Ungulate

Source: Wikipedia, the free encyclopedia.
For elephants and relatives, sometimes called ungulates or subungulates, see Paenungulata.

Ungulate
Temporal range: Paleocene–present
Image from top to left with
Brazilian tapir
.
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Clade: Pan-Euungulata
Mirorder: Euungulata
Waddell et al., 2001[1]
Orders and clades
Synonyms
  • Cetungulata
    Irwin and Wilson, 1993[3]

Ungulates (

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

entelodonts are omnivorous, while cetaceans and the extinct mesonychians
are carnivorous.

Etymology

Ungulate is from the Late Latin adjective ungulatus, 'hoofed'. Ungulatus is a diminutive form of Latin unguis, 'nail' (finger nail; toe nail).[6]

Classifications

History

Euungulata is a clade (or in some taxonomies, a grand order) of mammals. The two extant orders of ungulates are the

polyphyletic and now invalid clade. The three orders of Paenungulata are now considered a clade and grouped in the Afrotheria clade, while Euungulata is now grouped under the Laurasiatheria clade.[1]

In 2009, morphological

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.[13] This is a striking example of convergent evolution.[14]

There is now some dispute as to whether this smaller Euungulata is a

Fereuungulata or to the bats.[20] Other studies found the two orders not that closely related, as some place the perissodactyls as close relatives to bats and Ferae in Pegasoferae[21] and others place the artiodactyls as close relatives to bats.[22]

Taxonomy

Plains zebra
Black rhinoceros
Père David's deer
Hippopotamus
Blue whale
Common dolphin

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.

Phylogeny

Below is the general consensus of the phylogeny of the ungulate families.[22][23]

Euungulata
Perissodactyla

Equidae

Tapiridae

Rhinocerotidae

Artiodactyla

Evolutionary history

Uintatherium anceps
, a dinoceratan
Cladogram showing relationships within Euungulata[16]

condylarths;[24] the earliest known member of the group was the tiny Protungulatum,[25] 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.[26] 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.[27]

In Australia, the marsupial Chaeropus also developed hooves similar to those of artiodactyls,[28] an example of convergent evolution.

Perissodactyl evolution

See also: Evolution of the horse
Eurohippus parvulus, a mid- to late Eocene equid of Europe (Natural History Museum, Berlin
)
tusks.[29]

Perissodactyls were thought to have evolved from the

chalicotheres. The largest perissodactyl, an Asian rhinoceros called Paraceratherium, reached 15 tonnes (17 tons), more than twice the weight of an elephant.[32]

It has been found in a cladistic study that the

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

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.[35] A recent study based on bone collagen has found that at least litopterns and the notoungulates were closely related to the perissodactyls.[36]

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.[37] 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

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.[38] 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.[39]

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

Arctocyon, an arctocyonid

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),[30] 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.[30] Evidently these mammals soon evolved into two separate lineages: the mesonychians and the artiodactyls.

Anoplotherium commune
, an early artiodactyl with unusual features such as a long tail

The first artiodactyls looked like today's

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

See also: Evolution of cetaceans
Ambulocetus natans
, a stem whale

The traditional theory of cetacean evolution was that cetaceans were related to the

aquatic adaptation into the completely aquatic cetaceans.[40] 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 without leaving any descendants.[41]

The family Raoellidae is said to be the closest artiodactyl family to the cetaceans.[42][43] Consequentially, new theories in cetacean evolution hypothesize that whales and their ancestors escaped predation, not competition, by slowly adapting to the ocean.[44][45][46]

Mesonychian evolution

Restoration of Mesonyx

Mesonychians were depicted as "wolves on hooves" and were the first major mammalian predators, appearing in the Paleocene.

creodonts
.

Characteristics

Skeleton of a horse
The anatomy of a dolphin, showing its skeleton, major organs, tail, and body shape

Ungulates were in high diversity in response to

ocean depths; grasslands to deserts and some have been domesticated by humans
.

Anatomy

Ungulates have developed specialized adaptations, especially in the areas of cranial appendages, dentition, and leg morphology including the modification of the astragalus (one of the ankle bones at the end of the lower leg) with a short, robust head.

Hooves

See also: Hoof
Cloven hooves of roe deer (Capreolus capreolus), with dewclaws

The hoof is the tip of the toe of an ungulate mammal, strengthened by a thick horny (keratin) covering. The hoof consists of a hard or rubbery sole, and a hard wall formed by a thick nail rolled around the tip of the toe. Both the sole and the edge of the hoof wall normally bear the weight of the animal. Hooves grow continuously, and are constantly worn down by use. 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.[50] 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 a homologous characteristic that ungulates share with other mammals. This trait would have been passed down from a common ancestor. While the two orders of ungulates colloquial names were based on the number of toes of their members ("odd-toed" for the perissodactyls and "even-toed" for the terrestrial artiodactyls), it is not an accurate reason they were grouped. Tapirs have four toes in the front, yet they were members of the "odd-toed" order; peccaries and modern cetaceans were members of the "even-toed" order, yet peccaries have three toes in the front and whales were an extreme example as they have flippers instead of hooves. Scientists had classified them according to the distribution of their weight to their toes.

