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Ungulates ( ) are members of the diverse
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), lineal descendants - on a phylogenetic tree. R ...

clade
Ungulata which primarily consists of large mammals with
hooves Cloven hooves of roe deer (''Capreolus capreolus''), with prominent dewclaws A hoof ( or ), plural hooves ( or ) or hoofs , is the tip of a toe Toes are the digits (fingers) of the foot of a tetrapod. Animal Animals (also called M ...
. These include
odd-toed ungulate Odd-toed ungulates, mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary glands which in Female#Mammal ...
s such as
horse The horse (''Equus ferus caballus'') is a domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the reproduction and care of another group to ...

horse
s,
rhinoceros A rhinoceros (, , ), commonly abbreviated to rhino, is a member of any of the five extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'' ...

rhinoceros
es, and
tapir A tapir ( ) is a large, herbivorous A herbivore is an animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotro ...

tapir
s; and
even-toed ungulate The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...
s such as
cattle Cattle, taurine cattle, Eurasian cattle, or European cattle (''Bos taurus'' or ''Bos primigenius taurus'') are large domestication, domesticated Cloven hoof, cloven-hooved herbivores. They are a prominent modern member of the subfamily Bovinae ...

cattle
,
pig The pig (''Sus domesticus''), often called swine, hog, or domestic pig when distinguishing from other members of the genus '' Sus'', is an omnivorous An omnivore () is an animal that has the ability to eat and survive on both plant and ani ...

pig
s,
giraffe The giraffe is a tall African mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary glands which in Fe ...

giraffe
s,
camel A camel is an even-toed ungulate The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, ...

camel
s,
sheep Sheep (''Ovis aries'') are quadrupedal, ruminant mammals typically kept as livestock. Like all ruminants, sheep are members of the order (biology), order Artiodactyla, the even-toed ungulates. Although the name ''sheep'' applies to many species ...

sheep
,
deer Deer or true deer are ed s forming the Cervidae. The two main groups of deer are the , including the , the (wapiti), the , and the ; and the , including the (caribou), , the , and the . Male deer of all species (except the Chinese ) as we ...

deer
, and
hippopotamus The hippopotamus ( ; ''Hippopotamus amphibius''), also called the hippo, common hippopotamus or river hippopotamus, is a large, mostly herbivore, herbivorous, List of semiaquatic tetrapods, semiaquatic mammal and ungulate native to sub-Sahara ...

hippopotamus
es.
Cetacean Cetaceans (from la, cetus Cetus () is a constellation, sometimes called 'the whale' in English. The Cetus (mythology), Cetus was a sea monster in Greek mythology which both Perseus and Heracles needed to slay. Cetus is in the region of the s ...
s such as
whales Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. They are an informal grouping within the infraorder Cetacea, usually excluding dolphins and porpoises. Whales, dolphins and porpoises belong ...

whales
,
dolphins Dolphin is a common name of aquatic mammals within the infraorder Cetacea Cetaceans (from la, Cetus (mythology), cetus, lit=whale, from grc, κῆτος, translit=Cetus (mythology), kētos, lit = huge fish, sea monster) are aquatic mamma ...

dolphins
, and
porpoises
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
pig The pig (''Sus domesticus''), often called swine, hog, or domestic pig when distinguishing from other members of the genus '' Sus'', is an omnivorous An omnivore () is an animal that has the ability to eat and survive on both plant and ani ...

pig
s, are omnivorous, while some prehistoric species, such as
mesonychia Mesonychia ("middle claws") is an extinct taxon of small to large-sized carnivorous ungulates related to the cetartiodactyls. Mesonychids first appeared in the early Paleocene, went into a sharp decline at the end of the Eocene, and died out enti ...
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 Perissodactyla (odd-toed ungulates) and Artiodactyla (even-toed ungulates).
Hyracoidea (hyraxes)
Hyracoidea (hyraxes)
,
Sirenia (sea cows) (dugongs and manatees)
Sirenia (sea cows) (dugongs and manatees)
and
Proboscidea (elephants)
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 Laurasiatheria is a clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common ancestor and all its lineag ...

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
sengi Elephant shrews, also called jumping shrews or sengis, are small insectivore, insectivorous mammals native to Africa, belonging to the family Macroscelididae, in the order (biology), order Macroscelidea. Their traditional common English name "ele ...
s,
tenrec A tenrec is any species of mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communi ...

tenrec
s, and
golden mole Golden moles are small insectivorous burrowing mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary glan ...
s than to the perissodactyls and artiodactyls, and form the clade
Afrotheria Afrotheria ( from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Rom ...

