Ungulate
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Ungulates ( ) are members of the diverse Ungulata which primarily consists of large mammals with . These include s such as s, es, and s; and s such as , s, s, s, , , and es. s such as , , and 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 characteristics of other ungulates, but recent discoveries indicate that they were also descended from early . Ungulates are typically herbivorous and many employ specialized to allow them to digest cellulose. Some modern species, such as s, are omnivorous, while some prehistoric species, such as ns, were carnivorous.


Classifications


History

Ungulata is a clade (or in some taxonomies, a grand order) of mammals. The two orders of ungulates were the and . , and 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 clade. In 2009 morphological and molecular work found that aardvarks, hyraxes, sea cows, and elephants were more closely related to each other and to s, s, and s than to the perissodactyls and artiodactyls, and form the clade . Elephants, sea cows, and hyraxes were grouped together in the clade , while the aardvark has been considered as either a close relative to them or a close relative to sengis in the clade . This is a striking example of . There is now some dispute as to whether this smaller Ungulata is a (evolution-based) group, or merely a group () or (similar, but not necessarily related). Some studies have indeed found the n ungulates and paraxonian ungulates to form a monophyletic lineage, closely related to either the (the ns and the s) in the clade or to the s. Other studies found the two orders not that closely related, as some place the perissodactyls as close relatives to bats and Ferae in 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 n families and the families. See each family for the relationships of the species as well as the controversies in their respective article. * Ungulata (= Euungulata) ** (Mesaxonian ungulates) *** Hippomorpha **** : Horses, asses and zebras *** **** : Tapirs **** : Rhinoceroses ** (= ) (Paraxonian ungulates) *** **** : Camels and llamas *** Artiofabula **** ***** : Peccaries ***** : Pigs **** ***** ****** : Chevrotains ****** ******* : Pronghorn ******* : Giraffes and okapi ******* : Deer ******* : Musk deer ******* : Oxen and antelopes ***** ****** : Hippopotamuses ****** ******* ******** : Bowhead and right whales ******** : Pygmy right whale ******** : Rorquals ******* ******** ********* : Sperm whale ********* : Lesser sperm whales ******** ********* : Indian river dolphins ******** ********* : Beaked whales ******** ********* : Baiji (functionally extinct) ******** ********* : Amazonian river dolphins ********* : La Plata dolphin ******** ********* : Beluga and narwhal ********* : Porpoises ********* : Oceanic dolphins


Phylogeny

Below is the general consensus of the phylogeny of the ungulate families.


Evolutionary history

and include the majority of large land mammals. These two groups first appeared during the late , 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 of mammals known as the s; the earliest known member of the group was the tiny ', an ungulate that co-existed with the last of non-avian s 66 million years ago; however, many authorities do not consider it a true placental, let alone an ungulate. The enigmatic ns 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 '.Conference abstract (p. 99)
. Explanation and conclusions: .
In Australia, the ' also developed hooves similar to those of artiodactyls, an example of convergent evolution.


Perissodactyl evolution

Perissodactyls were said to have evolved from the , 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 , 55 million years ago (Mya), they had diversified and spread out to occupy several continents. s and s both evolved in North America; rhinoceroses appear to have developed in 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 and the bizarre s. The largest perissodactyl, an Asian rhinoceros called ', reached , more than twice the weight of an . It has been found in a cladistic study that the s and the ns - two lineages that have been previously classified as ns (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 s were known from the northern , from southern through , the and the Pacific coast of to the southern tip of . Their dental and skeletal form suggests desmostylians were aquatic s dependent on 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 s contain the somewhat tapir-like and , the mesaxonic s and the diverse . As a whole, meridiungulates were said to have evolved from animals like '. For a while their relationships with other ungulates were a mystery. Some have even challenged the of Meridiungulata by suggesting that the pyrotheres may be more closely related to other mammals, such as (an African order that were related to s) 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 s assigned to date from the early , 54 million years ago. They had been assigned to the genus ', but the 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 by the early . Fossils of ' found in North America date to this period. This small hornless ancestor resembled a or small more than a rhino. Three families, sometimes grouped together as the Rhinocerotoidea, evolved in the late Eocene: , and , thus creating an explosion of diversity unmatched for a while until environmental changes drastically eliminated several species. The first tapirids, such as ', appeared in the early . They appeared very similar to modern forms, but were about half the size, and lacked the proboscis. The first true tapirs appeared in the . By the , such genera as ' 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 . Perissodactyls were the dominant group of large terrestrial browsers right through the . However, the rise of grasses in the (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 (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, , which were unspecialized, superficially raccoon-like to bear-like omnivores from the Early (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

