Artiodactyla ( ; ) is an order of placental mammals. Typically, they are ungulates which bear weight equally on two (an even number) of their five toes (the third and fourth, often in the form of a hoof). The other three toes are either present, absent, vestigial, or pointing posteriorly. By contrast, most perissodactyls bear weight on an odd number of the five toes. Another difference between the two orders is that many artiodactyls (except for Suina) digest plant cellulose in one or more stomach chambers rather than in their intestine (as perissodactyls do). Molecular biology, along with fossil discoveries, has found that cetaceans (whales, dolphins, and porpoises) fall within this taxonomic branch, being most closely related to hippopotamuses. Some modern taxonomists thus apply the name Cetartiodactyla () to this group, while others opt to include cetaceans within the existing name of Artiodactyla. Some researchers use "even-toed ungulates" to exclude cetaceans and only include terrestrial artiodactyls, making the term paraphyletic in nature.
The roughly 270 land-based even-toed ungulate species include pigs, peccaries, hippopotamuses, antelopes, deer, giraffes, camels, llamas, alpacas, sheep, goats and cattle. Many are herbivores, but suids are omnivorous, and cetaceans are entirely carnivorous. Artiodactyls are also known by many extinct groups such as anoplotheres, cainotheriids, merycoidodonts, entelodonts, anthracotheres, basilosaurids, and palaeomerycids. Many artiodactyls are of great dietary, economic, and cultural importance to humans.
Evolutionary history
The oldest fossils of even-toed ungulates date back to the early Eocene (about 53 million years ago). Since these findings almost simultaneously appeared in Europe, Asia, and North America, it is very difficult to accurately determine the origin of artiodactyls. The earliest artiodactyls did not significantly affect ungulate locomotor disparity in North America, which suggests that artiodactyls did not outcompete more basal ungulates through competitive exclusion. The oldest artidactyl fossils are classified as belonging to the family Diacodexeidae; their best-known and best-preserved member is Diacodexis. These were small animals, some as small as a hare, with a slim build, lanky legs, and a long tail. Their hind legs were much longer than their front legs. The early to middle Eocene saw the emergence of the ancestors of most of today's mammals.
Two formerly widespread, but now extinct, families of even-toed ungulates were Entelodontidae and Anthracotheriidae. Entelodonts existed from the middle Eocene to the early Miocene in Eurasia and North America. They had a stocky body with short legs and a massive head, which was characterized by two humps on the lower jaw bone. Anthracotheres had a large, porcine (pig-like) build, with short legs and an elongated muzzle. This group appeared in the middle Eocene up until the Pliocene, and spread throughout Eurasia, Africa, and North America. Anthracotheres are thought to be the ancestors of hippos, and, likewise, probably led a similar aquatic lifestyle. Hippopotamuses appeared in the late Miocene and occupied Africa and Asia, but never got to the Americas.
The camels (Tylopoda) were, during large parts of the Cenozoic, limited to North America; early forms like Cainotheriidae occupied Europe. Among the North American camels were groups like the stocky, short-legged Merycoidodontidae. They first appeared in the late Eocene and developed a great diversity of species in North America. Only in the late Miocene or early Pliocene did they migrate from North America into Eurasia. The North American species became extinct around 10,000 years ago.
Suina (including pigs) have been around since the Eocene. In the late Eocene or the Oligocene, two families stayed in Eurasia and Africa; the peccaries, which became extinct in the Old World, exist today only in the Americas.
South America was settled by terrestrial even-toed ungulates only in the Pliocene, after the land bridge at the Isthmus of Panama formed some three million years ago. With only the peccaries, lamoids (or llamas), and various species of capreoline deer, South America has comparatively fewer artiodactyl families than other continents, except Australia, which has no (terrestrial) native species, and Antartica which has no terrestrial mammals at all.
Taxonomy and phylogeny
The classification of artiodactyls was hotly debated because ocean-dwelling cetaceans evolved from land-dwelling even-toed ungulates. Some semiaquatic even-toed ungulates (hippopotamuses) are more closely related to ocean-dwelling cetaceans than to other even-toed ungulates.
Phylogenetic classification only recognizes monophyletic taxa; that is, groups that descend from a common ancestor and include all of its descendants. To address this problem, the traditional order Artiodactyla and infraorder Cetacea are sometimes subsumed into the more inclusive Cetartiodactyla taxon. An alternative approach is to include both land-dwelling even-toed ungulates and ocean-dwelling cetaceans in a revised Artiodactyla taxon.
