clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
Ungulata which primarily consists of large mammals with
hooves
The hoof (plural: hooves) is the tip of a toe of an ungulate mammal, which is covered and strengthened with a thick and horny keratin covering. Artiodactyls are even-toed ungulates, species whose feet have an even number of digits, yet the rumi ...
. These include
odd-toed ungulate
Odd-toed ungulates, mammals which constitute the taxonomic order Perissodactyla (, ), are animals—ungulates—who have reduced the weight-bearing toes to three (rhinoceroses and tapirs, with tapirs still using four toes on the front legs) ...
s such as
horse
The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million y ...
s,
rhinoceros
A rhinoceros (; ; ), commonly abbreviated to rhino, is a member of any of the five extant species (or numerous extinct species) of odd-toed ungulates in the family Rhinocerotidae. (It can also refer to a member of any of the extinct species o ...
es, and
tapir
Tapirs ( ) are large, herbivorous mammals belonging to the family Tapiridae. They are similar in shape to a pig, with a short, prehensile nose trunk. Tapirs inhabit jungle and forest regions of South and Central America, with one species inhabit ...
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 (''Bos taurus'') are large, domesticated, cloven-hooved, herbivores. They are a prominent modern member of the subfamily Bovinae and the most widespread species of the genus ''Bos''. Adult females are referred to as cows and adult mal ...
,
pig
The pig (''Sus domesticus''), often called swine, hog, or domestic pig when distinguishing from other members of the genus '' Sus'', is an omnivorous, domesticated, even-toed, hoofed mammal. It is variously considered a subspecies of ''Sus s ...
s,
giraffe
The giraffe is a large African hoofed mammal belonging to the genus ''Giraffa''. It is the tallest living terrestrial animal and the largest ruminant on Earth. Traditionally, giraffes were thought to be one species, ''Giraffa camelopardalis ...
s,
camel
A camel (from: la, camelus and grc-gre, κάμηλος (''kamēlos'') from Hebrew or Phoenician: גָמָל ''gāmāl''.) is an even-toed ungulate in the genus ''Camelus'' that bears distinctive fatty deposits known as "humps" on its back. C ...
s,
sheep
Sheep or domestic sheep (''Ovis aries'') are domesticated, ruminant mammals typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus ''Ovis'', in everyday usage it almost always refers to domesticated s ...
,
deer
Deer or true deer are hoofed ruminant mammals forming the family Cervidae. The two main groups of deer are the Cervinae, including the muntjac, the elk (wapiti), the red deer, and the fallow deer; and the Capreolinae, including the reindeer ...
, and
hippopotamus
The hippopotamus ( ; : hippopotamuses or hippopotami; ''Hippopotamus amphibius''), also called the hippo, common hippopotamus, or river hippopotamus, is a large semiaquatic mammal native to sub-Saharan Africa. It is one of only two extan ...
es.
Cetacean
Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel them ...
s such as
whales
Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and ...
,
dolphins
A dolphin is an aquatic mammal within the infraorder Cetacea. Dolphin species belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontoporiidae (t ...
, and
porpoises
Porpoises are a group of fully aquatic marine mammals, all of which are classified under the family Phocoenidae, parvorder Odontoceti (toothed whales). Although similar in appearance to dolphins, they are more closely related to narwhals an ...
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
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 ...
. Ungulates are typically herbivorous and many employ specialized
gut bacteria
Gut microbiota, gut microbiome, or gut flora, are the microorganisms, including bacteria, archaea, fungi, and viruses that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the gut ...
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, domesticated, even-toed, hoofed mammal. It is variously considered a subspecies of ''Sus s ...
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 artiodactyls. Mesonychids first appeared in the early Paleocene, went into a sharp decline at the end of the Eocene, and died out entirely ...
ns, were carnivorous.
Etymology
Ungulate is from the
Late Latin
Late Latin ( la, Latinitas serior) is the scholarly name for the form of Literary Latin of late antiquity.Roberts (1996), p. 537. English dictionary definitions of Late Latin date this period from the , and continuing into the 7th century in t ...
adjective ''ungulatus'', "hoofed". ''Ungulatus'' is a
diminutive
A diminutive is a root word that has been modified to convey a slighter degree of its root meaning, either to convey the smallness of the object or quality named, or to convey a sense of intimacy or endearment. A (abbreviated ) is a word-formati ...
form of Latin ''unguis'', "nail" (finger nail; toe nail).
Laurasiatheria
Laurasiatheria ("laurasian beasts") is a superorder of placental mammals that groups together true insectivores ( eulipotyphlans), bats ( chiropterans), carnivorans, pangolins ( pholidotes), even-toed ungulates (artiodactyls), odd-toed ungulates ...
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 insectivorous mammals native to Africa, belonging to the family Macroscelididae, in the order Macroscelidea. Their traditional common English name "elephant shrew" comes from a pe ...
s,
tenrec
A tenrec is any species of mammal within the afrotherian family Tenrecidae endemic to Madagascar. Tenrecs are wildly diverse; as a result of convergent evolution some resemble hedgehogs, shrews, opossums, rats, and mice. They occupy aquatic, a ...
s, and
golden mole
Golden moles are small insectivorous burrowing mammals endemic to Sub-Saharan Africa. They comprise the family Chrysochloridae and as such they are taxonomically distinct from the true moles, family Talpidae, and other mole-like families, all o ...
s than to the perissodactyls and artiodactyls, and form the clade
Afrotheria
Afrotheria ( from Latin ''Afro-'' "of Africa" + ''theria'' "wild beast") is a clade of mammals, the living members of which belong to groups that are either currently living in Africa or of African origin: golden moles, elephant shrews (also know ...
