Evolution of the horse - AbsoluteAstronomy.com

The evolution of the horse pertains to the phylogenetic ancestry of the modern horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

from the small dog-sized, forest-dwelling Hyracotherium
Hyracotherium
Hyracotherium , also known as Eohippus or the dawn horse, is an extinct genus of very small perissodactyl ungulates that lived in the woodlands of the northern hemisphere, with species ranging throughout Asia, Europe, and North America during the early Tertiary Period and the early to mid Eocene...

over geologic time scale

Geologic time scale

The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth...

s. Paleozoologists

Paleozoology

Paleozoology, also spelled as palaeozoology , is the branch of paleontology or paleobiology dealing with the recovery and identification of multicellular animal remains from geological contexts, and the use of these fossils in the reconstruction of prehistoric environments and ancient...

have been able to piece together a more complete picture of the modern horse's evolutionary lineage than that of any other animal.

The horse belongs to an order

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

known as Perissodactyla

Odd-toed ungulate

An odd-toed ungulate is a mammal with hooves that feature an odd number of toes. Odd-toed ungulates comprise the order Perissodactyla . The middle toe on each hoof is usually larger than its neighbours...

, or "odd-toed ungulate

Odd-toed ungulate

s", which all share hoof

Hoof

A hoof , plural hooves or hoofs , is the tip of a toe of an ungulate mammal, strengthened by a thick horny covering. The hoof consists of a hard or rubbery sole, and a hard wall formed by a thick nail rolled around the tip of the toe. The weight of the animal is normally borne by both the sole...

ed feet and an odd number of toes on each foot, as well as mobile upper lip

Upper lip

The upper lip covers the anterior surface of the body of the maxilla.Its upper half is of usual skin color and has a depression at its centre, directly under the nasal septum, called the philtrum, while its lower half is a markedly different, red-colored skin tone more similar to the colour of the...

s and a similar tooth

Tooth

Teeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...

structure. This means that horses share a common ancestry with tapir

Tapir

A Tapir is a large browsing mammal, similar in shape to a pig, with a short, prehensile snout. Tapirs inhabit jungle and forest regions of South America, Central America, and Southeast Asia. There are four species of Tapirs: the Brazilian Tapir, the Malayan Tapir, Baird's Tapir and the Mountain...

s and rhinoceros

Rhinoceros

Rhinoceros , also known as rhino, is a group of five extant species of odd-toed ungulates in the family Rhinocerotidae. Two of these species are native to Africa and three to southern Asia....

es. The perissodactyls originally arose in the late Paleocene

Paleocene

The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...

, less than 10 million years after the Cretaceous-Tertiary extinction event

Cretaceous-Tertiary extinction event

The Cretaceous–Paleogene extinction event, formerly named and still commonly referred to as the Cretaceous-Tertiary extinction event, occurred approximately 65.5 million years ago at the end of the Maastrichtian age of the Cretaceous period. It was a large-scale mass extinction of animal and plant...

. This group of animals appears to have been originally specialized for life in tropical forests, but whereas tapirs and, to some extent, rhinoceroses, retained their jungle specializations, modern horses are adapted to life on drier land in the much-harsher climatic conditions of the steppe

Steppe

In physical geography, steppe is an ecoregion, in the montane grasslands and shrublands and temperate grasslands, savannas, and shrublands biomes, characterized by grassland plains without trees apart from those near rivers and lakes...

s. Other species of Equus

Equus (genus)

Equus is a genus of animals in the family Equidae that includes horses, donkeys, and zebras. Within Equidae, Equus is the only extant genus. Like Equidae more broadly, Equus has numerous extinct species known only from fossils. This article deals primarily with the extant species.The term equine...

are adapted to a variety of intermediate conditions.

The early ancestors of the modern horse walked on several spread-out toes, an accommodation to life spent walking on the soft, moist grounds of primeval forests. As grass

Grass

Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...

species began to appear and flourish, the equids' diets shifted from foliage to grasses, leading to larger and more durable teeth. At the same time, as the steppes began to appear, the horse's predecessors needed to be capable of greater speeds to outrun predators. This was attained through the lengthening of limbs and the lifting of some toes from the ground in such a way that the weight of the body was gradually placed on one of the longest toes, the third.

History of research

Indigenous modern horses died out in the New World at the end of the Pleistocene

Pleistocene

The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

, about 12 000 years ago, and thus were absent until the Spanish

Spain

Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...

brought domestic horses from Europe, beginning in 1493. Escaped horses quickly established large wild herds. The early naturalist Buffon

Georges-Louis Leclerc, Comte de Buffon

Georges-Louis Leclerc, Comte de Buffon was a French naturalist, mathematician, cosmologist, and encyclopedic author.His works influenced the next two generations of naturalists, including Jean-Baptiste Lamarck and Georges Cuvier...

suggested in the 1760s that this was an indication of inferiority of fauna in the New World, then later reconsidered this idea. William Clark's 1807 expedition to Big Bone Lick

Big Bone Lick State Park

Big Bone Lick State Park is located at Big Bone in Boone County, Kentucky. It is located on Beaver Road and between the communities of Beaverlick and Rabbit Hash. The name of the park comes from the Pleistocene megafauna fossils found there. The mammoths and other creatures are believed to have...

found "leg and foot bones of the Horses" which were included with other fossils sent to Thomas Jefferson

Thomas Jefferson

Thomas Jefferson was the principal author of the United States Declaration of Independence and the Statute of Virginia for Religious Freedom , the third President of the United States and founder of the University of Virginia...

and evaluated by the anatomist Caspar Wistar

Caspar Wistar (physician)

Caspar Wistar was an American physician and anatomist. He is sometimes referred to as Caspar Wistar the Younger, to distinguish him from his grandfather of the same name.-Biography:...

, but neither commented on the significance of this find.

The first equid fossil was found in the gypsum quarries in Montmartre

Montmartre

Montmartre is a hill which is 130 metres high, giving its name to the surrounding district, in the north of Paris in the 18th arrondissement, a part of the Right Bank. Montmartre is primarily known for the white-domed Basilica of the Sacré Cœur on its summit and as a nightclub district...

