Ungulates

Ungulates (meaning roughly "being pawed" or "hoofed animal") are several groups of mammals, most of which use the tips of their toes, usually hoofed, to sustain their whole body weight while moving. They make up several orders of mammals, of which six to eight survive. There is some dispute as to whether Ungulata are a cladistic (evolution-based) group, or merely a phenetic group or folk taxon (similar, but not necessarily related), because not all ungulates appear as closely related as once believed. Ungulata was considered an order which has been split into Perissodactyla, Artiodactyla, Tubulidentata, Hyracoidea, Sirenia, and Proboscidea. Members of the orders Perissodactyla, Artiodactyla, and Cetacea are called the 'true ungulates' to distinguish them from 'subungulates' (paenungulata) which include members from the Proboscidea, Sirenia, Hyracoidea, and Tubulidentata orders.

Commonly known examples of ungulates living today are the horse, zebra, donkey, cattle/bison, rhinoceros, camel, hippopotamus, goat, pig, sheep, giraffe, okapi, moose, deer, tapir, antelope, and gazelle.

Relationships
The Perissodactyla and Artiodactyla is 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 possess 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 hippopotami. 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; however, strictly speaking, this is not necessary, as it is possible simply to recognize Cetacea as a subgroup of Artiodactyla.

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 as well as genetic evidence supporting this interpretation. Some recent studies link Tubulidentata with the Paenungulata in the Pseudoungulata. Genetic studies indicate that these animals are not closely related to the artiodactyls and perissodactyls. Instead, the closest relatives of pseudungulates are the Afrosoricida; the Pseudungulata and Afrosoricida make up the Afrotheria.

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 have 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.

In most modern ungulates, the radius and ulna are fused along the length of the forelimb; early ungulates, such as the arctocyonids did not share this unique skeletal structure. The fusion of the radius and ulna prevents an ungulate from rotating its forelimb. Since this skeletal structure has no specific function in ungulates, it is considered to be a homologous characteristic that ungulates share with other mammals. This trait would have been passed down from a common ancestor.

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.

Recent developments
That these groups of mammals 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 orders of the extinct South-American ungulates, which arose when the continent was in isolation some time during the mid to late Paleocene, 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&mdash;assuming that the animals lumped by scientists into Condylarthra over the years are even related to one another.

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. This is 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 into so many unrelated clades.