 Encyclopædia Britannica/Perissodactyla

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​PERISSODACTYLA (i.e. odd-toed), the name proposed by Sir R. Owen for that division of ungulate mammals in which the toe corresponding to the middle (third) digit of the human hand and foot is symmetrical in itself, and larger than those on either side (when such are present). The Perissodactyla have been brigaded with the Artiodactyla (q.v.) to form the typical group of the ungulates, under the name of Diplarthra, or Ungulata Vera, and the features distinguishing the combined group from the less specialized members of the order Ungulata will be found under the heading of that order.
* Fig 1.—Bones of Right Fore-Foot of existing Perissodactyla.
* A, Tapir (Tapirus indicus)
* B, Rhinoceros (Rhinoceros sumatrensis)
* C, Horse (Equus caballus)
* U, ulna, R, radius, c, cuneiform, l, lunar; s, scaphoid; u, unciform; m, magnum, td, trapezoid, tm, trapezium.
The following are the leading characteristics by means of which the sub-order Perissodactyla is distinguished from the Artiodactyla. The cheek-teeth (premolars and molars) form a continuous series, with massive, quadrate, transversely ridged or complex crowns—the posterior premolars usually resembling the molars in structure. Crown of the last lower molar commonly bilobed. Dorso lumbar vertebrae never fewer than twenty-two, usually twenty-three in the existing species. Nasal bones expanded posteriorly. An alisphenoid canal. Femur with a third trochanter. The middle or third digit on both fore and hind feet larger than any of the others, and symmetrical in itself, the free border of the terminal phalanx being evenly rounded (see fig. 1) This may be the only functional toe, or the second and fourth may be subequally developed on each side. In the tapirs and many extinct forms the fifth toe also remains on the fore-limb, but its presence does not interfere with the symmetrical arrangement of the remainder of the foot on each side of the median line of the third or middle digit The astragalus has a pulley-like surface above for articulation into the tibia, but its lower surface is flattened and unites to a much greater extent with the navicular than with the cuboid, which bone is of comparatively less importance than in the Artiodactyles. In existing forms the calcaneum does not articulate with the lower end of the fibula. The stomach is simple, the caecum large and capacious, the placenta diffused, and the teats inguinal.
The Perissodactyla may be divided into the four following sections, namely the extinct Titanotheroidea, the Hippoidea, represented by the horse tribe and their ancestors, the Tapiroidea, typified by the tapirs, and the Rhinocerotoidea, which includes the modern rhinoceroses and their forerunners.
1. Titanotheres.—In the Titanotheroidea the dentition may be expressed by the formula i3.2 or 0/3.2 or 0, c1/1, p4/4–3, m3/3. There is usually a short gap between the canine and first premolar; the upper molars are short-crowned and transitional between the bunodont (tubercular) and selenodont (crescentic) types, with two outer concave tubercles and two inner conical ones; while the lower molars are crescentic, with three lobes in the last of the series. The skull is elongated, with the orbit not separated from the temporal fossa and the nasals, which may or may not carry horns, reaching at least as far forwards as the union of the premaxillae. The post-glenoid, post-tympanic and paroccipital processes of the skull are large, and there is an alisphenoid canal. There are four functional toes in front and three behind; while the calcaneum, unlike that of the other three groups, articulates with the fibula. The group is represented by the families Palaeosyopidae and Titanotheriidea in the Tertiary deposits of North America. Both families are described under the heading Titanotheriidea.
2. Horse Group.—In the Hippoidea there is generally the full series of 44 teeth, but the first premolar, which is always small, is often deciduous or even absent in the lower or in both jaws. The incisors are chisel-shaped, and the canines tend to become isolated, so as in the more specialized forms to occupy a more or less midway position in a longer or shorter gap between the incisors and premolars. In the upper molars the two outer columns or tubercles of the primitive tubercular molar coalesce to form an outer wall, from which proceed two crescentic transverse crests, the connexion between the crests and the wall being slight or imperfect, and the crests themselves sometimes tubercular. Each of the lower molars carries two crescentic ridges. In the earlier forms the cheek-teeth are low-crowned, but in the higher types they become high-crowned. The number of front toes ranges from four to one, and of hind ones from three to one. The post-glenoid, post-tympanic and paroccipital processes of the skull are large; the second of these being always distinct. Nasals long, normally without traces of horns.