, Paris

Paris

Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...

in the 1820s. The tooth was sent to the Paris Conservatory, where it was identified by Georges Cuvier

Georges Cuvier

Georges Chrétien Léopold Dagobert Cuvier or Jean Léopold Nicolas Frédéric Cuvier , known as Georges Cuvier, was a French naturalist and zoologist...

who identified it as a browsing equine related to the tapir

Tapir

. His sketch of the entire animal matched later skeletons found at the site.

During the Beagle survey expedition

Second voyage of HMS Beagle

The second voyage of HMS Beagle, from 27 December 1831 to 2 October 1836, was the second survey expedition of HMS Beagle, under captain Robert FitzRoy who had taken over command of the ship on its first voyage after her previous captain committed suicide...

the young naturalist Charles Darwin

Charles Darwin

Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...

had remarkable success with fossil hunting in Patagonia

Patagonia

Patagonia is a region located in Argentina and Chile, integrating the southernmost section of the Andes mountains to the southwest towards the Pacific ocean and from the east of the cordillera to the valleys it follows south through Colorado River towards Carmen de Patagones in the Atlantic Ocean...

. On 10 October 1833 at Santa Fe, Argentina

Santa Fe, Argentina

Santa Fe is the capital city of province of Santa Fe, Argentina. It sits in northeastern Argentina, near the junction of the Paraná and Salado rivers. It lies opposite the city of Paraná, to which it is linked by the Hernandarias Subfluvial Tunnel. The city is also connected by canal with the...

, he was "filled with astonishment" when he found a horse's tooth in the same stratum

Stratum

In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...

as fossil giant armadillo

Armadillo

Armadillos are New World placental mammals, known for having a leathery armor shell. Dasypodidae is the only surviving family in the order Cingulata, part of the superorder Xenarthra along with the anteaters and sloths. The word armadillo is Spanish for "little armored one"...

s, and wondered if it might have been washed down from a later layer, but concluded that this was "not very probable". After the expedition returned in 1836, the anatomist Richard Owen

Richard Owen

Sir Richard Owen, FRS KCB was an English biologist, comparative anatomist and palaeontologist.Owen is probably best remembered today for coining the word Dinosauria and for his outspoken opposition to Charles Darwin's theory of evolution by natural selection...

confirmed the tooth was from an extinct species which he subsequently named Equus curvidens, and remarked that "This evidence of the former existence of a genus, which, as regards South America, had become extinct, and has a second time been introduced into that Continent, is not one of the least interesting fruits of Mr. Darwin's palæontological discoveries."

In 1848 a study On the fossil horses of America by Joseph Leidy

Joseph Leidy

Joseph Leidy was an American paleontologist.Leidy was professor of anatomy at the University of Pennsylvania, and later was a professor of natural history at Swarthmore College. His book Extinct Fauna of Dakota and Nebraska contained many species not previously described and many previously...

systematically examined Pleistocene

Pleistocene

The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

horse fossils from various collections, including that of the Academy of Natural Sciences

Academy of Natural Sciences

The Academy of Natural Sciences of Drexel University, formerly Academy of Natural Sciences of Philadelphia, is the oldest natural science research institution and museum in the New World...

and concluded at least two ancient horse species had existed in North America: Equus curvidens and another which he named Equus americanus. A decade later, however, he found the latter name had already been taken and renamed it Equus complicatus. In the same year, he visited Europe and was introduced by Owen to Darwin.

The original sequence of species believed to have evolved into the horse was based on fossils discovered in North America in the 1870s by paleontologist Othniel Charles Marsh

Othniel Charles Marsh

Othniel Charles Marsh was an American paleontologist. Marsh was one of the preeminent scientists in the field; the discovery or description of dozens of news species and theories on the origins of birds are among his legacies.Born into a modest family, Marsh was able to afford higher education...

. The sequence, from Hyracotherium (popularly called Eohippus) to the modern horse (Equus), was popularized by Thomas Huxley

Thomas Huxley

Thomas Henry Huxley PC FRS was an English biologist, known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution....

and became one of the most widely-known examples of a clear evolutionary progression. The horse's evolutionary lineage became a common feature of biology textbooks, and the sequence of transitional fossil

Transitional fossil

A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...

s was assembled by the American Museum of Natural History

American Museum of Natural History

The American Museum of Natural History , located on the Upper West Side of Manhattan in New York City, United States, is one of the largest and most celebrated museums in the world...

into an exhibit which emphasized the gradual, "straight-line" evolution of the horse.

Since then, as the number of equid fossils has increased, the actual evolutionary progression from Hyracotherium to Equus has been discovered to be much more complex and multi-branched than was initially supposed. The straight, direct progression from the former to the latter has been replaced by a more elaborate model with numerous branches in different directions, of which the modern horse is only one of many. It was first recognized by George Gaylord Simpson

George Gaylord Simpson

George Gaylord Simpson was an American paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the modern evolutionary synthesis, contributing Tempo and mode in evolution , The meaning of evolution and The major features of...

in 1951 that the modern horse was not the "goal" of the entire lineage of equids, it is simply the only genus of the many horse lineages that has survived.

Detailed fossil information on the rate and distribution of new equid species has also revealed the progression between species was not as smooth and consistent as was once believed. Although some transitions, such as that of Dinohippus
Dinohippus
Dinohippus , is an extinct herbivorous mammal belonging to the tribe Equini, subfamily Equinae, which was endemic to North America from the late Hemphillian stage of the Miocene through the Zanclean stage of the Pliocene and in existence for approximately .-Taxonomy:Dinohippus was named by Quinn...

to Equus, were indeed gradual progressions, a number of others, such as that of Epihippus
Epihippus
Epihippus is an extinct genus of the modern horse family Equidae, that lived in the Eocene, from 46 to 38 million years ago.Epihippus is believed to have evolved from Orohippus , which continued the evolutionary trend of increasingly efficient grinding teeth. Epihippus had five grinding,...

to Mesohippus
Mesohippus
Mesohippus is an extinct genus of early horse. It lived some 40 to 30 million years ago from the late Eocene to the mid-Oligocene...