Afrotheria
. Elephants, sea cows, and hyraxes were grouped together in the clade
Paenungulata Paenungulata (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Rom ...

Paenungulata
, while the aardvark has been considered as either a close relative to them or a close relative to sengis in the clade
Afroinsectiphilia The Afroinsectiphilia (African insectivore A robber fly eating a hoverfly The giant anteater, a large insectivorous mammal An insectivore is a carnivorous plant or animal that eats insect Insects or Insecta (from Latin Latin ( ...
. This is a striking example of
convergent evolution Convergent evolution is the independent evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; eit ...
. There is now some dispute as to whether this smaller Ungulata is a
cladistic Cladistics (; ) is an approach to Taxonomy (biology), biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically ...
(evolution-based) group, or merely a
phenetic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...
group (
form taxon Form classification is the classification of organisms based on their morphology, which does not necessarily reflect their biological relationships. Form classification, generally restricted to palaeontology Paleontology, also spelled palaeon ...
) or folk taxon (similar, but not necessarily related). Some studies have indeed found the
mesaxonia Mesaxonians (near-synonymous with Panperissodactyla) a clade of ungulates whose weight is distributed on the third toe on all legs through the plane symmetry of their feet. For a while it was often seen to only contain the order Odd-toed ungulate, ...
n ungulates and paraxonian ungulates to form a monophyletic lineage, closely related to either the
Ferae Ferae , ("wild beasts") is a clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common ancestor and all ...

Ferae
(the
carnivora Carnivora is an order of placental Placentalia is one of the three extant subdivisions of the class of animals Mammalia Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch ...
ns and the
pangolin Pangolins, sometimes known as scaly anteaters, are mammals Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in t ...

pangolin
s) in the clade Fereuungulata or to the
bat Bats are mammal Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the po ...

bat
s. 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
pecora Pecora is an infraorder In biological classification In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biol ...

pecora
n families and the
baleen whale Baleen whales (systematic nameA systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter i ...

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 Equidae (sometimes known as the horse family) is the taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a spec ...
: Horses, asses and zebras ***
Ceratomorpha Odd-toed ungulates, mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary glands which in Female#Mammali ...
****
Tapiridae A tapir ( , or , ) is a large, herbivorous mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary gla ...

Tapiridae
: Tapirs ****
Rhinocerotidae A rhinoceros (, , ), commonly abbreviated to rhino, is a member of any of the five extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. T ...

Rhinocerotidae
: Rhinoceroses **
Artiodactyla The even-toed ungulates (Artiodactyla , ) are ungulate 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 , ...

Artiodactyla
(=
Cetartiodactyla The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing posteri ...
) (Paraxonian ungulates) ***
Tylopoda Tylopoda (meaning "calloused foot") is a suborder In biological classification, the order ( la, wikt:ordo#Latin, ordo) is # a taxonomic rank used in the classification of organisms and recognized by the nomenclature codes. The well-known ranks ...
****
Camelidae Camelids are members of the biological family (biology), family Camelidae, the only currently living family in the suborder Tylopoda. The extant taxon, extant members of this group are: dromedary, dromedary camels, Bactrian camels, wild Bactrian ...

Camelidae
: Camels and llamas *** Artiofabula ****
Suina Suina (also known as Suiformes) is a suborder In biological classification, the order ( la, wikt:ordo#Latin, ordo) is # a taxonomic rank used in the classification of organisms and recognized by the nomenclature codes. The well-known ranks in d ...
***** Tayassuidae: Peccaries *****
Suidae Suidae is a family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spatial or social territory, typically subject ...
: Pigs ****
Cetruminantia The Cetruminantia are a clade made up of the Cetancodontamorpha (or Whippomorpha) and their closest living relatives, the Ruminantia. Cetruminantia's placement within Artiodactyla can be represented in the following cladogram:(see e.g. Fig S10) ...
*****
Ruminantia Ruminantia is a taxon In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, ...