The section is divisible into the families Equidae and Palaeotheridae, of which the latter is extinct.
In the Equidae the premolars are generally 4/4 or 3/3. In the earlier short-crowned forms these teeth are unlike the molars, and the first of the series is separated by a gap from the second. In the high-crowned types, as well as in some of the intermediate ones, they become molar-like, and roots are not developed in the whole cheek-series till late. Orbit in higher forms closed by bone; and ridges of lower cheek-teeth terminating in large loops. Front toes 4, 3 or 1, hind; 3 or 1. (See Equidae and Horse.)
In the Palaeotheriidae the premolars may be 4/4 or 3/3, and are generally molar-like, while the first (when present) is always close to the second, all the cheek-teeth short-crowned and rooted, with or without cement. Outer walls of upper cheek-teeth W-shaped, and transverse crests oblique. Orbit open behind; and ridges of lower cheek-teeth generally terminating in small loops. Feet always 3-toed. (See Palaeotherium.)
3. Tapir Group.—In the Tapiroidea the dentition may be either the full 44, or lack the first premolar in the lower or in both jaws. The incisors are chisel-shaped; and (unlike the early Hippoidea) there is no gap between the first premolar, when present, and the second. The upper cheek-teeth are short-crowned and without cement, and show distinct traces of the primitive tubercles; the two outer columns form a more or less complete external wall, connected with the inner ones by a pair of nearly straight transverse crests; and the premolars are originally simpler than the molars. Lower cheek-teeth with two straight transverse ridges. Nasals long in early, but shorter in later forms, hornless; orbit open behind. Front toes, 4; hind toes, 3.
This group is also divided into two families, the Tapiridae and Lophiodontidae, the latter extinct.
In the Tapiridae the dentition may be reduced below the typical 44 by the loss of the first lower premolar. Hinder premolars either simple or molar like. Outer columns of upper molars similar, the hinder ones not flattened; ridges of lower molars oblique or directly transverse, a third ridge to the last molar in the earlier forms. The Lophiodontidae, which date from the Eocene, come very close to Hyracotherium in the horse-line; and it is solely on the authority of American palaeontologists that the division of these early forms into equoids and tapiroids is attempted. In North America the earliest representative of the group is Systemodon of the Lower Eocene, in which all the upper premolars are quite simple; while the molars are of a type which would readily develop into that of the modern tapirs, both outer columns being conical and of equal size. The absence of a gap between the lower canine and first premolar and between the latter and the following tooth is regarded as an essentially tapir-like feature Lophiodochoerus apparently represents this stage in the European Lower Eocene; Isectolophus, of the American Middle Eocene, represents a distinct advance, the last upper premolar becoming molar-like, while a second species from the Upper Eocene is still more advanced; the third lobe is, however, retained in the last lower molar. In the ​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 3/3, c 1/1, p 4/3, m 3/3, 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 4/4 or 3/3, 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 0/1 or 0 c 0 or 1/1 or 0 p 4 or 2/4 3 or 2 m 3/3. 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.
