View source for Artiodactyl

{{short description|Order of mammals}}
{{Missing information|whale traits and physical traits uniting whales with terrestrial artiodactyls|date=July 2023}}
{{Use dmy dates|date=November 2020}}
{{Automatic taxobox
| name = Artiodactyls
| image = <imagemap>
File:The Cetartiodactyla.jpg|300px
rect 0 0 640 1080 [[Northern giraffe]]
rect 640 0 1280 540 [[American bison]]
rect 640 1200 1280 540 [[Red deer]]
rect 1920 1080 1280 720 [[Orca]]
rect 1920 360 1280 720 [[Wild boar]]
rect 1920 0 1280 720 [[Dromedary]]
</imagemap>
| image_alt = Separate pictures of a giraffe, American bison, red deer, orca, wild boar, and camel
| image_caption = Clockwise from center: [[American bison]] (''Bison bison''), [[dromedary]] (''Camelus dromedarius''), [[wild boar]] (''Sus scrofa''), [[orca]] (''Orcinus orca''), [[red deer]] (''Cervus elaphus''), and [[northern giraffe]] (''Giraffa camelopardalis'')
| fossil_range = {{Fossil range|55.4|0}} Early [[Eocene]] – [[Holocene]]
| taxon = Artiodactyla
| authority = [[Richard Owen|Owen]], 1848
| display_parents = 5
| synonyms = Cetartiodactyla<br /><small>Montgelard ''et al.'', 1997</small>
| synonyms_ref = 
| subdivision_ranks = Subdivisions
| subdivision = *[[Tylopoda]]
*[[Artiofabula]]
**[[Suina]]
**[[Cetruminantia]]
***[[Ruminantia]]
***[[Whippomorpha]]
****[[Ancodonta]]
****[[Cetacea]] (historically excluded)
}}

'''Artiodactyls''' are [[placental mammal]]s belonging to the [[order (biology)|order]] '''Artiodactyla''' ({{IPAc-en|ˌ|ɑr|t|i|oʊ|ˈ|d|æ|k|t|ɪ|l|ə}} {{respell|AR|tee|oh|DAK|tih|lə}}; {{etymology|grc|''ἄρτιος'' ártios|even||''δάκτυλος'' dáktylos|finger, toe}}). Typically, they are [[ungulate]]s 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 [[perissodactyl]]s 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 new fossil discoveries, has found that cetaceans ([[whale]]s, [[dolphin]]s, and [[porpoise]]s) fall within this taxonomic branch, being most closely related to [[Hippopotamidae|hippopotamuses]]. Some modern taxonomists thus apply the name '''Cetartiodactyla''' ({{IPAc-en|s|ᵻ|ˌ|t|ɑr|t|i|oʊ|ˈ|d|æ|k|t|ɪ|l|ə}}) 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 [[Suidae|pig]]s, [[peccaries]], [[hippopotamidae|hippopotamuses]], [[antelope]]s, [[deer]], [[giraffe]]s, [[camel]]s, [[llama]]s, [[alpaca]]s, [[Ovis|sheep]], [[Capra (genus)|goat]]s and [[Bos|cattle]]. Many are herbivores, but [[Suidae|suids]] are omnivorous, and [[cetacea]]ns are entirely carnivorous. Artiodactyls are also known by many extinct groups such as [[Anoplotheriidae|anoplotheres]], [[Cainotheriidae|cainotheriids]], [[Merycoidodontoidea|merycoidodonts]], [[entelodonts]], [[anthracotheres]], [[basilosaurids]], and [[Palaeomerycidae|palaeomerycids]]. Many artiodactyls are of great dietary, economic, and cultural importance to humans.

==Evolutionary history==
{{See also|Evolution of cetaceans}}
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 principle|competitive exclusion]].<ref>{{Cite journal |last=Gould |first=François D. H. |date=10 November 2016 |title=Testing the Role of Cursorial Specializations as Adaptive Key Innovations in Paleocene-Eocene Ungulates of North America |url=https://link.springer.com/article/10.1007/s10914-016-9359-4 |journal=[[Journal of Mammalian Evolution]] |language=en |volume=24 |issue=4 |pages=453–463 |doi=10.1007/s10914-016-9359-4 |issn=1064-7554 |access-date=9 April 2025 |via=Springer Nature Link|url-access=subscription }}</ref> The oldest artidactyl fossils are classified as belonging to the family [[Diacodexeidae]];<ref name=Orliac2012>{{cite journal |last1= Orliac |first1= M.J.|last2= Benoit |first2= J. |last3= O'Leary |first3= M.A. |date= November 2012 |title= The inner ear of ''Diacodexis'', the oldest artiodactyl mammal |journal= Journal of Anatomy |volume= 21 |issue= 5 |pages= 417–426 |doi= 10.1111/j.1469-7580.2012.01562.x|pmid= 22938073|pmc= 3482349|s2cid= 2010691}}</ref><ref name=Prothero2007/><ref name=Boivin2018>{{cite journal |last1= Boivin |first1= M. |last2= Orliac |first2= M.J. |display-authors= etal |date= September 2018 |title= New material of ''Diacodexis'' (Mammalia, Artiodactyla) from the early Eocene of Southern Europe |journal= Geobios |volume= 51 |issue= 4 |pages= 285–306 |doi= 10.1016/j.geobios.2018.06.003 |bibcode= 2018Geobi..51..285B |s2cid= 134967454 |url= https://hal.archives-ouvertes.fr/hal-03052875/file/Boivin_etal_Geobios_2018.pdf |access-date= 16 October 2023 |archive-date= 2 March 2022 |archive-url= https://web.archive.org/web/20220302025914/https://hal.archives-ouvertes.fr/hal-03052875/file/Boivin_etal_Geobios_2018.pdf |url-status= live }}</ref> their best-known and best-preserved member is ''[[Diacodexis]]''.<ref name=Prothero2007>{{Cite book |first1=Jessica M. |last1=Theodor |title=Evolution of Artiodactyls |first2=Jörg |last2=Erfurt |last3=Grégoire Métais |date=2007-10-23 |publisher=Johns Hopkins University |isbn=978-0-8018-8735-2 |editor-first=Donald R. |editor-last=Prothero |pages=32–58 |chapter=The earliest artiodactyls: Diacodexeidae, Dichobunidae, Homacodontidae, Leptochoeridae and Raoellidae |editor-first2=Scott E. |editor-last2=Foss |chapter-url=https://books.google.com/books?id=qO8H_alEofAC&pg=PA32}}</ref> 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 [[Ypresian|early]] to middle Eocene saw the emergence of the ancestors of most of today's mammals.<ref name="spaulding2009" />

[[File:entelodon Sp Illustration.jpg|thumb|[[Entelodont]]s were stocky animals with a large head, and were characterized by bony bumps on the lower jaw.|alt=Two large boar-like creatures graze.]]

Two formerly widespread, but now extinct, families of even-toed ungulates were [[Entelodont]]idae 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 [[Snout|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.<ref name="spaulding2009" />

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 [[List of mammals of North America#Camelidae|North American species]] became extinct around 10,000 years ago.<ref>{{cite web|last1=Huffman |first1=Brent |title=Family Camelidae |url=https://www.ultimateungulate.com/Artiodactyla/Camelidae.html |access-date=11 November 2025}}</ref>

[[Suina]] (including [[pig]]s) 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]].{{citation needed|date=December 2024}}

[[File:MEPAN Sivatherium.jpg|thumb|''[[Sivatherium]]'' was a relative of giraffes with deer-like forehead [[ossicones]].|alt=A deer-like animal wanders through a clearing.]]

South America was [[Great American Interchange|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 [[llama]]s), and various species of [[Capreolinae|capreoline deer]], South America has [[List of mammals of South America#Order: Artiodactyla (even-toed ungulates and cetaceans)|comparatively fewer artiodactyl families]] than other continents, except Australia, which has no (terrestrial) native species.{{citation needed|date=December 2024}}

[[File:Anoplotherium commune quadrupedal.png|thumb|right|''[[Anoplotherium]]'' was the first fossil artiodactyl genus to be named, with a history dating back to 1804. It lived in Europe as part of the endemic family [[Anoplotheriidae]] during the late Eocene-earliest Oligocene.]]

