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 typicalmorphological characteristics of other ungulates, but recent discoveries indicate that they were also descended from earlyartiodactyls.[5] Ungulates are typically herbivorous and many employ specializedgut bacteria to enable them to digest cellulose, though some members may deviate from this: several species ofpigs and the extinctentelodonts are omnivorous, while cetaceans and the extinctmesonychians are carnivorous.
In 2009, morphological[7][8][9][10] and molecular[11][12] work found that aardvarks, hyraxes, sea cows, and elephants were more closely related to each other and tosengis,tenrecs, andgolden moles than to the perissodactyls and artiodactyls, and form the cladeAfrotheria. Elephants, sea cows, and hyraxes were grouped together in the cladePaenungulata, while the aardvark has been considered as either a close relative to them or a close relative to sengis in the cladeAfroinsectiphilia.[13] This is a striking example ofconvergent evolution.[14]
There is now some dispute as to whether this smaller Euungulata is acladistic (evolution-based) group, or merely aphenetic group (form taxon) orfolk taxon (similar, but not necessarily related). Some studies have indeed found themesaxonian ungulates and paraxonian ungulates to form a monophyletic lineage,[15][16][17] closely related to either theFerae (thecarnivorans and thepangolins)[18][19] in the cladeFereuungulata or to thebats.[20] Other studies found the two orders not that closely related, as some place the perissodactyls as close relatives to bats and Ferae inPegasoferae[21] and others place the artiodactyls as close relatives to bats.[22]
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 thepecoran families and thebaleen whale families. See each family for the relationships of the species as well as the controversies in their respective articles.
Speculativereconstruction of the controversialProtungulatumCladogram showing relationships within Euungulata[16]
Perissodactyla andArtiodactyla include the majority of large land mammals. These two groups first appeared during the latePaleocene, 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 anevolutionary grade of mammals known as thecondylarths.[24] The earliest known member of this group may have been the tinyProtungulatum, a mammal that co-existed with the last of non-aviandinosaurs 66 million years ago.[25] However, many authorities do not consider it a true placental, let alone an ungulate.[26] The enigmaticdinoceratans 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 toCarodnia.[27]
In Australia, the recently-extinctmarsupialChaeropus ("pig-footed bandicoot") also developed hooves similar to those of artiodactyls,[28] an example ofconvergent evolution.
Perissodactyls were thought to have evolved from thePhenacodontidae, 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).[30] By the start of theEocene, 55 million years ago (Mya), they had diversified and spread out to occupy several continents.Horses andtapirs both evolved in North America;[31] rhinoceroses appear to have developed inAsia 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 enormousbrontotheres and the bizarrechalicotheres. The largest perissodactyl, an Asian rhinoceros calledParaceratherium, reached 15 tonnes (17 tons), more than twice the weight of anelephant.[32]
It has been found in a cladistic study that theanthracobunids and thedesmostylians – two lineages that have been previously classified asAfrotherians (more specifically closer to elephants) – have been classified as a clade that is closely related to the perissodactyls.[2] The desmostylians were large amphibious quadrupeds with massive limbs and a short tail.[33][missing long citation] They grew to 1.8 metres (6 ft) in length and were thought to have weighed more than 200 kilograms (440 lb). Theirfossils were known from the northernPacific Rim,[34] from southernJapan throughRussia, theAleutian Islands and the Pacific coast ofNorth America to the southern tip ofBaja California. Their dental and skeletal form suggests desmostylians were aquaticherbivores dependent onlittoral 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 Americanmeridiungulates contain the somewhat tapir-likepyrotheres andastrapotheres, the mesaxoniclitopterns and the diversenotoungulates. As a whole, meridiungulates were said to have evolved from animals likeHyopsodus.[30] For a while their relationships with other ungulates were a mystery. Somepaleontologists have even challenged themonophyly of Meridiungulata by suggesting that the pyrotheres may be more closely related to other mammals, such asEmbrithopoda (an African order that were related toelephants) 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 knownfossils assigned toEquidae date from the earlyEocene, 54 million years ago. They had been assigned to the genusHyracotherium, but thetype 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 otherperissodactyls by the early Eocene. Fossils ofHyrachyus 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 thesuperfamily Rhinocerotoidea, evolved in the late Eocene:Hyracodontidae,Amynodontidae andRhinocerotidae, thus creating an explosion of diversity unmatched for a while until environmental changes drastically eliminated several species.
