ARTIODACTYL

a member of the mammalian order Artiodactyla, which consists of the even-toed ungulates. These large and medium-size herbivores, familiar from barnyard, field, and zoo, are distributed among nine families: pigs (Suidae), peccaries (Tayassuidae), hippopotamuses (Hippopotamidae), camels (Camelidae), chevrotains (Tragulidae), giraffes and okapi (Giraffidae), deer (Cervidae), pronghorn (Antilocapridae), and cattle, sheep, goats, and antelope (Bovidae). The artiodactyls were a distinct group by the end of the Eocene epoch (36.6 million years ago). They demonstrated skill at adapting to environmental changes during the Oligocene epoch, which ended about 23.7 million years ago, and have remained prominent ever since. The 150 living species of the order are widely distributed. Deer, originally Eurasian, have spread to the Americas. Their close cousins the giraffe and okapi are at home in Africa, as are the hippopotamuses. Various bovids (which number almost 100 species) and pigs flourish in the Western and Eastern hemispheres, and the two peccaries live in Central and South America. The larger camels are native to Asia and Africa, but smaller members of the family, such as llamas and vicuas, are found in South America. Three of the chevrotains live in the tropical forests of Asia, and one lives in Africa. The pronghorn, not a true antelope, lives in the grasslands of western America. The value of these mammals to man has always been great. They provide food in the form of meat and milk as well as clothing in the form of hides and wool, and in parts of the world camels are still used for transportation. The artiodactyls are distinguished from the perissodactylsthe odd-toed ungulates such as horses and rhinocerosesby the weight-bearing axis of the leg, which extends through the third and fourth toes together. (There is a fifth toe, but no first.) In the five-toed perissodactyls, the axis passes through the third, or central, toe. Another basic characteristic that identifies artiodactyls is the astragalus, an ankle bone that has rounded articulations at both ends and no constricted neck. This arrangement provides greater thrust when the animal runs, and, together with long legs and the ability to contract muscles quickly, it adds to the speed of most artiodactyls. The evolutionary success of the artiodactyls may be attributed to this and other adaptations of skeletal structures for swift flight and to the ability of members of the order to swallow food quickly and, in many cases, regurgitate it and chew it later. Antlers and horns are found in many members of the order. Antlers, the hard, branched outgrowths of bone in the skull, are common on most male deer and are found on female reindeer as well. Males use antlers, which are shed seasonally and regrown, to fight and to mark territory. Combatants usually seek to avoid inflicting serious injury. Bovids of both sexes often have horns, but they are not regenerated if lost. Hair or fur, the latter sometimes beautifully coloured or patterned, covers the bodies of all members of the order except the hippopotamuses. Advanced members of the order can bolt their food and then regurgitate and chew it, or ruminate, at a time of their choosing, such as when a threat from predators has passed. Most of these ruminants, who graze a variety of vegetation, have evolved a complex stomach containing four parts. Swallowed food passes from one part, the rumen, back into the mouth for complete mastication and is then swallowed again, after which it passes through the three other chambers, specialized for the digestion of a largely cellulose diet. Courtship among artiodactyls may be more or less elaborate, often beginning with the male sniffing the female's urine, perhaps to determine if she is in heat. Contact such as nuzzling or taps on the leg may precede mating. Gestation periods range from 4 to 14 months. A placenta joined to the mother's womb is the organ through which the fetus breathes, feeds, and eliminates waste. Single births are the rule, although some species of deer bear twins, and the number may range to five for the European wild pig. Reliance on milk lasts at least a few months, but all young eventually join in the endless search for food, to which snow, fire, and competition from other herbivores pose the chief problems. Adaptations to the search for food include the snout and tusks of pigs and the thickened skin that protects the warthog foraging under low bushes. Chevrotains, frequenting the thick undergrowth near water, have added roots and tubers to their diet. Another dietary