''Hipparion'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of three-toed, medium-sized

equine Equinae is a subfamily of the family Equidae, known from the Hemingfordian stage of the Early Miocene (16 million years ago) onwards. They originated in North America, before dispersing to every continent except Australia and Antarctica. They are ...

belonging to the extinct

tribe The term tribe is used in many different contexts to refer to a category of human social group. The predominant worldwide use of the term in English is in the discipline of anthropology. The definition is contested, in part due to conflict ...

Hipparionini Hipparionini is a tribe of three-toed horses in the subfamily Equinae. They had body forms similar to modern equines, with high-crowned teeth. They first appeared in North America during the Early Miocene around 17 million years ago, before migrat ...

, which lived about 10-5 million years ago. While the genus formerly included most hipparionines, the genus is now more narrowly defined as hipparionines from

North Africa North Africa (sometimes Northern Africa) is a region encompassing the northern portion of the African continent. There is no singularly accepted scope for the region. However, it is sometimes defined as stretching from the Atlantic shores of t ...

spanning the Late

Miocene The Miocene ( ) is the first epoch (geology), 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 mea ...

. ''Hipparion'' was a mixed-feeder who ate mostly grass, and lived in the

savanna A savanna or savannah is a mixed woodland-grassland (i.e. grassy woodland) biome and ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach th ...

h biome. ''Hipparion'' evolved from '' Cormohipparion'', and went extinct due to environmental changes like cooling

climate Climate is the long-term weather pattern in a region, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteoro ...

s and decreasing

atmospheric An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosphere ...

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

levels.

Taxonomy

"Hipparion" in sensu lato

The genus "Hipparion" was used for over a century as a

form classification 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 ...

to describe over a hundred species of Holartic hipparionines from the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Cormohipparion'' ''and

Neohipparion ''Neohipparion'' (Greek: "new" (neos), "pony" (hipparion)) is an extinction, extinct genus of equid, from the Neogene (Miocene to Pliocene) of North America and Central America. Fossils of this horse have been found in Texas, Florida, Kansas, ...

'' were proposed to further sort these species, typically based on differences in skull morphology''.'' These species are now known as "Hipparion" in

sensu lato ''Sensu'' is a Latin word meaning "in the sense of". It is used in a number of fields including biology, geology, linguistics, semiotics, and law. Commonly it refers to how strictly or loosely an expression is used in describing any particular co ...

(s.l.), or a broad sense.

''Hipparion'' in sensu stricto

''Hipparion'' in

sensu stricto ''Sensu'' is a Latin word meaning "in the sense of". It is used in a number of fields including biology, geology, linguistics, semiotics, and law. Commonly it refers to how strictly or loosely an expression is used in describing any particular c ...

(s.s.), or a strict sense, describes the genus of

Old World The "Old World" () is a term for Afro-Eurasia coined by Europeans after 1493, when they became aware of the existence of the Americas. It is used to contrast the continents of Africa, Europe, and Asia in the Eastern Hemisphere, previously ...

hipparionines from remains found in Eurasia (France, Greece, Turkey, Iran, and China) from the Late Miocene era (~10-5 Ma, or million years ago). The assignment of remains from elsewhere to the genus, such as North America and Africa, is uncertain.

Morphology

Fossil_Hipparion_in_Haining_Museum_02_2014-08

Limbs

''Hipparion'' generally resembled a smaller version of the modern horse, but was

tridactyl In biology, dactyly is the arrangement of digits (fingers and toes) on the hands, feet, or sometimes wings of a tetrapod animal. The term is derived from the Greek word () meaning "finger." Sometimes the suffix "-dactylia" is used. The derive ...

, or three-toed. It had two

vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...

outer toes on each limb in addition to its

hoof The hoof (: 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; the ruminants with ...

. In some species, these outer toes were functional.

Size

''Hipparion'' was typically medium in size, at about 1.4 m (4.6 ft) tall at the shoulder. The estimated body mass of ''Hipparion'' depends on the species, but ranges from about 135 to 200 kg (about 298 to 441 lbs).

Skull

''Hipparion'' had

hypsodont Hypsodont is a pattern of dentition characterized by with high crowns, providing extra material for wear and tear. Some examples of animals with hypsodont dentition are cows and horses; all animals that feed on gritty, fibrous material. The oppos ...

dentition (high- crowned teeth) for its

premolars The premolars, also called premolar teeth, or bicuspids, are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant in the permanent set of teeth, making eight premolars total in the mout ...

and

molars The molars or molar teeth are large, flat tooth, teeth at the back of the mouth. They are more developed in mammal, mammals. They are used primarily to comminution, grind food during mastication, chewing. The name ''molar'' derives from Latin, '' ...

