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:
Tylopoda - camels
Suiformes - pigs and peccaries (and formerly hippos)
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:
Cetancodonta - hippos, dolphins, and whales.
(är'tee-oh dak ti'lah)
Greek artios, complete, of numbers even; daktulos, a finger
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
) 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.
The Artiodactyla Family Tree
Branch lengths are not proportional to time
(From Price, Bininda-Emonds, and Gittleman, 2005)