, were relatively abrupt and sudden in geologic time

Geologic time scale

, taking place over only a few million years. Both anagenesis

Anagenesis

Anagenesis, also known as "phyletic change," is the evolution of species involving an entire population rather than a branching event, as in cladogenesis. When enough mutations have occurred and become stable in a population so that it is significantly differentiated from an ancestral population,...

(gradual change in an entire population's gene frequency) and cladogenesis

Cladogenesis

Cladogenesis is an evolutionary splitting event in a species in which each branch and its smaller branches forms a "clade", an evolutionary mechanism and a process of adaptive evolution that leads to the development of a greater variety of sister species...

(a population "splitting" into two distinct evolutionary branches) occurred, and many species coexisted with "ancestor" species at various times. The change in equids' traits was also not always a "straight line" from Hyracotherium to Equus: some traits reversed themselves at various points in the evolution of new equid species, such as size and the presence of facial fossa
Fossa (anatomy)
In anatomical terminology, fossa has come to mean a depression or hollow, in general, in a bone. Other parts of the body may be involved, for example, the antecubital fossa...

e, and it is only in retrospect that certain evolutionary trends can be recognized.

Hyracotherium

The earliest animal to bear recognizably horse-like anatomy was the Hyracotherium
Hyracotherium
Hyracotherium , also known as Eohippus or the dawn horse, is an extinct genus of very small perissodactyl ungulates that lived in the woodlands of the northern hemisphere, with species ranging throughout Asia, Europe, and North America during the early Tertiary Period and the early to mid Eocene...

("hyrax

Hyrax

A hyrax is any of four species of fairly small, thickset, herbivorous mammals in the order Hyracoidea. The rock hyrax Procavia capensis, the yellow-spotted rock hyrax Heterohyrax brucei, the western tree hyrax Dendrohyrax dorsalis, and the southern tree hyrax, Dendrohyrax arboreus live in Africa...

-like beast"). Its scientific name is derived from initial confusion over early partial fossils' relationship with living species: Richard Owen

Richard Owen

likened early Hyracotherium fossils "to a hare in one passage and to something between a hog and a hyrax in another". A later name for the Hyracotherium, "eohippus" ("dawn horse"), is also popular, though the earlier name takes precedence due to scientific naming conventions.

Hyracotherium lived in the Ypresian

Ypresian

In the geologic timescale the Ypresian is the oldest age or lowest stratigraphic stage of the Eocene. It spans the time between and , is preceded by the Thanetian age and is followed by the Eocene Lutetian age....

(early Eocene

Eocene

The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

), about 52 mya (million years ago). It was an animal approximately the size of a fox (250–450 mm in height), with a relatively short head and neck and a springy, arched back. It had 44 low-crowned teeth, in the typical arrangement of an omnivorous, browsing mammal: 3 incisors, 1 canine, 4 premolar

Premolar

The premolar teeth or bicuspids are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant, making eight premolars total in the mouth. They have at least two cusps. Premolars can be considered as a 'transitional tooth' during chewing, or...

s, and 3 molars on each side of the jaw. Its molars were uneven, dull, and bumpy, and used primarily for grinding foliage. The cusps of the molars were slightly connected in low crests. The Hyracotherium browsed on soft foliage and fruit, probably scampering between thickets in the mode of a modern muntjac

Muntjac

Muntjac, also known as Barking Deer and Mastreani Deer, are small deer of the genus Muntiacus. Muntjac are the oldest known deer, appearing 15–35 million years ago, with remains found in Miocene deposits in France, Germany and Poland....

; the Hyracotherium had a small brain, and possessed especially small frontal lobe

Frontal lobe

The frontal lobe is an area in the brain of humans and other mammals, located at the front of each cerebral hemisphere and positioned anterior to the parietal lobe and superior and anterior to the temporal lobes...

Its limbs were decently long relative to its body, already showing the beginnings of adaptations for running. However, all of the major leg bones were unfused, leaving the legs flexible and rotatable. Its wrist and hock joints were low to the ground. The forelimbs had developed five toes, out of which only four were equipped with a small proto-hoof; the large fifth "toe-thumb" was off the ground. The hind limbs had three out of the five toes equipped with small hooves, while the vestigial

Vestigial structure

Vestigiality describes homologous characters of organisms that have seemingly lost all or most of their original function in a species through evolution. These may take various forms such as anatomical structures, behaviors and biochemical pathways. Some of these disappear early in embryonic...

first and fifth toes did not touch the ground. Its feet were padded, much like a dog's, but with the small hooves on each toe in place of claws.

For a span of about 20 million years, the Hyracotherium thrived with few significant evolutionary changes. The most significant change was in the teeth, which began to adapt to the changing diet of Hyracotheria, as these early Equidae

Equidae

Equidae is the taxonomic family of horses and related animals, including the extant horses, donkeys, and zebras, and many other species known only from fossils. All extant species are in the genus Equus...

shifted from a mixed diet of fruits and foliage to one focused increasingly on browsing foods. During the Eocene, a Hyracotherium species (most likely Hyracotherium vassacciense) branched out into various new types of Equidae. Thousands of complete, fossilized skeletons of these animals have been found in the Eocene layers of North American strata, mainly in the Wind River

Wind River (Wyoming)

The Wind River is the name applied to the upper reaches of the Bighorn River in Wyoming in the United States. The Wind River is long. The two rivers are sometimes referred to as the Wind/Bighorn.-Course:...

basin in Wyoming

Wyoming

Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...

. Similar fossils have also been discovered in Europe, such as Propalaeotherium
Propalaeotherium
Propalaeotherium was an early genus of perissodactyl ancestral to the horse endemic to Europe and Asia during the Middle Eocene.Its name means "before Palaeotherium", as it is the ancestor of Palaeotherium, another relative of early horses...

(which is not considered ancestral to the modern horse).