. Elephants, sea cows, and hyraxes were grouped together in the clade
Paenungulata
Paenungulata (from Latin ''paene'' "almost" + ''ungulātus'' "having Hoof, hoofs") is a clade of "sub-Ungulate, ungulates", which groups three Extant taxon, extant mammal Order (biology), orders: Proboscidea (including elephants), Sirenia (sirenia ...
, 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 insectivores) is a clade that has been proposed based on the results of recent molecular phylogenetic studies. Many of the taxa within it were once regarded as part of the order Insectivora, but Insectivora is now ...
. This is a striking example of
convergent evolution
Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
.
There is now some dispute as to whether this smaller Ungulata is a
cladistic
Cladistics (; ) is an approach to 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 shared derived char ...
(evolution-based) group, or merely a
phenetic
In biology, phenetics ( el, phainein – to appear) , also known as taximetrics, is an attempt to classify organisms based on overall similarity, usually in morphology or other observable traits, regardless of their phylogeny or evolutionary rel ...
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, reflects uncertainty; the goal of sc ...
) or
folk taxon
A folk taxonomy is a vernacular name, naming system, as distinct from Taxonomy (general), scientific taxonomy. Folk biological classification is the way people traditionally describe and organize their natural surroundings/the world around them, ...
(similar, but not necessarily related). Some studies have indeed found the
mesaxonia
Mesaxonia (near-synonymous with Panperissodactyla) is 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 Perissodactyla ...
n ungulates and paraxonian ungulates to form a monophyletic lineage, closely related to either the
Ferae
Ferae ( , , "wild beasts") is a mirorder of placental mammalsMalcolm C. McKenna, Susan K. Bell: ''Classification of Mammals: Above the Species Level'' in Columbia University Press, New York (1997), 631 Seiten. that groups together clades Pan-Ca ...
(the
carnivora
Carnivora is a Clade, monophyletic order of Placentalia, placental mammals consisting of the most recent common ancestor of all felidae, cat-like and canidae, dog-like animals, and all descendants of that ancestor. Members of this group are f ...
ns and the
pangolin
Pangolins, sometimes known as scaly anteaters, are mammals of the order Pholidota (, from Ancient Greek ϕολιδωτός – "clad in scales"). The one extant family, the Manidae, has three genera: '' Manis'', '' Phataginus'', and '' Smut ...
s) in the clade
Fereuungulata
Ferungulata ("wild beasts and ungulates") is a grandorder of placental mammals that groups together mirorder Ferae and clade Pan-Euungulata. It has existed in two guises, a traditional one based on morphological analysis and a revised one taki ...
or to the
bat
Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most bi ...
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
Pegasoferae ("winged beast") is a proposed clade of mammals based on genomic research in molecular systematics by Nishihara, Hasegawa and Okada (2006).
To the surprise of the authors, their data led them to propose a clade that includes bats (or ...
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 of even-toed hoofed mammals with ruminant digestion. Most members of Pecora have cranial appendages projecting from their frontal bones; only two extant genera lack them, ''Hydropotes'' and ''Moschus''. The name “Pecor ...
n families and the
baleen whale
Baleen whales (systematic name Mysticeti), also known as whalebone whales, are a parvorder of carnivorous marine mammals of the infraorder Cetacea (whales, dolphins and porpoises) which use keratinaceous baleen plates (or "whalebone") in their ...
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 family of horses and related animals, including the extant horses, asses, and zebras, and many other species known only from fossils. All extant species are in the genus '' Equus'', ...
: Horses, asses and zebras
***
Ceratomorpha
Odd-toed ungulates, mammals which constitute the taxonomic order Perissodactyla (, ), are animals—ungulates—who have reduced the weight-bearing toes to three (rhinoceroses and tapirs, with tapirs still using four toes on the front legs) ...
****
Tapiridae
Tapirs ( ) are large, herbivorous mammals belonging to the family Tapiridae. They are similar in shape to a pig, with a short, prehensile nose trunk. Tapirs inhabit jungle and forest regions of South and Central America, with one species inhabi ...
: Tapirs
****
Rhinocerotidae
A rhinoceros (; ; ), commonly abbreviated to rhino, is a member of any of the five extant species (or numerous extinct species) of odd-toed ungulates in the family Rhinocerotidae. (It can also refer to a member of any of the extinct species ...
: Rhinoceroses
**
Artiodactyla
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 ...
(=
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 poster ...
) (Paraxonian ungulates)
***
Tylopoda
Tylopoda (meaning "calloused foot") is a suborder of terrestrial herbivorous even-toed ungulates belonging to the order Artiodactyla. They are found in the wild in their native ranges of South America and Asia, while Australian feral camels are ...
****
Camelidae
Camelids are members of the biological family Camelidae, the only currently living family in the suborder Tylopoda. The seven extant members of this group are: dromedary camels, Bactrian camels, wild Bactrian camels, llamas, alpacas, vicuñas, ...
: Camels and llamas
*** Artiofabula
****
Suina
Suina (also known as Suiformes) is a suborder of omnivorous, non-ruminant artiodactyl mammals that includes the domestic pig and peccaries. A member of this clade is known as a suine. Suina includes the family Suidae, termed suids, known in Eng ...
Suidae
Suidae is a family of artiodactyl mammals which are commonly called pigs, hogs or swine. In addition to numerous fossil species, 18 extant species are currently recognized (or 19 counting domestic pigs and wild boars separately), classified into ...
: 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 fig S10) ...
*****
Ruminantia
Ruminants ( suborder Ruminantia) are hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The ...
******
Tragulidae
The asterisk ( ), from Late Latin , from Ancient Greek , ''asteriskos'', "little star", is a typographical symbol. It is so called because it resembles a conventional image of a heraldic star.
