Figure 1. The foot of artiodactyls is symmetrical about an axis between the third and fourth toes. Artiodactyls walk on the tips of their toes. The forefoot of (a) a four-toed protoceratid, Protoceras, and (b) a two-toed camelid, Poebrotherium. From Gregory WK (1951) Evolution Emerging: A Survey of Changing Patterns from Primeval Life to Man, vol. 2. New York: Macmillan.

Figure 2. The specialized astragalus of artiodactyls with its double pulley, compared with a carnivore. An astragalus of (a) a dog, Canis, and (b) a pig, Sus. Bar, 1 cm. After O'Leary MA and Rose KD (1995) Journal of Vertebrate Paleontology 15(2): 401–430.

Figure 3. The types of tooth morphology found in artiodactyls. (a) A bunodont dentition from Choeropotamus, with blunt, bulbous cusps. (b) A selenodont dentition from Cranioceras showing crescent-shaped crests. From Gregory WK (1951) Evolution Emerging: A Survey of Changing Patterns from Primeval Life to Man, vol. 2. New York: Macmillan.

Figure 4. Types of cranial ornamentation in artiodactyls: (a) The extinct protoceratid Cranioceras, (b) the giraffe, Giraffa, (c) the red deer, Cervus, (d) the American pronghorn, Antilocapra and (e) the springbok, Antidorcas. (a) From Gregory WK (1951) Evolution Emerging: A Survey of Changing Patterns from Primeval Life to Man, vol. 2. New York: Macmillan. (b–e) From Modell W (1969) Scientific American April.

Figure 5. Phylogeny and fossil occurrences of major lineages of Artiodactyla during the Cenozoic Era. Epochs and radiometric ages are marked on the left. Numbered boxes indicate the higher taxonomic categories listed: 1, Ruminantia; 2, Tylopoda; 3, Suiformes; 4, Pecora; 5, Oreodonta. MYA, millions of years ago.

Figure 6. The variety of extinct artiodactyls. (a) The cainothere Cainotherium, from the Oligocene of Europe, approximately 30 cm long. (b) The early ruminant Leptomeryx, from the Oligocene of North America, approximately 60 cm long. (c) The camelid Oxydactylus, from the Miocene of North America, approximately 2 m long. (d) The anthracothere Anthracotherium from the Miocene of Europe, approximately 1.5 m long. (e) The oreodont Merycoidodon, from the Oligocene of North America, approximately 1 m long. (f) The peccary Platygonus, from the Pleistocene of North America, approximately 1 m long. (g) The entelodont Dinohyus, from the Miocene of North America, approximately 3 m long. (a) and (g) After Gregory WK (1951) Evolution Emerging: A Survey of Changing Patterns from Primeval Life to Man, vol. 2. New York: Macmillan. (b)–(f) From Gregory WK (1951) Evolution Emerging: A Survey of Changing Patterns from Primeval Life to Man, vol. 2. New York: Macmillan.