Artiodactyls are the closest living relatives of the Cetacea (whales and dolphins), although whales may be more closely related to an extinct group of mammals, the mesonychids. It seems odd to think of the whales being closely related to terrestrial ungulates, but biologists have long appreciated that artiodactyls and whales share several unusual morphologies despite their vastly divergent ways of life. One such feature is that lumps of tissue, called umbilical pearls, are found along the umbilical cord between the fetus and the mother. Male whales and artiodactyls share distinctive features of the penis, which is composed primarily of fibroelastic tissue, unlike the penis of other mammals, largely made up of spongy tissue. Because the penis is fibroelastic and not spongy, the mechanism of erection of the penis differs from most mammals, utilizing specialized retractor muscles, which are found in both artiodactyls and cetaceans.
I. Diagnostic Characters
Artiodactyls are even-toed ungulates because they have two or four toes on each foot. They have elongated legs and stand on the tips of their toes, which bear hooves instead of nails. Artiodactyls are characterized by paraxonic foot symmetry, in which the main axis of weight bearing of the foot passes between the third and the fourth digits. The ankle of artiodactyls is uniquely specialized. The astragalus, the main bone transferring force from the leg to the foot, has a deeply grooved, pulley-shaped surface on both the tibial and foot ends, whereas other mammals have only the tibial pulley. The double-pulley joint strongly limits lateral rotation of the foot on the hind leg, permitting only fore and aft movements of the leg.
II. Taxonomy
Artiodactyla includes nine families today: the Suidae (pigs), Tayassuidae (peccaries), Hippopotamidae (hippos), Camelidae (camels and llamas). Tragulidae (mouse deer), Giraffidae (giraffe and okapi), Antilocapridae (pronghorn antelope), Cervidae (deer), and Bovidae (cows, sheep, goats, and antelope). Of the 211 extant artiodactyl species, close to 65% are in the Bovidae. The families Hippopotamidae, Tragulidae, Giraffidae, and Antilocapridae are very low in diversity, with one or two species each.
III. Distribution and Ecology
Living artiodactyls are native to North America, Eurasia, Africa, and South America, and they have been introduced to other areas. Artiodactyls occupy a wide range of habitats, from deserts to taiga, and show a wide range of dietary specialization, social structure, locomotor modes, and habitat preferences. However, the majority of artiodactyls are terrestrial, and only the hippo is semiaquatic. There is a considerable range of body sizes, from 0.7 to 3000 kg. Most artiodactyls are herbivores, except the pigs and peccaries, which also eat small prey and eggs. The pigs, peccaries, hippos, and mouse deer are nocturnal, whereas most other species are diurnal. Most artiodactyls bear precocial young in small litters, although pigs have large litters of relatively helpless young who are reared in nests.
IV. Notable Anatomy
The most notable anatomical system of artiodactyls is foregut rumination, which allows ruminant artiodactyls (bovids, cervids, antilocaprids, giraffids, and tragulids) the ability to break down cellulose, a plant material ordinarily indigestible to vertebrates.
This is accomplished by symbiotic bacteria housed in an elaborate system of chambers within the stomach (part of the foregut). Ruminants break down the plant material by swallowing the food, which is fermented by the bacteria in the first stomach chamber, the rumen. The cud, or fermented food, is later regurgitated, chewed, and swallowed again, passing into the rest of the digestive tract. Bacteria break down the cellulose from the food into components that can be digested by the ruminant. All ruminants have some ability to use foregut rumination, and it is also found to a more limited degree in the camels.
V. Phylogeny
Artiodactyla is a diverse group today, but it also contains a number of extinct types. This diversity has made it difficult to understand the interrelationships within this group. Artiodactyls have been thought to be monophyletic based on their morphology, but recent molecular studies indicate that whales may be closely related to hippos, which implies that whales actually belong within Artiodactyla.
Most morphologists place the living families into three major groups: the Suiformes (pigs, peccaries, and hippos), the Ty-lopoda (camels, protoceratids, and oromerycids), and the Ru-minantia (bovids, cervids, antilocaprids, giraffids, and tragulids). Tylopoda and Ruminantia are usually considered to be more closely related to one another than either is to the Suiformes, based on their dental and gut morphology (see Fig. 1 for summary). However, molecular evidence linking whales to hippos also suggests that the Ruminantia may be more closely related to Suiformes and whales than to the Tylopoda. This hypothesis conflicts strongly with morphological data from living and extinct artiodactyls and whales. Further investigation may help clarify the relationships between whales and artiodactyls, but it is unlikely that molecular analyses of the extinct forms will be feasible.
VI. Fossil Record
The oldest artiodactyls are known from the earliest Eocene of Europe, North America, and Pakistan, about 56 million years ago, around the same time as the earliest fossil whales. The oldest fossil artiodactyls, usually referred to the genus Diacodexis, were rabbit-sized omnivores, but during the middle Eocene many other new groups evolved (see Fig. 1). The majority of these lineages became extinct, including some which were enormously diverse and successful for long periods of time, such as the oreodonts, an endemic North American group of sheep-sized herbivores. A number of these fossil groups are usually described as pig-like, meaning that they have long trunks and short legs, with low-crowned teeth suitable for an omnivorous diet. These groups include the entelodonts, which were very large animals, with skulls as long as 1 m in length and with large bony bumps on the facial region. The anthracotheres are another of these groups, and they are usually thought to be related to the hippos. Other groups were obviously herbivores, including the oreodonts, and seem to be related either to the
Figure 1 Phijlogeny and fossil occurrences of major lineages of Artiodactyla during the Cenozoic era. Epochs and radiometric ages are marked on the left.
Tylopoda (oromerycids, protoceratids, and xiphodontids) or to the Ruminantia (hypertragulids, leptomerycids, gelocids, dro-momerycids, and paleomerycids). A number of these groups had very complex antlers or horns, especially the extinct protoceratids, some of whom had large slingshot-shaped horns on the snout. Many extinct members of the Dromomerycidae, An-tilocapridae, and Giraffidae had much more elaborate cranial ornamentation than any species alive today.
Another successful group was the Camelidae, which became very diverse during the Miocene in North America. The camels and llamas emigrated out of North America and were the only members of the family to survive the extinctions of large mammals in the Pleistocene. The bovids underwent a rapid radiation during the Miocene and Pliocene. The cervids diversified during the Pleistocene, and both groups are diverse today. It is especially important to realize that the modern geographic distribution of the order Artiodactyla is very different from what it was at different times in the past and has been heavily influenced by human activity.
