Ungulate

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Ungulate
Temporal range: Paleocene-Recent, 65–0Ma
Possible Late Cretaceous-Present
Donkey, Equus africanus
Common Bottlenose Dolphin, Tursiops truncatus
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Infraclass: Eutheria
Superorder: Laurasiatheria
(unranked): Ungulata
Orders

Perissodactyla
Artiodactyla (including Cetacea)
see text

Ungulates (pronounced /ˈʌŋɡjʊlts/) are a diverse group of large mammals, most of which use the tips of their toes, usually hoofed, to sustain their whole body weight while 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 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.[1] 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.[citation needed]

Classification[edit]

History[edit]

Ungulata, which was formerly considered an order, has been split into the following: Perissodactyla (odd-toed ungulates), Artiodactyla (even-toed ungulates), Tubulidentata (aardvarks), Hyracoidea (hyraxes), Sirenia (dugongs and manatees), and Proboscidea (elephants) as the fossil record seemed to connect them.[2]

However, recent morphological[3][4][5][6] and molecular[7][8] work has found aardvarks, hyraxes, sea cows, and elephants to be more closely related to sengis, tenrecs, and golden moles than to the perissodactyls and artiodactyls and form Afrotheria. Elephants, sea cows, and hyraxes are 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.[9] This is a striking example of convergent evolution.[10]

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,[11][12][13] closely related to either the feraens (the carnivorans) and the pangolins[14][15] in the clade Fereuungulata or to the bats.[16] Other studies found the two orders not that closely related, as some place the perissodactyls as close relatives to bats and feraens in Pegasoferae[17] and others place the artiodactyls as close relatives to bats.[18]

Taxonomy[edit]

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 are 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.

Evolution[edit]

Skeletal drawing of Toxodon
Cladogram showing relations within Ungulata[12]

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 are descended an evolutionary grade of mammals known as the condylarths;[20] the earliest known member of the group was the tiny Protungulatum,[21] an ungulate that co-existed with the last of non-avian dinosaurs 66 million years ago.[22] 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. Another somewhat disputed group are the South American meridiungulates, which contain the somewhat tapir-like pyrotheres and astrapotheres, the mesaxonic litopterns and the diverse notoungulates; As a whole, meridiungulates are said to have evolved from animals like Hyopsodus.[23] How they are related to the other ungulates is also a mystery, with suggesting a relationship to the artiodactyls,[24] to the dinoceratans,[25] or another distant lineage such as Afrotheria. 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 are related to elephants) than to other South American ungulates.[26]

Perissodactyl evolution[edit]

Restoration of Eurohippus parvulus, a mid- to late Eocene equid of Europe (Museum für Naturkunde, Berlin)
The thick dermal armour of the Rhinoceros evolved at the same time as shearing tusks[27]

Perissodactyls are said to have evolved from the Phenacodontidae, small, sheep-sized animals that already shown signs of anatomical features that will be inherited by future descendents (the reduction of digit I and V for example).[28] 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;[29] 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 15 tonnes (17 tons), more than twice the weight of an elephant.[30]

The oldest known fossils assigned to Equidae date from the early Eocene, 54 million years ago. They formerly were assigned to the genus Hyracotherium, but the type species of that genus now is regarded to be 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.[31] 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.[32] 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 are 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.[33]

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[edit]

Restoration of Mesonyx

The artiodactyls are 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 hoofs),[34] and long, heavy tails. Their primitive anatomy makes it unlikely that they were able to run down preys, but with their powerful proportions, claws, and long canines, they may have been able to overpower smaller animals in surprise attacks.[35] Evidently these mammals would soon evolved into two sepereate lineages: the mesonychians and the artiodactyls.

Mesonychians are depicted as "wolves on hooves" and were the first major mammalian predators, appearing in the Paleocene.[36] 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 hoofs 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 its digits (digitigrade locomotion).[36] Mesonychians fared very poorly at the close of the Eocene epoch, with only one genus, Mongolestes,[37] surviving into the Early Oligocene epoch, as climate change and fierce competition from the better adapted creodonts.

The first artiodactyls look 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 ventured out into the seas.

