Ungulate

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Ungulates (meaning roughly "hoofed" or "hoofed animal") make up several orders of mammals, of which six to eight survive. There is some dispute as to whether ungulate should be treated as an actual cladistic (evolution-based) group, or merely a phenetic group (similar, but not necessarily related), in light of the fact that all ungulates do not appear to be as closely related as once believed (see below). Ungulata was formerly considered an order which has been split into Perissodactyla and Artiodactyla. Members of these two orders are called the 'true ungulates' to distinguish them from 'subungulates' which include members from the Proboscidea, Sirenia, and Hyracoidea orders.<ref>Mammology: adaptation, diversity, and ecology, Feldhammer, George A. 1999, p. 312</ref> Ungulates include:

True Ungulates:

Artiodactyla: even-toed ungulates, such as cattle, sheep, goats, pigs, antelope, and many others
Cetacea: whales and dolphins
Perissodactyla: odd-toed ungulates such as horses and rhinos

Subungulates:

(Disputed:)

1 Relationships
2 Recent developments
3 See also
4 References

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Relationships

The Perissodactyla and Artiodactyla make up the largest portion of ungulates, and also comprise the majority of large land mammals. These two groups first appeared during the late Paleocene and early Eocene (about 54 million years ago), rapidly spreading to a wide variety of species on numerous continents, and have developed in parallel since that time.

Although whales and dolphins (Cetacea) do not posess most of the typical morphological characteristics of ungulates, recent discoveries have suggested that they are likely descended from early artiodactyls, and thus are directly related to other even-toed ungulates such as cattle and hippopotamuses. (As a result of these discoveries, a new order of Cetartiodactyla has also been proposed to include the members of Artiodactyla and Cetacea, to reflect their common ancestry.)

The Hyracoidea, Sirenia and Proboscidea are the Paenungulata. The Tubulidentata are also thought to be Ungulates. The Macroscelidea have been interpreted as Ungulates, and there is dental evidence supporting this interpretation. The Macroscelidea and Tubilidentata have recently been united with the Paenungulata in the Pseudungulata.

Suggestions that Cetaceans and Hyracoids are not closest to at least some other ungulates are out of favour, and so is the suggestion that the aardvark is related to South American Xenarthrans.

Ungulate groups represented in the fossil record include the Embrithopods, Demostylians, Mesonychids, Condylarths and various South American and Paleogene lineages.

In addition to hooves, most ungulates developed reduced canine teeth, bunodont molars (molars with low, rounded cusps), and an astragalus (one of the ankle bones at the end of the lower leg) with a short, robust head.

Ungulates diversified rapidly in the Eocene, but are thought to date back as far as the late Cretaceous. Most ungulates are herbivores, but a few are omnivores or even predators: the Mesonychia and whales.

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Recent developments

That these groups of mammal are most closely related to each other has occasionally been questioned on anatomical and genetic grounds. Molecular phylogenetic studies have suggested that Perissodactyla and Cetartiodactyla are closest to Carnivora and Pholidota rather than to the Pseudungulata.

The Pseudungulata are by some scientists united with the Afrosoricida in the cohort or super-order Afrotheria based on molecular and DNA analysis. This means they are not related to other ungulates.

The extinct South-American ungulates, evolved when the continent was in isolation, are united in the super-order Meridiungulata. They are by some thought to be unrelated to the other ungulates. Instead, they are united with the Afrotheria and the Xenarthra in the supercohort Atlantogenata.

The position of other extinct ungulates is unclear. Embrithopods, Desmostylians and other related groups are seen as relatives of the Paenungulata, thus members of the Afrotheria. The Condylarths are, as a result, no longer seen as the ancestors of all ungulates. Instead, it is now believed the condylarths are members of the cohort Laurasiatheria. So it seems that, of all the ungulates, only the Perissiodacyla and Artiodactyla descended from the condylarths.

As a result of all this, it seems the typical Ungulate morphology originated three times independently: in the Meridiungulata, the Afrotheria and the "true" ungulates in the Laurasiatheria. A great example of convergent evolution. This is met with scepticism by some scientists, who say there is no morphological evidence to split the ungulates up in so many unrelated clades.

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