Computer scientists and mathematicians often v ...
Antilocapridae
The Antilocapridae are a family of artiodactyls endemic to North America. Their closest extant relatives are the giraffids with which they comprise the superfamily Giraffoidea. Only one species, the pronghorn (''Antilocapra americana''), is li ...
: Pronghorn
*******
Giraffidae
The Giraffidae are a family of ruminant artiodactyl mammals that share a common ancestor with deer and bovids. This family, once a diverse group spread throughout Eurasia and Africa, presently comprises only two extant genera, the giraffe (one or ...
: Giraffes and okapi
*******
Cervidae
Deer or true deer are hoofed ruminant mammals forming the family Cervidae. The two main groups of deer are the Cervinae, including the muntjac, the elk (wapiti), the red deer, and the fallow deer; and the Capreolinae, including the reindeer ...
: Deer
*******
Moschidae
Moschidae is a family of pecoran even-toed ungulates, containing the musk deer (''Moschus'') and its extinct relatives. They are characterized by long 'saber teeth' instead of horns, antlers or ossicones, modest size (''Moschus'' only reaches ; ...
: Musk deer
*******
Bovidae
The Bovidae comprise the biological family of cloven-hoofed, ruminant mammals that includes cattle, bison, buffalo, antelopes, and caprines. A member of this family is called a bovid. With 143 extant species and 300 known extinct species, ...
: Cattle 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 and Andrewsarchus. All Whippomorphs are descendan ...
******
Hippopotamidae
Hippopotamidae is a family of stout, naked-skinned, and semiaquatic artiodactyl mammals, possessing three-chambered stomachs and walking on four toes on each foot. While they resemble pigs physiologically, their closest living relatives are t ...
: Hippopotamuses
******
Cetacea
Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel them ...
*******
Mysticeti
Baleen whales (systematic name Mysticeti), also known as whalebone whales, are a parvorder of carnivorous marine mammals of the infraorder Cetacea (whales, dolphins and porpoises) which use keratinaceous baleen plates (or "whalebone") in their ...
********
Balaenidae
Balaenidae () is a family of whales of the parvorder Mysticeti that contains two living genera: the right whales (genus ''Eubalaena''), and in a separate genus, the closely related bowhead whale (genus ''Balaena'').
Evolutionary history
Bale ...
: Bowhead and right whales
********
Cetotheriidae
Cetotheriidae is a family of baleen whales (parvorder Mysticeti). The family is known to have existed from the Late Oligocene to the Early Pleistocene before going extinct. Although some phylogenetic studies conducted by recovered the living py ...
: Pygmy right whale
********
Balaenopteridae
Rorquals () are the largest group of baleen whales, which comprise the family Balaenopteridae, containing ten extant species in three genera. They include the largest animal that has ever lived, the blue whale, which can reach , and the fin wha ...
: Rorquals
*******
Odontoceti
The toothed whales (also called odontocetes, systematic name Odontoceti) are a parvorder of cetaceans that includes dolphins, porpoises, and all other whales possessing teeth, such as the beaked whales and sperm whales. Seventy-three species of ...
********
Physeteroidea
Physeteroidea is a superfamily that includes three extant species of whales: the sperm whale, in the genus ''Physeter'', and the pygmy sperm whale and dwarf sperm whale, in the genus ''Kogia''. In the past, these genera have sometimes been united ...
Kogiidae
Kogiidae is a family comprising at least two extant species of Cetacea, the Pygmy sperm whale, pygmy (''Kogia breviceps)'' and dwarf sperm whale, dwarf (''K. sima)'' sperm whales. As their common names suggest, they somewhat resemble sperm whales ...
: Lesser sperm whales
********
Platanistoidea
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 paraphyletic group within the infraorder Cetacea. Extant riv ...
********* Platanistidae: Indian river dolphins
********
Ziphioidea
Beaked whales (systematic name Ziphiidae) are a family of cetaceans 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 24 species are reasonably well-k ...
*********
Ziphiidae
Beaked whales (systematic name Ziphiidae) are a family of cetaceans 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 24 species are reasonably well-kn ...
: Beaked whales
********
Lipotoidea
Lipotidae is a family of river dolphins containing the possibly extinct baiji of China and the fossil genus ''Parapontoporia'' from the Late Miocene and Pliocene of the Pacific coast of North America. The genus '' Prolipotes'', which is based ...
*********
Lipotidae
Lipotidae is a family of river dolphins containing the possibly extinct baiji of China and the fossil genus ''Parapontoporia'' from the Late Miocene and Pliocene of the Pacific coast of North America. The genus '' Prolipotes'', which is based on ...
: 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 paraphyletic group within the infraorder Cetacea. Extant riv ...
*********
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, but the family formerly had a wider presence across the Atlantic Ocean.
Iniidae a ...
: 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, which can reach 6 tons, while the smallest is the vaquita.
Taxonomy
Based on McGowen ''et al''., ...
*********
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 ...
: Beluga and narwhal
*********
Phocoenidae
Porpoises are a group of fully aquatic marine mammals, all of which are classified under the family Phocoenidae, parvorder Odontoceti (toothed whales). Although similar in appearance to dolphins, they are more closely related to narwhals an ...
: Porpoises
*********
Delphinidae
Oceanic dolphins or Delphinidae are a widely distributed family of dolphins that live in the sea. Close to forty extant species are recognised. They include several big species whose common names contain "whale" rather than "dolphin", such as the ...
: Oceanic dolphins
Phylogeny
Below is the general consensus of the phylogeny of the ungulate families.
Artiodactyla
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 ...
include the majority of large land mammals. These two groups first appeared during the late
Paleocene
The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...
