Editing Ungulate (section)

===Artiodactyl evolution===
[[File:Arctocyon DB.jpg|thumb|''[[Arctocyon]]'', an [[Arctocyonidae|arctocyonid]]]]
The artiodactyls were thought to have evolved from a small group of condylarths, [[Arctocyonidae]], which were unspecialized, superficially raccoon-like to bear-like omnivores from the Early [[Paleocene]] (about 65 to 60&nbsp;million years ago). They had relatively short limbs lacking specializations associated with their relatives (e.g. reduced side digits, fused bones, and hooves),<ref name="Jehle-Condylarths">Jehle, Martin [http://www.paleocene-mammals.de/condylarths.htm#Phenacodontidae "Condylarths: Archaic hoofed mammals"] in ''Paleocene mammals of the world''</ref> and long, heavy tails. Their primitive anatomy makes it unlikely that they were able to run down prey, but with their powerful proportions, claws, and long canines, they may have been able to overpower smaller animals in surprise attacks.<ref name="Jehle-Condylarths"/> Evidently these mammals soon evolved into two separate lineages: the [[mesonychia]]ns and the artiodactyls.

[[File:Anoplotherium commune.jpg|thumb|left|Skeleton of ''[[Anoplotherium commune]]'', an early artiodactyl with unusual features such as a long tail]]
The first artiodactyls looked like today's [[chevrotain]]s or pigs: small, short-legged creatures that ate [[leaves]] and the soft parts of [[plants]]. By the Late Eocene (46&nbsp;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 perissodactyls were much more successful and far more numerous. Artiodactyls 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. While most artiodactyls were taking over the niches left behind by several extinct perissodactyls, one lineage of artiodactyls began to venture out into the seas.

====Cetacean evolution====
{{See also|Evolution of cetaceans}}
[[File:Ambulocetus natans.jpg|thumb|Skeleton of ''[[Ambulocetus natans]]'', a stem whale]]
The traditional theory of cetacean evolution was that cetaceans were related to the [[Mesonychia|mesonychian]]. These animals had unusual triangular teeth very similar to those of primitive cetaceans.  This is why scientists long believed that cetaceans evolved from a form of mesonychian. Today, many scientists believe cetaceans evolved from the same stock that gave rise to hippopotamuses. This hypothesized ancestral group likely split into two branches around {{mya|54|million years ago}}.<ref name="Genomes" /> One branch would [[Evolution of cetaceans|evolve into cetaceans]], possibly beginning about {{mya|52|million years ago}} with the proto-whale ''[[Pakicetus]]'' and other early cetacean ancestors collectively known as [[Archaeoceti]], which eventually underwent [[aquatic adaptation]] into the completely aquatic [[cetacea]]ns.<ref name="Cetartiodactyla">{{cite journal |last=Boisserie |first=Jean-Renaud |author2=Lihoreau, F. |author3=Brunet, M. |date=February 2005 |title= The position of Hippopotamidae within Cetartiodactyla |journal= [[Proceedings of the National Academy of Sciences]] |volume= 102 |issue= 5 |pages= 1537–1541|doi= 10.1073/pnas.0409518102 |pmc=547867|bibcode = 2005PNAS..102.1537B |pmid= 15677331 |doi-access=free }}</ref> The other branch became the [[anthracotheriidae|anthracotheres]], a large family of four-legged beasts, the earliest of whom in the late [[Eocene]] would have resembled skinny hippopotamuses with comparatively small and narrow heads. All branches of the anthracotheres, except that which evolved into [[Hippopotamidae]], became extinct during the [[Pliocene]] without leaving any descendants.<ref name="ScienceNews">{{Cite web | title = Scientists find missing link between the dolphin, whale and its closest relative, the hippo | date = 2005-01-25 | access-date = 2007-06-18 | url = http://www.sciencenewsdaily.org/story-2806.html | work = Science News Daily | url-status = dead | archive-url = https://web.archive.org/web/20070304214747/http://www.sciencenewsdaily.org/story-2806.html | archive-date = 2007-03-04 }}</ref>

The family [[Raoellidae]] is said to be the closest artiodactyl family to the cetaceans.<ref>{{cite journal |last1=Thewissen |first1=J. G. M. |year=2007 |title=Whales originated from aquatic artiodactyls in the Eocene epoch of India |journal=Nature |volume=450 |issue=7173 |pages=1190–1194 |doi=10.1038/nature06343 |url= http://repository.ias.ac.in/4642/1/316.pdf|pmid=18097400 |last2=Cooper |first2=LN |last3=Clementz |first3=MT |last4=Bajpai |first4=S|author4-link=Sunil Bajpai |last5=Tiwari |first5=BN |bibcode=2007Natur.450.1190T|s2cid=4416444 }}</ref><ref>{{cite magazine
|url=http://www.sciam.com/article.cfm?id=closest-whale-cousin
|title=Closest Whale Cousin—A Fox-Size Deer? Researchers split on closest evolutionary kin to whales and dolphins
|first=JR
|last=Minkel 
|magazine=Scientific American
|date=2007-12-19}}</ref> Consequentially, new theories in cetacean evolution hypothesize that whales and their ancestors escaped predation, not competition, by slowly adapting to the ocean.<ref name=GuardianIndohyus>{{cite news |url=https://www.theguardian.com/science/2007/dec/19/whale.deer?gusrc=rss&feed=networkfront |title=Whales may be descended from a small deer-like animal  |access-date=2007-12-21 |author= Sample, Ian|date=December 19, 2007|work=[[Guardian Unlimited]] |location=London}}</ref><ref name=TheLoom>{{cite web |url=http://scienceblogs.com/loom/2007/12/19/whales_from_so_humble_a_beginn.php |title=The Loom : Whales: From So Humble A Beginning... |access-date=2007-12-21 |author=Zimmer, Carl |author-link=Carl Zimmer |date=December 19, 2007 |publisher=[[ScienceBlogs]] |archive-date=2007-12-21 |archive-url=https://web.archive.org/web/20071221103928/http://scienceblogs.com/loom/2007/12/19/whales_from_so_humble_a_beginn.php |url-status=dead }}</ref><ref name=PharyngulaIndohyus>{{cite web |url=http://scienceblogs.com/pharyngula/2007/12/indohyus.php |title=Pharyngula: Indohyus |access-date=2007-12-21 |author=Myers, P.Z. |author-link=PZ Myers |date=December 19, 2007 |work=[[Pharyngula (blog)|Pharyngula]] |publisher=[[ScienceBlogs]] |archive-date=2007-12-20 |archive-url=https://web.archive.org/web/20071220115757/http://scienceblogs.com/pharyngula/2007/12/indohyus.php |url-status=dead }}</ref>