Ungulates are members of the diverse clade Euungulata, which primarily consists of large mammals with hooves. Once part of the taxon "Ungulata" along with paenungulates and tubulidentates, as well as several extinct taxa, "Ungulata" has since been determined to be a polyphyletic grouping based on molecular data. As a result, true ungulates have since been reclassified to the newer clade Euungulata in 2001 within the clade Laurasiatheria, while Paenungulata and Tubulidentata have been reclassified to the distant clade Afrotheria. Alternatively, some authors use the name Ungulata to designate the same clade as Euungulata.
Living ungulates are divided into two orders: Odd-toed Perissodactyla including equines, rhinoceroses, and tapirs. Even-toed Artiodactyla including cattle, antelope, pigs, giraffes, camels, sheep, deer, and hippopotamuses, among others.
Cetaceans such as whales, dolphins, and porpoises are also classified as artiodactyls, although they do not have hooves.
Most terrestrial ungulates use the hoofed tips of their toes to support their body weight while standing or moving. Two other orders of ungulates, Notoungulata and Litopterna, both native to South America, became extinct at the end of the Pleistocene, around 12,000 years ago.
The term means, roughly, "being hoofed" or "hoofed animal". As a descriptive term, "ungulate" normally excludes cetaceans as they do not possess most of the typical morphological characteristics of other ungulates, but they were also descended from early artiodactyls. Ungulates are typically herbivorous and many employ specialized gut bacteria to enable them to digest cellulose, though some members may deviate from this: several species of pigs and the extinct entelodonts are omnivorous, while cetaceans are carnivorous.
Etymology Ungulate is from the Late Latin adjective , from , a diminutive of (finger nail; toe nail).
Classifications History Euungulata is a clade (or in some taxonomies, a grand order) of mammals. The two extant orders of ungulates are the Perissodactyla (odd-toed ungulates) and Artiodactyla (even-toed ungulates). Hyracoidea (hyraxes), Sirenia (sea cows, dugongs and manatees), Proboscidea (elephants) and Tubulidentata (aardvarks) were in the past grouped within the clade "Ungulata", later found to be a polyphyletic and invalid. The three orders of Paenungulata are considered a clade and grouped in the Afrotheria clade, along with Tubulidentata, while Euungulata is grouped under the Laurasiatheria clade.
In 2009, morphological and molecular work found that aardvarks, hyraxes, sea cows, and elephants were more closely related to each other and to sengis, tenrecs, and golden moles than to the perissodactyls and artiodactyls, and form the clade Afrotheria. Elephants, sea cows, and hyraxes were grouped together in the clade Paenungulata, while the aardvark has been considered as either a close relative to them or a close relative to sengis in the clade Afroinsectiphilia. This is a striking example of convergent evolution.
There is some dispute as to whether this smaller Euungulata is a cladistic (evolution-based) group, or merely a phenetic group (form taxon) or folk taxon (similar, but not necessarily related). Some studies have indeed found the mesaxonian ungulates and paraxonian ungulates to form a monophyletic lineage, closely related to either the Ferae (the carnivorans and the pangolins) in the clade Fereuungulata or to the bats. Other studies found the two orders not that closely related, as some place the perissodactyls as close relatives to bats and Ferae in Pegasoferae and others place the artiodactyls as close relatives to bats.
Taxonomy
Below is a simplified taxonomy (assuming that ungulates do indeed form a natural grouping) with the extant families, in order of the relationships. Keep in mind that there were still some grey areas of conflict, such as the case with the relationship between the pecoran families and the baleen whale families. See each family for the relationships of the species as well as the controversies in their respective articles. Euungulata Perissodactyla (Mesaxonian ungulates) Hippomorpha Equidae: Horses, asses and zebras Ceratomorpha Tapiridae: Tapirs Rhinocerotidae: Rhinoceroses Artiodactyla (= Cetartiodactyla) (Paraxonian ungulates) Tylopoda Camelidae: Camels and llamas Artiofabula Suina Tayassuidae: Peccaries Suidae: Pigs Cetruminantia Ruminantia Tragulidae: Chevrotains Cervoidea Antilocapridae: Pronghorn Giraffidae: Giraffes and okapi Cervidae: Deer Moschidae: Musk deer Bovidae: Cattle and antelopes Whippomorpha Hippopotamidae: Hippopotamuses Cetacea Mysticeti Balaenidae: Bowhead and right whales Cetotheriidae: Pygmy right whale Balaenopteridae: Rorquals Odontoceti Physeteroidea Physeteridae: Sperm whale Kogiidae: Lesser sperm whales Platanistoidea Platanistidae: Indian river dolphins Ziphioidea Ziphiidae: Beaked whales Lipotoidea Lipotidae: Baiji (functionally extinct) Inioidea Iniidae: Amazonian river dolphins Pontoporiidae: La Plata dolphin Delphinoidea Monodontidae: Beluga and narwhal Phocoenidae: Porpoises Delphinidae: Oceanic dolphins
Phylogeny Below is the general consensus of the phylogeny of the ungulate families.