, 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
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 strictly phylogenetic unit.
Definition
An evolutionary grade is a group of ...
of mammals known as the
condylarth
Condylarthra is an informal group – previously considered an order – of extinct placental mammals, known primarily from the Paleocene and Eocene epochs. They are considered early, primitive ungulates. It is now largely considered to be a wast ...
s; the earliest known member of the group was the tiny ''
Protungulatum
''Protungulatum'' ('first ungulate') is a extinct genus of pan-euungulate mammals within extinct family Protungulatidae, and also one of the earliest known placental mammals in the fossil record, that lived in North America from the Late Creta ...
'', 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 period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s 66 million years ago; however, many authorities do not consider it a true placental, let alone an ungulate. The enigmatic
dinocerata
Dinocerata (from the Greek (), "terrible", and (), "horn") is an extinct order of plant-eating hoofed mammals with horns and protuberant canine teeth.
Classification
A 2015 phylogenetic study recovered Dinocerata as closely related to '' ...
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
''Carodnia'' is an extinct genus of South American ungulate known from the Early Eocene of Brazil, Argentina, and Peru.
''Carodnia'' is placed in the order ''Xenungulata'' together with ''Etayoa'' and '' Notoetayoa''.
''Carodnia'' is the larg ...
marsupial
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a po ...
''
Chaeropus
''Chaeropus'', known as the pig-footed bandicoots, is a genus of small mammals that became extinct during the 20th century. They were unique marsupials, of the order Peramelemorphia (bandicoots and bilbies), with unusually thin legs, yet were a ...
'' also developed hooves similar to those of artiodactyls, an example of convergent evolution.
Perissodactyl evolution
Perissodactyls were thought to have evolved from the
Phenacodontidae
Phenacodontidae is an extinct family (biology), family of large herbivorous mammals traditionally placed in the “wastebasket taxon” Condylarthra, which may instead represent early-stage Perissodactyla, perissodactyls. They lived in the Paleo ...
, 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
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
, 55 million years ago (Mya), they had diversified and spread out to occupy several continents.
Horse
The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million y ...
s and
tapir
Tapirs ( ) are large, herbivorous mammals belonging to the family Tapiridae. They are similar in shape to a pig, with a short, prehensile nose trunk. Tapirs inhabit jungle and forest regions of South and Central America, with one species inhabit ...
s both evolved in North America; rhinoceroses appear to have developed in
Asia
Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an area ...
from tapir-like animals and then colonised the Americas during the middle Eocene (about 45 Mya). Of the approximately 15 families, only three survive (McKenna and Bell, 1997; Hooker, 2005). These families were very diverse in form and size; they included the enormous
brontotheres
Brontotheriidae is a family of extinct mammals belonging to the order Perissodactyla, the order that includes horses, rhinoceroses, and tapirs. Superficially, they looked rather like rhinos, although they were actually more closely related to ...
and the bizarre
chalicothere
Chalicotheres (from Greek '' chalix'', "gravel" and '' therion'', "beast") are an extinct clade of herbivorous, odd-toed ungulate (perissodactyl) mammals that lived in North America, Eurasia, and Africa from the Middle Eocene until the Early Plei ...
s. The largest perissodactyl, an Asian rhinoceros called ''
Paraceratherium
''Paraceratherium'' is an extinct genus of hornless rhinoceros. It is one of the largest terrestrial mammals that has existed and lived from the early to late Oligocene epoch (34–23 million years ago). The first fossils were discovered ...
'', reached , more than twice the weight of an
elephant
Elephants are the largest existing land animals. Three living species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. They are the only surviving members of the family Elephantidae an ...
.
It has been found in a cladistic study that the
anthracobunid
Anthracobunidae is an extinct family of stem perissodactyls that lived in the early to middle Eocene period. They were originally considered to be a paraphyletic family of primitive proboscideans possibly ancestral to the Moeritheriidae and the ...
s and the
desmostylia
The Desmostylia (from Greek δεσμά ''desma'', "bundle", and στῦλος ''stylos'', "pillar") are an extinct order of aquatic mammals that existed from the early Oligocene (Rupelian) to the late Miocene (Tortonian) ().
Desmostylians are t ...
ns - two lineages that have been previously classified as
Afrotheria
Afrotheria ( from Latin ''Afro-'' "of Africa" + ''theria'' "wild beast") is a clade of mammals, the living members of which belong to groups that are either currently living in Africa or of African origin: golden moles, elephant shrews (also know ...
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
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s were known from the northern
Pacific Rim
The Pacific Rim comprises the lands around the rim of the Pacific Ocean. The ''Pacific Basin'' includes the Pacific Rim and the islands in the Pacific Ocean. The Pacific Rim roughly overlaps with the geologic Pacific Ring of Fire.
List of co ...
, from southern
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
through
Russia
Russia (, , ), or the Russian Federation, is a List of transcontinental countries, transcontinental country spanning Eastern Europe and North Asia, Northern Asia. It is the List of countries and dependencies by area, largest country in the ...
, the
Aleutian Islands
The Aleutian Islands (; ; ale, Unangam Tanangin,”Land of the Aleuts", possibly from Chukchi language, Chukchi ''aliat'', "island"), also called the Aleut Islands or Aleutic Islands and known before 1867 as the Catherine Archipelago, are a cha ...
and the Pacific coast of
North America
North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
to the southern tip of
Baja California
Baja California (; 'Lower California'), officially the Free and Sovereign State of Baja California ( es, Estado Libre y Soberano de Baja California), is a state in Mexico. It is the northernmost and westernmost of the 32 federal entities of Mex ...