Ruminantia
****** Tragulidae: Chevrotains ****** *******
Antilocapridae The Antilocapridae are a family of artiodactyls endemic to North America North America is a continent entirely within the Northern Hemisphere and almost all within the Western Hemisphere. It can also be described as the northern subco ...
: Pronghorn *******
Giraffidae The Giraffidae are a family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being of its members and of societ ...
: Giraffes and okapi *******
Cervidae Deer or true deer are hoof A hoof ( or ), plural hooves ( or ) or hoofs , is the tip of a toe Toes are the digits (fingers) of the foot of a tetrapod. Animal Animals (also called Metazoa) are multicellular eukaryotic organis ...

Cervidae
: Deer *******
Moschidae Moschidae is a family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families is to maintain the well ...
: Musk deer *******
Bovidae The Bovidae comprise the biological family Family ( la, familia, plural ') is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy; it is classified between order (biology), order and genus. A family may be divided into su ...
: Oxen and antelopes *****
Whippomorpha Whippomorpha or Cetancodonta is a group of animals that contains all living cetaceans (whales, dolphins, etc.) and hippopotamuses, as well as their extinct relatives, I.E Entelodont, Entelodonts, Andrewsarchus. All Whippomorphs are descendants of ...

Whippomorpha
******
Hippopotamidae :''This is the article on the family Hippopotamidae; for the main article on hippos, see Hippopotamus The hippopotamus ( ; ''Hippopotamus amphibius''), also called the hippo, common hippopotamus or river hippopotamus, is a large, mostly herbi ...

Hippopotamidae
: Hippopotamuses ******
Cetacea Cetaceans (from la, cetus Cetus () is a constellation, sometimes called 'the whale' in English. The Cetus (mythology), Cetus was a sea monster in Greek mythology which both Perseus and Heracles needed to slay. Cetus is in the region of the ...

Cetacea
*******
Mysticeti Baleen whales (systematic name Mysticeti), also known as whalebone whales, form a parvorder of the infraorder Cetacea (whales, dolphins and porpoises). They are a widely range (biology), distributed and diverse parvorder of carnivorous marine mam ...

Mysticeti
********
Balaenidae Balaenidae is a family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being of its members and of society. Id ...
: Bowhead and right whales ********
Cetotheriidae Cetotheriidae is a family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being of its members and of society ...
: Pygmy right whale ********
Balaenopteridae Rorquals (Balaenopteridae) are the largest group of baleen whale Baleen whales (systematic name Mysticeti), also known as whalebone whales, form a parvorder of the infraorder Cetacea (whales, dolphins and porpoises). They are a widely range (b ...
: Rorquals *******
Odontoceti The toothed whales (also called odontocetes, systematic nameA systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance A chemical substance is a form of matter In classical physics an ...
********
Physeteroidea Physeteroidea is a superfamily that, today, includes three extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscri ...
********* Physeteridae: Sperm whale *********
Kogiidae Kogiidae is a family comprising at least two extant species of Cetacea, the pygmy (''Kogia breviceps)'' and dwarf (''K. sima)'' sperm whales. As their common names suggest, they somewhat resemble sperm whale#REDIRECT Sperm whale The sperm ...
: Lesser sperm whales ********
Platanistoidea River dolphins are a polyphyletic group of fully aquatic mammals that reside exclusively in freshwater or brackish water Brackish water, also sometimes termed brack water, is water occurring in a natural environment having more salinity F ...
*********
Platanistidae Platanistidae is a Family (biology), family of river dolphins containing the extant Platanista, South Asian river dolphin but also extinct relatives from marine deposits in the Neogene. The Amazon river dolphin, Yangtze river dolphin, and francis ...
: Indian river dolphins ********
Ziphioidea Beaked whales are a group of noted as being one of the least known groups of mammals because of their deep-sea habitat and apparent low abundance. Only three or four of the 22 species are reasonably well-known. s and s were subject to commercial ...
*********
Ziphiidae Beaked whales are a group of cetaceans Cetaceans (from la, Cetus (mythology), cetus, lit=whale, from grc, κῆτος, translit=Cetus (mythology), kētos, lit = huge fish, sea monster) are aquatic mammals constituting the infraorder Cetacea ...
: Beaked whales ********
Lipotoidea Lipotidae is a family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families is to maintain the well ...
*********
Lipotidae Lipotidae is a family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families is to maintain the well ...
: Baiji (functionally extinct) ********
Inioidea River dolphins are a polyphyletic group of fully aquatic mammals that reside exclusively in freshwater or brackish water. They are an informal grouping of dolphins, which itself is a paraphyly, paraphyletic group within the infraorder Cetacea. ...
*********
Iniidae Iniidae is a family of river dolphins containing one living genus, ''Inia'', and four extinct genera. The extant genus inhabits the river basins of South America South America is a continent entirely in the Western Hemisphere and mostly ...
: Amazonian river dolphins ********* Pontoporiidae: La Plata dolphin ********
Delphinoidea Delphinoidea is the largest group of toothed whales with 66 genera in 6 families. The largest living member of the superfamily is the killer whale The killer whale or orca (''Orcinus orca'') is a toothed whale belonging to the oceanic dol ...
*********
Monodontidae The cetacean family Monodontidae comprises two living whale species, the narwhal and the beluga whale and at least four extinct species, known from the fossil record. Beluga and Narwhal are native to coastal regions and pack ice around the Arctic O ...
: Beluga and narwhal *********
Phocoenidae Porpoises are a group of fully aquatic marine mammals, similar in appearance to a dolphin, all of which are classified under the family Phocoenidae, parvorder Odontoceti (toothed whales). They are, however, more closely related to narwhals and B ...
: Porpoises *********
Delphinidae Oceanic dolphins or Delphinidae are a widely distributed family of dolphins that live in the sea. Thirty extant species are described. They include several big species whose common names contain "whale" rather than "dolphin", such as the Globic ...
: Oceanic dolphins