==Taxonomy and phylogeny==
[[File:Thomas Herbert Maguire - Richard Owen 1850.png|thumb|[[Richard Owen]] coined the term "even-toed ungulate".|alt=Portrait of Richard Owen]]

The classification of artiodactyls was hotly debated because ocean-dwelling [[cetacea]]ns evolved from land-dwelling even-toed ungulates. Some [[semiaquatic]] even-toed ungulates ([[hippopotamus]]es) are more closely related to ocean-dwelling cetaceans than to other even-toed ungulates.<ref name="montgelard97"/>

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.<ref name="montgelard97">{{Cite journal |last1=Montgelard |first1=Claudine |last2=Catzeflis |first2=Francois M. |last3=Douzery |first3=Emmanuel |year=1997 |title=Phylogenetic relationships of artiodactyls and cetaceans as deduced from the comparison of cytochrome b and 12S rRNA mitochondrial sequences |journal=Molecular Biology and Evolution |volume=14 |issue=5 |pages=550–559 |doi=10.1093/oxfordjournals.molbev.a025792 |pmid=9159933 |doi-access=}}</ref> An alternative approach is to include both land-dwelling even-toed ungulates and ocean-dwelling cetaceans in a revised Artiodactyla taxon.<ref name="spaulding2009" />

===Classification===
{{main|List of artiodactyls}}
'''Order Artiodactyla'''/'''Clade Cetartiodactyla'''<ref name="spaulding2009">{{Cite journal |last1=Spaulding |first1=M |last2=O'Leary |first2=MA |last3=Gatesy |first3=J |year=2009 |editor-last=Farke |editor-first=Andrew Allen |title=Relationships of Cetacea (Artiodactyla) Among Mammals: Increased Taxon Sampling Alters Interpretations of Key Fossils and Character Evolution |journal=PLOS ONE |volume=4 |issue=9 |article-number=e7062 |bibcode=2009PLoSO...4.7062S |doi=10.1371/journal.pone.0007062 |pmc=2740860 |pmid=19774069 |doi-access=free}}</ref><ref>{{Cite book |last1=Groves |first1=Colin P. |title=Ungulate Taxonomy |last2=Grubb |first2=Peter |date=2011 |publisher=Johns Hopkins University Press |isbn=978-1-4214-0093-8 |location=Baltimore, Maryland |page=25}}</ref>
*'''Basal and [[incertae sedis]]'''
** Family {{Abbr|†|Extinct}}[[Diacodexeidae]]
** Family {{Abbr|†|Extinct}}[[Dichobunidae]]
** Family {{Abbr|†|Extinct}}[[Cebochoerus|Cebochoeridae]]
** Family {{Abbr|†|Extinct}}[[Leptochoeridae]]
** Family {{Abbr|†|Extinct}}[[Homacodontidae]]
** '''Superfamily Anoplotheroidea''' (possible tylopoda or stem-ruminantia)
*** Family {{Abbr|†|Extinct}}[[Mixtotheriidae]]
*** Family {{Abbr|†|Extinct}}[[Robiacina|Robiacinidae]]
*** Family {{Abbr|†|Extinct}}[[Cainotheriidae]]
*** Family {{Abbr|†|Extinct}}[[Anoplotheriidae]]
*** Family {{Abbr|†|Extinct}}[[Amphimerycidae]]?
*** Family {{Abbr|†|Extinct}}[[Xiphodontidae]]?
**'''Superfamily [[Merycoidodontoidea]]''' (oreodonts, possible tylopoda or stem-ruminantia)
*** Family {{Abbr|†|Extinct}} [[Merycoidodontidae]]
*** Family {{Abbr|†|Extinct}} [[Agriochoerus|Agriochoeridae]]
** Family {{Abbr|†|Extinct}}[[Protoceratidae]] (possible tylopoda or stem-ruminantia)
**'''Suborder [[Tylopoda]]'''
*** Family [[Camelidae]]: [[camel]]s, [[llama]]s, [[alpaca]]s, [[vicuña]]s, and [[guanaco]]s (7 extant and 13 extinct species)
*** Family {{Abbr|†|Extinct}}[[Oromerycidae]] 
** '''Clade [[Artiofabula]]'''
***'''Suborder [[Suina]]'''
**** Family [[Suidae]]: pigs (19 species)
**** Family [[Tayassuidae]]: peccaries (4 species)
**** Family {{Abbr|†|Extinct}}[[Sanitheriidae]]
****Family {{Abbr|†|Extinct}}[[Doliochoeridae]]
*** '''Clade [[Cetruminantia]]'''
****'''Clade [[Cetancodontamorpha]]'''
***** Genus {{Abbr|†|Extinct}}''[[Andrewsarchus]]''?
***** Family {{Abbr|†|Extinct}}[[Helohyidae]]?
***** Family {{Abbr|†|Extinct}}[[Entelodontidae]]?
***** '''Suborder [[Whippomorpha]]'''
****** Family {{Abbr|†|Extinct}}[[Raoellidae]]
****** '''Infraorder [[Ancodonta]]'''
******* Family {{Abbr|†|Extinct}}[[Choeropotamidae]]
******* Family {{Abbr|†|Extinct}}[[Anthracotheriidae]] – paraphyletic to Hippopotamidae
******* Family [[Hippopotamidae]]: hippos (two species)
****** '''Infraorder [[Cetacea]]''': whales (about 90 species)
*******'''Parvorder {{Abbr|†|Extinct}}[[Archaeoceti]]'''
******** Family {{Abbr|†|Extinct}}[[Pakicetidae]]
******** Family {{Abbr|†|Extinct}}[[Ambulocetidae]]
******** Family {{Abbr|†|Extinct}}[[Remingtonocetidae]]
******** Family {{Abbr|†|Extinct}}[[Basilosauridae]]
******* '''Clade [[Neoceti]]'''
******** '''Parvorder [[Mysticeti]]''': baleen whales
********* Superfamily Balaenoidea: [[right whale]]s
********** Family [[Balaenidae]]: greater [[right whale]]s (four species)
********** Family [[Cetotheriidae]]: [[pygmy right whale]] (one species)
********* Superfamily Balaenopteroidea: large [[baleen whale]]s
********** Family [[Balaenopteridae]]: [[Balaenoptera|slender-back rorquals]] and [[humpback whale]] (eight species)
********** Family [[Eschrichtiidae]]: [[gray whale]] (one species)
******** '''Parvorder [[Odontoceti]]''': toothed whales
********* Superfamily [[Delphinoidea]]: [[oceanic dolphin]]s, [[porpoise]]s, and others
********** Family [[Delphinidae]]: [[Oceanic dolphin|oceanic true dolphins]] (38 species)
********** Family [[Monodontidae]]: [[Monodontidae|Arctic whales]]; [[narwhal]] and [[Beluga whale|beluga]] (two species)
********** Family [[Phocoenidae]]: [[porpoise]]s (six species)
********* Superfamily [[Physeteroidea]]: [[Physeteroidea|sperm whales]]
********** Family [[Kogiidae]]: [[Kogiidae|lesser sperm whales]] (two species)
********** Family [[Physeteridae]]: [[sperm whale]] (one species)
********* Superfamily [[Platanistoidea]]: [[river dolphin]]s
********** Family [[Iniidae]]: [[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 whale]]s (22 species)
**** '''Total-group Ruminantia'''
***** '''Suborder [[Ruminantia]]'''
****** '''Infraorder [[Tragulina]]''' (paraphyletic)
******* Family {{Abbr|†|Extinct}}[[Archaeomerycidae]]
******* Family {{Abbr|†|Extinct}}[[Leptomeryx|Leptomerycidae]]
******* Family {{Abbr|†|Extinct}}[[Hypertragulidae]]
******* Family {{Abbr|†|Extinct}}[[Praetragulus|Praetragulidae]]
******* Family {{Abbr|†|Extinct}}[[Lophiomeryx|Lophiomerycidae]]
******* Family {{Abbr|†|Extinct}}[[Gelocidae]]
******* Family {{Abbr|†|Extinct}}[[Bachitherium|Bachitheriidae]]
******* Family [[Tragulidae]]: chevrotains (ten species)
****** '''Infraorder [[Pecora]]'''
******* Family [[Antilocapridae]]: [[pronghorn]] (one species)
******* Family {{Abbr|†|Extinct}}[[Hoplitomerycidae]]
******* Superfamily [[Giraffoidea]]
******** Family {{Abbr|†|Extinct}}[[Palaeomerycidae]]
******** Family {{Abbr|†|Extinct}}[[Climacoceratidae]]
******** Family [[Giraffidae]]: [[okapi]] and four species of [[giraffe]] (five species total)
******* Superfamily Cervoidea
******** Family {{Abbr|†|Extinct}}[[Dromomerycidae]]?
******** Family [[Cervidae]]: [[deer]] (49 species)
******* Superfamily Bovoidea
******** Family [[Moschidae]]: [[musk deer]] (7 species)
******** Family [[Bovidae]]: [[Bovini|cattle]], [[Capra (genus)|goat]]s, [[Ovis|sheep]] and [[antelope]]s (135 species)

===Research history===
[[File:Humpback Whale underwater shot.jpg|thumb|Molecular and morphological studies confirmed that [[cetacea]]ns are the closest living relatives of [[hippopotamus]]es.|alt=Humpback whale swimming under water]]