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

in ''Paleocene mammals of the world''
Early mesonychids had five digits on their feet, which probably rested flat on the ground during walking ( 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 ( locomotion). Mesonychians fared very poorly at the close of the Eocene epoch, with only one genus, ', surviving into the Early epoch, as the climate changed and fierce competition arose from the better adapted s. The first artiodactyls looked like today's s or pigs: small, short-legged creatures that ate and the soft parts of . By the Late Eocene (46 million years ago), the three modern suborders had already developed: (the group); (the group); and (the and 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 , and it is presumably at that time that they developed their complex s, 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 s. 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 , possibly beginning about with the proto-whale ' and other early cetacean ancestors collectively known as , which eventually underwent into the completely aquatic ns. The other branch became the , a large family of four-legged beasts, the earliest of whom in the late would have resembled skinny hippopotamuses with comparatively small and narrow heads. All branches of the anthracotheres, except that which evolved into , became extinct during the without leaving any descendants. The family is said to be the closest artiodactyl family to the cetaceans. Consequentially, new theories in cetacean evolution hypothesize that whales and their ancestors escaped predation, not competition, by slowly adapting to the ocean.


Characteristics

Ungulates were in high diversity in response to and events; the majority of ungulates lack a . Terrestrial ungulates were for the most part herbivores, with some of them being . However, there were exceptions to this as pigs, peccaries, hippos and s were known to have an omnivorous diet. Some cetaceans were the only modern ungulates that were carnivores; baleen whales consume significantly smaller animals in relation to their body size, such as small species of fish and ; toothed whales, depending on the species, can consume a wide range of species: , fish, s, and other species of mammals such as and other whales. In terms of ecosystem ungulates have colonized all corners of the planet, from s to the ; s to s and some have been domesticated by s.


Anatomy

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


Hooves

The hoof is the tip of a of an ungulate , strengthened by a thick horny () covering. The hoof consists of a hard or rubbery sole, and a hard wall formed by a thick 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 were constantly worn down by use. In most modern ungulates, the and were fused along the length of the forelimb; early ungulates, such as the , 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 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 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 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 dewclaws that were located further up on the leg. The earliest cetaceans (the ), also have this characteristic in the addition of also having both an and in the ankle, which were further diagnostic traits of artiodactyls. In modern cetaceans, the front limbs have become 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 ). The main method of moving is an up-and-down motion with the tail fin, called the , which is used for , 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 and specialized , including bunodont (low, rounded cusps) and (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 about 25 million years. As forest biomes declined, grasslands spread, opening new 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 . Some ungulates completely lack upper incisors and instead have a to assist in browsing. It can be found in camels, ruminants, and some toothed whales; modern baleen whales were remarkable in that they have 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.


Cranial appendages

Ungulates evolved a variety of cranial appendages that today can be found in (with the exception of musk deer). In oxen and antelope, the size and shape of the 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 . The unique horn structure is the only unambiguous morphological feature of bovids that distinguishes them from other . Male horn development has been linked to sexual selection, while the presence of horns in females is likely due to natural selection. The horns of females were usually smaller than those of males, and were sometimes of a different shape. The horns of female bovids were thought to have evolved for defense against s or to express territoriality, as nonterritorial females, which were able to use for predator defense, often do not have horns. Rhinoceros horns, unlike those of other horned mammals, only consist of . The horns rest on the nasal ridge of the animals skull. s were unique to cervids and found mostly on males: only have antlers on the females, and these were normally smaller than those of the males. Nevertheless, fertile from other species of deer have the capacity to produce antlers on occasion, usually due to increased testosterone levels. Each antler grows from an attachment point on the skull called a pedicle. While an antler is growing, it is covered with highly called velvet, which supplies oxygen and nutrients to the growing bone. Antlers were considered one of the most exaggerated cases of male secondary sexual traits in the animal kingdom, and grow faster than any other mammal bone. Growth occurs at the tip, and is initially , 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 s and the antlers fall off at some point. As a result of their fast growth rate, antlers were 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. s were -like (or -like) protuberances that can be found on the heads of giraffes and male s today. They were similar to the horns of s and , save that they were derived from ossified ,"The Nashville Zoo at Grassmere - Animals :: Masai Giraffe". The Nashville Zoo at Grassmere, n.d. Web. 15 Feb. 2010. and that the ossicones remain covered in and , rather than horn. Antlers (such as on ) were 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. were 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 ous sheath which is shed and regrown on an annual basis. Unlike the horns of the family Bovidae, the horn sheaths of the pronghorn were branched, each sheath possessing a forward-pointing tine (hence the name pronghorn). The horns of males were well developed.


See also

*


References


External links


Your Guide to the World's Hoofed Mammals
- The Ultimate Ungulate Page * {{Authority control
Taxa named by Carl Linnaeus Taxonomy (biology), Taxa named by Carl Linnaeus (1707−1778), an 18th-century Swedish taxonomist, botanist, and zoologist. Known as the "father of modern taxonomy" – from his inventing and developing binomial nomenclature; the taxonomy (biology) ...