Belongs within: Euungulata.
Contains: Bunomerycidae, Suina, Cetancodontamorpha, Ruminantiamorpha, Merycoidodontoidea, Choeropotamidae, Anoplotherioidea, Oromerycidae, Camelidae.
The Artiodactyla are a clade of mammals containing the even-toed hoofed mammals, as well as the cetaceans which are nested well within this group. Most recent studies place the cetaceans closest to the Hippopotamidae among living artiodactyls, with the two in turn often sister to the Ruminantia. Previously, the ruminants had been thought more closely related to the Camelidae due to shared characters such as stomach divided into three or four chambers and rear teeth with crescent-shaped cusps. However, modern molecular analyses mostly place camelids as the sister group to all other living artiodactyls.
The Artiodactyla have an extensive fossil record going back to the Eocene, but the affinities of many basal taxa are currently uncertain in the light of the aforementioned changes in our understanding of artiodactyl phylogeny. The Cainotheriidae are small, rabbit-sized animals known from the late Eocene to the middle Miocene of Europe that may be distantly related to the camels. The Amphimerycidae of the Eocene and Oligocene of Europe were very small artiodactyls with very elongated premolars. The Dichobuninae and Hyperdichoduninae are other groups of small artiodactyls (or possibly stem-artiodactyls) from the Eocene of Europe. The Dichobuninae have a bulbous dentition and moderately elongated snouts. The Hyperdichobuninae have molarised, crescentiform premolars and molars with a tendency to broaden the distal side of the latter. The Antiacodontidae are known from the Eocene of North America.
Where do you put your camels?
Published 5 February 2017
In any discussion of the conflicts that may exist between morphological and molecular data in phylogenetic analysis, hippos and whales are bound to come up sooner or later. The claim in the late 1990s that these are each other’s closest living relatives (and hence, that whales are nested fairly deeply within the artiodactyls, or even-toed hoofed mammals) was greeted with amazement, incredulity and more than a little skepticism. The story even caught the interest of the general public through news stories like this one, meaning that many people who are otherwise unfamiliar with the trivia of mammalian phylogeny may have picked up this detail. Since then, the whale-hippo relationship has been tested, re-tested and examined again, using every data source available. But the insertion of the whales was not the only way that molecular data mixed up the artiodactyl family tree. There was also the question of where one put the camels.
Based on anatomical data, it had previously been generally agreed that camelids (including camels and llamas) were most closely related to the ruminants, the group including such artiodactyls such as cattle, deer or giraffes. Both camelids and ruminants regurgitate cud pellets from the stomach back to the mouth in order to break their food more efficiently*, and both camelids and ruminants have a stomach divided into chambers with food only travelling to the rear section of the stomach after it has been re-chewed. They do differ in that whereas ruminants have the stomach divided into four distinct chambers, camelids only possess three; the rear two chambers (the abomasum and omasum) are not clearly differentiated in camelids. They were also united by features of the dentition, such as the presence of distinctly crescent-shaped cusps on the rear teeth. This latter feature lead the camelid+ruminant grouping to commonly be referred to as the Selenodontia (the ‘moon-teeth’).
*Yes, giraffes do chew cud. Yes, the cud does travel all the way between the stomach and the mouth each time.
However, the advent of molecular analyses cast doubt on this long-accepted arrangement. Instead of supporting the expected Selenodontia clade, molecular analyses placed camelids as the sister group to all other artiodactyls, with the ruminants instead being sister to the whales+hippos clade (with a pigs+peccaries clade the next clade out). This implied that the shared features of camelids and ruminants had arisen convergently (or else all other artiodactyls had reverted to a state considered more plesiomorphic for the group as a whole). In support for such a proposition, one might point to the ecological similarities in play. Camelids and ruminants are more specialist browsers than the non-selenodont artiodactyls, which are commonly more omnivorous (pigs and peccaries) or even carnivorous (whales).
However, even if one is willing to credit that the ‘selenodont’ characters may have been the result of similar dietary pressures, one must also consider the issue that there are a number of fossil artiodactyl groups with selenodont or quasi-selenodont features. Examples of these include the Protoceratidae, a North American group that has commonly made some sort of appearance in popular books on fossil animals due to the weird home arrangements of some species, and the semi-bipedal Anoplotherium. In an influential morphological study of artiodactyl relationships, Gentry & Hooker (1988) referred to the possibility that some of these groups might be members of the selenodont stem, sitting outside the exclusive camelid+ruminant clade. Obviously, if selenodonts were not monophyletic, fossil ‘selenodonts’ might be aligned to either camelids or ruminants, but they couldn’t be connected to both. Most studies that posited selenodont polyphyly, however, looked at living taxa only and did not consider extinct groups.
The most detailed study that I’ve found so far that considers the relationship between data from fossil taxa and from molecular sources in artiodactyl phylogeny is that published by Spaulding et al. (2009). This combined analysis of both morphological and molecular data produced results that were largely concordant with the latter, generally supporting placement of camelids as the sister group to all other artiodactyls (it’s worth noting, mind you, that the size of the molecular data set used was considerably larger than that for the morphology, and an analysis of their morphological data only resulted in selenodont monophyly). The various ‘proto-selenodonts’ were scattered to the stems of various Recent clades. Protoceratids, for instance, were associated with ruminants rather than with camelids*. There are still a number of groups that remain yet to be analysed, but they’ve made a start.