specialization is that of the reindeer, which prefer lichens during the winter. The preference of different species of herbivores for different plants permits a number of species to flourish in the same geographic area. Some species have limited ranges, but others travel widely, frequently in seasonal migrations. Herds or solitary individuals may move from south to north or from low altitudes to the mountains as the snow recedes. Many species form herds, usually as protection from predators. Predation takes its heaviest toll among the young, the old, and the sick. Older members of the herd lead the way to food, water, and breeding sites. In general the herds consist of females, their offspring, and the older dominant males as guardians. any member of the mammalian order Artiodactyla, or even-toed ungulates, which includes the pigs (see table), peccaries, hippopotamuses, camels, chevrotains, deer, giraffes, pronghorn, antelopes, sheep (see table), goats (see table), and cattle (see beef and dairy tables). It is one of the larger mammal orders, containing about 150 species, a total that may be somewhat reduced with continuing revision of their classification. Many artiodactyls are well-known to man, and the order as a whole is of more economic and cultural benefit than any other group of mammals. The much larger order of rodents (Rodentia) affects man primarily in a negative way, by competing with him or impeding his economic and cultural progress. Additional reading I.W. Cornwall, Bones for the Archaeologist, rev. ed. (1974), on the morphology and identification of bones including artiodactyls; J. Dorst and P. Dandelot, A Field Guide to the Larger Mammals of Africa, 2nd ed. (1972), giving summarized descriptions, habits, ecology, and distribution maps for all African artiodactyls; R.F. Ewer, Ethology of Mammals (1968, reissued 1973), a comprehensive text with much information on artiodactyl behaviour; V. Geist, The Evolution of Horn-Like Organs, Behaviour, 27:175214 (1966), on the functional implications of horn shape; G.G. Simpson, The Principles of Classification and a Classification of Mammals, Bull. Am. Mus. Nat. Hist., vol. 85 (1945), which forms the basis for most modern classifications of artiodactyls; T. Haltenorth in W. Kukenthal and T. Krumbach, Handbuch der Zoologie, vol. 8, pp. 1167, Klassifikation der Sugetiere: Artiodactyla (1963), a weighty classification of artiodactyls (in German); V.G. Heptner, A.A. Nasimovic, and A.G. Bannikov (eds.), Die Sugetiere der Sowjetunion, vol. 1, Paarhufer und Unpaarhufer (1966), a massive work on Eurasian ungulates, most of it dealing with artiodactyls, originally published in Russian in 1961; A. Keast, Comparisons of the Contemporary Mammalian Faunas of the Southern Continents, Q. Rev. Biol., 44:121167 (1969), a review of zoogeography and adaptations, with further references; P.S. Martin and H.E. Wright (eds.), Pleistocene Extinctions: The Search for a Cause (1967), a collection of essays on Pleistocene extinctions involving artiodactyls; D. Morris, The Mammals (1965), a general account with illustrations; G.B. Schaller, The Deer and the Tiger (1967, reprinted 1984), a study of the life of Indian artiodactyls; C.A. Spinage, The Book of the Giraffe (1968), much information well presented for the general reader; W.P. Taylor (ed.), The Deer of North America (1956, reissued 1969), on all aspects of the life of North American deer; J.Z. Young, The Life of Vertebrates, 3rd ed. (1981), containing a chapter on artiodactyls; and F.E. Zeuner, A History of Domesticated Animals (1963), a useful source for much information difficult to find elsewhere. Alan William Gentry Evolution and paleontology The artiodactyls can be traced back to a probable descent from a group of early generalized mammals called condylarths, and were certainly distinct by the Eocene Epoch, which ended about 38,000,000 years ago. Fossil artiodactyls can be more or less convincingly classified in three suborders; the more primitive Suiformes, centred around pigs, the Tylopoda, centred on camels, and the Ruminantia or ruminants. The most primitive artiodactyls are the suiform group Palaeodonta, which had four functional toes on each foot, primitive, low-cusped cheek teeth, and the typical artiodactyl astragalus. The artiodactyls became more prominent in the Oligocene (between about 38,000,000 and 26,000,000 years ago) with a decline of the then dominant perissodactyls, and the later history of artiodactyls appears as successive waves of groups, each better adapted than its predecessors to the changing environment. In the suiform line, the