Ungulates

Ungulates ( ) are members of the diverse clade Ungulata which primarily consists of large mammals with hooves. These include odd-toed ungulates such as horses, rhinoceroses, and tapirs; and even-toed ungulates such as cattle, pigs, giraffes, camels, sheep, deer, and hippopotamuses. Cetaceans such as whales, dolphins, and porpoises are also classified as even-toed ungulates, 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.

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 recent discoveries indicate that they were also descended from early artiodactyls. Ungulates are typically herbivorous and many employ specialized gut bacteria to allow them to digest cellulose. Some modern species, such as pigs, are omnivorous, while some prehistoric species, such as mesonychians, were carnivorous.

Etymology

Ungulate is from the Late Latin adjective ''ungulatus'', "hoofed". ''Ungulatus'' is a diminutive form of Latin ''unguis'', "nail" (finger nail; toe nail).

Classifications

History

Ungulata is a clade (or in some taxonomies, a grand order) of mammals. The two orders of ungulates were the Perissodactyla (odd-toed ungulates) and Artiodactyla (even-toed ungulates). Hyracoidea (hyraxes), Sirenia (sea cows) (dugongs and manatees) and Proboscidea (elephants) were in the past included in a superorder called Paenungulata which was grouped with the ungulata. These three orders were now considered a clade and grouped in the Afrotheria clade while Ungulata is now 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 now some dispute as to whether this smaller Ungulata 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 relationship of 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 article.

* Ungulata (= 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 the group was the tiny ''Protungulatum'', an ungulate that co-existed with the last of non-avian dinosaurs 66 million years ago; 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 marsupial ''Chaeropus'' 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 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 grasses 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. 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

The artiodactyls were thought to have evolved from a small group of condylarths, Arctocyonidae, which were unspecialized, superficially raccoon-like to bear-like omnivores from the Early Paleocene
(about 65 to 60 million years ago). They had relatively short limbs lacking specializations associated with their relatives (e.g. reduced side digits, fused bones, and hooves),Jehle, Marti
"Condylarths: Archaic hoofed mammals"
in ''Paleocene mammals of the world'' and long, heavy tails. Their primitive anatomy makes it unlikely that they were able to run down prey, but with their powerful proportions, claws, and long canines, they may have been able to overpower smaller animals in surprise attacks. Evidently these mammals soon evolved into two separate lineages: the mesonychians and the artiodactyls.

Mesonychians were depicted as "wolves on hooves" and were the first major mammalian predators, appearing in the Paleocene.Jehle, Marti
"Carnivores, creodonts and carnivorous ungulates: Mammals become predators"
in ''Paleocene mammals of the world'' Early mesonychids had five digits on their feet, which probably rested flat on the ground during walking (plantigrade locomotion), but later mesonychids had four digits that ended in tiny hooves on all of their toes and were increasingly well adapted to running. Like running members of the even-toed ungulates, mesonychids (''Pachyaena'', for example) walked on their digits (digitigrade locomotion). Mesonychians fared very poorly at the close of the Eocene epoch, with only one genus, ''Mongolestes'', surviving into the Early Oligocene epoch, as the climate changed and fierce competition arose from the better adapted creodonts.

The first artiodactyls looked like today's chevrotains or pigs: small, short-legged creatures that ate leaves and the soft parts of plants. By the Late Eocene (46 million years ago), the three modern suborders had already developed: Suina (the pig group); Tylopoda (the camel group); and Ruminantia (the goat and cattle group). Nevertheless, artiodactyls were far from dominant at that time: the perissodactyls were much more successful and far more numerous. Artiodactyls survived in niche roles, usually occupying marginal habitats, and it is presumably at that time that they developed their complex digestive systems, which allowed them to survive on lower-grade food. While most artiodactyls were taking over the niches left behind by several extinct perissodactyls, one lineage of artiodactyls began to venture out into the seas.

Cetacean evolution

The traditional theory of cetacean evolution was that cetaceans were related to the mesonychids. These animals had unusual triangular teeth very similar to those of primitive cetaceans. This is why scientists long believed that cetaceans evolved from a form of mesonychid. Today, many scientists believe cetaceans evolved from the same stock that gave rise to hippopotamuses. This hypothesized ancestral group likely split into two branches around . One branch would evolve into cetaceans, possibly beginning about with the proto-whale ''Pakicetus'' and other early cetacean ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans. The other branch became the anthracotheres, a large family of four-legged beasts, the earliest of whom in the late Eocene would have resembled skinny hippopotamuses with comparatively small and narrow heads. All branches of the anthracotheres, except that which evolved into Hippopotamidae, became extinct during the Pliocene

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