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 [[whale]]s and [[hippopotamus]]es is the paraphyletic group Artiodactyla.<ref name=Gatesy1996/><ref name=Amrine2003/>

Dan Graur and Desmond Higgins were among the first to come to this conclusion, and included a paper published in 1994.<ref>{{Cite journal |last1=Graur |first1=Dan |last2=Higgins |first2=Desmond G. |year=1994 |title=Molecular Evidence for the Inclusion of Cetaceans within the Order Artiodactyla |url=http://mbe.library.arizona.edu/data/1994/1103/3grau.pdf |journal=Molecular Biology and Evolution |pages=357–364 |archive-url=https://web.archive.org/web/20160305180005/http://mbe.library.arizona.edu/data/1994/1103/3grau.pdf |archive-date=5 March 2016 |access-date=23 August 2015}}</ref> However, they did not recognize hippopotamuses and classified the [[ruminant]]s as the sister group of cetaceans. Subsequent studies established the close relationship between hippopotamuses and cetaceans; these studies were based on [[Casein|casein genes]],<ref name=Gatesy1996>{{Cite journal |last1=Gatesy |first1=John |last2=Hayashi |first2=Cheryl |last3=Cronin |first3=Mathew A. |last4=Arctander |first4=Peter |year=1996 |title=Evidence from milk casein genes that cetaceans are close relatives of hippopotamid artiodactyls |journal=Molecular Biology and Evolution |volume=13 |issue=7 |pages=954–963 |doi=10.1093/oxfordjournals.molbev.a025663 |pmid=8752004 |doi-access=free}}</ref> [[Short interspersed nuclear element|SINEs]],<ref>{{Cite journal |last=Shimamura |first=M. |year=1997 |title=Molecular evidence from retroposons that whales form a clade within even-toed ungulates |journal=Nature |volume=388 |issue=6643 |pages=666–670 |bibcode=1997Natur.388..666S |doi=10.1038/41759 |pmid=9262399 |doi-access=free |s2cid=4429657}} {{closed access}}</ref> [[fibrinogen]] sequences,<ref>{{Cite journal |last=Gatesy |first=John |year=1997 |title=More DNA Support for a Cetacea/Hippopotamidae Clade: The Blood-Clotting Protein Gene y-Fibrinogen |journal=Molecular Biology and Evolution |volume=14 |issue=5 |pages=537–543 |doi=10.1093/oxfordjournals.molbev.a025790 |pmid=9159931 |doi-access=free}}</ref> [[cytochrome]] and [[rRNA]] sequences,<ref name=montgelard97/><ref name="Agnarsson-2008">{{Cite journal |last1=Agnarsson |first1=Ingi |last2=May-Collado |first2=Laura J. |year=2008 |title=The phylogeny of Cetartiodactyla: The importance of dense taxon sampling, missing data, and the remarkable promise of cytochrome b to provide reliable species-level phylogenies |url=https://www.researchgate.net/publication/5262443 |journal=Molecular Phylogenetics and Evolution |volume=48 |issue=3 |pages=964–85 |doi=10.1016/j.ympev.2008.05.046 |pmid=18590827 |bibcode=2008MolPE..48..964A |access-date=3 September 2018 |archive-date=4 September 2020 |archive-url=https://web.archive.org/web/20200904193253/https://www.researchgate.net/publication/5262443_The_phylogeny_of_Cetartiodactyla_The_importance_of_dense_taxon_sampling_missing_data_and_the_remarkable_promise_of_cytochrome_b_to_provide_reliable_species-level_phylogenies |url-status=live }}</ref> [[RBP3|IRBP]] (and [[Von Willebrand factor|vWF]]) gene sequences,<ref>{{Cite journal |last1=Gatesy |first1=John |last2=Milinkovitch |first2=Michel |last3=Waddell |first3=Victor |last4=Stanhope |first4=Michael |year=1999 |title=Stability of Cladistic Relationships between Cetacea and Higher-Level Artiodactyl Taxa |journal=Systematic Biology |volume=48 |issue=1 |pages=6–20 |doi=10.1080/106351599260409 |pmid=12078645 |doi-access=free}}</ref> [[adrenergic receptor]]s,<ref>{{Cite journal |last1=Madsen |first1=Ole |last2=Willemsen |first2=Diederik |last3=Ursing |first3=Björn M. |last4=Arnason |first4=Ulfur |last5=de Jong |first5=Wilfried W. |year=2002 |title=Molecular Evolution of the Mammalian Alpha 2B Adrenergic Receptor |journal=Molecular Biology and Evolution |volume=19 |issue=12 |pages=2150–2160 |doi=10.1093/oxfordjournals.molbev.a004040 |pmid=12446807 |doi-access=free}}</ref> and [[apolipoprotein]]s.<ref name=Amrine2003>{{Cite journal |last1=Amrine-Madsen |first1=Heather |last2=Koepfli |first2=Klaus-Peter |last3=Wayne |first3=Robert K. |last4=Springer |first4=Mark S. |year=2003 |title=A new phylogenetic marker, apolipoprotein B, provides compelling evidence for eutherian relationships |url=https://www.researchgate.net/publication/10647632 |journal=Molecular Phylogenetics and Evolution |volume=28 |issue=2 |pages=225–240 |doi=10.1016/s1055-7903(03)00118-0 |pmid=12878460 |bibcode=2003MolPE..28..225A |access-date=3 September 2018 |archive-date=4 September 2020 |archive-url=https://web.archive.org/web/20200904193254/https://www.researchgate.net/publication/10647632_A_new_phylogenetic_marker_apolipoprotein_B_provides_compelling_evidence_for_eutherian_relationships |url-status=live |doi-access=free }}</ref>

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 [[archaeocete]]s ("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 bone|talus]] (ankle bone) with a double-rolled joint surface,{{Clarify|reason=What is a 'double-rolled joint surface'?|date=September 2016}} previously thought to be unique to even-toed ungulates, were also in early cetaceans.<ref name="MamEv">{{Cite book |last1=Savage |first1=R. J. G. |url=https://archive.org/details/mammalevolutioni0000sava |title=Mammal Evolution: an illustrated guide |last2=Long |first2=M. R. |publisher=Facts on File |year=1986 |isbn=978-0-8160-1194-0 |location=New York |pages=[https://archive.org/details/mammalevolutioni0000sava/page/208 208] |url-access=registration}}</ref> The [[mesonychia]]ns, 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.{{citation needed|date=December 2024}}

[[File:Hippopotamus amphibius.JPG|thumb|[[Hippopotamus]]es are a geologically young group, which raises questions about their origin.|alt=A hippo splashes in the water]]

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 [[skull]]s, 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.<ref name="Agnarsson-2008" /><ref name="Price-2005">{{Cite journal |last1=Price |first1=Samantha A. |last2=Bininda-Emonds |first2=Olaf R. P. |last3=Gittleman |first3=John L. |year=2005 |title=A complete phylogeny of the whales, dolphins and even-toed hoofed mammals (Cetartiodactyla) |url=https://www.researchgate.net/publication/7665216 |journal=Biological Reviews |volume=80 |issue=3 |pages=445–73 |doi=10.1017/s1464793105006743 |pmid=16094808 |s2cid=45056197 |access-date=3 September 2018 |archive-date=4 September 2020 |archive-url=https://web.archive.org/web/20200904193253/https://www.researchgate.net/publication/7665216_A_complete_phylogeny_of_the_whales_dolphins_and_even-toed_hoofed_mammals_Cetartiodactyla |url-status=live }}</ref> 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.<ref>{{cite book|url=https://books.google.com/books?id=DhchVG_rbQ8C&pg=PA216|publisher=Johns Hopkins University Press|isbn=978-0-8018-8022-3|date=2005|title=The Rise of Placental Mammals: Origins and Relationships of the Major Extant Clades|first1=Kenneth|last1=D. Rose|editor=J. David Archibald|page=216|access-date=20 June 2024|archive-date=21 June 2024|archive-url=https://web.archive.org/web/20240621034329/https://books.google.com/books?id=DhchVG_rbQ8C&pg=PA216#v=onepage&q&f=false|url-status=live}}</ref> [[Henri de Blainville]] recognized the similar anatomy of the limbs of pigs and hippos,{{when|date=July 2016}} and British zoologist [[Richard Owen]] coined the term "even-toed ungulates" and the scientific name "Artiodactyla" in 1848.<ref>{{Cite journal |last=Owen |first=Richard |author-link=Richard Owen |date=1848 |title=Description of Teeth and portions of Jaws of two extinct Anthracotherioid Quadrupeds (Hyopotamus vectianus and Hyop. bovinus) discovered by the Marchioness of Hastings in the Eocene Deposits on the N.W. coast of the Isle of Wight: with an attempt to {{as written|develope}} Cuvier's idea of the Classification of Pachyderms by the Number of their Toes |url=https://www.biodiversitylibrary.org/page/37048577 |journal=Quarterly Journal of the Geological Society of London |volume=4 |issue=1 |pages=103–141 |doi=10.1144/GSL.JGS.1848.004.01-02.21 |access-date=16 February 2024 |archive-date=6 November 2023 |archive-url=https://web.archive.org/web/20231106171352/https://www.biodiversitylibrary.org/page/37048577 |url-status=live }}</ref>