Classification
Order Artiodactyla/Clade Cetartiodactyla Basal and incertae sedis Family Diacodexeidae Family Dichobunidae Family Cebochoeridae Family Leptochoeridae Family Homacodontidae Superfamily Anoplotheroidea (possible tylopoda or stem-ruminantia) Family Mixtotheriidae Family Robiacinidae Family Cainotheriidae Family Anoplotheriidae Family Amphimerycidae? Family Xiphodontidae? Superfamily Merycoidodontoidea (oreodonts, possible tylopoda or stem-ruminantia) Family Merycoidodontidae Family Agriochoeridae Family Protoceratidae (possible tylopoda or stem-ruminantia) Suborder Tylopoda Family Camelidae: camels, llamas, alpacas, vicuñas, and guanacos (7 extant and 13 extinct species) Family Oromerycidae Clade Artiofabula Suborder Suina Family Suidae: pigs (19 species) Family Tayassuidae: peccaries (4 species) Family Sanitheriidae Family Doliochoeridae Clade Cetruminantia Clade Cetancodontamorpha Genus Andrewsarchus? Family Helohyidae? Family Entelodontidae? Suborder Whippomorpha Infraorder Ancodonta Family Choeropotamidae Family Anthracotheriidae – paraphyletic to Hippopotamidae Family Hippopotamidae: hippos (two species) Clade Cetaceamorpha Family †Raoellidae Infraorder Cetacea: whales (about 90 species) Parvorder Archaeoceti – paraphyletic Family †Kalakocetidae Family Pakicetidae Family Ambulocetidae Family Remingtonocetidae Family Protocetidae – paraphyletic Family Basilosauridae Clade Neoceti Parvorder Mysticeti: baleen whales Superfamily Balaenoidea: right whales Family Balaenidae: greater right whales (four species) Family Cetotheriidae: pygmy right whale (one species) Superfamily Balaenopteroidea: large baleen whales Family Balaenopteridae: slender-back rorquals and humpback whale (eight species) Family Eschrichtiidae: gray whale (one species) Parvorder Odontoceti: toothed whales Superfamily Delphinoidea: oceanic dolphins, porpoises, and others Family Delphinidae: oceanic true dolphins (38 species) Family Monodontidae: Arctic whales; narwhal and beluga (two species) Family Phocoenidae: porpoises (six species) Superfamily Physeteroidea: sperm whales Family Kogiidae: lesser sperm whales (two species) Family Physeteridae: sperm whale (one species) Superfamily Platanistoidea: river dolphins Family Iniidae: South American river dolphins (two species) Family Lipotidae: Chinese river dolphin (one species, possibly extinct) Family Platanistidae: South Asian river dolphin (one species) Family Pontoporiidae: La Plata dolphin (one species) Superfamily Ziphioidea Family Ziphiidae: beaked whales (22 species) Total-group Ruminantia Suborder Ruminantia Infraorder Tragulina (paraphyletic) Family Archaeomerycidae Family Leptomerycidae Family Hypertragulidae Family Praetragulidae Family Lophiomerycidae Family Gelocidae Family Bachitheriidae Family Tragulidae: chevrotains (ten species) Infraorder Pecora Family Antilocapridae: pronghorn (one species) Family Hoplitomerycidae Superfamily Giraffoidea Family Palaeomerycidae Family Climacoceratidae Family Giraffidae: okapi and four species of giraffe (five species total) Superfamily Cervoidea Family Dromomerycidae? Family Cervidae: deer (49 species) Superfamily Bovoidea Family Moschidae: musk deer (7 species) Family Bovidae: cattle, goats, sheep and antelopes (135 species)
Research history
In the 1990s, biological systematics used not only morphology and fossils to classify organisms, but also molecular biology. Molecular biology involves sequencing an organism's DNA and RNA and comparing the sequence with that of other living beings; the more similar they are, the more closely they are related. Comparison of even-toed ungulate and cetaceans genetic material has shown that the closest living relatives of whales and hippopotamuses is the paraphyletic group Artiodactyla.
Dan Graur and Desmond Higgins were among the first to come to this conclusion, and included a paper published in 1994. However, they did not recognize hippopotamuses and classified the ruminants as the sister group of cetaceans. Subsequent studies established the close relationship between hippopotamuses and cetaceans; these studies were based on casein genes, SINEs, fibrinogen sequences, cytochrome and rRNA sequences, IRBP (and vWF) gene sequences, adrenergic receptors, and apolipoproteins.
In 2001, the fossil limbs of a Pakicetus (amphibioid cetacean the size of a wolf) and Ichthyolestes (an early whale the size of a fox) were found in Pakistan. They were both archaeocetes ("ancient whales") from about 48 million years ago (in the Eocene). These findings showed that archaeocetes were more terrestrial than previously thought, and that the special construction of the talus (ankle bone) with a double-rolled joint surface, previously thought to be unique to even-toed ungulates, were also in early cetaceans. The mesonychians, another type of ungulate, did not show this special construction of the talus, and thus was concluded to not have the same ancestors as cetaceans.
The oldest cetaceans date back to the early Eocene (53 million years ago), whereas the oldest known hippopotamus dates back only to the Miocene (15 million years ago). The hippopotamids are descended from the anthracotheres, a family of semiaquatic and terrestrial artiodactyls that appeared in the late Eocene, and are thought to have resembled small- or narrow-headed hippos. Research is therefore focused on anthracotheres (family Anthracotheriidae); one dating from the Eocene to Miocene was declared to be "hippo-like" upon discovery in the 19th century. A study from 2005 showed that the anthracotheres and hippopotamuses had very similar skulls, but differed in the adaptations of their teeth. It was nevertheless believed that cetaceans and anthracothereres descended from a common ancestor, and that hippopotamuses developed from anthracotheres. A study published in 2015 confirmed this, but also revealed that hippopotamuses were derived from older anthracotherians. The newly introduced genus Epirigenys from Eastern Africa is thus the sister group of hippos.
Historical classification of Artiodactyla Linnaeus postulated a close relationship between camels and ruminants as early as the mid-1700s. Henri de Blainville recognized the similar anatomy of the limbs of pigs and hippos, and British zoologist Richard Owen coined the term "even-toed ungulates" and the scientific name "Artiodactyla" in 1848.