. Their dental and skeletal form suggests desmostylians were aquatic
herbivore
A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...
s dependent on
littoral
The littoral zone or nearshore is the part of a sea, lake, or river that is close to the shore. In coastal ecology, the littoral zone includes the intertidal zone extending from the high water mark (which is rarely inundated), to coastal areas ...
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 meridiungulates contain the somewhat tapir-like pyrotheres and
astrapotheres
Astrapotheria is an extinct order of South American and Antarctic hoofed mammals that existed from the late Paleocene to the Middle Miocene, ."The uruguaytheriine Astrapotheriidae from the rich middle Miocene Honda Group of the upper Magdalen ...
, the mesaxonic
litoptern
Litopterna (from grc, λῑτή πτέρνα "smooth heel") is an extinct order of fossil hoofed mammals from the Cenozoic era. The order is one of the five great orders of South American ungulates that were endemic to the continent, until th ...
s and the diverse
notoungulates
Notoungulata is an extinct order of mammalian ungulates that inhabited South America from the early Paleocene to the Holocene, living from approximately 61 million to 11,000 years ago. Notoungulates were morphologically diverse, with forms resemb ...
. As a whole, meridiungulates were said to have evolved from animals like ''
Hyopsodus
''Hyopsodus'' is a genus of extinct early ungulate mammal of the family Hyopsodontidae, a group associated with or basal to the Perissodactyla. Fossils of this genus have been found in the Eocene of North America, especially the Bighorn Ba ...
''. For a while their relationships with other ungulates were a mystery. Some
paleontologists
Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...
have even challenged the
monophyly
In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...
of Meridiungulata by suggesting that the pyrotheres may be more closely related to other mammals, such as
Embrithopoda
Embrithopoda ("heavy-footed") is an order of extinct mammals known from Asia, Africa and Eastern Europe. Most of the embrithopod genera are known exclusively from jaws and teeth dated from the late Paleocene to the late Eocene; however, the ord ...
(an African order that were related to
elephant
Elephants are the largest existing land animals. Three living species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. They are the only surviving members of the family Elephantidae an ...
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
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s assigned to
Equidae
Equidae (sometimes known as the horse family) is the taxonomic family of horses and related animals, including the extant horses, asses, and zebras, and many other species known only from fossils. All extant species are in the genus '' Equus'', ...
date from the early
Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
, 54 million years ago. They had been assigned to the genus ''
Hyracotherium
''Hyracotherium'' ( ; "hyrax-like beast") is an extinct genus of very small (about 60 cm in length) perissodactyl ungulates that was found in the London Clay formation. This small, fox-sized animal was once considered to be the earliest know ...
'', but the
type species
In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...
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
Odd-toed ungulates, mammals which constitute the taxonomic order Perissodactyla (, ), are animals—ungulates—who have reduced the weight-bearing toes to three (rhinoceroses and tapirs, with tapirs still using four toes on the front legs) o ...
by the early
Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
. Fossils of '' Hyrachyus eximus'' found in North America date to this period. This small hornless ancestor resembled a
tapir
Tapirs ( ) are large, herbivorous mammals belonging to the family Tapiridae. They are similar in shape to a pig, with a short, prehensile nose trunk. Tapirs inhabit jungle and forest regions of South and Central America, with one species inhabit ...
or small
horse
The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million y ...
more than a rhino. Three families, sometimes grouped together as the
superfamily
SUPERFAMILY is a database and search platform of structural and functional annotation for all proteins and genomes. It classifies amino acid sequences into known structural domains, especially into SCOP superfamilies. Domains are functional, str ...
Rhinocerotoidea, evolved in the late Eocene:
Hyracodontidae
The Hyracodontidae are an extinct family of rhinoceroses endemic to North America, Europe, and Asia during the Eocene through early Oligocene, living from 48.6 to 26.3 million years ago (Mya), existing about .
The Hyracodontidae thrived in the r ...
,
Amynodontidae
Amynodontidae ("defensive tooth") is a family of extinct perissodactyls related to true rhinoceroses. They are commonly portrayed as semiaquatic hippo-like rhinos but this description only fits members of the Metamynodontini; other groups of a ...
and
Rhinocerotidae
A rhinoceros (; ; ), commonly abbreviated to rhino, is a member of any of the five extant species (or numerous extinct species) of odd-toed ungulates in the family Rhinocerotidae. (It can also refer to a member of any of the extinct species ...
, thus creating an explosion of diversity unmatched for a while until environmental changes drastically eliminated several species.
The first tapirids, such as ''
Heptodon
''Heptodon'' is an extinct genus of tapir-type herbivore of the family Helaletidae endemic to North America during the Early Eocene. It lived from 50.3—48.6 Annum, mya, existing for approximately .
''Heptodon'' was about in length, and closely ...
'', appeared in the early
Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''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
The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...
. By the
Miocene
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
, such genera as ''
Miotapirus
''Miotapirus harrisonensis'' is an extinct species of tapir lived during the early Miocene Epoch some 20 million years ago in North America.
Physically ''Miotapirus'' was virtually identical to its modern relatives; with a length of 2 m (6 f ...
'' 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
The Great American Biotic Interchange (commonly abbreviated as GABI), also known as the Great American Interchange and the Great American Faunal Interchange, was an important late Cenozoic paleozoogeographic biotic interchange event in which lan ...
.
Perissodactyls were the dominant group of large terrestrial browsers right through the
Oligocene
The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...
. However, the rise of grasses in the
Miocene
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
(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
The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...
(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
Arctocyonidae (from Greek ''arktos'' ''kyôn'', "bear/dog-like") has been defined as an extinct family of unspecialized, primitive mammals with more than 20 genera. Animals assigned to this family were most abundant during the Paleocene, but ext ...