*Another result of their analysis that is not directly relevant to the selenodont question, but cannot go unremarked upon, is that their tree indicates that Andrewsarchus, a lead contender for the title of largest terrestrial mammalian carnivore ever, might some sort of giant entelodont. I don’t know how much I should read into this—not all of Andrewsarchus‘ potential relatives were included in Spaulding et al.‘s analysis—but that’s the sort of result that one just wants to be true.
Systematics of Artiodactyla
Synapomorphies (from Spaulding et al. 2009, for Artiodactylamorpha): Sustentacular facet wide; contact of distal astragalus with cuboid present; ectal facet primary orientation lateral; astragalus with lateral edges of proximal and distal trochlea aligned; astragalar head arc to dorsoplantar wide, ~200 degrees; astragalar neck as wide as tibial trochlea.
Artiodactyla (see below for synonymy)SOG09
|--+--SuinaSOG09
| `--Cetruminantia [Cetruminantiamorpha]SOG09
| |--CetancodontamorphaSOG09
| `--RuminantiamorphaSOG09
`--CamelidamorphaSOG09
|--MerycoidodontoideaSOG09
`--+--ChoeropotamidaeGT09
`--+--+--AnoplotherioideaGT09
| `--AmphimerycidaeSM93
| |--Amphimeryx Pomel 1848GT09, SM93
| `--Pseudamphimeryx Stehlin 1910SM93
`--+--+--OromerycidaeG01
| `--CamelidaeTC07
`--CainotheriidaeGH88
|--Oxacron Filhol 1884SM93
|--Plesiomeryx Gervais 1873SM93
|--Paroxacron Hurzeler 1936SM93
`--Cainotherium Bravard 1828SOG09, SM93
|--C. communeD07
|--C. geoffroyiD07
`--C. laticurvatumD07
Artiodactyla incertae sedis:
Heptacodon Marsh 1894H96
`--H. pellionis Storer 1983H96
SiamotragulusCJ03
WasatchiaGH88
ChorlakkiaGH88
TexodonGH88
ArretotheriumMS56
|--A. acridens Douglass 1901MS56
|--A. fricki Macdonald & Schultz 1956MS56
`--A. leptodus (Matthew 1909)DRM86
Masritherium Fourtau 1918B78
`--*M. depereti Fourtau 1918B78
Hyoboops Trouessart 1904B78
|--*H. palaeindicus (Lydekker 1883)B78
|--H. africanus (Andrews 1914) [=Merycops africanus]B78
`--H. moneyi (Fourtau 1918) [=Brachyodus moneyi]B78
AnthracothemaS68
Aumelasia Sudre 1980SL00
Protodichobune Lemoine 1878SL00
`--*P. oweni Lemoine 1878SL00
MesselobunodonGH88
DichobuninaeGH88
|--Dichobune Cuvier 1822GH88
|--Metriotherium Filhol 1882GH88
`--Meniscodon Rütimeyer 1888GH88
`--M. europaeus (Ruetimeyer 1888) [=Phenacodus europaeus]T90
AntiacodontidaeS96
|--AuxontodonS96
| |--A. pattersoniP96
| `--A. processusS96
`--Antiacodon Marsh 1872 [incl. Sarcolemur Cope 1875]C77
|--*A. venustus Marsh 1872G52
|--A. crassus Cope 1875 [=Sarcolemur crassus]C77
|--‘*Sarcolemur’ furcatusC77
`--A. pygmaeusG52 [=Sarcolemur pygmaeusC77]
HyperdichobuninaeGH88
|--Hyperdichobune Stehlin 1910GH88
`--Mouillacitherium Filhol 1882GH88
Titanotylopus Barbour & Schultz 1934D07
|--T. nebraskensisD07
`--T. spatulusD07
‘Proviverra’ americanus Scott 1892V65
Probrachyodus Xu & Chiu 1962SM93
Bakalovia Nikolov & Heissig 1985SM93
Anthracosenex Zdansky 1930SM93
Prominatherium Teller 1884SM93
Ulausuodon Hu 1963SM93
Gobiohyus Matthew & Granger 1925SM93
|--G. orientalis Matthew & Granger 1925SL00
|--G. pressidensHUG17
`--G. robustusHUG17
BunomerycidaeGH88
Mesomeryx grangeriP96
ApriculusH96
Portax pictusT99
Kemas hylocriasB41
Nomen nudum: Sarcolemur bicuspis Loomis 1907G52
Artiodactyla [Ancodonta, Anthracotheriidae, Anthracotherioidea, Bunodontia, Cameloidea, Cetartiodactyla, Choeropotamoidea, Dichobunidae, Dichobunoidea, Helohyidae, Merycotheria, Neoselenodontia, Palaeodonta, Selenodontia, Suiformes, Tylopoda, Whippomorpha]SOG09
*Type species of generic name indicated
References
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