Evolutionary history
Perissodactyla and Artiodactyla include the majority of large land mammals. These two groups first appeared during the late Paleocene, rapidly spreading to a wide variety of species on numerous continents, and have developed in parallel since that time. Some scientists believed that modern ungulates were descended from an evolutionary grade of mammals known as the condylarths. The earliest known member of this group may have been the tiny Protungulatum, a mammal that co-existed with the last of non-avian dinosaurs 66 million years ago.Jehle, Martin "Condylarths: Archaic hoofed mammals" in Paleocene mammals of the world However, many authorities do not consider it a true placental, let alone an ungulate. The enigmatic dinoceratans were among the first large herbivorous mammals, although their exact relationship with other mammals is still debated with one of the theories being that they might just be distant relatives to living ungulates; the most recent study recovers them as within the true ungulate assemblage, closest to Carodnia. Conference abstract (p. 99) . Explanation and conclusions: .
In Australia, the recently extinct marsupial Chaeropus ("pig-footed bandicoot") also developed hooves similar to those of artiodactyls, an example of convergent evolution.
Perissodactyl evolution
Perissodactyls were thought to have evolved from the Phenacodontidae, small, sheep-sized animals that were already showing signs of anatomical features that their descendants would inherit (the reduction of digit I and V for example). By the start of the Eocene, 55 million years ago (Mya), they had diversified and spread out to occupy several continents. Horses and tapirs both evolved in North America; rhinoceroses appear to have developed in Asia from tapir-like animals and then colonised the Americas during the middle Eocene (about 45 Mya). Of the approximately 15 families, only three survive (McKenna and Bell, 1997; Hooker, 2005). These families were very diverse in form and size; they included the enormous brontotheres and the bizarre chalicotheres. The largest perissodactyl, an Asian rhinoceros called Paraceratherium, reached , more than twice the weight of an elephant.
It has been found in a cladistic study that the anthracobunids and the desmostylians – two lineages that have been previously classified as Afrotherians (more specifically closer to elephants) – have been classified as a clade that is closely related to the perissodactyls. The desmostylians were large amphibious quadrupeds with massive limbs and a short tail. They grew to in length and were thought to have weighed more than . Their fossils were known from the northern Pacific Rim, from southern Japan through Russia, the Aleutian Islands and the Pacific coast of North America to the southern tip of Baja California. Their dental and skeletal form suggests desmostylians were aquatic herbivores dependent on littoral habitats. Their name refers to their highly distinctive molars, in which each cusp was modified into hollow columns, so that a typical molar would have resembled a cluster of pipes, or in the case of worn molars, volcanoes. They were the only marine mammals to have gone extinct.
The South American meridiungulates contain the somewhat tapir-like pyrotheres and astrapotheres, the mesaxonic litopterns and the diverse notoungulates. As a whole, meridiungulates were said to have evolved from animals like Hyopsodus. For a while their relationships with other ungulates were a mystery. Some paleontologists have even challenged the monophyly of Meridiungulata by suggesting that the pyrotheres may be more closely related to other mammals, such as Embrithopoda (an African order that were related to elephants) than to other South American ungulates. A recent study based on bone collagen has found that at least litopterns and the notoungulates were closely related to the perissodactyls.
The oldest known fossils assigned to Equidae date from the early Eocene, 54 million years ago. They had been assigned to the genus Hyracotherium, but the type species of that genus is now considered not a member of this family, but the other species have been split off into different genera. These early Equidae were fox-sized animals with three toes on the hind feet, and four on the front feet. They were herbivorous browsers on relatively soft plants, and were already adapted for running. The complexity of their brains suggest that they already were alert and intelligent animals. Later species reduced the number of toes, and developed teeth more suited for grinding up grass and other tough plant food.
Rhinocerotoids diverged from other perissodactyls by the early Eocene. Fossils of Hyrachyus eximus found in North America date to this period. This small hornless ancestor resembled a tapir or small horse more than a rhino. Three families, sometimes grouped together as the superfamily Rhinocerotoidea, evolved in the late Eocene: Hyracodontidae, Amynodontidae and Rhinocerotidae, thus creating an explosion of diversity unmatched for a while until environmental changes drastically eliminated several species.
The first tapirids, such as Heptodon, appeared in the early Eocene.Ballenger, L. and Myers, P. (2001). "Family Tapiridae", Animal Diversity Web. Retrieved November 22, 2007. They appeared very similar to modern forms, but were about half the size, and lacked the proboscis. The first true tapirs appeared in the Oligocene. By the Miocene, such genera as Miotapirus were almost indistinguishable from the extant species. Asian and American tapirs were believed to have diverged around 20 to 30 million years ago; and tapirs migrated from North America to South America around 3 million years ago, as part of the Great American Interchange.
Perissodactyls were the dominant group of large terrestrial browsers right through the Oligocene. However, the rise of grasses in the Miocene (about 20 Mya) saw a major change: the artiodactyl species with their more complex stomachs were better able to adapt to a coarse, low-nutrition diet, and soon rose to prominence. Nevertheless, many perissodactyl species survived and prospered until the late Pleistocene (about 10,000 years ago) when they faced the pressure of human hunting and habitat change.
Artiodactyl evolution