Internal morphology (mainly the stomach and the molars) were used for classification. [[Suines]] (including pigs) and hippopotamuses have molars with well-developed roots and a simple stomach that digests food. Thus, they were grouped together as non-ruminants (porcine). All other even-toed ungulates have molars with a [[selenodont]] construction (crescent-shaped cusps) and have the ability to [[ruminant|ruminate]], which requires regurgitating food and re-chewing it. Differences in stomach construction indicated that rumination evolved independently between [[tylopod]]s and [[ruminant]]s; therefore, tylopods were excluded from [[Ruminantia]].<ref name="Price-2005"/>

The taxonomy that was widely accepted by the end of the 20th century was:<ref name="Price-2005"/>

{{Clade|style=font-size:100%;line-height:100%
  |label1={{nowrap|Even-toed ungulates }}
  |1={{Clade
    |label1=[[Suina]]
    |1={{Clade
      |1=[[Suidae]] [[File:Recherches pour servir à l'histoire naturelle des mammifères (Pl. 80) (white background).jpg|50 px]]
      |2=[[Hippopotamidae]]  [[File:Voyage en Abyssinie Plate 2 (white background).jpg|50 px]]
  }}
    |label2=[[Selenodont]]
    |2={{Clade
      |1=[[Tylopoda]] [[File:Cladogram of Cetacea within Artiodactyla (Camelus bactrianus).png|50 px]]
      |label2=[[Ruminant]]s
      |2={{Clade
        |1=[[Tragulidae]] [[File:Tragulus napu - 1818-1842 - Print - Iconographia Zoologica - Special Collections University of Amsterdam - (white background).jpg|50 px]]
        |2=[[Pecora]] [[File:Walia ibex illustration white background.png|50 px]]
    }}
  }}
}}
}}

====Historical classification of Cetacea====
[[File:Mesonyx.jpg|thumb|The [[mesonychia]]ns were long considered ancestors of [[whale]]s.|alt=An illustration of a mesonychian, which looks like a wolf-like animal]]

Modern cetaceans are highly adapted sea creatures which, morphologically, have little in common with land mammals; they are similar to other [[marine mammal]]s, such as [[pinniped|seals]] and [[sea cow]]s, due to [[convergent evolution]]. However, they evolved from originally terrestrial mammals. The most likely ancestors were long thought to be mesonychians; large, carnivorous animals from the early Cenozoic ([[Paleocene]] and [[Eocene]]), which had [[hooves]] instead of [[claw]]s on their feet. Their molars were adapted to a carnivorous diet, resembling the teeth in modern [[toothed whale]]s, and, unlike other mammals, had a uniform construction.<ref>{{Cite journal |last1=Ando |first1=Konami |last2=Fujiwara |first2=Shin-ichi |date=2016-07-10 |title=Farewell to life on land – thoracic strength as a new indicator to determine paleoecology in secondary aquatic mammals |journal=Journal of Anatomy |volume=229 |issue=6 |pages=768–777 |doi=10.1111/joa.12518 |issn=0021-8782 |pmc=5108153 |pmid=27396988}}</ref>

The then suspected relations were as follows:<ref name="Price-2005" /><ref>{{Cite book |first1=Malcolm C. |last1=McKenna |title='Classification of Mammals - Above the Species Level |first2=Susan K. |last2=Bell |publisher=Columbia University Press |year=1997 |isbn=978-0-231-11013-6}}{{Page needed|date=September 2016}}</ref>

{{Clade|style=font-size:100%;line-height:100%
       |label1=[[Paraxonia]]
       |1={{Clade
           |1=[[Artiodactyla]] [[File:Walia ibex illustration white background.png|50 px]]
           |label2=Cetaceamorpha
           |2={{Clade
               |1=[[Mesonychia]] {{Abbr|†|Extinct}}[[File:Synoplotherium112DB.jpg|50 px]]
               |2=[[Cetacea]] [[File:Bowhead-Whale1 (16273933365).jpg|50 px]]
             }}
          }}
       }}

===Inner systematics===
Molecular findings and morphological indications suggest that artiodactyls, as traditionally defined, are paraphyletic with respect to cetaceans. Cetaceans are deeply nested within the former; the two groups together form a [[monophyletic]] taxon, for which the name Cetartiodactyla is sometimes used. Modern nomenclature divides Artiodactyla (or Cetartiodactyla) in four subordinate taxa, camelids (Tylopoda), pigs and peccaries (Suina), ruminants (Ruminantia), and hippos plus cetaceans (Whippomorpha).

The presumed lineages within Artiodactyla can be represented in the following [[cladogram]]:<ref>{{Cite journal |last=Beck |first=N.R. |year=2006 |title=A higher-level MRP supertree of placental mammals |journal=BMC Evol Biol |volume=6 |page=93 |doi=10.1186/1471-2148-6-93 |pmc=1654192 |pmid=17101039 |doi-access=free }}</ref><ref name="O'Leary2013">{{Cite journal |last1=O'Leary |first1=M.A. |last2=Bloch |first2=J.I. |last3=Flynn |first3=J.J. |last4=Gaudin |first4=T.J. |last5=Giallombardo |first5=A. |last6=Giannini |first6=N.P. |last7=Goldberg |first7=S.L. |last8=Kraatz |first8=B.P. |last9=Luo |first9=Z.-X. |last10=Meng |first10=J. |last11=Ni |first11=X. |last12=Novacek |first12=M.J. |last13=Perini |first13=F.A. |last14=Randall |first14=Z.S. |last15=Rougier |first15=G.W. |year=2013 |title=The Placental Mammal Ancestor and the Post-K-Pg Radiation of Placentals |journal=Science |volume=339 |issue=6120 |pages=662–667 |bibcode=2013Sci...339..662O |doi=10.1126/science.1229237 |pmid=23393258 |hdl-access=free |last16=Sargis |first16=E.J. |last17=Silcox |first17=M.T. |last18=Simmons |first18=N.B. |last19=Spaulding |first19=M. |last20=Velazco |first20=P.M. |last21=Weksler |first21=M. |last22=Wible |first22=J.R. |last23=Cirranello |first23=A.L. |s2cid=206544776 |hdl=11336/7302}}</ref><ref name="Song2012">{{Cite journal |last1=Song |first1=S. |last2=Liu |first2=L. |last3=Edwards |first3=S.V. |last4=Wu |first4=S. |year=2012 |title=Resolving conflict in eutherian mammal phylogeny using phylogenomics and the multispecies coalescent model |journal=Proceedings of the National Academy of Sciences |volume=109 |issue=37 |pages=14942–14947 |bibcode=2012PNAS..10914942S |doi=10.1073/pnas.1211733109 |pmc=3443116 |pmid=22930817 |doi-access=free}}</ref><ref name="dos Reis2012">{{Cite journal |last1=dos Reis |first1=M. |last2=Inoue |first2=J. |last3=Hasegawa |first3=M. |last4=Asher |first4=R.J. |last5=Donoghue |first5=P.C.J. |last6=Yang |first6=Z. |year=2012 |title=Phylogenomic datasets provide both precision and accuracy in estimating the timescale of placental mammal phylogeny |journal=Proceedings of the Royal Society B: Biological Sciences |volume=279 |issue=1742 |pages=3491–3500 |doi=10.1098/rspb.2012.0683 |pmc=3396900 |pmid=22628470 |doi-access=free}}</ref><ref name="Upham2019">{{Cite journal |last1=Upham |first1=N.S. |last2=Esselstyn |first2=J.A. |last3=Jetz |first3=W. |year=2019 |title=Inferring the mammal tree: Species-level sets of phylogenies for questions in ecology, evolution, and conservation |journal=PLOS Biology |volume=17 |issue=12 |article-number=e3000494 |doi=10.1371/journal.pbio.3000494 |pmc=6892540 |pmid=31800571 |doi-access=free}}(see e.g. Fig S10)</ref>

