Temporal range: 55–0 Ma Early Eocene-Holocene
|Bones of right fore feet of existing Artiodactyla: From left to right: pig (Sus scrofa), red deer (Cervus elaphus), and camel (Camelus bactrianus). U = ulna, R = radius, c = cuneiform, l = lunar, s = scaphoid, u = unciform, m = magnum, td = trapezoid. In the sheep and the camel, the long compound bone, supporting the two main (or only) toes is the cannon bone.|
The even-toed ungulates (order Artiodactyla) are ungulates (hoofed animals) whose weight is borne approximately equally by the third and fourth toes, rather than mostly or entirely by the third as in odd-toed ungulates (perissodactyls), such as horses.
The name Artiodactyla comes from (Greek: ἄρτιος (ártios), "even", and δάκτυλος (dáktylos), "finger/toe"), so the name "even-toed" is a translation of the description. This group includes pigs (family Suidae), peccaries (family Tayassuidae), hippopotamuses (family Hippopotamidae), camels (genus Camelus in family Camelidae), llamas (genus Lama in family Camelidae), chevrotains or mouse deer (family Tragulidae), deer (family Cervidae), giraffes (family Giraffidae), pronghorn (genus Antilocapra in family Antilocapridae), antelopes (sections of family Bovidae including subfamily Antilopinae), goat-antelopes (which include sheep, goats and others; subfamily Caprinae in family Bovidae), and cattle (genus Bos in family Bovidae). The group excludes whales (Cetacea), although DNA sequence and anatomical data indicate they share a common ancestor, making the group paraphyletic. The phylogenetically accurate group is called Cetartiodactyla (from Cetacea + Artiodactyla).
Of the roughly 220 artiodactyl species, many are of great dietary, economic, and cultural importance to humans.
As with many extant mammal groups, even-toed ungulates first appeared during the early Eocene (about 55 million years ago). In form, they were rather like today's chevrotains: small, short-legged creatures that ate leaves and the soft parts of plants. By the late Eocene (~46 million years ago), the four modern suborders had already developed: Suina (the pig group); Tylopoda (the camel group); Ruminantia (the goat and cattle group, and the Cetruminantia, which still includes the extant hippos and both whale groups). Nevertheless, artiodactyls were far from dominant at that time; the odd-toed ungulates (ancestors of today's horses and rhinoceroses) were much more successful and far more numerous. Even-toed ungulates survived in niche roles, usually occupying marginal habitats, and presumably at that time they developed their complex digestive systems, which allowed them to survive on lower-grade food.
The appearance of grasses during the Eocene, and their subsequent spread during the Miocene (about 20 million years ago), allowed a major change; grasses are very difficult to digest, and the even-toed ungulates, with their highly developed stomachs, were better able to adapt to this coarse, low-nutrient diet, and soon replaced the odd-toed ungulates as the dominant terrestrial herbivores. Now-extinct Artiodactyla that developed during the Miocene include the genera Ampelomeryx, Tauromeryx, and Triceromeryx.
This classification is based on Spaulding et al., 2009 and the extant families recognised by Mammal Species of the World published in 2005. As a result of this Cetacea has been reduced from order status to infraorder status, with the Mysticeti and Odontoceti as parvorders.
Alternately, some researchers have used Cetartiodactyla instead as the order name. It is a merger between Cetacea and Artiodactyla and is supported by the IUCN Cetacean Specialist Group  and by the Taxonomy Committee  of the Society for Marine Mammalogy, the largest international association of marine mammal scientists in the world. Use of the Order Cetartiodactyla, instead of Cetacea with Suborders Odontoceti and Mysticeti, is favored by most evolutionary mammalogists working with molecular data. However, some authors have argued that there should be no change in the ordinal name and it is best to keep the old name,  not to mention that Artiodactyla has more hits than Cetartiodactyla on Google Scholar The other extreme is adopted by Skinner & Chumumba (2005), who count the artiodactyl suborders as orders.
- Order Artiodactyla/Cetartiodactyla
- Suborder Tylopoda
- Clade Artiofabula
- Suborder Suina
- Clade Cetruminantia
- Clade Cetancodontamorpha
- Genus †Andrewsarchus?
