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The even-toed ungulates form the order Artiodactyla, the group that contains the pigs, peccaries, hippopotamuses, camels, chevrotains (mouse deers), deer, giraffes, pronghorn, antelopes, sheep, goats, and cattle. They are ungulates whose weight is borne (if they have more than two toes) about equally by the third and fourth toes, rather than mostly or entirely by the third as in perissodactyls. Another key distinguishing feature is the shape of the astragalus (a bone in the hock joint), which has a double-pulley structure in artiodactyls, giving the foot greater flexibility.^1]

There are about 220 artiodactyl species, including many that are of great nutritional, economic and cultural importance to humans.

Evolutionary History

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.

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). 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 it is presumably at that time that 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) saw a major change: grasses are very difficult to eat and the even-toed ungulates with their highly-developed stomachs were better able to adapt to this coarse, low-nutrition diet, and soon replaced the odd-toed ungulates as the dominant terrestrialh erbivores. Now-extinct Artiodactyla which developed during the Miocene include the species Ampelomeryx, Tauromeryx, Triceromeryx, and others.

Evolution

Suina

Suina (pigs and peccaries) are artiodactyls that retain four toes of fairly equal size, have simpler molars, short legs, and often have enlarged canine teeth that form tusks. In general, they are omnivores and have a simple stomach, except for the two hippopotamus species and the babirusa which are herbivores.^[2] Hippopotamidae have been considered a member of Suina, however, recent morphological and genetic research suggests that hippos are more closely related to whales.^[3]

Camelids and Ruminantia

Camelids and Ruminantia tend to be longer-legged, to walk on only the central two 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. They have evolved a highly developed digestive process in which partly-digested food is regurgitated and re-chewed (chewing the cud or cudding). This complex digestion takes place in a multi-chambered stomach, the rumen itself. It allows them to use fermentation by microorganisms to digest cellulose, a plant material which animals cannot digest directly.^[2]

Cetaceans

One group of artiodactyls (which molecular biology suggests were most closely related to Hippopotamidae) returned to the sea to become whales. Thus Artiodactyla without Cetacea is a paraphyletic group. For this reason, the term Cetartiodactyla was coined to refer to the group containing both artiodactyls and whales.^[4]

Classification

The following classification uses systematics laid out by McKenna and Bell in 1997,^[5] and the extant families recognised by Mammal Species of the World published in 2005.^[6] Currently the cetaceans and even-toed ungulates have been placed in Cetartiodactyla as sister groups, although DNA analysis has shown cetaceans evolved from within Artiodactyl. The most recent theory into the origins of hippopotamidae suggests that hipposa nd whales shared a common semi-aquatic ancestor that branched off from other artiodactyls around 60 million years ago.^[7][3] This hypothesized ancestral group likely split into two branches around 54 million years ago.^[8] 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.^[4]

• Order Artiodactyla
  • Suborder Suina
    • Family Suidae: pigs (19 species)
    • Family Tayassuidae: peccaries (4 species)
    • Family †Entelodontidae
    • Family †Raoellidae
    • Family †Choeropotamidae
    • Family †Sanitheriidae
  • Suborder Tylopoda
    • Family †Anoplotheriidae
    • Family †Anthracotheriidae
    • Family Hippopotamidae: hippos (2 species)
    • Family †Dichobunidae
    • Family †Cebochoeridae
    • Family †Helohyidae
    • Family †Cainotheriidae
    • Family †Merycoidodontidae
    • Family †Agriochoeridae
    • Family †Protoceratidae
    • Family Camelidae: camels and llamas (4 species)
    • Family †Oromerycidae
    • Family †Xiphodontidae
  • Suborder Ruminantia
    • Infraorder Tragulina
      • Family †Amphimerycidae
      • Family †Prodremotheriidae
      • Family †Hypertragulidae
      • Family †Praetragulidae
      • Family Tragulidae: chevrotains (6 species)
      • Family †Leptomerycidae
      • Family †Archaeomerycidae
      • Family †Lophiomerycidae
    • Infraorder Pecora
      • Family Moschidae: musk deer (4 species)
      • Family Cervidae: deer (49 species)
      • Family Antilocapridae: pronghorn (2 species)
      • Superfamily Giraffoidea
        • Family Giraffidae: Giraffe and Okapi (1 species)
        • Family †Climacoceratidae
      • Family Bovidae: cattle, goats, sheep, and antelope (135 species)
      • Family †Gelocidae
      • Family †Palaeomerycidae
      • Family †Hoplitomerycidae