, with a crown height of about 60 mm (2.36 in). ''Hipparion'' had isolated protocones in the upper molars, meaning a

cusp A cusp is the most pointed end of a curve. It often refers to cusp (anatomy), a pointed structure on a tooth. Cusp or CUSP may also refer to: Mathematics * Cusp (singularity), a singular point of a curve * Cusp catastrophe, a branch of bifu ...

of the teeth called a protocone was not connected to a tooth crest called a protoloph. ''Hipparion'' is also characterized by its facial fossa, or deep depression in the skull, located high on the head in front of the

orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...

Life

Habitat and diet

''Hipparion'' lived in the Old World Savannah

Biome A biome () is a distinct geographical region with specific climate, vegetation, and animal life. It consists of a biological community that has formed in response to its physical environment and regional climate. In 1935, Tansley added the ...

, or OWSB, consisting of

woodland A woodland () is, in the broad sense, land covered with woody plants (trees and shrubs), or in a narrow sense, synonymous with wood (or in the U.S., the '' plurale tantum'' woods), a low-density forest forming open habitats with plenty of sunli ...

s to

grassland A grassland is an area where the vegetation is dominance (ecology), dominated by grasses (Poaceae). However, sedge (Cyperaceae) and rush (Juncaceae) can also be found along with variable proportions of legumes such as clover, and other Herbaceo ...

s. ''Hipparion'' ate a mixed-feed diet, mostly consisting of

grass Poaceae ( ), also called Gramineae ( ), is a large and nearly ubiquitous family (biology), family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos, the grasses of natural grassland and spe ...

. This diet is indicated by fossil evidence of microscopic

wear Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical (e.g., erosion) or chemical (e.g., corrosion). The study of wear and related processes is referred to as tribology. Wear in ...

patterns of scratches and pits on the enamel of ''Hipparion'''s teeth, observed using

scanning electron microscopy A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that ...

(SEM).

Lifespan

''Hipparion'' achieved skeletal maturity and possibly

sexual maturity Sexual maturity is the capability of an organism to reproduce. In humans, it is related to both puberty and adulthood. ''Puberty'' is the biological process of sexual maturation, while ''adulthood'', the condition of being socially recognized ...

at about 3 years old. Fossils of ''Hipparion'' individuals are up to 10 years old at death.

Evolution and extinction

Evolution

''Hipparion'' likely evolved from a species of ''Cormohipparion'' during the Late Miocene, about 11.4–11.0 Ma''.'' This species, ''C. occidentale,'' came to Eurasia and Africa from North America. The last common ancestor of ''Hipparion'' and the modern horse was ''

Merychippus ''Merychippus'' is an extinct proto-horse of the family Equidae that was endemic to North America during the Miocene, 15.97–5.33 million years ago. It had three toes on each foot and is the first horse known to have Grazing (behaviour), grazed ...

.''

Extinction

In the Old World, ''Hipparion'' experienced population decline and extinction down a North to South gradient, as did many other Miocene vertebrates. This trend is believed to be due to environmental changes caused by global cooling and decreasing carbon dioxide levels in the atmosphere.

Species

References

{{Taxonbar, from=Q971275 Cenozoic mammals of Asia Cenozoic mammals of North America Miocene horses Pliocene horses Pleistocene horses Miocene genus first appearances Pleistocene genus extinctions Cenozoic mammals of Europe Cenozoic mammals of Africa Prehistoric placental genera Fossil taxa described in 1832 Fossils of Tanzania Fossils of Thailand Fossils of Italy Fossils of Romania Fossils of Ukraine Fossils of Algeria Fossils of the United States Fossils of Spain Fossils of Iran Fossils of Austria Fossils of Russia Fossils of Tunisia Fossils of Myanmar Fossils of Nepal Fossils of China Fossils of Hungary Fossils of India Fossils of Turkey Fossils of Iraq Fossils of Ethiopia Fossils of Slovakia Fossils of Syria Fossils of Kazakhstan Fossils of Georgia (country) Fossils of Kyrgyzstan Fossils of Mongolia Fossils of Azerbaijan Fossils of South Africa Fossils of Lebanon Hipparionini