- Family †Entelodontidae
- Suborder Whippomorpha
- Superfamily Dichobunoidea - paraphyletic to Cetacea and Raoellidae
- Infraorder Cetacea: whales (about 90 species)
- Family †Pakicetidae
- Family †Ambulocetidae
- Family †Remingtonocetidae
- Family †Basilosauridae
- Parvorder Mysticeti: baleen whales
- Superfamily Balaenoidea: Right whales
- Superfamily Balaenopteroidea: Large baleen whales
- Parvorder Odontoceti: toothed whales
- Superfamily Delphinoidea: oceanic dolphins, porpoises, and others
- Superfamily Physeteroidea: Sperm whales
- Superfamily Platanistoidea: River dolphins
- Superfamily Ziphioidea
- Family †Raoellidae
- Infraorder Ancodonta
- Clade Ruminantiamorpha
- Suborder Ruminantia
- Infraorder Tragulina
- Infraorder Pecora
- Family †Gelocidae
- Family †Palaeomerycidae
- Family Antilocapridae: pronghorn (one species)
- Family †Climacoceratidae
- Family Giraffidae: giraffe and okapi (two species)
- Family †Hoplitomerycidae
- Family Cervidae: deer (49 species)
- Family †Leptomerycidae
- Family Moschidae: musk deer (seven species)
- Family Bovidae: cattle, buffalo, goat-antelope, antelope, and others (135 species)
- Suborder Ruminantia
- Clade Cetancodontamorpha
Anatomy, physiology, and morphology
The even-toed ungulates stand on an even number of toes; the group's three suborders differ in other characteristics. Suina (pigs and peccaries) have retained four toes of fairly equal size, have simpler molars, short legs, and often have enlarged canine teeth that form tusks. Camelids and Ruminantia tend to be longer-legged, to walk on only the two central toes (though the outer two may survive as rarely used dew claws) and to have more complex cheek teeth that are well-suited to grinding up tough grasses.
Diet and feeding
The ancestors of the even-toed ungulates were omnivores that preferred plant material; now, even-toed ungulates are generally herbivorous, although species in the suborder Suina are, like their primitive ancestors, omnivores. Larger stomachs and longer intestines have evolved because plant material is more difficult to digest than meat.
Tylopoda (camels, llamas, and alpacas) and chevrotains have three-chambered stomachs, while the rest of Ruminantia have four-chambered stomachs. The handicap of a heavy digestive system has increased selective pressure for limb bone adaptations to escape predators. Most species within Suina have a simple two-chambered stomach that allows an omnivorous diet, the babirusa, however, is a herbivore. They have extra maxillary teeth to allow for the proper mastication of plant material. Most of the fermentation occurs with the help of cellulolytic microorganisms within the caecum. Peccaries, however, have a complex stomach that contains four compartments. Microbial fermentation with the formation of high volatile fatty acid levels has been observed in the fore stomach; it has been proposed that their complex fore stomach is a means to slow digestive passage and increase digestive efficiency. Hippopotamuses have three-chambered stomachs and do not ruminate. They consume around 68 kg of grass and other plant matter each night. They may cover large distances (up to 20 miles) to obtain their food, which they digest with the help of microbes that produce cellulase. Their closest living relatives, the whales, are obligate carnivores.
Rumination occurs in the ruminants (Ruminantia and Tylopoda), whereby food is regurgitated and rechewed then broken down by microbes in the stomach. After ingestion of plant material, it is mixed with saliva in the rumen and reticulum and separates into layers of solid and liquid material. The solids lump together to form a bolus (also known as the cud), this is regurgitated by reticular contractions while the glottis is closed. When the bolus enters the mouth, the fluid is squeezed out with the tongue and reswallowed. The bolus is chewed slowly to completely mix it with saliva and to break down the particle size. Ingested food passes to the 'fermentation chamber' (rumen and reticulum) where it is kept in continual motion by rhythmic contractions of this organ. Cellulytic microbes (bacteria, protozoa, and fungi) produce cellulase, which is needed to break down the cellulose found in plant material. Without this mutual symbiosis, ruminants would find plant material indigestible.
Habitat and distribution
Relationship with humans
The even-toed ungulates are of more economic and cultural benefit than any other group of mammals. Clear evidence exists of antelope being used for food 2 million years ago in the Olduvai Gorge, part of the Great Rift Valley. Cro-Magnons relied heavily on reindeer for food, skins, tools, and weapons; with dropping temperatures and increased reindeer numbers at the end of the Pleistocene, they became the prey of choice. By around 12,500 years ago, reindeer remains accounted for 94% of bones and teeth found in a cave above the Céou River.
Humans have hunted many species of artiodactyls without regulation. This has caused half of the even-toed ungulates to be near extinction, especially in areas with decreased economic development. Conservation efforts to increase local population growths have been undertaken. Some have been so effective, population control has been enforced. The even-toed ungulate has experienced habitat loss in addition to climate change. Climate change has forced many species to move poleward. An example would be moose, which are heat intolerant, whose southernmost populations have declined sharply in response to increased temperatures.
There are 168 artiodactyl species on the IUCN Red List of Threatened Species. Seven are listed as extinct, two as “extinct in the wild”, 26 as “endangered”, one as “near threatened”, and 73 as “lower risk”. Information is lacking for the 13 other species.
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