Orohippus

Approximately 50 million years ago, in the early-to-middle Eocene, Hyracotherium smoothly transitioned into Orohippus
Orohippus
Orohippus is an extinct ancestor of the modern horse that lived in the Eocene .thumb|left|Restoration...

over a gradual series of changes. Although its name means "mountain horse", Orohippus was not a true horse and did not live in the mountains. It resembled Hyracotherium in size, but had a slimmer body, an elongated head, slimmer forelimbs, and longer hind legs, all of which are characteristics of a good jumper. Although Orohippus was still pad-footed, the vestigial outer toes of Hyracotherium were not present in the Orohippus; there were four toes on each forelimb, and three on each hind leg.

The most dramatic change between Hyracotherium and Orohippus was in the teeth: the first of the premolar teeth were dwarfed, the last premolar shifted in shape and function into a molar, and the crests on the teeth became more pronounced. Both of these factors gave the teeth of Orohippus greater grinding ability, suggesting that Orohippus ate tougher plant material.

Epihippus

In the mid-Eocene, about 47 million years ago, Epihippus
Epihippus
Epihippus is an extinct genus of the modern horse family Equidae, that lived in the Eocene, from 46 to 38 million years ago.Epihippus is believed to have evolved from Orohippus , which continued the evolutionary trend of increasingly efficient grinding teeth. Epihippus had five grinding,...

, a genus which continued the evolutionary trend of increasingly efficient grinding teeth, evolved from Orohippus. Epihippus had five grinding, low-crowned cheek teeth with well-formed crests. A late species of Epihippus, sometimes referred to as Duchesnehippus intermedius, had teeth similar to Oligocene

Oligocene

The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...

equids, although slightly less developed. Whether Duchesnehippus was a subgenus of Epihippus or a distinct genus is disputed.

Mesohippus

In the late Eocene and the early stages of the Oligocene

Oligocene

epoch (32–24 mya), the climate of North America became drier, and the earliest grass

Grass

es began to evolve. The forests were yielding to flatlands, home to grasses and various kinds of brush. In a few areas these plains were covered in sand

Sand

Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

, creating the type of environment resembling the present-day prairie

Prairie

Prairies are considered part of the temperate grasslands, savannas, and shrublands biome by ecologists, based on similar temperate climates, moderate rainfall, and grasses, herbs, and shrubs, rather than trees, as the dominant vegetation type...

s.

In response to the changing environment, the then-living species of Equidae also began to change. In the late Eocene, they began developing tougher teeth and becoming slightly larger and leggier, allowing for faster running speeds in open areas, and thus for evading predators in non-wooded areas. About 40 mya, Mesohippus
Mesohippus
Mesohippus is an extinct genus of early horse. It lived some 40 to 30 million years ago from the late Eocene to the mid-Oligocene...

("middle horse") suddenly developed in response to strong new selective

Selection

In the context of evolution, certain traits or alleles of genes segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the...

pressures to adapt, beginning with the species Mesohippus celer and soon followed by Mesohippus westoni.

In the early Oligocene, Mesohippus was one of the more widespread mammals in North America. It walked on three toes on each of its front and hind feet (the first and fifth toes remained, but were small and not used in walking). The third toe was stronger than the outer ones, and thus more weighted; the fourth front toe was diminished to a vestigial nub. Judging by its longer and slimmer limbs, Mesohippus was an agile animal.

Mesohippus was slightly larger than Epihippus, about 610 mm (24") at the shoulder. Its back was less arched, and its face, snout, and neck were somewhat longer. It had significantly larger cerebral hemisphere

Cerebral hemisphere

A cerebral hemisphere is one of the two regions of the eutherian brain that are delineated by the median plane, . The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter called the cerebral cortex that is...

s, and had a small, shallow depression on its skull called a fossa
Fossa
Fossa may refer to:*Fossa dei Leoni, an Italian football supporters association of Milan*La Fossa, an Italian rap group*Formaggio di fossa, a sheep's milk cheese from Sogliano al Rubicone, Italy*Cacio di Fossa, a hard, sharp sheep's milk Italian cheese...

, which in modern horses is quite detailed. The fossa serves as a useful marker for identifying an equine fossil's species. Mesohippus had six grinding "cheek teeth", with a single premolar in front—a trait all descendant Equidae would retain. Mesohippus also had the sharp tooth crests of Epihippus, improving its ability to grind down tough vegetation.

Miohippus

Around 36 million years ago, soon after the development of Mesohippus, Miohippus
Miohippus
Miohippus was a genus of prehistoric horse existing longer than most Equidae. Miohippus lived in what is now North America during the Oligocene approximately 32-25 million years ago. While descending genera of this species lived during the Miocene period, the Miohippus was a horse of the Oligocene...

("lesser horse") emerged, the earliest species being Miohippus assiniboiensis. Like Mesohippus, Miohippus's evolution was relatively abrupt, though a few transitional fossils linking the two genera have been found. It was once believed that Mesohippus had anagenetically

Anagenesis

evolved into Miohippus by a gradual series of progressions, but new evidence has shown that Miohippus's evolution was cladogenetic

Cladogenesis

: a Miohippus population split off from the main Mesohippus genus, coexisted with Mesohippus for around 4 million years, and then over time came to replace Mesohippus.

Miohippus was significantly larger than its predecessors, and its ankle joints had subtly changed. Its facial fossa was larger and deeper, and it also began to show a variable extra crest in its upper cheek teeth, a trait that became a characteristic feature of equine teeth.

Miohippus ushered in a major new period of diversification in Equidae. While Mesohippus died out in the mid-Oligocene, Miohippus continued to thrive, and in the early Miocene

Miocene

The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

(24–5.3 mya), it began to rapidly diversify and speciate. It branched out into two major groups, one of which adjusted to the life in forests once again, while the other remained suited to life on the prairies.

