Relationships
Perissodactyla and Artiodactyla comprise the largest portion of ungulates, and also include 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 indicate that they are descended from early artiodactyls, and thus are directly related to other even-toed ungulates such as cattle, with hippopotamuses being their closest living relatives. As a result of these discoveries, the new order Cetartiodactyla has been proposed to include the members of Artiodactyla and Cetacea, to reflect their common ancestry; however, strictly speaking, this is merely a matter of nomenclature, since it is possible simply to recognize Cetacea as a subgroup of Artiodactyla.
Hyracoidea, Sirenia and Proboscidea comprise Paenungulata. Aardvarks are also thought to be ungulates. Some recent studies link Tubulidentata with Paenungulata in Pseudoungulata. Macroscelideans have been interpreted as pseudoungulates as well, based on dental as well as genetic evidence. Genetic studies indicate that these animals are not closely related to artiodactyls and perissodactyls; their closest relatives are afrosoricidans. Pseudungulata, Macroscelidea and Afrosoricida together make up Afrotheria.
Ungulate groups represented in the fossil record include afrotherian embrithopods and demostylians, artiodactyl-related 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. Some completely lack upper incisors and instead have a dental pad to assist in browsing.
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 (mesonychids and whales).
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