The first tapirids, such asHeptodon, 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 theOligocene. By theMiocene, such genera asMiotapirus 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 theGreat 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 latePleistocene (about 10,000 years ago) when they faced the pressure of human hunting and habitat change.
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 EarlyPaleocene (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: themesonychians and the artiodactyls.
Skeleton ofAnoplotherium commune, an early artiodactyl with unusual features such as a long tail
The first artiodactyls looked like today'schevrotains or pigs: small, short-legged creatures that ateleaves and the soft parts ofplants. By the Late Eocene (46 million years ago), the three modern suborders had already developed:Suina (thepig group);Tylopoda (thecamel group); andRuminantia (thegoat andcattle 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 marginalhabitats, and it is presumably at that time that they developed their complexdigestive 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.
The traditional theory of cetacean evolution was that cetaceans were related to themesonychian. 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 mesonychian. Today, many scientists believe cetaceans evolved from the same stock that gave rise to hippopotamuses. This hypothesized ancestral group likely split into two branches around54 million years ago.[5] One branch wouldevolve into cetaceans, possibly beginning about52 million years ago with the proto-whalePakicetus and other early cetacean ancestors collectively known asArchaeoceti, which eventually underwentaquatic adaptation into the completely aquaticcetaceans.[40] The other branch became theanthracotheres, a large family of four-legged beasts, the earliest of whom in the lateEocene would have resembled skinny hippopotamuses with comparatively small and narrow heads. All branches of the anthracotheres, except that which evolved intoHippopotamidae, became extinct during thePliocene without leaving any descendants.[41]
The familyRaoellidae 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]
Mesonychians were depicted as "wolves on hooves" and were the first major mammalian predators, appearing in the Paleocene.[47] Early mesonychians had five digits on their feet, which probably rested flat on the ground during walking (plantigrade locomotion), but later mesonychians 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, mesonychians (Pachyaena, for example) walked on their digits (digitigrade locomotion).[47] Mesonychians fared very poorly at the close of the Eocene epoch, with only one genus,Mongolestes,[48] surviving into the EarlyOligocene epoch, as the climate changed and fierce competition arose from the better adaptedcreodonts.
Ungulates were in high diversity in response tosexual selection andecological events; most ungulates lack acollar bone.[49] Terrestrial ungulates were for the most part herbivores, with some of them beinggrazers. However, there were exceptions to this as pigs, peccaries, hippos andduikers 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 andkrill; toothed whales, depending on the species, can consume a wide range of species:squid, fish,sharks, and other species of mammals such asseals and other whales. In terms of ecosystem ungulates have colonized all corners of the planet, frommountains to theocean depths;grasslands todeserts and some have been domesticated byhumans.
Ungulates have developed specialized adaptations, especially in the areas of cranial appendages, dentition, and leg morphology including the modification of theastragalus (one of the ankle bones at the end of the lower leg) with a short, robust head.
Cloven hooves ofroe deer (Capreolus capreolus), with dewclaws
The hoof is the tip of thetoe of an ungulatemammal, strengthened by a thick horny (keratin) covering. The hoof consists of a hard or rubbery sole, and a hard wall formed by a thicknail 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, theradius andulna are fused along the length of the forelimb; early ungulates, such as thearctocyonids, 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; themeridiungulates 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 thedewclaws that were located further up on the leg. The earliest cetaceans (thearchaeocetes), also had this characteristic in the addition of also having both anastragalus andcuboid bone in the ankle, which were further diagnostic traits of artiodactyls.[51]
In modern cetaceans, the front limbs had becomepectoral 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 asatavism). The main method of moving is an up-and-down motion with the tail fin, called thefluke, which is used forpropulsion, 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.
Most ungulates have developed reducedcanine teeth and specializedmolars, including bunodont (low, rounded cusps) andhypsodont (high crowned) teeth. The development of hypsodonty has been of particular interest as this adaptation was strongly associated with the spread of grasslands during theMiocene about 25 million years ago. As forest biomes declined, grasslands spread, opening newniches 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 theGrit, not grass hypothesis.[52]
Some ungulates completely lack upper incisors and instead have adental 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 havebaleen 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]
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 incervoids (with the exception of musk deer). In oxen and antelope, the size and shape of thehorns varies 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 ofkeratin. The unique horn structure is the only unambiguous morphological feature of bovids that distinguishes them from otherpecorans.[56][57] Male horn development has been linked to sexual selection,[58][59] while the presence of horns in females is likely due to natural selection.[58][60] 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 againstpredators or to express territoriality, as nonterritorial females, which are able to usecrypsis 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 arecaribou and reindeer, whose antlers are normally smaller than males'. Nevertheless, fertiledoes 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 highlyvascularskin 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 ascartilage 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 byosteoclasts 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 maleokapis. They are similar to the horns ofantelopes andcattle save that they are derived from ossifiedcartilage,[66] and that the ossicones remain covered inskin andfur 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.
