Temporal range: 54–0Ma Early Eocene - Recent
|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 (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.
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, peccaries, hippopotamuses, camels, llamas, chevrotains (mouse deer), deer, giraffes, pronghorn, antelopes, sheep, goats, and cattle. The group excludes whales (Cetacea) even though DNA sequence data indicate that they share a common ancestor, making the group paraphyletic. The phylogenetically accurate group is Cetartiodactyla (from Cetacea + Artiodactyla).
There are about 220 artiodactyl species, including many that are of great dietary, economic, and cultural importance to humans.
As with many mammal groups, even-toed ungulates first appeared during the Early Eocene (about 54 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 three modern suborders had already developed: Suina (the pig group); Tylopoda (the camel group); and Ruminantia (the goat and cattle group, properly called Cetruminantia since it includes the whales). Nevertheless, artiodactyls were far from dominant at that time; the odd-toed ungulates (ancestors of today's horses and rhinos) 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. Currently, the cetaceans and even-toed ungulates have been placed in Cetartiodactyla as sister groups, although DNA analysis has shown cetaceans evolved from within Artiodactyla. The most recent theory into the origins of Hippopotamidae suggests hippos and whales shared a common semiaquatic ancestor that branched off from other artiodactyls around 60 million years ago. This hypothesized ancestral group likely split into two branches around 54 million years ago. One branch would evolve into cetaceans, possibly beginning with the proto-whale Pakicetus from 52 million years ago, with other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans.
- Order Artiodactyla/Cetartiodactyla
- Suborder Tylopoda
- Suborder Suina
- Suborder Cetruminantia
- unranked Cetancodontamorpha
- unranked Ruminantiamorpha
- Infraorder Tragulina
- Infraorder Pecora
- Family Antilocapridae: pronghorn (one species)
- Family Giraffidae: giraffe and okapi (two species)
- Family †Climacoceratidae
- Family Moschidae: musk deer (seven species)
- Family †Leptomerycidae
- Family Cervidae: deer (49 species)
- Family †Gelocidae
- Family †Palaeomerycidae
- Family †Hoplitomerycidae
- Family Bovidae: cattle, goats, sheep, and antelope (135 species)
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 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 (pigs and peccaries) 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 a three-chambered stomach, 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 proper mastication of plant material. Most of the fermentation occurs in the caecum with the help of cellulolytic microorganisms. 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 a three-chambered stomach and do not ruminate, they consume grass during the night and may cover large distances (up to 20 miles) to feed. They eat around 68 kg of food each night, also relying on microbes to break down plant material with 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. There is clear evidence 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 percent 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 even-toed ungulates to be near extinction, especially in areas with decreased economic development. There have been conservation efforts to increase local population growths. Some have been so effective that 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 that moose are heat intolerant and have extremely decreased population its southernmost distribution in response to increased temperatures. There are 168 artiodactyl species on the IUCN Red List of Threatened Species. Two are listed as “extinct” in the wild. Twenty-six species are listed as “endangered.” One species is “near threatened” and 73 species are listed as “lower risk.” There is information lacking for the thirteen other species.
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