{{clade
    |style=font-size:100%;line-height:100%
    |label1='''Artiodactyla'''
    |1={{clade
        |1=[[Tylopoda]] (camels) [[File:Cladogram of Cetacea within Artiodactyla (Camelus bactrianus).png|50 px]]
        |label2=[[Artiofabula]]
        |2={{clade
            |1=[[Suina]] (pigs) [[File:Recherches pour servir à l'histoire naturelle des mammifères (Pl. 80) (white background).jpg|50 px]]
            |label2=[[Cetruminantia]]
            |2={{clade
                |label1=[[Ruminantia]] (ruminants)
                |1={{clade
                    |1=[[Tragulidae]] (mouse deer) [[File:Tragulus napu - 1818-1842 - Print - Iconographia Zoologica - Special Collections University of Amsterdam - (white background).jpg|50 px]]
                    |2=[[Pecora]] (horn bearers) [[File:Walia ibex illustration white background.png|50 px]]
              }}
                |label2=[[Cetancodonta]]
                |sublabel2=(or [[Whippomorpha]])
                |2={{clade
                    |1=[[Hippopotamidae]] (hippopotamuses) [[File:Voyage en Abyssinie Plate 2 (white background).jpg|50 px]]
                    |2=[[Cetacea]] (whales) [[File:Bowhead-Whale1 (16273933365).jpg|50 px]]
              }}
          }}
       }}
    }}
 }}
[[File:Camelus dromedarius in Singapore Zoo.JPG|thumb|[[Camel]]s, like this dromedary, are now considered a sister group to [[Artiofabula]].|alt=A camel standing in a zoo.]]
[[File:Antilocapra americana.jpg|thumb|The [[pronghorn]] is the only extant antilocaprid.|alt=A pronghorn]]

The four summarized Artiodactyla taxa are divided into ten extant families:<ref>{{Cite book |title=Mammal Species of the World |publisher=Johns Hopkins University Press |year=2005 |isbn=978-0-8018-8221-0 |editor-last=Wilson, D. E. |edition=3rd |pages=111–184 |editor-last2=Reeder, D. M.}}</ref>
* The camelids ([[Tylopoda]]) comprise only one family, [[Camelidae]]. This is a species-poor artiodactyl suborder of North American origin<ref name="Cui2007">{{Cite journal |last1=Cui |first1=P. |last2=Ji |first2=R. |last3=Ding |first3=F. |last4=Qi |first4=D. |last5=Gao |first5=H. |last6=Meng |first6=H. |last7=Yu |first7=J. |last8=Hu |first8=S. |last9=Zhang |first9=H. |year=2007 |title=A complete mitochondrial genome sequence of the wild two-humped camel (''Camelus bactrianus ferus''): an evolutionary history of Camelidae |journal=BMC Genomics |volume=8 |issue=1 |page=241 |doi=10.1186/1471-2164-8-241 |pmc=1939714 |pmid=17640355 |doi-access=free }}</ref> that is well adapted to extreme habitats; the [[dromedary]] and [[Bactrian camel|Bactrian]] camels in the Old World deserts and the [[guanaco]]s, llamas, [[vicuña]]s, and [[alpaca]]s in South American high mountain regions.
* The pig-like creatures ([[Suina]]) are made up of two families:
** The pigs ([[Suidae]]) are limited to the Old World. These include the [[wild boar]] and the domesticated form, the [[domestic pig]].
** The peccaries ([[Tayassuidae]]) are named after glands on their belly and are indigenous to Central and South America.
* The ruminants ([[Ruminantia]]) consist of six families:
** The mouse deer ([[Tragulidae]]) are the smallest and most primitive even-toed-ruminants; they inhabit forests of Africa and Asia.
** The giraffe-like creatures ([[Giraffidae]]) are composed of two species, the giraffe and the [[okapi]].
** The musk deer ([[Moschidae]]) is indigenous to East Asia.
** The antilocaprids ([[Antilocapridae]]) of North America comprise only one extant species, the [[pronghorn]].
** The deer ([[Cervidae]]) are made up of about 45 species, which are characterized by a pair of [[antler]]s (generally only in males). They are widespread across Europe, Asia, and the Americas. This group includes, among other species, the [[red deer]], [[moose]], [[roe deer]], [[muntjac]]s, and [[reindeer]] (caribou).
** The bovids ([[Bovidae]]) are the most species-rich family. Among them are [[Bos|cattle]], [[Ovis|sheep]], [[Caprinae|caprine]]s, and [[antelope]]s, and more.
* The [[Whippomorpha|whippomorphs]] include hippos and cetaceans:
** The hippos ([[Hippopotamidae]]) comprise two species, the [[common hippo]] and the [[pygmy hippo]].
** The [[cetacea]]ns comprise 72 species in two parvorders, the toothed whales ([[Odontoceti]]) and the baleen whales ([[Mysticeti]]).

Although deer, musk deer, and pronghorns have traditionally been summarized as cervids (Cervioidea), molecular studies provide different, and inconsistent, results, so the question of phylogenetic systematics of infraorder [[Pecora]] (the horned ruminants) for the time being, is not yet answered.

[[File:Indohyus BW.jpg|thumb|Reconstruction of ''[[Indohyus]]''|alt=Illustration of an Indohyus, a mouse-like mammal]]

==Anatomy==
[[File:Blue Duiker skeleton.jpg|thumb|A [[blue duiker]] skeleton on display at the [[Museum of Osteology]]]]

Apart from cetaceans, artiodactyls are generally [[quadruped]]s. Two major body types are known: suinids and hippopotamuses are characterized by a stocky body, short legs, and a large head; camels and ruminants, though, have a more slender build and lanky legs. Size varies considerably; the smallest member, the mouse deer, often reaches a body length of only {{Convert|45|cm|sp=us}} and a weight of {{convert|1.5|kg}}. The largest member, the hippopotamus, can grow up to {{convert|5|m|sp=us}} in length and weigh {{convert|4.5|MT|ST|0}}, and the giraffe can grow to be {{convert|5.5|m|sp=us}} tall and {{convert|4.7|m|sp=us}} in body length. All even-toed ungulates display some form of [[sexual dimorphism]]: the males are consistently larger and heavier than the females. In the majority of deer species, only the males grow antlers, and the horns of [[bovines]] are usually small or not present in females. Male [[Indian antelope]]s have a much darker coat than females. In cetaceans the two forelimbs have evolved into flippers, and the hind legs lost to external view;<ref name="Thewissen">{{cite web | title=Highlights of Cetacean Embryology | url=https://www.aquaticmammalsjournal.org/wp-content/uploads/2018/11/44_6_thewissen.pdf | access-date=2025-11-12}}</ref> the largest cetaceans, up to {{cvt|30|m}} long, are considerably larger than any other artiodactyls.

Almost all even-toed ungulates have fur, with the exception being the nearly hairless hippopotamus. Fur varies in length and coloration depending on the habitat. Species in cooler regions can shed their coat in summer. Camouflaged coats come in colors of yellow, gray, brown, or black tones. Cetaceans are largely hairless, apart from a few small facial [[whiskers]].<ref name="Thewissen"/>

===Limbs===
[[File:Tragulus javanicus.jpg|thumb|The [[mouse deer]] is the smallest even-toed ungulate.|alt=A mouse deer, which looks like a mouse with tiny stilt-like deer legs.]]

Even-toed ungulates bear their name because they have an even number of [[toe]]s (two or four). In some peccaries, the [[hind legs]] have a reduction in the number of toes to three. The central axis of the leg is between the third and fourth toe. The first toe is missing in modern artiodactyls, and can only be found in now-extinct genera. The second and fifth toes are adapted differently between species:

<gallery mode="packed">
File:Hippopotamus-foot.jpg|Hippopotamuses have all four toes pointing out.
File:Sow-foot.jpg|In pigs and other biungulates, the second and fifth toes are directed backwards.
File:Dromedar-Fuss.jpg|When camels have only two toes present, the claws are transformed into nails.
</gallery>

When camels have only two toes present, the [[claw]]s are transformed into [[Nail (anatomy)|nails]] (while both are made of [[keratin]], claws are curved and pointed while nails are flat and blunt).<ref>{{Cite web |date=2014-01-07 |title=Claws Out: Things You Didn't Know About Claws |url=http://www.thomsonsafaris.com/blog/animal-claws-out |access-date=2016-09-24 |website=Thomson Safaris |language=en-US |archive-date=24 September 2016 |archive-url=https://web.archive.org/web/20160924184519/http://www.thomsonsafaris.com/blog/animal-claws-out/ |url-status=live }}</ref> These claws consist of three parts: the plate (top and sides), the sole (bottom), and the bale (rear). In general, the claws of the forelegs are wider and blunter than those of the hind legs, and they are farther apart. Aside from camels, all even-toed ungulates put just the tip of the foremost [[phalanx bone|phalanx]] on the ground.<ref>{{Cite book |last=Salomon |first=Franz-Viktor |title=Anatomy for veterinary medicine (Musculoskeletal system) |collaboration=et al. |year=2008 |isbn=978-3-8304-1075-1 |editor-last=Salomon |editor-first=F.-V. |pages=22–234}}</ref>

[[File:Hand skeletons with Artiodactyls highlighted.png|alt=Six hand skeletons|thumb|250x250px| Diagrams of hand skeletons of various mammals, left to right: [[orangutan]], [[dog]], [[pig]], [[cow]], [[tapir]], and [[horse]]. Highlighted are the even-toed ungulates pig and cow.]]

