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PERISSODACTYLA (i.e. odd-toed)
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See also:PERISSODACTYLA (i.e. See also:odd-toed) , the name proposed by See also:Sir R. See also:Owen for that See also:division of ungulate mammals in which the toe corresponding to the See also:middle (third) See also:digit of the human See also:hand and See also:foot is symmetrical in itself, and larger than those on either See also:side (when such are See also:present). The Perissodactyla have been brigaded with the See also:Artiodactyla (q.v.) to See also:form the typical See also:group of the ungulates, under the name of Diplarthra, or See also:Ungulata See also:Vera, and the features distinguishing the combined group from the less specialized members of the See also:order Ungulata will be found under the heading of that order. The following are the leading characteristics by means of which the sub-order Perissodactyla is distinguished from the \rtiodactyla. The cheek-See also:teeth (premolars and molars) form a A B c S 7 A, See also:Tapir (Tapirus indicus). B, See also:Rhinoceros (Rhinoceros sumatrensis). C, See also:Horse (Equus See also:cabal/us). U, atria; R, See also:radius; c, See also:cuneiform; ',lunar; s, scaphoid; u, unciform; in, magnum; td, trapezoid; tm, trapezium. continuous See also:series, with massive, quadrate, transversely ridged or complex crowns—the posterior premolars usually resembling the molars in structure. See also:Crown of the last See also:lower molar commonly bilobed. Dorso-lumbar vertebrae never fewer than twenty-two, usually twenty-three in the existing See also:species. Nasal bones See also:expanded posteriorly. An alisphenoid See also:canal. Femur with a third trochanter. The middle or third digit on both fore and See also:hind feet larger than any of the others, and symmetrical in itself, the See also:free border of the terminal See also:phalanx being evenly rounded (see fig. I). This may be the only functional toe, or the second and See also:fourth may be subequally See also:developed on each side. In the tapirs and many See also:extinct forms the fifth toe also remains on the fore-See also:limb, but its presence does not interfere with the symmetrical arrangement of the See also:remainder of the foot on each side of the median See also:line of the third or middle digit. The astragalus has a See also:pulley-like See also:surface above for See also:articulation into the See also:tibia, but its lower surface is flattened and unites to a much greater extent with the navicular than with the cuboid, which See also: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 See also:stomach is See also:simple, the caecum large and capacious, the See also: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 See also:modern rhinoceroses and their forerunners. 1. Titanotheres.—In the Titanotheroidea the dentition may be expressed by the See also:formula i9:uo(o, c), pTg-3, ml. There is usually a See also:short See also: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 See also:outer See also:concave tubercles and two inner conical ones; while the lower molars are crescentic, with three lobes in the last of the series. The See also:skull is elongated, with the See also: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 See also:union of the premaxillae. The See also: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 See also:groups, articulates with the fibula. The group is represented by the families Palaeosyopidae and See also:Titanotheriidae in the See also:Tertiary deposits of See also:North See also:America. Both families are described under the heading TITANOTHERIIDAE. 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 See also:deciduous or even absent in the lower or in both jaws. The incisors are See also: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 See also:primitive tubercular molar coalesce to form an outer See also: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 See also: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 postglenoid, post-tympanic and paroccipital processes of the skull are large; the second of these being always distinct. Nasals See also:long, normally without traces of horns. The See also:section is divisible into the families See also:Equidae and Palaeotheriidae, of which the latter is extinct. In the Equidae the premolars are generally 1 or gg. 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 See also:late. Orbit in higher forms closed by bone; and ridges of lower cheek-teeth terminating in large loops. Front toes 4, 3 or I, hind; 3 or I. (See EQUIDAE and HORSE.) In the Palaeotheriidae the premolars may be 1 or g, and are generally molar-like, while the first (when present) is always See also:close to the second; all the cheek-teeth short-crowned and rooted, with or without See also: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 See also: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 See also: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 See also:complete See also: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 See also:ridge to the last molar in the earlier forms. The Lophiodontidae, which date from the See also:Eocene, come very close to Hyracotherium in the horse-line; and it is solely on the authority of See also:American palaeontologists that the division of these early fcrms 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 See also:size. The See also: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 See also:stage in the See also: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 See also:lobe is, however, retained in the last lower molar. In the are only three front toes, and the limbs are long and adapted for See also:running. In the Amynodontidae, represented by the North American Middle Eocene Amynodon and Metamynodon, the premolars may be either f or t, making