, which were unspecialized, superficially raccoon-like to bear-like omnivores from the Early
Paleocene
The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...
(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 artiodactyls. Mesonychids first appeared in the early Paleocene, went into a sharp decline at the end of the Eocene, and died out entirely ...
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
151px, Portion of a human skeleton, showing plantigrade habit
In terrestrial animals, plantigrade locomotion means walking with the toes and metatarsals flat on the ground. It is one of three forms of locomotion adopted by terrestrial mammals. T ...
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
In terrestrial vertebrates, digitigrade () locomotion is walking or running on the toes (from the Latin ''digitus'', 'finger', and ''gradior'', 'walk'). A digitigrade animal is one that stands or walks with its toes (metatarsals) touching the groun ...
locomotion). Mesonychians fared very poorly at the close of the Eocene epoch, with only one genus, ''
Mongolestes
''Mongolestes'' ("Mongolian robber") is an extinct genus of mesonychid known from the 'Ulan Gochu' formation of Inner Mongolia, and likely originated in Asia. It was the last surviving representative of Mesonychia and became extinct in the early O ...
'', surviving into the Early
Oligocene
The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...
epoch, as the climate changed and fierce competition arose from the better adapted
creodont
Creodonta ("meat teeth") is a former order of extinct carnivorous placental mammals that lived from the early Paleocene to the late Miocene epochs in North America, Europe, Asia and Africa. Originally thought to be a single group of animals ance ...
s.
The first artiodactyls looked like today's
chevrotain
Chevrotains, or mouse-deer, are small even-toed ungulates that make up the family Tragulidae, the only extant members of the infraorder Tragulina. The 10 extant species are placed in three genera, but several species also are known only ...
s or pigs: small, short-legged creatures that ate
leaves
A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...
and the soft parts of
plants
Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...
. By the Late Eocene (46 million years ago), the three modern suborders had already developed:
Suina
Suina (also known as Suiformes) is a suborder of omnivorous, non-ruminant artiodactyl mammals that includes the domestic pig and peccaries. A member of this clade is known as a suine. Suina includes the family Suidae, termed suids, known in Eng ...
(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, domesticated, even-toed, hoofed mammal. It is variously considered a subspecies of ''Sus s ...
group);
Tylopoda
Tylopoda (meaning "calloused foot") is a suborder of terrestrial herbivorous even-toed ungulates belonging to the order Artiodactyla. They are found in the wild in their native ranges of South America and Asia, while Australian feral camels are ...
(the
camel
A camel (from: la, camelus and grc-gre, κάμηλος (''kamēlos'') from Hebrew or Phoenician: גָמָל ''gāmāl''.) is an even-toed ungulate in the genus ''Camelus'' that bears distinctive fatty deposits known as "humps" on its back. C ...
group); and
Ruminantia
Ruminants ( suborder Ruminantia) are hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The ...
(the
goat
The goat or domestic goat (''Capra hircus'') is a domesticated species of goat-antelope typically kept as livestock. It was domesticated from the wild goat (''C. aegagrus'') of Southwest Asia and Eastern Europe. The goat is a member of the a ...
and
cattle
Cattle (''Bos taurus'') are large, domesticated, cloven-hooved, herbivores. They are a prominent modern member of the subfamily Bovinae and the most widespread species of the genus ''Bos''. Adult females are referred to as cows and adult mal ...
group). Nevertheless, artiodactyls were far from dominant at that time: the perissodactyls were much more successful and far more numerous. Artiodactyls survived in niche roles, usually occupying marginal
habitats
In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
, and it is presumably at that time that they developed their complex
digestive system
The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). Digestion involves the breakdown of food into smaller and smaller compone ...
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
mesonychid
Mesonychia ("middle claws") is an extinct taxon of small- to large-sized carnivorous ungulates related to artiodactyls. Mesonychids first appeared in the early Paleocene, went into a sharp decline at the end of the Eocene, and died out entirely ...
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 evolve into cetaceans, possibly beginning about with the proto-whale ''
Pakicetus
''Pakicetus'' is an extinct genus of amphibious cetacean of the family Pakicetidae, which was endemic to Pakistan during the Eocene, about 50 million years ago. It was a wolf-like animal, about to long, and lived in and around water where it a ...
'' and other early cetacean ancestors collectively known as
Archaeoceti
Archaeoceti ("ancient whales"), or Zeuglodontes in older literature, is a paraphyletic group of primitive cetaceans that lived from the Early Eocene to the late Oligocene (). Representing the earliest cetacean radiation, they include the initia ...
, which eventually underwent
aquatic adaptation
Several groups of tetrapods have undergone secondary aquatic adaptation, an evolutionary transition from being purely terrestrial to living at least part of the time in water. These animals are called "secondarily aquatic" because although their a ...
into the completely aquatic
cetacea
Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel them ...
ns. The other branch became the
anthracotheres
Anthracotheriidae is a paraphyletic family of extinct, hippopotamus-like artiodactyl ungulates related to hippopotamuses and whales. The oldest genus, ''Elomeryx'', first appeared during the middle Eocene in Asia. They thrived in Africa and Euras ...
, a large family of four-legged beasts, the earliest of whom in the late
Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
would have resembled skinny hippopotamuses with comparatively small and narrow heads. All branches of the anthracotheres, except that which evolved into
Hippopotamidae
Hippopotamidae is a family of stout, naked-skinned, and semiaquatic artiodactyl mammals, possessing three-chambered stomachs and walking on four toes on each foot. While they resemble pigs physiologically, their closest living relatives are t ...
, became extinct during the
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58
The family
Raoellidae
The Raoellidae, previously grouped within Helohyidae, are an extinct family of semiaquatic digitigrade artiodactyls in the clade Whippomorpha. Fossils of raoellids are found in Eocene strata of South and Southeast Asia.