Phylogeny

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


Evolutionary history

Perissodactyla and
Artiodactyla The even-toed ungulates (Artiodactyla , ) are ungulate 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 , ...

Artiodactyla
include the majority of large land mammals. These two groups first appeared during the late
Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch In chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronology Chronology (from Latin Latin (, or , ) is a classical language bel ...
, 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 In alpha taxonomy, a grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade A clade (; from grc, , ''klados'', "branch"), also known ...
of mammals known as the
condylarth Condylarthra is an informal group – previously considered an order – of extinct placental mammal Placentalia is one of the three extant subdivisions of the class of animals Mammalia; the other two are Monotremata and Marsupialia. The pla ...
s; the earliest known member of the group was the tiny '' Protungulatum'', an ungulate that co-existed with the last of non-avian
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 annum, million years ago, although the exact origin and timing of the evolution ...

dinosaur
s 66 million years ago; however, many authorities do not consider it a true placental, let alone an ungulate. The enigmatic
dinocerata Image:Eobasileus Uintatherium.jpg, ''Eobasileus'' (left) and ''Uintatherium'' (right). Dinocerata (from the Ancient Greek, Greek (), "terrible", and (), "horn") is an extinct order of herbivore, plant-eating, hoofed mammals with horns and protu ...
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 '' 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 said 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
tapir A tapir ( ) is a large, herbivorous A herbivore is an animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotro ...

tapir
s 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 Brontotheriidae, 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 Anthracobunidae, anthracobunids and the desmostylians - two lineages that have been previously classified as
Afrotheria Afrotheria ( from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Rom ...

Afrotheria
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 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 Meridiungulata, meridiungulates contain the somewhat tapir-like Pyrotheria, pyrotheres and Astrapotheria, astrapotheres, the mesaxonic Litopterna, litopterns and the diverse Notoungulata, 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 Equidae (sometimes known as the horse family) is the taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a spec ...
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, Hyrachyus eximus'' found in North America date to this period. This small hornless ancestor resembled a
tapir A tapir ( ) is a large, herbivorous A herbivore is an animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotro ...

tapir
or small
horse The horse (''Equus ferus caballus'') is a domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the reproduction and care of another group to ...

horse
more than a rhino. Three families, sometimes grouped together as the Taxonomic rank, superfamily Rhinocerotoidea, evolved in the late Eocene: Hyracodontidae, Amynodontidae and
Rhinocerotidae A rhinoceros (, , ), commonly abbreviated to rhino, is a member of any of the five extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. T ...

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 The Paleocene, ( ) or Palaeocene, is a geological epoch In chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronology Chronology (from Latin Latin (, or , ) is a classical language bel ...
(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
mesonychia Mesonychia ("middle claws") is an extinct taxon of small to large-sized carnivorous ungulates related to the cetartiodactyls. Mesonychids first appeared in the early Paleocene, went into a sharp decline at the end of the Eocene, and died out enti ...
ns 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 Suina (also known as Suiformes) is a suborder In biological classification, the order ( la, wikt:ordo#Latin, ordo) is # a taxonomic rank used in the classification of organisms and recognized by the nomenclature codes. The well-known ranks in d ...
(the
pig The pig (''Sus domesticus''), often called swine, hog, or domestic pig when distinguishing from other members of the genus '' Sus'', is an omnivorous An omnivore () is an animal that has the ability to eat and survive on both plant and ani ...

pig
group);
Tylopoda Tylopoda (meaning "calloused foot") is a suborder In biological classification, the order ( la, wikt:ordo#Latin, ordo) is # a taxonomic rank used in the classification of organisms and recognized by the nomenclature codes. The well-known ranks ...
(the
camel A camel is an even-toed ungulate The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, ...

camel
group); and
Ruminantia Ruminantia is a taxon In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, ...