In even-toed ungulates, the bones of the stylopodium (upper arm or thigh bone) and [[zygopodium]]s (tibia and fibula) are usually elongated. The muscles of the limbs are predominantly localized, which ensures that artiodactyls often have very slender legs. A [[clavicle]] is never present, and the [[scapula]] is very agile and swings back and forth for added mobility when running. The special construction of the legs causes the legs to be unable to rotate, which allows for greater stability when running at high speeds. In addition, many smaller artiodactyls have a very flexible body, contributing to their speed by increasing their stride length.

===Head===
Many even-toed ungulates have a relatively large head. The skull is elongated and rather narrow; the [[frontal bone]] is enlarged near the back and displaces the [[parietal bone]], which forms only part of the side of the cranium (especially in ruminants).

====Horns and antlers====
[[File:Gemsbok Kgalagadi.jpg|thumb|Outgrowths of the frontal bone characterize most forehead weapons carriers, such as the [[gemsbok]] and its horns.|alt=A gemsbok, a type of antelope]]

Four families of even-toed ungulates have cranial appendages. These [[Pecora]] (with the exception of the [[musk deer]]), have one of four types of cranial appendages: true horns, [[antler]]s, [[ossicone]]s, or [[pronghorn]]s.<ref name="DeMiguel">{{Cite journal |last1=DeMiguel |first1=Daniel |last2=Azanza |first2=Beatriz |last3=Morales |first3=Jorge |year=2014 |title=Key innovations in ruminant evolution: a paleontological perspective |journal=Integrative Zoology |volume=9 |issue=4 |pages=412–433 |doi=10.1111/1749-4877.12080 |pmid=24148672}}</ref>

''True horns'' have a bone core that is covered in a permanent sheath of keratin, and are found only in the [[bovids]]. ''Antlers'' are bony structures that are shed and replaced each year; they are found in deer (members of the family [[Cervidae]]). They grow from a permanent outgrowth of the frontal bone called the pedicle and can be branched, as in the [[white-tailed deer]] (''Odocoileus virginianus''), or palmate,<ref>[https://en.wiktionary.org/wiki/palmate palmate]</ref> as in the [[moose]] (''Alces alces''). ''Ossicones'' are permanent bone structures that fuse to the frontal or parietal bones during an animal's life and are found only in the [[Giraffidae]]. ''Pronghorns'', while similar to horns in that they have keratinous sheaths covering permanent bone cores, are deciduous.{{Clarify|reason=Deciduous as in shed seasonally, or deciduous as in temporary (like baby teeth)?|date=September 2016}}<ref name="JS">{{Cite journal |last1=Janis |first1=C.M. |last2=Scott |first2=K.M. |year=1987 |title=The Interrelationships of Higher Ruminant Families with Special Emphasis on the Members of the Cervoidea |url=http://digitallibrary.amnh.org/dspace/handle/2246/5180 |journal=American Museum Novitates |issue=2893 |pages=1–85 |archive-url=https://web.archive.org/web/20141006085126/http://digitallibrary.amnh.org/dspace/handle/2246/5180 |archive-date=6 October 2014 |access-date=28 February 2016 |hdl=2246/5180}}</ref>

All these cranial appendages can serve for posturing, battling for mating privilege, and for defense. In almost all cases, they are sexually dimorphic, and are often found only on the males. One exception is the species ''Rangifer tarandus'', known as [[reindeer]] in Europe or caribou in North America, where both sexes can grow antlers yearly, though the females' antlers are typically smaller and not always present.

====Teeth====
[[File:Hirscheber1a.jpg|thumb|The [[Canine tooth|canines]] of Suina (here, a [[babirusa]] pig) develop into [[tusk]]s.|alt=A babirusa with elongated lower canines that curve up, forming tusks.]]

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|- style="background-color: #eeeeee;"
! colspan="2" style="border-bottom: solid 1px gray; width: 8em;" | [[Dental formula]]
! style="border-bottom: solid 1px gray; width: 12.5%" | [[Incisor|I]]
! style="border-bottom: solid 1px gray; width: 12.5%" | [[Canine tooth|C]]
! style="border-bottom: solid 1px gray; width: 12.5%" | [[Premolar|P]]
! style="border-bottom: solid 1px gray; width: 12.5%" | [[Molar (tooth)|M]]
|-
| rowspan="2" style="text-align: right;" | 30–44
| rowspan="2" | =
| style="border-bottom: solid 1px black; color: #ffaa00;" | 0–3
| style="border-bottom: solid 1px black; color: #00dd00;" | 0–1
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| style="border-bottom: solid 1px black; color: #0000ff;" | 3
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| style="color: #ffaa00;" | 1–3
| style="color: #00dd00;" | 1
| style="color: #dd00dd;" | 2–4
| style="color: #0000ff;" | 3
|}

There are two trends in terms of teeth within Artiodactyla. The Suina and hippopotamuses have a relatively large number of teeth (with some pigs having 44); their dentition is more adapted to a squeezing [[mastication]], which is characteristic of [[omnivore]]s. Camels and ruminants have fewer teeth; there is often a yawning [[diastema]], a designated gap in the teeth where the molars are aligned for crushing plant matter.

The [[incisor]]s are often reduced in ruminants, and are completely absent in the [[upper jaw]]. The canines are enlarged and tusk-like in the Suina, and are used for digging in the ground and for defense. In ruminants, the males' upper canines are enlarged and used as a weapon in certain species (mouse deer, musk deer, [[water deer]]); species with frontal weapons are usually missing the upper canines. The lower canines of ruminants resemble the incisors, so that these animals have eight uniform teeth in the frontal part of the [[lower jaw]].

The molars of porcine have only a few bumps. In contrast, camels and ruminants have bumps that are crescent-shaped cusps ([[selenodont]]).

====Senses====
Artiodactyls have a well-developed sense of smell and sense of hearing. Unlike many other mammals, they have a poor sense of sight, and find moving objects much easier to see than stationary ones. Similar to many other prey animals, their eyes are on the sides of the head, giving them an almost panoramic view.

===Digestive system===
[[File:Warthog Face 001.jpg|thumb|alt=A warthog.|Pigs, such as this [[common warthog]], have a simple sack-shaped stomach.]]
The [[ruminant]]s (Ruminantia) ruminate their food, regurgitating and re-chewing it. Ruminants' mouths often have additional [[salivary gland]]s, and the [[oral mucosa]] is often heavily calloused to avoid injury from hard plant parts and to allow easier transport of roughly chewed food. Their stomachs are divided into three to four sections: the [[rumen]], the [[reticulum (anatomy)|reticulum]], the [[omasum]], and the [[abomasum]].<ref name="EoM">{{Cite book |last1=Janis, C. |url=https://archive.org/details/encyclopediaofma00mals_0/page/498 |title=The Encyclopedia of Mammals |last2=Jarman, P. |publisher=Facts on File |year=1984 |isbn=978-0-87196-871-5 |editor-last=Macdonald, D. |location=New York |pages=[https://archive.org/details/encyclopediaofma00mals_0/page/498 498–499] |url-access=registration}}</ref> After the food is ingested, it is mixed with saliva in the [[rumen]] and [[reticulum (anatomy)|reticulum]] and separates into layers of solid versus liquid material. The solids lump together to form a [[bolus (digestion)|bolus]] (also known as the [[cud]]); this is regurgitated by reticular contractions while the [[glottis]] is closed. When the bolus enters the mouth, the fluid is squeezed out with the tongue and re-swallowed. The bolus is chewed slowly to completely mix it with saliva and to break it down. Ingested food passes to the "fermentation chamber" (rumen and reticulum), where it is kept in continual motion by rhythmic contractions. Cellulytic [[microbe]]s ([[bacteria]], [[protozoa]], and [[fungi]]) produce [[cellulase]], which is needed to break down the cellulose found in plant material.<ref name="EoM" /> This form of digestion has two advantages: plants that are indigestible to other species can be digested and used, and the duration of the actual food consumption shortened; the animal spends only a short time out in the open with its head to the ground; rumination can take place later, in a sheltered area.<ref name="Shively1985">{{Cite journal |last=Shively, C. L. |display-authors=etal |year=1985 |title=Some Aspects of the Nutritional Biology of the Collared Peccary |journal=The Journal of Wildlife Management |volume=49 |issue=3 |pages=729–732 |doi=10.2307/3801702 |jstor=3801702}}</ref>
[[File:Columbian White-tailed Deer (52574194795).jpg|alt=The face of a deer with antlers|thumb|As with all [[ruminant]]s, [[deer]] have such a multi-chambered stomach, which is used for better digestion of plant food.]]
[[Tylopoda]] (camels, llamas, and alpacas) and [[chevrotain]]s have three-chambered stomachs, while the rest of [[Ruminantia]] have four-chambered stomachs. The handicap of a heavy digestive system has increased selective pressure towards limbs that allow the animal to quickly escape predators.<ref name="britannica" /> Most species within [[Suina]] have a simple two-chambered stomach that allows for an [[omnivorous]] diet. The [[babirusa]], however, is a herbivore,<ref name="EoM" /> and has extra [[maxillary teeth]] to allow for proper mastication of plant material. Most of the fermentation occurs with the help of cellulolytic microorganisms within the [[caecum]] of the [[large intestine]]. [[Peccaries]] have a complex stomach that contains four compartments.<ref name="Shively1985" /> Their fore stomach has fermentation carried out by microbes and has high levels of [[volatile fatty acid]]; it has been proposed that their complex fore-stomach is a means to slow digestive passage and increase digestive efficiency.<ref name="Shively1985" /> Hippopotamuses have three-chambered stomachs and do not ruminate. They consume around {{Convert|68|kg}} of grass and other plant matter each night. They may cover distances up to {{Convert|20|mi|km|sp=us|order=flip}} to obtain food, which they digest with the help of microbes that produce cellulase. Their closest living relatives, the [[whale]]s, are [[obligate carnivore]]s.