An exceptionally complete ...
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
Sexual selection is a mode of natural selection in which members of one biological sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of t ...
and
ecological
Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...
events; the majority of ungulates lack a
collar bone
The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the righ ...
. Terrestrial ungulates were for the most part herbivores, with some of them being grazers. However, there were exceptions to this as pigs, peccaries, hippos and
duiker
A duiker is a small to medium-sized brown antelope native to sub-Saharan Africa, found in heavily wooded areas. The 22 extant species, including three sometimes considered to be subspecies of the other species, form the subfamily Cephalophina ...
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
krill
Krill are small crustaceans of the order Euphausiacea, and are found in all the world's oceans. The name "krill" comes from the Norwegian
Norwegian, Norwayan, or Norsk may refer to:
*Something of, from, or related to Norway, a country in n ...
; toothed whales, depending on the species, can consume a wide range of species:
squid
True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting t ...
, fish,
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimo ...
s, and other species of mammals such as
seals
Seals may refer to:
* Pinniped, a diverse group of semi-aquatic marine mammals, many of which are commonly called seals, particularly:
** Earless seal, or "true seal"
** Fur seal
* Seal (emblem), a device to impress an emblem, used as a means of a ...
and other whales. In terms of ecosystem ungulates have colonized all corners of the planet, from
mountain
A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited Summit (topography), summit area, and ...
s to the
ocean depths
The deep sea is broadly defined as the ocean depth where light begins to fade, at an approximate depth of 200 metres (656 feet) or the point of transition from continental shelves to continental slopes. Conditions within the deep sea are a combin ...
;
grassland
A grassland is an area where the vegetation is dominated by grasses (Poaceae). However, sedge (Cyperaceae) and rush (Juncaceae) can also be found along with variable proportions of legumes, like clover, and other herbs. Grasslands occur natur ...
s to
desert
A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About on ...
s and some have been domesticated by
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
s.
Anatomy
Ungulates have developed specialized adaptations, especially in the areas of cranial appendages, dentition, and leg morphology including the modification of the
astragalus
''Astragalus'' is a large genus of over 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae and the subfamily Faboideae. It is the largest genus of plants in terms of described species. The genus is native to tempe ...
(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
Toes are the digits (fingers) of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being ''digitigrade''. Humans, and other animals that walk on the soles of their feet, are described as being ''plan ...
of an ungulate
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
, strengthened by a thick horny (
keratin
Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...
) covering. The hoof consists of a hard or rubbery sole, and a hard wall formed by a thick
nail
Nail or Nails may refer to:
In biology
* Nail (anatomy), toughened protective protein-keratin (known as alpha-keratin, also found in hair) at the end of an animal digit, such as fingernail
* Nail (beak), a plate of hard horny tissue at the tip ...
rolled around the tip of the toe. The weight of the animal is normally borne by both the sole and the edge of the hoof wall. Hooves grow continuously, and are constantly worn down by use. In most modern ungulates, the
radius
In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...
and
ulna
The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...
are fused along the length of the forelimb; early ungulates, such as the 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
meridiungulates
South American native ungulates, commonly abbreviated as SANUs, are extinct ungulate-like mammals of controversial affinities that were indigenous to South America prior to the Great American Biotic Interchange. They comprise five major groups c ...
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
Archaeoceti ("ancient whales"), or Zeuglodontes in older literature, is a paraphyletic group of primitive cetaceans that lived from the Early Eocene to the late Oligocene (). Representing the earliest cetacean radiation, they include the initial ...
), also have this characteristic in the addition of also having both an
astragalus
''Astragalus'' is a large genus of over 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae and the subfamily Faboideae. It is the largest genus of plants in terms of described species. The genus is native to tempe ...
and
cuboid bone
In the human body, the cuboid bone is one of the seven tarsal bones of the foot.
Structure
The cuboid bone is the most lateral of the bones in the distal row of the tarsus. It is roughly cubical in shape, and presents a prominence in its inf ...
in the ankle, which were further diagnostic traits of artiodactyls.
In modern cetaceans, the front limbs have become
pectoral fins
Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...
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
In biology, an atavism is a modification of a biological structure whereby an ancestral genetic trait reappears after having been lost through evolutionary change in previous generations. Atavisms can occur in several ways; one of which is when ...
). The main method of moving is an up-and-down motion with the tail fin, called the
fluke
Fluke may refer to:
Biology
* Fluke (fish), a species of marine flatfish
* Fluke (tail), the lobes of the tail of a cetacean, such as dolphins or whales, ichthyosaurs, mosasaurs
Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ...
, which is used for
propulsion
Propulsion is the generation of force by any combination of pushing or pulling to modify the translational motion of an object, which is typically a rigid body (or an articulated rigid body) but may also concern a fluid. The term is derived from ...
, 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
In mammalian oral anatomy, the canine teeth, also called cuspids, dog teeth, or (in the context of the upper jaw) fangs, eye teeth, vampire teeth, or vampire fangs, are the relatively long, pointed teeth. They can appear more flattened howeve ...
and specialized
molars
The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...
, including bunodont (low, rounded cusps) and
hypsodont
Hypsodont is a pattern of dentition with high-crowned teeth and enamel extending past the gum line, providing extra material for wear and tear. Some examples of animals with hypsodont dentition are cows and horses; all animals that feed on gritt ...