Kalobatippus

The forest-suited form was Kalobatippus (or Miohippus intermedius, depending on whether it was a new genus or species), whose second and fourth front toes were long, well-suited travel on the soft forest floors. Kalobatippus probably gave rise to Anchitherium
Anchitherium
Anchitherium was a fossil horse with a three-toed hoof.Anchitherium was a browsing horse that originated in the early Miocene of North America and subsequently dispersed to Europe and Asia, where it gave rise to the larger bodied genus Sinohippus...

, which travelled to Asia via the Bering Strait

Bering Strait

The Bering Strait , known to natives as Imakpik, is a sea strait between Cape Dezhnev, Chukotka Autonomous Okrug, Russia, the easternmost point of the Asian continent and Cape Prince of Wales, Alaska, USA, the westernmost point of the North American continent, with latitude of about 65°40'N,...

land bridge

Land bridge

A land bridge, in biogeography, is an isthmus or wider land connection between otherwise separate areas, over which animals and plants are able to cross and colonise new lands...

, and from there to Europe. In both North America and Eurasia, larger-bodied genera evolved from Anchitherium; Sinohippus in Eurasia and Hypohippus
Hypohippus
Hypohippus is an extinct genus of three-toed Horse, which lived 17 - 11 million years ago. It was the size of a pony, and fossils of it have been found in Nebraska, Colorado, and Montana....

and Megahippus
Megahippus
Megahippus is an extinct equid genus belonging to the subfamily Anchitheriinae. As with other members of this subfamily, Megahippus is more primitive than the living horses. Fossil remains of Megahippus have been found across the U.S., from Montana to Florida.-References:* *...

in North America. Hypohippus became extinct by the late Miocene.

Parahippus

The Miohippus population that remained on the steppes is believed to be ancestral to Parahippus
Parahippus
Parahippus is an extinct relative of the modern horse, very similar to Miohippus, but slightly larger, at around tall, at the withers....

, a North American animal about the size of a small pony

Pony

A pony is a small horse . Depending on context, a pony may be a horse that is under an approximate or exact height at the withers, or a small horse with a specific conformation and temperament. There are many different breeds...

, with a prolonged skull and a facial structure resembling the horses of today. Its third toe was stronger and larger, and carried the main weight of the body. Its four premolars resembled the molar teeth and the first were small and almost nonexistent. The incisive teeth of Parahippus, like those of its predecessors, had a crown as humans do; however, the top incisors had a trace of a shallow crease marking the beginning of the core/cup.

Merychippus

In the middle of the Miocene epoch, the grazer Merychippus
Merychippus
Merychippus is an extinct proto-horse of the family Equidae that was endemic to North America during the Miocene from 20.43—10.3 Ma living for approximately .It had three toes on each foot and is the first horse known to have grazed...

flourished. Merychippus had wider molars than its predecessors, which are believed to have been used for crunching the hard grasses of the steppes. The hind legs, which were relatively short, had side toes equipped with small hooves, but they probably only touched the ground when running. Merychippus
Merychippus
Merychippus is an extinct proto-horse of the family Equidae that was endemic to North America during the Miocene from 20.43—10.3 Ma living for approximately .It had three toes on each foot and is the first horse known to have grazed...

radiated into at least 19 additional grassland species.

Hipparion

Three lineages within Equidae are believed to be descended from the numerous varieties of Merychippus: Hipparion
Hipparion
Hipparion is an extinct genus of horse living in North America, Asia, Europe, and Africa during the Miocene through Pleistocene ~23 Mya—781,000 years ago, existing for...

, Protohippus
Protohippus
Protohippus is an extinct genus of horse....

and Pliohippus
Pliohippus
Pliohippus is an extinct genus of Equidae, the "horse family". Pliohippus arose in the middle Miocene, around 12 million years ago, probably from Calippus. It was similar in appearance to Equus, but had two long extra toes on both sides of the hoof, externally barely visible as callused stubs...

. The most different from Merychippus was Hipparion. The main difference was in the structure of tooth enamel

Tooth enamel

Tooth enamel, along with dentin, cementum, and dental pulp is one of the four major tissues that make up the tooth in vertebrates. It is the hardest and most highly mineralized substance in the human body. Tooth enamel is also found in the dermal denticles of sharks...

: in comparison with other Equidae, the inside, or tongue

Tongue

The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste , as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly...

side, had a completely isolated parapet

Parapet

A parapet is a wall-like barrier at the edge of a roof, terrace, balcony or other structure. Where extending above a roof, it may simply be the portion of an exterior wall that continues above the line of the roof surface, or may be a continuation of a vertical feature beneath the roof such as a...

. A complete and well-preserved skeleton of the North American Hipparion shows an animal the size of a small pony. They were very slim, rather like antelope

Antelope

Antelope is a term referring to many even-toed ungulate species indigenous to various regions in Africa and Eurasia. Antelopes comprise a miscellaneous group within the family Bovidae, encompassing those old-world species that are neither cattle, sheep, buffalo, bison, nor goats...

s, and were adapted to life on dry prairies. On its slim legs, Hipparion had three toes equipped with small hooves, but the side toes did not touch the ground.

In North America, Hipparion and its relatives (Cormohipparion
Cormohipparion
The extinct Cormohipparion was originally described as a new genus of horse, and assigned to the tribe Hipparionini. However it was soon argued that the partial material fell within the range of morphological variation seen in Hipparion, and that the members of Cormohipparion belonged instead...

, Nannippus
Nannippus
Nannippus is an extinct genus of horse endemic to North America during the Miocene through Pliocene, ~13.3—3.3 Ma, living approximately .Nannippus lived as far south as Central Mexico to as far north as Canada , to California in the west, North Carolina and Florida Nannippus is an extinct genus...

, Neohipparion, and Pseudhipparion), proliferated into many kinds of equid

Equidae

s, at least one of which managed to migrate to Asia and Europe during the Miocene epoch. (European Hipparion differs from American Hipparion in its smaller body size – the best-known discovery of these fossils was near Athens.)