Anatomy, Physiology and Morphology

Behaviour

Development Through Life Stages

Diet and Feeding

Even-toed ungulates are mostly herbivorous; larger stomachs and longer intestines have evolved because plant food is less easily digested than meat.^[9] The handicap of a heavy digestive system has increased selective pressure for limb bone adaptations to escape predators.^[9]

Habitat and Distribution

Even-toed ungulates are found on every continent but Antarctica; they were introduced to Australia and New Zealand by humans.^[10]

Relationship With Humans

The even-toed ungulates are of more economic and cultural benefit than any other group of mammals.^[9] There is clear evidence of antelopes being used for food 2 million years ago in the Olduvai Gorge, part of the Great Rift Valley.^[9] 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.^[11]

Cattle today are the basis of a multi-billion dollar industry worldwide. The international trade in beef for 2000 was over $30 billion and represented only 23 percent of world beef production.^[12]

Conservation

The Order Artiodactyla is a member of the Mirorder Eparctocyona. Here is the complete "parentage" of Artiodactyla:

• Domain: Eukaryota Whittaker & Margulis,1978 - eukaryotes
  • Kingdom: Animalia Linnaeus, 1758 - animals
    • Subkingdom: Bilateria (Hatschek, 1888) Cavalier-Smith, 1983 - bilaterians
      • Branch: Deuterostomia Grobben, 1908 - Deuterostomes
        • Infrakingdom: Chordonia (Haeckel, 1874) Cavalier-Smith, 1998
          • Phylum: Chordata Bateson, 1885 - Chordates
            • Subphylum: Vertebrata Cuvier, 1812 - Vertebrates
              • Infraphylum: Gnathostomata Auct. - Jawed Vertebrates
                • Superclass: Tetrapoda Goodrich, 1930 - Tetrapods
                  • Class: Mammalia C. Linnaeus, 1758 - Mammals
                    • Subclass: Theriiformes (Rowe, 1988) M.c. Mckenna & S.k. Bell, 1997
                      • Infraclass: Holotheria (Wible Et Al., 1995) M.c. Mckenna & S.k. Bell, 1997
                        • Superlegion: Trechnotheria Mckenna, 1975
                          • Legion: Cladotheria Mckenna, 1975
                            • Sublegion: Zatheria Mckenna, 1975
                              • Infralegion: Tribosphenida (Mckenna, 1975) M.c. Mckenna & S.k. Bell, 1997
                                • Supercohort: Theria (Parker & Haswell, 1897) M.c. Mckenna & S.k. Bell, 1997 - Therians
                                  • Cohort: Placentalia (Owen, 1837) M.c. Mckenna & S.k. Bell, 1997 - Placentals
                                    • Magnorder: Epitheria (Mckenna, 1975) M.c. Mckenna & S.k. Bell, 1997
                                      • Superorder: Preptotheria (Mckenna, 1975) Mckenna, in Stucky & Mckenna, in Benton, Ed., 1993
                                        • Grandorder: Ungulata (un-goo-LAH-tuh) (C. Linnaeus, 1766) Mckenna, 1975
                                          • Mirorder: Eparctocyona Mckenna, 1975
                                            • Order: Artiodactyla Owen, 1848 - Even-Toed Ungulates