Unlike other even-toed ungulates, pigs have a simple sack-shaped [[stomach]].<ref name="EoM" /> Some artiodactyls, such as [[white-tailed deer]], lack a [[gall bladder]].<ref>{{Cite book |last=Hewitt |first=David G |url=https://books.google.com/books?id=k4DOBQAAQBAJ&q=artiodactyla+gall+bladder&pg=PA5 |title=Biology and Management of White-tailed Deer |date=2011-06-24 |publisher=CRC Press |isbn=978-1-4822-9598-6 |access-date=17 October 2020 |archive-date=21 June 2024 |archive-url=https://web.archive.org/web/20240621034332/https://books.google.com/books?id=k4DOBQAAQBAJ&q=artiodactyla+gall+bladder&pg=PA5#v=onepage&q=artiodactyla%20gall%20bladder&f=false |url-status=live }}</ref>

===Genitourinary system===
{{multiple image|perrow=2|total_width=230px|footer=[[Genitourinary system]] of a [[bull]]|
image1=Animal castration - a book for the use of students and practitioners (1914) (14597296668).jpg
|image2=The comparative anatomy of the domesticated animals (1908) (20676871591).jpg
}}
The penises of even-toed ungulates have an S-shape at rest and lie in a pocket under the skin on the belly.<ref>{{Cite book |last=Lombardi |first=Julian |url=https://books.google.com/books?id=tXvjBwAAQBAJ&pg=PA102 |title=Comparative Vertebrate Reproduction |date=2012-12-06 |publisher=Springer Science & Business Media |isbn=978-1-4615-4937-6 |language=en}}</ref> The [[corpus cavernosum penis|corpora cavernosa]] are only slightly developed; and an [[erection]] mainly causes this curvature to extend, which leads to an extension, but not a thickening, of the [[penis]]. Cetaceans have similar penises.<ref name="Gille" /> In some even-toed ungulates, the penis contains a structure called the ''urethral process''<ref>Spinage, C. A. "[http://www.reproduction-online.org/content/18/3/445.full.pdf Reproduction in the Uganda defassa waterbuck, Kobus defassa ugandae Neumann]". Journal of reproduction and fertility 18.3 (1969): 445–457.</ref><ref>Yong, Hwan-Yul. "[http://society.kisti.re.kr/sv/SV_svpsbs03V.do?method=download&cn1=JAKO200912763358599 Reproductive System of Giraffe (Giraffa camelopardalis).] {{Webarchive|url=https://web.archive.org/web/20190425142112/http://society.kisti.re.kr/sv/SV_svpsbs03V.do?method=download&cn1=JAKO200912763358599 |date=25 April 2019 }}" Journal of Embryo Transfer 24.4 (2009): 293–295.</ref><ref>Sumar, Julio. "[https://books.google.com/books?id=jA_LBAAAQBAJ&dq=%22Reproductive+physiology+in+South+American%22&pg=PA81 Reproductive physiology in South American Camelids] {{Webarchive|url=https://web.archive.org/web/20230330184147/https://books.google.com/books?id=jA_LBAAAQBAJ&dq=%22Reproductive%20physiology%20in%20South%20American%22&pg=PA81 |date=30 March 2023 }}". Genetics of Reproduction in Sheep (2013): 81.</ref> or ''penile vermiform appendix''.<ref>{{Cite book |last=Lombardi |first=Julian |url=https://www.google.com/books/edition/Comparative_Vertebrate_Reproduction/tXvjBwAAQBAJ?hl=en&gbpv=1&pg=PA104&printsec=frontcover |title=Comparative Vertebrate Reproduction |date=2012-12-06 |publisher=Springer Science & Business Media |isbn=978-1-4615-4937-6 |language=en}}</ref>

The [[testicle]]s are located in the [[scrotum]] and thus outside the [[abdominal cavity]]. The [[ovaries]] of many females descend as the testicles descend of many male mammals, and are close to the [[pelvic inlet]] at the level of the fourth [[lumbar vertebra]]. The [[uterus]] has two horns ([[Uterine horns|uterus bicornis]]).<ref name="Gille">{{Cite book |last=Uwe Gille |title=urinary and sexual apparatus, urogenital Apparatus. In: F.-V. Salomon and others (eds.): Anatomy for veterinary medicine |year=2008 |isbn=978-3-8304-1075-1 |pages=368–403}}</ref>

===Other===
[[File:Lightmatter japanese serows.jpg|thumb|The [[Japanese serow]] has glands in the eyes that are clearly visible.|alt=Two Japanese serows (goat-antelopes) sit together.]]

The number of [[mammary gland]]s is variable and correlates, as in all mammals, with [[litter size]]. Pigs, which have the largest litter size of all even-toed ungulates, have two rows of [[teat]]s lined from the armpit to the groin area. In most cases, however, even-toed ungulates have only one or two pairs of teats. In some species these form an [[udder]] in the groin region.

[[Exocrine gland|Secretory glands]] in the skin are present in virtually all species and can be located in different places, such as in the eyes, behind the horns, the neck, or back, on the feet, or in the anal region.

Artiodactyls have a [[carotid rete]] heat exchange that enables them, unlike perissodactyls which lack one, to regulate their brain temperature independently of their bodies. It has been argued that its presence explains the greater success of artiodactyls compared to perissodactyls in being able to adapt to diverse environments from the Arctic Circle to deserts and tropical savannahs.<ref name="Mitchell Lust 2008 pp. 415–418">{{cite journal |last1=Mitchell |first1=G |last2=Lust |first2=A |date=2008-08-23 |title=The carotid rete and artiodactyl success |journal=Biology Letters |volume=4 |issue=4 |pages=415–418 |doi=10.1098/rsbl.2008.0138 |issn=1744-9561 |pmc=2610139 |pmid=18426746}}</ref>

==Lifestyle==

=== Distribution and habitat ===
Artiodactyls are native to almost all parts of the world, with the exception of [[Oceania]] and [[Antarctica]]. Humans have introduced different artiodactyls worldwide as hunting animals.<ref name="Pough2005">{{Cite book |last1=Pough |first1=F. W. |title=Vertebrate Life |last2=Janis |first2=C. M. |last3=Heiser |first3=J. B. |publisher=Pearson |year=2005 |isbn=978-0-13-127836-3 |edition=7th |page=539 |chapter=Major Lineages of Mammals |orig-date=1979}}</ref> Artiodactyls inhabit almost every habitat, from [[tropical rainforest]]s and [[steppe]]s to [[desert]]s and high [[Mountain range|mountain regions]]. The greatest [[biodiversity]] prevails in open habitats such as [[grassland]]s and open forests.

===Social behavior===
[[File:Impalas and Giraffes Benh.jpg|thumb|Artiodactyls, like [[impala]]s and [[giraffe]]s, live in groups.|alt=Two giraffes stand, surrounded by impalas (a type of antelope).]]

The social behavior of even-toed ungulates varies from species to species. Generally, there is a tendency to merge into larger groups, but some live alone or in pairs. Species living in groups often have a [[Social hierarchy|hierarchy]], both among males and females. Some species also live in [[Harem (zoology)|harem groups]], with one male, several females, and their common offspring. In other species, the females and juveniles stay together, while males are solitary or live in bachelor groups and seek out females only during [[mating season]].