earlier palaeodonts are succeeded by other groups such as the entelodonts, giant pigs of the European and North American Oligocene, characterized by very large skulls (some nearly a metre [three feet] long), very small brains, and a large, bony flange below the eyes. The functionally two-toed ruminants succeeded four-toed suiforms in the Miocene, and within the Old World ruminants of the bovid subfamily Caprinae, the zenith of the tribe Caprini, for example, followed that of the mainly Pliocene tribe Ovibovini. The artiodactyls had an interesting history in North America through the Tertiary Period. Some forms, such as the entelodonts, were shared with the Old World, but others were characteristic of North America. One very prominent New World family was the merycoidodonts (or oreodonts), which lasted until the early Pliocene (about 6,000,000 years ago). They had somewhat piglike proportions, short faces, a large upper canine and a caniniform first lower premolar, and selenodont molars. A close relative, Agriochoerus, had clawed feet, the function of which remains uncertain. Camelids evolved in North America and, at or toward the end of the Tertiary, spread into South America and into the Old World. By the end of the Pleistocene they all became extinct in their homeland, just as horses did. The hypertragulids were a mainly Oligocene group of chevrotain-like forms related to the Protoceratidae. The latter had horns above their noses, a position unique among artiodactyls, as well as in the usual position. The North American Miocene (26,000,000 to 7,000,000 years ago) produced some ruminants, such as Blastomeryx, that are hard to distinguish from the early palaeomerycine relatives of giraffes and deer in the Old World, which, with the North American groups, constitute the family Palaeomerycidae. Some developed horns, and the dromomerycine Cranioceras even had a third horn above the back of its skull. During the Miocene and Pliocene there finally appeared relatives of the surviving pronghorn, an example being Merycodus. Many of these North American groups have parallels with Old World groups, and the subject of North American artiodactyl evolution is of great interest. Only further finds will indicate whether Blastomeryx, the dromomerycines, Merycodus, and the pronghorns evolved from hypertragulids already in North America or sprang from some immigrant ruminant and, if the latter, whether the supposed hypertragulid Leptomeryx could be such an immigrant ruminant. It is uncertain whether the hypertragulids are nearer the tragulines or the camels, and how close the oreodonts are to the anthracotheres. Of the great New World radiation there survived after the Pleistocene only three or four camelid species and the pronghorn (deer and bovids in the Americas are immigrants), whereas in the Old World as little as 200 years ago, Eurasia and Africa had abundant deer and antelopes. Until the Miocene there were some archaic artiodactyls in Europe, the xiphodonts, which have cautiously been taken as tylopods, and the cainotheres and anoplotheres, which are classified near anthracotheres. A possible ruminant ancestor was Archaeomeryx from the upper Eocene of China, a small animal that already had a fused naviculo-cuboid bone in the ankle. Tragulids occurred in Africa and Eurasia back to the Miocene, and the more advanced gelocids are known from the upper Eocene and lower Oligocene. At the end of the Oligocene, the first ruminants began to appear with teeth more advanced than those of tragulids. From early in the Miocene they began to be recognizable as giraffes, deer, or antelopes, although the last were relatively uncommon before the late Miocene. Much remains to be learned about the detailed early history of these groups. Several different giraffids lived in later Miocene and early Pliocene times, but the group has since declined to only two species. Deer gradually acquired more complicated antlers, which became very large in some lineages. Different subfamilies of bovids originated in Eurasia and Africa, and it is of zoogeographic interest that representatives of African subfamilies have been found as fossils in northern India and Pakistan. Classification Annotated classification The following classification is principally based on that of American paleontologist George Gaylord Simpson, with alterations in the bovid subfamilies, in the placing of early relatives of giraffes and deer in a giraffoid subfamily Palaeomerycinae, and in the placing of hypertragulids and protoceratids with camels. Groups indicated by the dagger () are known only as fossils.