the See also: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 See also:family, the post-glenoid See also:process of the skull is broad; the whole skull be'_ng 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 See also:variations, by the formula i i ar oo c i o~ o Oligocene of both hemispheres appears Protapirus, which ranges well into the See also: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 See also:appearance in the Upper Miocene, and continues till the present See also:day. The characters of the genus may be expressed as follows in a more detailed manner. The dentition is i 8, c p 3, m $, 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 See also: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 See also:column. The other upper premolars and molars all forrped on the same See also: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 See also: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 See also:ordinary See also:standing position. Hind-feet with the typical perissodactyle arrangement of three toes—the middle one being the largest, the two others nearly equal. See also:Nose and upper See also:lip elongated into a flexible, See also:mobile snout or short See also: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 See also:hair. Tapirs are See also:common to the See also:Malay countries and tropical America; two species from the latter See also:area differ from the See also:rest in having a See also:vertical bony See also: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 See also:compression and consequent lengthening of the outer columns, of which the hinder is See also: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 See also:age, in which the first lower pre-molar 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 See also:Europe the group is represented by the long-known and typical genus Lophiodon with three premolars in each See also: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 See also: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 specialized 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 See also:cranial; the orbit is freely open behind; and the premaxillae tend to be reduced and fused 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 Triplo- pus. 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. Thereas oo zz p m The first upper incisor, when present, has an as~~z antero-posteriorly elongated crown, but the second is small; when fully developed, the lower canine is a large forwardly directed tusk-like tooth with See also: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 See also: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 See also:double. In North America rhinoceroses became extinct before the close of the See also:Pliocene See also:period; but in the Old World, although their See also:geographical See also:distribution has become greatly restricted, at least five well-marked species survive. The group is unknown in See also:South America. 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 B B, Rhinoceros sondaicus. 6, Postero-See also:internal See also:pillar or column. 7, Anterior valley. 8, Median valley. 9, Posterior valley. to, See also:Accessory valley. Crista. 12, Crochet. others gradually increasing in size up to the penultimate. The upper molars present a characteristic See also:pattern of crown, having a much-developed See also: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 projecting into it from the anterior surface of the posterior transverse ridge or from the wall, the development of which is a useful See also: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 See also:head is large, and the skull elongated, and elevated posteriorly into a transverse occipital See also: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 See also:meeting in the middle line in front, often rudimentary. Tympanics small, not forming a bulla. See also:Brain-cavity small for the size of the skull. Vertebrae: cervical, 7; dorsal i9–2o; lumbar, 3; sacral, 4; caudal, about 22. 2 e a 9 A, Rhinoceros unicornis. i, Anterior surface. 2, Posterior surface. 3, Internal surface. 4, External surface (wall or dorsum). 5, Antero-internal pillar or column. 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, See also: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 See also:great length (three or even four feet), are composed of a solid See also: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 inter-mediate mass of cells which grow up from the interspaces between the papillae. It results from this that the See also:horn has the appearance of a mass of agglutinated hairs, which, in the newly growing See also:part at the See also:base, readily fray out on destruction of the softer intermediate substance; but the See also:fibres differ from true hairs in growing from a free papilla of the deem, and not within a follicular involution of the same. Considerable difference of See also:opinion exists with regard to the best See also: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 Ronrotherium 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 See also: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 See also:Asia indicates another subgenus, represented in the European Miocene by R. sansaniensis and in the See also: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 See also:African species, representing the subgenus Diceros, in which there are two horns but no front teeth. Tc this group belongs the extinct European and See also:Asiatic woolly rhinoceros, Rhinoceros (Diceros) antiquitatis, of See also:Pleistocene age, of which the frozen bodies are sometimes found in See also:Siberia, and R. (D.) pachygnathus of the Lower Pliocene of See also: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. Additional information and CommentsThere are no comments yet for this article.
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