Perissodactyls have a mesaxonic foot, meaning that the weight is distributed on the third toe on all legs thanks to the plane symmetry of their feet. There has been a reduction of toes from the common ancestor, with the classic example being horses with their single hooves. In consequence, there was an alternative name for the perissodactyls the nearly obsolete Mesaxonia. Perissodactyls were not the only lineage of mammals to have evolved this trait; the

meridiungulates
have evolved mesaxonic feet numerous times.

Terrestrial artiodactyls have a paraxonic foot, meaning that the weight is distributed on the third and the fourth toe on all legs. The majority of these mammals have cloven hooves, with two smaller ones known as the

astragalus and cuboid bone in the ankle, which were further diagnostic traits of artiodactyls.[51]

Oklahoma City, Oklahoma

In modern cetaceans, the front limbs had become

pectoral fins and the hind parts were internal and reduced. Occasionally, the genes that code for longer extremities cause a modern cetacean to develop miniature legs (known as atavism). The main method of moving is an up-and-down motion with the tail fin, called the fluke, which is used for propulsion
, while the pectoral fins together with the entire tail section provide directional control. All modern cetaceans still retain their digits despite the external appearance suggesting otherwise.

Teeth

Most ungulates have developed reduced

canine teeth and specialized molars, including bunodont (low, rounded cusps) and hypsodont (high crowned) teeth. The development of hypsodonty has been of particular interest as this adaptation was strongly associated with the spread of grasslands during the Miocene about 25 million years ago. As forest biomes declined, grasslands spread, opening new niches for mammals. Many ungulates switched from browsing diets to grazing diets, and possibly driven by abrasive silica in grass, hypsodonty became common. However, recent evidence ties the evolution of hypsodonty to open, gritty habitats and not the grass itself. This is termed the Grit, not grass hypothesis.[52]

Some ungulates completely lack upper incisors and instead have a dental pad to assist in browsing.[53][54] It can be found in camels, ruminants, and some toothed whales; modern baleen whales were remarkable in that they have baleen instead to filter out the krill from the water. On the other spectrum teeth have been evolved as weapons or sexual display seen in pigs and peccaries, some species of deer, musk deer, hippopotamuses, beaked whales and the Narwhal, with its long canine tooth.[55]

Cranial appendages

See also: Horn (anatomy), Ossicone, and Antler
Velvet covers a growing antler and provides it with blood, supplying oxygen and nutrients.

Ungulates have evolved a variety of cranial appendages that can be found in

predators or to express territoriality, as nonterritorial females, which are able to use crypsis for predator defense, often lack horns.[60]

Rhinoceros horns, unlike those of other horned mammals, consist only of keratin. These horns rest on the nasal ridge of the animal's skull.

Antlers are unique to cervids and found mostly on males: the only cervid females with antlers are caribou and reindeer, whose antlers are normally smaller than males'. Nevertheless, fertile does of other species of deer have the capacity to produce antlers on occasion, usually due to increased testosterone levels.[61] Each antler grows from an attachment point on the skull called a pedicle. While an antler is growing it is covered with highly vascular skin called velvet, which supplies oxygen and nutrients to the growing bone.[62] Antlers are considered one of the most exaggerated cases of male secondary sexual traits in the animal kingdom,[63] and grow faster than any other mammal bone.[64] Growth occurs at the tip, initially as cartilage that is then mineralized to become bone. Once the antler has achieved its full size, the velvet is lost and the antler's bone dies. This dead bone structure is the mature antler. In most cases, the bone at the base is destroyed by osteoclasts and the antlers eventually fall off.[62] As a result of their fast growth rate antlers place a substantial nutritional demand on deer; they thus can constitute an honest signal of metabolic efficiency and food gathering capability.[65]

Ossicones are horn-like (or antler-like) protuberances found on the heads of giraffes and male okapis. They are similar to the horns of antelopes and cattle save that they are derived from ossified cartilage,[66] and that the ossicones remain covered in skin and fur rather than horn.

Pronghorn cranial appendages are unique. Each "horn" of the pronghorn is composed of a slender, laterally flattened blade of bone that grows from the frontal bones of the skull, forming a permanent core. As in the Giraffidae, skin covers the bony cores, but in the pronghorn it develops into a keratinous sheath that is shed and regrown on an annual basis. Unlike the horns of the family Bovidae, the horn sheaths of the pronghorn are branched, each sheath possessing a forward-pointing tine (hence the name pronghorn). The horns of males are well developed.

See also

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External links

Look up ungulate in Wiktionary, the free dictionary.
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