Understanding the evolutionary beginnings of the early artiodactyls, like that of the early ungulates, is hampered by an incomplete fossil record. Also, the artiodactyls appeared abruptly, along with early perissodactyls, without any clear intermediate forms between the early ungulates and the early artiodactyls. Some aspects of the evolutionary story are difficult to follow because the characteristics used to assign taxonomic position do not fossilize.
For example, modern artiodactyls are divided into three suborders-
However, the teeth of Diacodexis were still bunodont (low-
They were also small, being probably no more than 11 lb (5 kg) with long tails and teeth, suggesting an omnivorous diet. There is a tentatively identified arctocyonid similar to Chriacus, a primitive oxyclaenid condylarth from the Paleocene, as being closest to the oldest artiodactyls so far discovered. The family Dichobunidae, to which Diacodexis probably belonged, is the most primitive group of artiodactyls discovered so far. They are placed in the suborder Paleodonta, along with the closely related Leptochoeridae and the Entelodontidae. The entelodontids were much more advanced than either of the other two families and resembled giant pigs. In one genus, Archaeotherium, their elongated skulls had characteristic processes protruding from the jugal bone, as well as bony knobs on the lower jaw, reminiscent of the modern African warthog (Phacochoerus africanus). The incisors were blunt and heavy, while the canines were robust and capable of inflicting serious injury. The small molars were almost piglike and, along with the premolars, were well spaced along the jaw. Their limbs had between two and three digits, with separate metapodials, although the ulna and radius were fused. Paleodonts have been found mainly in Europe, but also North America. They appeared in the early Eocene and became extinct by the end of the Miocene. The suborder Ancodonta, another group of primitive, presumably non-
Two families, the Agriochoeridae and Merycoidodontidae, are grouped together in the suborder Oreodontae. Their remains have been found only in deposits from Central and North America. The Merycoidodontidae were a very diverse group of small-
Toward the end of the Eocene, the world’s climate began to change and by the beginning of the Oligocene epoch 38 mya, the Northern and Southern Hemispheres experienced definite seasons. Seasonality of climate resulted in significant and predictable variation in the growth and abundance of plants. Under these new conditions, both plants and the herbivores feeding on them evolved rapidly. Artiodactyls especially began to diversify and many large species evolved, with all but the pigs and peccaries becoming obligate herbivores. Molars of herbivorous artiodactyls evolved selenodont (crescent-
Since the order Cetartiodactyle is not yet accepted, the Artiodactyla is considered to be divided into three suborders with 10 families. The suborder Suina contains three families: the Suidae (pigs), Tayassuidae (peccaries and javelinas), and Hippopotamidae (hippopotamuses). The Tylopoda contains only the family Camelidae (camels and llamas), while the suborder Ruminantia is comprised of the Tragulidae (mouse deer and chevrotains), Giraffidae (giraffe and okapi), Cervidae (deer), Antilocapridae (pronghorn), and Bovidae (antelopes, cattle, sheep, and goats). The modern Artiodactyla include a total of 79–81 genera and 217–223 species. A meaningful number of subspecies is difficult because many are disputed, but there are probably more than 800 recognized.