The even-toed ungulates are the most successful group of large herbivores on earth today, having outpaced the formerly widespread perissodactyls in the Oligocene. Artiodactyls are indigenous to every zoogeographic region (including several species on Sulawesi and other islands in the Australasian region); they are not native to the continents of Antarctica and Australia, but many species have been introduced into areas outside of their natural range, including Australia, New Guinea, and the islands of Oceania. The approximately 240 ungulate members of this order show incredible diversity in size, form, dietary preferences, and climatic tolerance. This order contains the majority of domesticated mammal species, including cattle, reindeer, camels, pigs, goats, and sheep.

Recent molecular evidence has radically reorder the classification of this order, notably proving that whales and dolphins (Order Cetacea) belong WITHIN this order. Some authors have proposed renaming the order "Cetartiodactyla" to reflect the new combination, while other have suggested retaining the two orders, but group them in a superorder, also titled the Cetartiodactyla. However, the general consensus among the scientific community is to retain the name Artiodactyla (which has priority) for the combined order of even-toed ungulates and cetaceans. Since they don't have hooves, whales and dolphins aren't included in this website - but you can check out the links section for some great cetacean websites!

There are three well-established suborders:

1. Tylopoda - camels
2. Suiformes - pigs and peccaries (and formerly hippos)
3. Ruminantia - the ruminants, containing the majority of modern artiodactyl species
    
   Like the suggested hybridized name Cetartiodactyla, the close allying of the hippopotamuses and cetaceans has created the need for a new suborder:
4. Cetancodonta - hippos, dolphins, and whales.

The evolutionary history of the ungulate members of the Artiodactyla is relatively well known, since, due to their large bones, fossils are plentiful. However, the inclusion of cetaceans - a group with a relatively scant fossil history - in this order has caused much debate. The first artiodactyl fossils (including the rabbit-sized Diacodexis and Protodichobune) appear around 54 million years ago, in the early Eocene deposits of North America and Europe. These early even-toed ungulates had the full placental complement of low-crowned teeth (44 in total), four distinct toes on each foot, and no cranial appendages. Arising at a time when perissodactyls dominated the large herbivorous niches, artiodactyls remained relatively unspecialized until the Oligocene, when an explosive radiation is apparent (primarily in Eurasia). This order has remained abundant and diverse since that time (18 extinct families of ungulate artiodactyls are known in addition to the ten modern families), while the formerly highly successful Perissodactyla have continually declined.

The highly specialized cetaceans are believed to have evolved from the Archaeoceti, a group of primitive whales known from fossils as early as 50 million years ago. The Archaeocetes were formerly thought to have arisen from Mesonychian ungulates based on morphological features. However, Mesonychians are known from fossil deposits 60 million years old - much earlier than the first artiodactyls. While it is now generally agreed that these shared characters are convergent, the fossil history of the cetaceans has yet to be fully resolved in relation to the evolution of the artiodactyls.

The primary distinguishing feature of all of the ungulates within this order is the paraxonic limb structure, in which the symmetry of the foot passes between the two middle digits (III and IV). The first digit (the "thumb" or pollex in the hand and the hallux on the hind limb) is absent in all modern artiodactyls, with the result that all species possess an even number of toes on each foot (with the exception of the Tayassuidae, in which the hind foot only has three digits). Two main types of foot structure are recognized: a "cloven hoof" with two weight-bearing toes, and a spreading foot with four digits. In all cases the third and fourth digits are well developed, while the second and fifth are reduced, vestigial, or absent. All ungulate artiodactyls have pulley-shaped articulating surfaces on both ends of the astragalus (fossil evidence of cetaceans indicates that primitive whales also possessed this feature).

The nasal bones in the skulls of the artiodactyls are not expanded caudally, nor is there an alisphenoid canal. All species have a postorbital bar. Teeth are variable, but the upper incisors are always reduced or absent. Canines are usually small or not present at all, although in some species they are greatly enlarged into tusks. Two main types of molars are recognized - the brachyodont (low-crowned) teeth of the pigs, peccaries, and hippos, and the hypsodont (high-crowned) teeth of the camels and some ruminants.