Even-toed ungulate
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{{Taxobox
| name = Even-toed ungulates
| fossil_range = Early [[Eocene]] - Recent
| image = Masai Giraffe right-rear foot.jpg
| image_caption = Right-rear foot of a [[Masai Giraffe]] (''Giraffa camelopardalis tippelskirchi'') at the [[San Diego Zoo]]
| regnum = [[Animal]]ia
| phylum = [[Chordate|Chordata]]
| classis = [[Mammalia]]
| infraclassis = [[Eutheria]]
| superordo = [[Laurasiatheria]]
| ordo = '''Artiodactyla'''[[paraphyly|*]]
| ordo_authority = [[Richard Owen|Owen]], 1848
| subdivision_ranks = Families
| subdivision =
[[Antilocapridae]]<br>
[[Bovidae]]<br>
[[Camelidae]]<br>
[[Cervidae]]<br>
[[Giraffidae]]<br>
[[Hippopotamidae]]<br>
[[Moschidae]]<br>
[[Suidae]]<br>
[[Tayassuidae]]<br>
[[Tragulidae]]<br>
[[Leptochoeridae]] [[extinction|†]]<br>
[[Dichobunidae]] †<br>
[[Cebochoeridae]] †<br>
[[Entelodontidae]] †<br>
[[Anoplotheriidae]] †<br>
[[Anthracotheriidae]] †<br>
[[Cainotheriidae]] †<br>
[[Agriochoeridae]] †<br>
[[Merycoidodontidae]] †<br>
[[Leptomerycidae]] †<br>
[[Protoceratidae]] †<br>
[[Oromerycidae]] †<br>
[[Xiphodontidae]] †<br>
[[Amphimerycidae]] †<br>
[[Helohyidae]] †<br>
[[Gelocidae]] †<br>
[[Dromomerycidae]] †<br>
[[Raoellidae]] †<br>
[[Choeropotamidae]] †<br>
[[Sanitheriidae]] †<br>
[[Climacoceratidae]] †<br>
[[Palaeomerycidae]] †<br>
}}
The '''even-toed ungulates''' form the [[mammal]] [[order (biology)|order]] '''Artiodactyla'''. They are [[ungulate]]s 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 [[Talus bone|astragalus]] (a bone in the [[Hock (zoology)|hock]] joint), which has a double-pulley structure in artiodactyls, giving the foot greater flexibility.<ref name=MamEv>{{cite book |author= Savage, RJG, & Long, MR|year=1986 |title= Mammal Evolution: an illustrated guide|publisher= Facts on File|location=New York|pages= 208|isbn= 0-8160-1194-X}}</ref>
There are about 220 artiodactyl [[species]], including many that are of great nutritional, [[economic]] and cultural importance to humans.
== Evolution ==
As with many animal groups, even-toed ungulates first appeared during the Early [[Eocene]]
(about 54 million years ago). In form they were rather like today's [[chevrotain]]s: 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 ungulate]]s (ancestors of today's [[horse]]s and [[Rhinoceros|rhino]]s) were much more successful and far more numerous. Even-toed ungulates survived in niche roles, usually occupying marginal [[habitat (ecology)|habitats]], and it is presumably at that time that they developed their complex [[digestive system]]s, which allowed them to survive on lower-grade food.
The appearance of [[Poaceae|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 [[stomach]]s were better able to adapt to this coarse, low-[[nutrition]] diet, and soon replaced the odd-toed ungulates as the dominant [[Landform|terrestrial]] [[herbivore]]s. Now-extinct Artiodactyla which developed during the [[Miocene]] include the species [[Ampelomeryx]], [[Tauromeryx]], [[Triceromeryx]], and others.
=== Suina ===
Suina ([[pigs]] and [[peccary|peccaries]]) are artiodactyls that retain four toes of fairly equal size, have simpler [[molar (tooth)|molars]], short legs, and often have enlarged [[canine teeth]] that form [[tusk]]s. In general, they are [[omnivore]]s and have a simple stomach, except for the two [[hippopotamus]] species and the [[babirusa]] which are herbivores.<ref name=EoM>{{cite book |editor=Macdonald, D.|author= Janis, Christine & Jarman, Peter|year=1984 |title= The Encyclopedia of Mammals|publisher= Facts on File|location=New York|pages= 498-499|isbn= 0-87196-871-1}}</ref> [[Hippopotamidae]] have been considered a member of [[Suina]], however, recent morphological and genetic research suggests that hippos are more closely related to [[whales]].<ref name="DNA">{{Cite journal | url = http://mbe.oxfordjournals.org/cgi/content/abstract/14/5/537 | title = More DNA support for a Cetacea/Hippopotamidae clade: the blood-clotting protein gene gamma-fibrinogen | author = Gatesy, J. | journal = [[Molecular Biology and Evolution]] | volume = 14 | pages = 537–543 | pmid = 9159931}}</ref>
=== Camelids and Ruminantia ===
[[Camelid]]s 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 [[regurgitation|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.<ref name=EoM/>
=== Cetaceans ===
One group of artiodactyls (which [[molecular biology]] suggests were most closely related to Hippopotamidae) returned to the sea [[Evolution of cetaceans|to become whales]]. Thus Artiodactyla without [[Cetacea]] is a [[Paraphyly|paraphyletic group]]. For this reason, the term '''[[Cetartiodactyla]]''' was coined to refer to the group containing both artiodactyls and whales.<ref name="Cetartiodactyla">{{cite journal |last=Boisserie |first=Jean-Renaud |coauthors= Fabrice Lihoreau and Michel Brunet |year= 2005 |month= February |title= The position of Hippopotamidae within Cetartiodactyla |journal= [[Proceedings of the National Academy of Sciences]] |volume= 102 |issue= 5 |pages= 1537–1541|url= http://www.pnas.org/cgi/content/full/102/5/1537|accessdate= 2007-06-09 |doi= 10.1073/pnas.0409518102 |pmid= 15677331}}</ref>
== Classification ==
* '''[[Cetartiodactyla]]''' (unranked [[clade]], higher than Artiodactyla)
** Family [[Hippopotamidae]]: hippos
** '''Order''' [[Cetaceans]]: whales, dolphins and porpoises
** '''ORDER ARTIODACTYLA'''
*** '''Suborder [[Suina]]'''
**** Family [[Suidae]]: pigs
**** Family [[Tayassuidae]]: peccaries
*** '''Suborder [[Tylopoda]]'''
**** Family [[Camelidae]]: camels and [[llama]]s
*** '''Suborder [[Ruminantia]]'''
**** '''Infraorder [[Pecora]]'''
***** Family [[Antilocapridae]]: [[pronghorn]]
***** Family [[Bovidae]]: cattle, goats, [[sheep]], and [[antelope]]
***** Family [[Cervidae]]: [[deer]]
***** Family [[Giraffidae]]: [[Giraffe]] and [[Okapi]]
***** Family [[Moschidae]]: musk deer
**** '''Infraorder [[Tragulina]]'''
***** Family [[Tragulidae]]: chevrotains
== See also ==
* [[Odd-toed ungulate]]
==References==
{{commonscat|Artiodactyla}}
{{Wikispecies|Artiodactyla}}
{{reflist}}
{{Mammals}}
{{Artiodactyla}}
[[Category:Even-toed ungulates| ]]
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