Pliohippus

Pliohippus
Pliohippus
Pliohippus is an extinct genus of Equidae, the "horse family". Pliohippus arose in the middle Miocene, around 12 million years ago, probably from Calippus. It was similar in appearance to Equus, but had two long extra toes on both sides of the hoof, externally barely visible as callused stubs...

arose from Callippus in the middle Miocene, around 12 mya. It was very similar in appearance to Equus
Equus (genus)
Equus is a genus of animals in the family Equidae that includes horses, donkeys, and zebras. Within Equidae, Equus is the only extant genus. Like Equidae more broadly, Equus has numerous extinct species known only from fossils. This article deals primarily with the extant species.The term equine...

, though it had two long extra toes on both sides of the hoof, externally barely visible as callused stubs. The long and slim limbs of Pliohippus reveal a quick-footed steppe animal.

Until recently, Pliohippus was believed to be the ancestor of present-day horses because of its many anatomical similarities. However, though Pliohippus was clearly a close relative of Equus, its skull had deep facial fossae, whereas Equus had no fossae at all. Additionally, its teeth were strongly curved, unlike the very straight teeth of modern horses. Consequently, it is unlikely to be the ancestor of the modern horse; instead, it is a likely candidate for the ancestor of Astrohippus
Astrohippus
Astrohippus is an extinct member of the Equidae tribe Equini, the same tribe that contains the only living equid genus, Equus. Fossil remains have been found in the central United States, Florida, and the Mexican states of Chihuahua, Jalisco, and Guanajuato...

.

Dinohippus

Dinohippus
Dinohippus
Dinohippus , is an extinct herbivorous mammal belonging to the tribe Equini, subfamily Equinae, which was endemic to North America from the late Hemphillian stage of the Miocene through the Zanclean stage of the Pliocene and in existence for approximately .-Taxonomy:Dinohippus was named by Quinn...

was the most common species of Equidae in North America during the late Pliocene

Pliocene

The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...

. It was originally thought that Dinohippus was monodactyl, but a 1981 fossil find in Nebraska shows that some were tridactyl.

Plesippus

Plesippus
Plesippus
Plesippus is often considered an intermediary stage between Dinohippus and the present day genus, Equus.The famous fossils found near Hagerman, Idaho were originally thought to be a part of the genus Plesippus Hagerman Fossil Beds , a Pliocene site, dating about 3.5 Ma ago...

is often considered an intermediate stage between Dinohippus and the extant genus, Equus.

The famous fossils

Hagerman Horse

The Hagerman horse , also called the Hagerman zebra or the American zebra, was a North American species of equid from the Pliocene period and the Pleistocene period. It was one of the oldest horses of the genus Equus. Discovered in 1928 in Hagerman, Idaho, it is believed to have been like the...

found near Hagerman, Idaho were originally thought to be a part of the genus Plesippus. Hagerman Fossil Beds

Hagerman Fossil Beds National Monument

Hagerman Fossil Beds National Monument near Hagerman, Idaho, contains the largest concentration of Hagerman Horse fossils in North America. The fossil horses for which the Monument is famous have been found in only one locale in the northern portion of the Monument called the Hagerman Horse Quarry...

(Idaho) is a Pliocene site, dating to about 3.5 mya. The fossilized remains were originally called Plesippus shoshonensis, but further study by paleontologists determined that fossils represented the oldest remains of the genus Equus. Their estimated average weight was 425 kg, roughly the size of an Arabian horse

Arabian horse

The Arabian or Arab horse is a breed of horse that originated on the Arabian Peninsula. With a distinctive head shape and high tail carriage, the Arabian is one of the most easily recognizable horse breeds in the world. It is also one of the oldest breeds, with archaeological evidence of horses...

.

At the end of the Pliocene, the climate in North America began to cool significantly and most of the animals were forced to move south. One population of Plesippus moved across the Bering land bridge into Eurasia around 2.5 Mya.

Equus

The genus Equus, which includes all extant equines, is believed to have evolved from Dinohippus
Dinohippus
Dinohippus , is an extinct herbivorous mammal belonging to the tribe Equini, subfamily Equinae, which was endemic to North America from the late Hemphillian stage of the Miocene through the Zanclean stage of the Pliocene and in existence for approximately .-Taxonomy:Dinohippus was named by Quinn...

, via the intermediate form Plesippus
Plesippus
Plesippus is often considered an intermediary stage between Dinohippus and the present day genus, Equus.The famous fossils found near Hagerman, Idaho were originally thought to be a part of the genus Plesippus Hagerman Fossil Beds , a Pliocene site, dating about 3.5 Ma ago...

. One of the oldest species is Equus simplicidens
Hagerman Horse
The Hagerman horse , also called the Hagerman zebra or the American zebra, was a North American species of equid from the Pliocene period and the Pleistocene period. It was one of the oldest horses of the genus Equus. Discovered in 1928 in Hagerman, Idaho, it is believed to have been like the...

, described as zebra-like with a donkey-shaped head. The oldest material to date is ~3.5 million years old from Idaho, USA. The genus appears to have spread quickly into the Old World, with the similarly aged Equus livenzovensis documented from western Europe and Russia.

Molecular phylogenies indicate that the most recent common ancestor of all modern equids (members of the genus Equus) lived ~5.6 (3.9-7.8) mya. The oldest divergencies are the Asian hemiones (subgenus E. (Asinus
Asinus
The subgenus Asinus encompasses four species and several subspecies of Equidae characterized by long ears, a lean, straight-backed build, a scant tail, and a reputation for considerable toughness and endurance....

)), including the Kulan

Kulan

Khulan was an empress of Genghis Khan and head of the second Court of Genghis Khan. Her status in the Mongol Empire was second only to Grand Empress Borte.-Biography:...

, Onager

Onager

The Onager is a large member of the genus Equus of the family Equidae native to the deserts of Syria, Iran, Pakistan, India, Israel and Tibet...

, and Kiang

Kiang

The kiang is the largest of the wild asses. It is native to the Tibetan Plateau, where it inhabits montane and alpine grasslands. Its current range is restricted to Ladakh in Jammu and Kashmir, plains of the Tibetan plateau and northern Nepal along the Tibetan border...