Ruminantia
(the goat and
cattle Cattle, taurine cattle, Eurasian cattle, or European cattle (''Bos taurus'' or ''Bos primigenius taurus'') are large domestication, domesticated Cloven hoof, cloven-hooved herbivores. They are a prominent modern member of the subfamily Bovinae ...

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 habitat (ecology), 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 Evolution of cetaceans, 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 anthracotheriidae, 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 :''This is the article on the family Hippopotamidae; for the main article on hippos, see Hippopotamus The hippopotamus ( ; ''Hippopotamus amphibius''), also called the hippo, common hippopotamus or river hippopotamus, is a large, mostly herbi ...

Hippopotamidae
, became extinct during the Pliocene without leaving any descendants. The family Raoellidae 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 sexual selection and ecological events; the majority of ungulates lack a clavicle, collar bone. Terrestrial ungulates were for the most part herbivores, with some of them being grazing, grazers. However, there were exceptions to this as pigs, peccaries, hippos and duikers 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 krill; toothed whales, depending on the species, can consume a wide range of species: squid, fish, sharks, and other species of mammals such as Pinniped, seals and other whales. In terms of ecosystem ungulates have colonized all corners of the planet, from mountains to the 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 talus bone, astragalus (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 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 (anatomy), 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 were constantly worn down by use. In most modern ungulates, the Radius (bone), radius and ulna were fused along the length of the forelimb; early ungulates, such as the Arctocyonidae, arctocyonids, 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 Meridiungulata, 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 dewclaws that were located further up on the leg. The earliest cetaceans (the archaeocetes), also have this characteristic in the addition of also having both an Astragalus (bone), astragalus and cuboid bone in the ankle, which were further diagnostic traits of artiodactyls. In modern cetaceans, the front limbs have 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 :wiktionary:fluke, fluke, which is used for Animal locomotion, 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 molar (tooth), 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. As forest biomes declined, grasslands spread, opening new Ecological niche, 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. Some ungulates completely lack upper incisors and instead have a dental pad to assist in browsing. 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.


Cranial appendages

Ungulates evolved a variety of cranial appendages that today can be found in Pecora, cervoids (with the exception of musk deer). In oxen and antelope, the size and shape of the horn (anatomy), 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 distinguishes them from other pecorans. 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 predators or to express territoriality, as nonterritorial females, which were able to use crypsis for predator defense, often do not have horns. Rhinoceros horns, unlike those of other horned mammals, only consist of keratin. The horns rest on the nasal ridge of the animals skull. Antlers were unique to cervids and found mostly on males: only reindeer, caribou and reindeer have antlers on the females, and these were normally smaller than those of the males. Nevertheless, fertile wiktionary:doe#English, does 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 Blood vessel, vascular skin 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 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. 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. Ossicones were Horn (anatomy), horn-like (or antler-like) protuberances that can be found on the heads of giraffes and male okapis today. They were similar to the horns of antelopes and
cattle Cattle, taurine cattle, Eurasian cattle, or European cattle (''Bos taurus'' or ''Bos primigenius taurus'') are large domestication, domesticated Cloven hoof, cloven-hooved herbivores. They are a prominent modern member of the subfamily Bovinae ...

cattle
, save that they were derived from ossified cartilage,"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 skin and fur, rather than horn. Antlers (such as on
deer Deer or true deer are ed s forming the Cervidae. The two main groups of deer are the , including the , the (wapiti), the , and the ; and the , including the (caribou), , the , and the . Male deer of all species (except the Chinese ) as we ...

deer
) 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. Pronghorn 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 keratinous 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

* Altungulata


References


External links


Your Guide to the World's Hoofed Mammals
- The Ultimate Ungulate Page * {{Authority control Ungulates, Extant Paleocene first appearances Taxa named by Carl Linnaeus