Many artiodactyls are [[Territory (animal)|territorial]] and mark their territory, for example, with [[Territory (animal)#Scent marking|glandular secretions or urine]]. In addition to year-round sedentary species, there are animals that [[Animal migration|migrate]] seasonally.

There are [[diurnality|diurnal]], [[crepuscular]], and [[nocturnal]] artiodactyls. Some species' pattern of wakefulness varies with season or habitat.

===Reproduction and life expectancy===
Generally, even-toed ungulates tend to have long [[gestation period]]s, smaller litter sizes, and more highly developed newborns. As with many other mammals, species in [[temperate]] or [[Polar climate|polar]] regions have a fixed [[mating season]], while those in [[tropical]] areas breed year-round. They carry out [[Polygyny in animals|polygynous]] mating behavior, meaning a male mates with several females and suppresses all competition.
[[File:Blue-common wildebeast calf (5985600718).jpg|alt=Blue wildebeest calf|left|thumb|Most artiodactyls, such as the [[blue wildebeest]], are born with hair.]]
The length of the gestation period varies from four to five months for pigs, deer, and musk deer; six to ten months for hippos, deer, and bovines; ten to thirteen months with camels; and fourteen to fifteen months with giraffes. Most deliver one or two babies, but some pigs can deliver up to ten.

The newborns are [[precocial]] (born relatively mature) and come with open eyes and are hairy (with the exception of the hairless hippos). Juvenile deer and pigs have striped or spotted coats; the pattern disappears as they grow older. The juveniles of some species spend their first weeks with their mother in a safe location, where others may be running and following the herd within a few hours or days.

Life expectancy is typically twenty to thirty years; as in many mammals, smaller species often have a shorter lifespan than larger species. The artiodactyls with the longest lifespans are the hippos, cows, and camels, which can live 40 to 50 years.

===Predators and parasites===
Artiodactyls have different [[natural predator]]s depending on their size and habitat. There are several [[carnivore]]s that prey on them, including [[large cat]]s ([[lion]]s, [[tiger]]s, and relatives) and [[bear]]s. Other predators are [[crocodile]]s, [[wolves]] and [[dog]]s, large [[raptor (bird)|raptors]], and for small species and young animals, large [[snake]]s. For cetaceans, possible predators include sharks, polar bears, and other cetaceans; in the latter is the orca, the top predator of the oceans.<ref>{{Cite web |date=3 August 2021 |title=Killer Whale |url=https://www.fisheries.noaa.gov/species/killer-whale |access-date=26 August 2021 |publisher=NOAA Fisheries |archive-date=26 August 2021 |archive-url=https://web.archive.org/web/20210826012616/https://www.fisheries.noaa.gov/species/killer-whale |url-status=live }}</ref>

[[Parasites]] include [[nematode]]s, [[botflies]], [[flea]]s, [[lice]], or [[fluke (parasite)|flukes]], but they have debilitating effects only when the infestation is severe.{{citation needed|date=May 2021}}

==Interactions with humans==

===Domestication===
{{See also|Domestication of animals}}
[[File:Ardiak bazkan ari dira.JPG|thumb|Some artiodactyls, like [[sheep]], have been domesticated for thousands of years.|alt=Sheep on a farm]]

Artiodactyls have been hunted by primitive humans for various reasons: for meat or [[Fur clothing|fur]], as well as to use their bones and teeth as weapons or tools. Their [[domestication]] began around 8000 BCE. To date, humans have domesticated [[goat]]s, [[sheep]], [[cattle]], camels, llamas, alpacas, and [[pig]]s. Initially, livestock was used primarily for food, but they began being used for work activities around 3000 BCE.<ref name="britannica">{{Cite encyclopedia |year=2008 |title=Artiodactyl |encyclopedia=Encyclopædia Britannica Online |publisher=[[Encyclopædia Britannica, Inc.]] |url=https://www.britannica.com/EBchecked/topic/37203/artiodactyl |access-date=2008-10-17 |archive-date=28 April 2015 |archive-url=https://web.archive.org/web/20150428121228/http://www.britannica.com/EBchecked/topic/37203/artiodactyl |url-status=live }}</ref> Clear evidence exists of antelope being used for food 2 million years ago in the [[Olduvai Gorge]], part of the [[Kenyan Rift Valley|Great Rift Valley]].<ref name="britannica" /><ref>{{Cite news |last=McKie |first=Robin |date=2012-09-22 |title=Humans hunted for meat 2 million years ago |work=The Guardian |url=https://www.theguardian.com/science/2012/sep/23/human-hunting-evolution-2million-years |access-date=2015-10-26}}</ref> [[Cro-Magnon]]s relied heavily on reindeer for food, skins, tools, and weapons; with dropping temperatures and increased reindeer numbers at the end of the Pleistocene, they became the prey of choice. Reindeer remains accounted for 94% of bones and teeth found in a cave above the river [[Céou]] that was inhabited around 12,500 years ago.<ref>{{Cite web |year=2003 |title=Bones From French Cave Show Neanderthals, Cro-Magnon Hunted Same Prey |url=https://www.sciencedaily.com/releases/2003/09/030923065212.htm |access-date=17 October 2008 |publisher=ScienceDaily |archive-date=22 October 2008 |archive-url=https://web.archive.org/web/20081022110102/http://www.sciencedaily.com/releases/2003/09/030923065212.htm |url-status=live }}</ref> In general, most even-toed ungulates can be consumed as a [[kosher]] meat, with the principal exception of [[Suina]] (pigs, etc.) and [[hippopotamidae|hippopotamids]], which are even-toed but do not [[ruminants|chew the cud]], and of [[Cetacea]], which, for the purpose of [[Halakha|rabbinic law]], are considered to be scaleless fish, and thus not kosher.

Today, artiodactyls are kept primarily for their meat, milk, and [[wool]], fur, or [[hide (skin)|hide]] for clothing. Domestic cattle, the [[water buffalo]], the [[yak]], and camels are used for work, as rides, or as [[pack animal]]s.<ref>{{Cite book |last=Clay, J. |title=World Agriculture and the Environment: A Commodity-by-Commodity Guide to Impacts and Practices |publisher=Island Press |year=2004 |isbn=978-1-55963-370-3 |location=Washington, D.C., US}}</ref>{{Page needed|date=September 2016}}

===Threats===
[[File:Ur-painting.jpg|thumb|The [[aurochs]] has been extinct since the 17th century.|alt=Painting of an aurochs]]

The endangerment level of each even-toed ungulate is different. Some species are [[synanthropic]] (such as the [[wild boar]]) and have spread into areas that they are not indigenous to, either having been brought in as farm animals or having run away as people's pets. Some artiodactyls also benefit from the fact that their predators (e.g., the [[Tasmanian tiger]]) were severely decimated by ranchers, who saw them as competition.<ref name="Pough2005" />

Conversely, many artiodactyls have declined significantly in numbers, and some have even gone extinct, largely due to [[over-hunting]], and, more recently, [[habitat destruction]]. Extinct species include several gazelles, the [[aurochs]], the [[Malagasy hippopotamus]], the [[bluebuck]], and [[Schomburgk's deer]]. Two species, the [[scimitar-horned oryx]] and [[Père David's deer]], are [[extinct in the wild]]. 14 species are considered [[critically endangered]], including the [[addax]], the [[kouprey]], the [[wild Bactrian camel]], [[Przewalski's gazelle]], the [[saiga]], and the [[pygmy hog]]. 24 species are considered [[endangered]].<ref>{{Cite web |title=Cetartiodactyla |url=http://www.iucnredlist.org/search |access-date=12 March 2007 |archive-date=5 August 2011 |archive-url=https://web.archive.org/web/20110805065610/http://www.iucnredlist.org/apps/redlist/search |url-status=live }}</ref><ref>{{Cite web |title=Artiodactyla |url=http://eol.org/pages/7678/details |access-date=15 November 2014 |publisher=Encyclopedia of Life |archive-date=13 October 2014 |archive-url=https://web.archive.org/web/20141013110619/http://eol.org/pages/7678/details |url-status=live }}</ref>

== See also ==
* {{Portal-inline|Mammals}}
{{Clear}}

==References==
{{reflist|25em}}

==External links==
{{Commons category|Artiodactyla}}
{{Wikispecies|Artiodactyla}}
{{wiktionary | artiodactyl}}
* [http://www.ultimateungulate.com/NewTaxonomy.html Ungulate Taxonomy: A new perspective from Groves and Grubb (2011)]

{{Mammals}}
{{Artiodactyla}}

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{{DEFAULTSORT:Even-Toed Ungulate}}
[[Category:Artiodactyls| ]]
[[Category:Extant Ypresian first appearances]]