| Fig. 2.—Grinding Surface of moderately worn Right Upper Second Molars of Rhinoceros. |||
| A, Rhinoceros unicornis. | B, Rhinoceros sondaicus. |
| 1, Anterior surface. 2, Posterior surface. 3, Internal surface. 4, External surface (wall or dorsum). 5, Antero-internal pillar or column. 6, Postero-internal pillar or column. | 7, Anterior valley.  8, Median valley.  9, Posterior valley. 10, Accessory valley.  11, Crista. 12, Crochet. |
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. ​Limbs stout, and of moderate length. Three completely developed toes, with distinct broad rounded hoofs on each foot. Teats two, inguinal. Eyes small. Ears of moderate size, oval, erect, prominent, placed near the occiput. Skin very thick, in many species thrown into massive folds. Hairy covering scanty. One or two median horns on the face. When one is present it is situated over the conjoined nasal bones; when two, the hinder one is over the frontals. These horns, which are of a more or less conical form and usually recurved, and often grow to a great length (three or even four feet), are composed of a solid mass of hardened epidermic cells growing from a cluster of long dermal papillae. The cells formed on each papilla constitute a distinct horny fibre, like a thick hair, and the whole is cemented together by an intermediate mass of cells which grow up from the interspaces between the papillae. It results from this that the horn has the appearance of a mass of agglutinated hairs, which, in the newly growing part at the base, readily fray out on destruction of the softer intermediate substance, but the fibres differ from true hairs in growing from a free papilla of the derm, and not within a follicular involution of the same. Considerable difference of opinion exists with regard to the best classification of the family, some authorities including most of the species in the typical genus Rhinoceros, while others recognize quite a number of sub-families and still more genera. Here the family is divided into two groups Rhinocerotinae and Elasmotheriinae, the latter including only Elasmotherium, and the former all the rest. In the Lower Oligocene of Europe we have Ronzotherium and in that of America Leptaceratherium (Trigonias), which were primitive species with persistent upper canines and three-toed fore-feet. Possibly they belonged to the Amynodontidae, but they may have been related to the Upper Oligocene Diceratherium, in which the nasal bones formed a transverse pair; this genus being common to Europe and North America. Caenopus is an allied American type. Hornless rhinoceroses, with five front-toes, ranging from the Oligocene to the Lower Pliocene in Europe, represent the genus Aceratherium, which may also occur in America, as it certainly does in India. With the short-skulled, short-footed, three-toed and generally horned rhinoceroses ranging in Europe and America from the Lower Miocene to the Lower Pliocene, typified by the European R. goldfursi and R. brachypus, we may consider the genus Rhinoceros to commence; these species constituting the subgenus Teleoceras. The living R. (Dicerorhinus) sumatrensis of south-eastern Asia indicates another subgenus, represented in the European Miocene by R. sansaniensis and in the Indian Pliocene by R. platyrhinus, in which two horns are combined with the presence of upper incisors and lower canines. Next we have the living African species, representing the subgenus Diceros, in which there are two horns but no front teeth. To this group belongs the extinct European and Asiatic woolly rhinoceros, Rhinoceros (Diceros) antiquitatis, of Pleistocene age, of which the frozen bodies are sometimes found in Siberia, and R. (D.) pachygnathus of the Lower Pliocene of Greece. Finally the Great Indian rhinoceros R. unicornis, the Javan R. sondaicus, and the Lower Pliocene Indian R. sivalensis and R. palaeindicus, represent Rhinoceros proper, in which front teeth are present, but there is only one horn. (See Rhinoceros).
The subfamily Elasmotheriinae is represented only by the huge E. sibircum of the Siberian Pleistocene, in which the premolars were reduced to 2/2 while front-teeth were probably wanting, and the cheek teeth developed tall crowns, without roots, but with cement in the valleys, and the enamel of the central parts curiously crimped. A hump on the forehead probably indicates the existence of a large frontal horn.
Literature.—J. L. Wortman and C. Earle, “Ancestors of the Tapir from the Lower Miocene of Dakota,” Bull. Amer. Mus. vol. v. art. 11. (1893); H. F. Osborn, “Phylogeny of the Rhinoceroses of Europe,” op. cit. vol. xiii art. 19 (1900); O. Thomas, “Notes on the Type Specimen of Rhinoceros lasiotis, with Remarks on the Generic Position of the Living Species of Rhinoceros,” Proc. Zool. Soc. (London, 1901). (R. L.*)