(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
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
about 25 million years. As forest biomes declined, grasslands spread, opening new niches for mammals. Many ungulates switched from browsing diets to grazing diets, and possibly driven by abrasive silica in grass, hypsodonty became common. However, recent evidence ties the evolution of hypsodonty to open, gritty habitats and not the grass itself. This is termed the
Grit, not grass hypothesis The grit, not grass hypothesis is an evolutionary hypothesis that explains the evolution of high-crowned teeth, particularly in New World mammals. The hypothesis is that the ingestion of gritty soil is the primary driver of hypsodont tooth developm ...
.
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
Baleen is a filter-feeding system inside the mouths of baleen whales. To use baleen, the whale first opens its mouth underwater to take in water. The whale then pushes the water out, and animals such as krill are filtered by the baleen and ...
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 have evolved a variety of cranial appendages that today can be found in cervoids (with the exception of musk deer). In oxen and antelope, the size and shape of the
horns Horns or The Horns may refer to:
* Plural of Horn (instrument), a group of musical instruments all with a horn-shaped bells
* The Horns (Colorado), a summit on Cheyenne Mountain
* ''Horns'' (novel), a dark fantasy novel written in 2010 by Joe Hill ...
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 of
keratin
Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...
. The unique horn structure is the only unambiguous morphological feature of bovids that distinguishes them from other
pecorans
Pecora is an infraorder of even-toed hoofed mammals with ruminant digestion. Most members of Pecora have cranial appendages projecting from their frontal bones; only two extant genera lack them, ''Hydropotes'' and ''Moschus''. The name “Pecor ...
. 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 are usually smaller than those of males and are sometimes of a different shape. The horns of female bovids are thought to have evolved for defense against
predator
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
s or to express territoriality, as nonterritorial females, which are able to use
crypsis
In ecology, crypsis is the ability of an animal or a plant to avoid observation or detection by other animals. It may be a predation strategy or an antipredator adaptation. Methods include camouflage, nocturnality, subterranean lifestyle and ...
for predator defense, often lack horns.
Rhinoceros horns, unlike those of other horned mammals, consist only of
keratin
Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...
. These horns rest on the nasal ridge of the animal's skull.
Antler
Antlers are extensions of an animal's skull found in members of the Cervidae (deer) family. Antlers are a single structure composed of bone, cartilage, fibrous tissue, skin, nerves, and blood vessels. They are generally found only on male ...
s are unique to cervids and found mostly on males: the only cervid females with antlers are caribou and reindeer, whose antlers are normally smaller than males'. Nevertheless, fertile does of other species of deer have the capacity to produce antlers on occasion, usually due to increased testosterone levels. Each antler grows from an attachment point on the skull called a pedicle. While an antler is growing it is covered with highly
vascular
The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away f ...
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...
called velvet, which supplies oxygen and nutrients to the growing bone. Antlers are 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, initially as
cartilage
Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...
that is then mineralized to become bone. Once the antler has achieved its full size the velvet is lost and the antler's bone dies. This dead bone structure is the mature antler. In most cases the bone at the base is destroyed by
osteoclast
An osteoclast () is a type of bone cell that breaks down bone tissue. This function is critical in the maintenance, repair, and remodeling of bones of the vertebral skeleton. The osteoclast disassembles and digests the composite of hydrated pro ...
s and the antlers eventually fall off. 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.
Ossicone
Ossicones are columnar or conical skin-covered bone structures on the heads of giraffes, male okapi, and some of their extinct relatives. Ossicones are distinguished from the superficially similar structures of horns and antlers by their unique ...
s are
horn
Horn most often refers to:
*Horn (acoustic), a conical or bell shaped aperture used to guide sound
** Horn (instrument), collective name for tube-shaped wind musical instruments
*Horn (anatomy), a pointed, bony projection on the head of various ...
-like (or
antler
Antlers are extensions of an animal's skull found in members of the Cervidae (deer) family. Antlers are a single structure composed of bone, cartilage, fibrous tissue, skin, nerves, and blood vessels. They are generally found only on male ...
-like) protuberances found on the heads of giraffes and male
okapi
The okapi (; ''Okapia johnstoni''), also known as the forest giraffe, Congolese giraffe, or zebra giraffe, is an artiodactyl mammal that is endemic to the northeast Democratic Republic of the Congo in central Africa. It is the only species i ...
s. They are similar to the horns of
antelope
The term antelope is used to refer to many species of even-toed ruminant that are indigenous to various regions in Africa and Eurasia.
Antelope comprise a wastebasket taxon defined as any of numerous Old World grazing and browsing hoofed mammals ...
s and
cattle
Cattle (''Bos taurus'') are large, domesticated, cloven-hooved, herbivores. They are a prominent modern member of the subfamily Bovinae and the most widespread species of the genus ''Bos''. Adult females are referred to as cows and adult mal ...
save that they are derived from ossified
cartilage
Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...
,"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
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...
and
fur
Fur is a thick growth of hair that covers the skin of mammals. It consists of a combination of oily guard hair on top and thick underfur beneath. The guard hair keeps moisture from reaching the skin; the underfur acts as an insulating blanket t ...
rather than horn.
Pronghorn
The pronghorn (, ) (''Antilocapra americana'') is a species of artiodactyl (even-toed, hoofed) mammal indigenous to interior western and central North America. Though not an antelope, it is known colloquially in North America as the American a ...
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
keratin
Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...
ous sheath that is shed and regrown on an annual basis. Unlike the horns of the family Bovidae, the horn sheaths of the pronghorn are branched, each sheath possessing a forward-pointing tine (hence the name pronghorn). The horns of males are well developed.
See also
*
Altungulata
Altungulata or Pantomesaxonia (''sensu'' and later authors) is an invalid clade (mirorder) of ungulate mammals comprising the perissodactyls, hyracoids, and tethytheres (sirenians, proboscideans, and related extinct taxa).
The name "Pantomesa ...