), followed by the African zebras (subgenera E. (Dolichohippus), and E. (Hippotigris)). All other modern forms including the domesticated horse (and many fossil Pliocene

Pliocene

and Pleistocene

Pleistocene

The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

forms) belong to the subgenus E. (Equus
Equus
Equus may refer to:* Equus , a genus of animals including horses, donkeys, zebras and onagers* Equus , a play by Peter Shaffer* Equus , a film adaptation of the Peter Shaffer play...

) which diverged ~4.8 (3.2-6.5) million years ago.

Pleistocene horse fossils have been assigned to a multitude of species, with over 50 species of equines described from the Pleistocene

Pleistocene

The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

of North America alone, although the taxonomic validity of most of these has been called into question. Recent genetic work on fossils has found evidence for only three genetically divergent

Genetic divergence

Genetic divergence is the process in which two or more populations of an ancestral species accumulate independent genetic changes through time, often after the populations have become reproductively isolated for some period of time...

equid lineages in Pleistocene North and South America. These results suggest that all North American fossils of caballine-type horses (which also include the domesticated horse

Horse

and Przewalski's Horse

Przewalski's Horse

Przewalski's Horse or Dzungarian Horse, is a rare and endangered subspecies of wild horse native to the steppes of central Asia, specifically China and Mongolia.At one time extinct in the wild, it has been reintroduced to its native habitat in Mongolia at the Khustain Nuruu...

of Europe and Asia), as well as South American fossils traditionally placed in the subgenus E. (Amerhippus) belong to the same species: E. ferus
Wild Horse
The wild horse is a species of the genus Equus, which includes as subspecies the domesticated horse as well as the undomesticated Tarpan and Przewalski's Horse. The Tarpan became extinct in the 19th century, and Przewalski's Horse was saved from the brink of extinction and reintroduced...

. Remains attributed to a variety of species and lumped as New World stilt-legged horses (including E. francisci, E. tau, E. quinni and potentially N. American Pleistocene fossils previously attributed to E. cf. hemiones, and E. (Asinus) cf. kiang) likely all belong to a second species endemic to N. America, which despite a superficial resemblance to species in the subgenus E. (Asinus) (and hence occasionally referred to as North American Ass) is closely related to E. ferus. Surprisingly, the third species, endemic to S. America, and traditionally referred to as Hippidion
Hippidion
Hippidion was a Welsh pony-sized horse that lived in South America during the Pleistocene epoch, between two million and 10,000 years ago....

, originally believed to be descended from Pliohippus, was shown to be a third species in the genus Equus, closely related to the New World stilt-legged horse. The temporal and regional variation in body size and morphological features within each lineage indicates extraordinary intraspecific

Intraspecific

Intraspecific is a term used in biology to describe behaviors, biochemical variations and other issues within individuals of a single species, thereby contrasting with interspecific. For example:* Intraspecific antagonism...

plasticity. Such environment-driven adaptative changes would explain why the taxonomic diversity of Pleistocene equids has been overestimated on morphoanatomical grounds.

According to these results, it appears that the genus Equus evolved from a Dinohippus-like ancestor ~4-7 mya. It rapidly spread into the Old World and there diversified into the various species of asses and zebras. A North American lineage of the subgenus E. (Equus) evolved into the New World stilt-legged horse (NWSLH). Subsequently, populations of this species entered South America as part of the Great American Interchange

Great American Interchange

The Great American Interchange was an important paleozoogeographic event in which land and freshwater fauna migrated from North America via Central America to South America and vice versa, as the volcanic Isthmus of Panama rose up from the sea floor and bridged the formerly separated continents...

shortly after the formation of the Isthmus of Panama and evolved into the form currently referred to as "Hippidion" ~2.5 million years ago. "Hippidion" is thus unrelated to the morphologically similar Pliohippus
Pliohippus
Pliohippus is an extinct genus of Equidae, the "horse family". Pliohippus arose in the middle Miocene, around 12 million years ago, probably from Calippus. It was similar in appearance to Equus, but had two long extra toes on both sides of the hoof, externally barely visible as callused stubs...

, which presumably went extinct during the Miocene

Miocene

. Both the NWSLH and "Hippidium" show adaptations to dry, barren ground, whereas the shortened legs of "Hippidion" may have been a response to sloped terrain. In contrast, the geographic origin of the closely related modern E. ferus is not resolved. However, genetic results on extant and fossil material of Pleistocene

Pleistocene

The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

age indicate two clades, potentially subspecies, one of which had a holarctic

Holarctic

The Holarctic ecozone refers to the habitats found throughout the northern continents of the world as a whole. This region is divided into the Palearctic, consisting of Northern Africa and all of Eurasia, with the exception of Southeast Asia and the Indian subcontinent, and the Nearctic,...

distribution spanning from Europe through Asia and across North America and would become the founding stock of the modern domesticated horse. The other population appears to have been restricted to N. America. One or more N. American populations of E. ferus entered S. America ~1.0-1.5 million years ago, leading to the forms currently known as "E. (Amerhippus)", which represent an extinct geographic variant or race of E. ferus, however.

Pleistocene extinctions

Digs in western Canada have unearthed clear evidence that horses existed in North America until about 12,000 years ago. However, all Equidae in North America ultimately became extinct. The causes of this extinction (simultaneous with the extinctions of a variety of other American megafauna

Megafauna

In terrestrial zoology, megafauna are "giant", "very large" or "large" animals. The most common thresholds used are or...

) have been a matter of debate. Given the suddenness of the event and the fact that these mammals had been flourishing for millions of years previously, something quite unusual must have happened. There are two main hypotheses. The first attributes extinction to climate change

Climate change

Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...

. For example, in Alaska

Alaska

Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...

, beginning approximately 12,500 years ago, the grasses characteristic of a steppe

Steppe

ecosystem gave way to shrub tundra

Tundra

In physical geography, tundra is a biome where the tree growth is hindered by low temperatures and short growing seasons. The term tundra comes through Russian тундра from the Kildin Sami word tūndâr "uplands," "treeless mountain tract." There are three types of tundra: Arctic tundra, alpine...