Cetacean evolution[edit]

Skeleton of Ambulocetus natans, a stem whale

The traditional theory of cetacean evolution was that cetaceans were related to carnivorous ungulates called 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 54 million years ago.[1] One branch would evolve into cetaceans, possibly beginning about 52 million years ago with the proto-whale Pakicetus and other early cetacean ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans.[38] 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.[39] The family Raoellidae is said to be the closest artiodactyl family to the cetaceans.[40][41] Consequentially, new theories in cetacean evolution hypothesize that whales and their ancestors escaped predation, not competition, by slowly adapting to the ocean.[42][43][44]

Characteristics[edit]

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

Ungulates are in high diversity in response to sexual selection and ecological events; the majority of ungulates lack a collar bone.[45] Terrestrial ungulates are for the most part herbivores, with some of them being grazers. However there are exceptions to this as pigs, peccaries, hippos and duikers are known to have an omnivorous diet. Some cetaceans are the only modern ungulates that are carnivores; baleen whales consume significantly smaller animals in relation to their body size, such as small species of fish and krill; toothed whales, depending on the species, can consume a wide range of species: squid, fish, sharks, and other species of mammals such as seals and other whales. In terms of ecosystem ungulates have colonized all corners of the planets, from mountains to the ocean depths; grasslands to deserts and have been domesticated by humans.

Anatomy[edit]

Hooves[edit]

The hoof is the tip of a 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. The weight of the animal is normally borne by both the sole and the edge of the hoof wall. 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.[46] 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. While the two orders of ungulates colloquial names are 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 are grouped. Tapirs have four toes in the front, yet they are members of the "odd-toed" order; peccaries and modern cetaceans are members of the "even-toed" order, yet peccaries have three toes in the front and whales are an extreme example as they have flippers instead of hooves. Scientists had classified them according to the distribution of their weight to their toes.

The fossil horse Merychippus, with a diagram of its digits

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. It should be noted that there has been 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. Though it should be aware that the perissodactyls were not the only lineage of mammals to have evolved this trait as the meridiungulates had evolved mesaxonic feet numerous times.

Cloven hooves of Roe Deer (Capreolus capreolus), with dew claws

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 dewclaws that are located further up on the leg. The earliest cetaceans (the archaeocetes), also have this characteristic in the addition of also having both an astragalus and cuboid bone in the ankle, which are further diagnostic traits of artiodactyls.[47]

In modern cetaceans, the front limbs have become pectoral fins and the hind parts are 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, 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[edit]

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.[48][49] It can be found in camels, ruminants, and some toothed whales; modern baleen whales are 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.[50]

Cranial appendages[edit]

Velvet covers a growing antler and provides it with blood, supplying oxygen and nutrients.

Ungulates had evolved a variety of cranial appendages, today can be found in cervoids (with the exception of musk deer). In oxen and antelope, the size and shape of the horns vary greatly, but the basic structure is always a pair of simple bony protrusions without branches, often having a spiral, twisted or fluted form, each covered in a permanent sheath of keratin. The unique horn structure is the only unambiguous morphological feature of bovids that distinguish them from other pecorans.[51][52] Male horn development has been linked to sexual selection,[53][54] while the presence of horns in females is likely due to natural selection.[53][55] The horns of females are usually smaller than those of males, and are sometimes of a different shape. The horns of female bovids are thought to have evolved for defense against predators or to express territoriality, as nonterritorial females, which are able to use crypsis for predator defense, often do not have horns.[55]

Rhinoceros horns, unlike those of other horned mammals, only consist of keratin. The horns rest on the nasal ridge of the animals skull.

Antlers are unique to cervids and found mostly on males: only caribou and reindeer have antlers on the females, and these are normally smaller than those of the males. Nevertheless, fertile does from other species of deer have the capacity to produce antlers on occasion, usually due to increased testosterone levels.[56] 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.[57] Antlers are considered one of the most exaggerated cases of male secondary sexual traits in the animal kingdom,[58] and grow faster than any other mammal bone.[59] Growth occurs at the tip, and is initially cartilage, which is 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 fall off at some point.[57] As a result of their fast growth rate, antlers are considered a handicap since there is an incredible nutritional demand on deer to re-grow antlers annually, and thus can be honest signals of metabolic efficiency and food gathering capability.[60]

Ossicones are horn-like (or antler-like) protuberances that can be found on the heads of giraffes and male okapis today. They are similar to the horns of antelopes and cattle, save that they are derived from ossified cartilage,[61] and that the ossicones remain covered in skin and fur, rather than horn. Antlers (such as on deer) are derived from bone tissue: when mature, the skin and fur covering of the antlers, termed "velvet", is sloughed and scraped off to expose the bone of the antlers.

Pronghorn are unique when compared to their relatives. 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 which 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[edit]

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