^abPeter J. Waddell, Hirohisa Kishino, Rissa Ota (2001). "A Phylogenetic Foundation for Comparative Mammalian Genomics".Genome Informatics12: 141–154,doi:10.11234/gi1990.12.141.
^Irwin, D.M. and Wilson, A.C. (1993). "Limitations of molecular methods for establishing the phylogeny of mammals, with special reference to the position of elephants". In: F.S. Szalay, M.J. Novacek, and M.C. McKenna (eds.),Mammal Phylogeny: Placentals. pp. 257–267, Springer-Verlag, New York.
^Sanchez-Villagra, M. R.; Narita, Y.; Kuratani, S. (2007). "Thoracolumbar vertebral number: the first skeletal synapomorphy for afrotherian mammals".Syst Biodivers.5:1–17.doi:10.1017/S1477200006002258.S2CID85675984.
^Nery, M. F.; González, D. M. J.; Hoffmann, F. G.; Opazo, J. C. (2012). "Resolution of the laurasiatherian phylogeny: Evidence from genomic data".Molecular Phylogenetics and Evolution.64 (3):685–689.doi:10.1016/j.ympev.2012.04.012.PMID22560954.
^Archibald, J. David; Zhang, Yue; Harper, Tony; Cifelli, Richard L. (2011). "Protungulatum, Confirmed Cretaceous Occurrence of an Otherwise Paleocene Eutherian (Placental?) Mammal".Journal of Mammalian Evolution.18 (3):153–161.doi:10.1007/s10914-011-9162-1.S2CID16724836.
^Shockey, B.J. & Anaya, F. (2004). "Pyrotherium macfaddeni, sp. nov. (late Oligocene, Bolivia) and the pedal morphology of pyrotheres".Journal of Vertebrate Paleontology.24 (2):481–488.Bibcode:2004JVPal..24..481S.doi:10.1671/2521.S2CID83680724.
^Palmer, D., ed. (1999).The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 255.ISBN978-1-84028-152-1.
^Ballenger, L. and Myers, P. (2001)."Family Tapiridae",Animal Diversity Web. Retrieved November 22, 2007.
^Ashley, M.V.; Norman, J.E.; Stross, L. (1996). "Phylogenetic analysis of the perissodactyl family tapiridae using mitochondrial cytochrome c oxidase (COII) sequences".Journal of Mammalian Evolution.3 (4):315–326.doi:10.1007/BF02077448.S2CID24948320.
^The Illustrated Encyclopedia of the Animal Kingdom. p. 7
^Janis, Christine M.; Scott, Kathleen M. and Jacobs, Louis L. (1998)Evolution of Tertiary Mammals of North America, Volume 1. Cambridge: Cambridge University Press. pp. 322-23.ISBN9780521355193
^Jardine, Phillip E.; Janis, Christine M.; Sahney, Sarda; Benton, Michael J. (2012). "Grit not grass: Concordant patterns of early origin of hypsodonty in Great Plains ungulates and Glires".Palaeogeography, Palaeoclimatology, Palaeoecology.365–366:1–10.Bibcode:2012PPP...365....1J.doi:10.1016/j.palaeo.2012.09.001.
^Ezenwa, V.; Jolles, A. (2008). "Horns honestly advertise parasite infection in male and female African buffalo".Animal Behaviour.75 (6):2013–2021.doi:10.1016/j.anbehav.2007.12.013.S2CID49240459.
^abHall, Brian K. (2005)."Antlers".Bones and Cartilage: Developmental and Evolutionary Skeletal Biology. Academic Press. pp. 103–114.ISBN978-0-12-319060-4. Retrieved2010-11-08.
.jpg)
.png)
_(white_background).jpg)
.jpg)
_Hangul_white_background.png)
_(14562088237)_white_background.jpg)
.jpg)
.png)
.png)