, which was covered with unpalatable plants. Another hypothesis suggests extinction was linked to overexploitation

Overexploitation

Overexploitation, also called overharvesting, refers to harvesting a renewable resource to the point of diminishing returns. Sustained overexploitation can lead to the destruction of the resource...

of naive prey by newly arrived humans. Extinctions were roughly simultaneous with the end of the most recent glacial advance and the appearance of the big-game-hunting Clovis culture

Clovis culture

The Clovis culture is a prehistoric Paleo-Indian culture that first appears 11,500 RCYBP , at the end of the last glacial period, characterized by the manufacture of "Clovis points" and distinctive bone and ivory tools...

. Several studies have indicated that humans probably arrived in Alaska at the same time or shortly before the local extinction of horses. Additionally, it has been proposed that the steppe-tundra vegetation transition in Beringia may have been a consequence, rather than a cause, of the extinction of megafaunal grazers.

In Eurasia, horse fossils began occurring frequently again in archaeological sites in Kazakhstan

Kazakhstan

Kazakhstan , officially the Republic of Kazakhstan, is a transcontinental country in Central Asia and Eastern Europe. Ranked as the ninth largest country in the world, it is also the world's largest landlocked country; its territory of is greater than Western Europe...

and the southern Ukraine

Ukraine

Ukraine is a country in Eastern Europe. It has an area of 603,628 km², making it the second largest contiguous country on the European continent, after Russia...

about 6,000 years ago. From then on, it is probable that domesticated horses

Domestication of the horse

There are a number of hypotheses on many of the key issues regarding the domestication of the horse. Although horses appeared in Paleolithic cave art as early as 30,000 BCE, these were truly wild horses and were probably hunted for meat. How and when horses became domesticated is disputed...

as well as the knowledge of capturing, taming, and rearing horses spread relatively quickly, with wild mares from several wild populations being incorporated en route.

Return to the Americas

Horses only returned to the Americas with Christopher Columbus

Christopher Columbus

Christopher Columbus was an explorer, colonizer, and navigator, born in the Republic of Genoa, in northwestern Italy. Under the auspices of the Catholic Monarchs of Spain, he completed four voyages across the Atlantic Ocean that led to general European awareness of the American continents in the...

in 1493. These were Iberian horse

Iberian horse

The Iberian horse is a title given to a number of horse breeds native to the Iberian peninsula. At present, 17 horse breeds are recognized by FAO as characteristic of the Iberian Peninsula....

s first brought to Hispaniola

Hispaniola

Hispaniola is a major island in the Caribbean, containing the two sovereign states of the Dominican Republic and Haiti. The island is located between the islands of Cuba to the west and Puerto Rico to the east, within the hurricane belt...

and later to Panama

Panama

Panama , officially the Republic of Panama , is the southernmost country of Central America. Situated on the isthmus connecting North and South America, it is bordered by Costa Rica to the northwest, Colombia to the southeast, the Caribbean Sea to the north and the Pacific Ocean to the south. The...

, Mexico

Mexico

The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...

, Brazil

Brazil

Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

, Peru

Peru

Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....

, Argentina

Argentina

Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...

, and, in 1538, Florida

Florida

Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...

. The first horses to return to the main continent were 16 specifically identified horses brought by Hernan Cortes

Hernán Cortés

Hernán Cortés de Monroy y Pizarro, 1st Marquis of the Valley of Oaxaca was a Spanish Conquistador who led an expedition that caused the fall of the Aztec Empire and brought large portions of mainland Mexico under the rule of the King of Castile in the early 16th century...

. Subsequent explorers, such as Coronado

Francisco Vásquez de Coronado

Francisco Vásquez de Coronado y Luján was a Spanish conquistador, who visited New Mexico and other parts of what are now the southwestern United States between 1540 and 1542...

and De Soto brought ever-larger numbers, some from Spain and other from breeding establishments set up by the Spanish in the Caribbean. Later, as Spanish missions were founded on the mainland, horses would eventually be lost or stolen, and proliferated into large herds of feral horse

Feral horse

A feral horse is a free-roaming horse of domesticated ancestry. As such, a feral horse is not a wild animal in the sense of an animal without domesticated ancestors. However, some populations of feral horses are managed as wildlife, and these horses often are popularly called "wild" horses...

s that became known as mustang

Mustang (horse)

A Mustang is a free-roaming horse of the North American west that first descended from horses brought to the Americas by the Spanish. Mustangs are often referred to as wild horses, but there is intense debate over terminology...

s.

The indigenous peoples of the Americas

Indigenous peoples of the Americas

The indigenous peoples of the Americas are the pre-Columbian inhabitants of North and South America, their descendants and other ethnic groups who are identified with those peoples. Indigenous peoples are known in Canada as Aboriginal peoples, and in the United States as Native Americans...

did not have a specific word for horses, and came to refer to them in various languages as a type of dog or deer (in one case, "elk-dog").

Toes

The ancestors of the horse came to walk only on the end of the third toe and both side toes. Skeletal remnants show obvious wear on the back of both sides of metacarpal and metatarsal bones, commonly called the “splint bones”. They are the remnants of the second and the fourth toe. Modern horses retain the splint bones; it is often believed that they are a useless attachment, but they in fact play an important role in supporting the carpal joints (front knee) and even the tarsal joints (hock).

Teeth

Throughout the phylogenetic development, the teeth of the horse underwent significant changes. The type of the original omnivorous

Omnivore

Omnivores are species that eat both plants and animals as their primary food source...

teeth with short, "bumpy" molars, with which the prime members of the evolutionary line distinguished themselves, gradually changed into the teeth common to herbivorous

Herbivore

Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...

mammals. They became long (as much as 100 mm), roughly cubical molars equipped with a flat grinding surface. In conjunction with the teeth, during the horse’s evolution the elongation of the facial part of the skull is apparent, and can also be observed in the backward set eyeholes. In addition, the relatively short neck of the equine ancestors became longer with equal elongation of the legs. Finally, the size of the body grew as well.

External links

: An excerpted collection of images of horse fossils

Evolution of Horses

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