''Merychippus'' is an extinct proto-

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 mi ...

of the family

Equidae Equidae (commonly known as the horse family) is the Taxonomy (biology), taxonomic Family (biology), family of Wild horse, horses and related animals, including Asinus, asses, zebra, zebras, and many extinct species known only from fossils. The fa ...

that was endemic to

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

during the

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 ...

, 15.97–5.33

million years ago Million years ago, abbreviated as Mya, Myr (megayear) or Ma (megaannum), is a unit of time equal to (i.e. years), or approximately 31.6 teraseconds. Usage Myr is in common use in fields such as Earth science and cosmology. Myr is also used w ...

. It had three toes on each foot and is the first horse known to have grazed.

Discovery and naming

''Merychippus'' was named by Joseph Leidy (1856). Numerous authors assigned the type species – ''Merychippus insignis'' – to ''

Protohippus ''Protohippus'' is an extinct three-toed genus of horse. It was roughly the size of a modern donkey. Fossil evidence suggests that it lived during the Late Miocene (Clarendonian to Hemphillian The Hemphillian North American Stage on the geologic ...

'', but this is ignored. It was assigned to the

by Leidy (1856) and Carroll (1988), and to the

Equinae 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 ar ...

by MacFadden (1998) and Bravo-Cuevas and Ferrusquía-Villafranca (2006). Its name means "

ruminant Ruminants are herbivorous grazing or browsing artiodactyls belonging to the suborder Ruminantia that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microb ...

horse", but current evidence does not support ''Merychippus'' ruminating.

Description

''Merychippus'' lived in groups. It was about tall and at the time it was the tallest equine to have existed. Its muzzle was longer, deeper jaw, and eyes wider apart than any other horse-like animal to date. The brain was also much larger, making it smarter and more agile. ''Merychippus'' was the first equine to have the distinctive head shape of today's horses. The Miocene was a time of drastic change in environment, with

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 transforming into grass plains.MacFadden, B. J.. 1992. Fossil Horses: Systematics, Paleobiology, and Evolution of the Family Equidae. Cambridge University Press, Cambridge. This led to evolutionary changes in the hooves and teeth of

equid Equidae (commonly known as the horse family) is the Taxonomy (biology), taxonomic Family (biology), family of Wild horse, horses and related animals, including Asinus, asses, zebra, zebras, and many extinct species known only from fossils. The fa ...

s. A change in surface from soft, uneven mud to hard

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 meant there was less need for increased

surface area The surface area (symbol ''A'') of a solid object is a measure of the total area that the surface of the object occupies. The mathematical definition of surface area in the presence of curved surfaces is considerably more involved than the d ...

. The foot was fully supported by

ligament A ligament is a type of fibrous connective tissue in the body that connects bones to other bones. It also connects flight feathers to bones, in dinosaurs and birds. All 30,000 species of amniotes (land animals with internal bones) have liga ...

s, and the middle toe developed into a

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 ...

that did not have a pad on the bottom. In some ''Merychippus'' species, the side toes were larger, whereas in others, they had become smaller and only touched the ground when running. The transformation into plains also meant ''Merychippus'' began consuming more

phytolith Phytoliths (from Greek language, Greek, "plant stone") are rigid, microscopic mineral deposits found in some plant tissues, often persisting after the decay of the plant. Although some use "phytolith" to refer to all mineral secretions by plants, ...

rich plants. This led to the presence of

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 ...

teeth. Such teeth range from medium to intense crown height, are curved, covered in large amounts of cement, and are characteristic of

grazing In agriculture, grazing is a method of animal husbandry whereby domestic livestock are allowed outdoors to free range (roam around) and consume wild vegetations in order to feed conversion ratio, convert the otherwise indigestible (by human diges ...

animals Equid size also increased, with ''Merychippus'' ranging, on average, between 71 and 100.6 kg.MacFadden, B. J. 1986. Fossil horses from “Eohippus” (''Hyracotherium'') to ''Equus'': scaling, Cope's Law, and the evolution of body size. ''Paleobiology'', 12:355–369.

Classification

By the end of the Miocene era, ''Merychippus'' was one of the first quick grazers. It gave rise to at least 19 different species of grazers, which can be categorized into three major groups. This burst of diversification, termed an

adaptive radiation In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, alters biotic int ...

, is often known as the "Merychippine radiation". The first was a series of three-toed grazers known as

hipparion ''Hipparion'' is an extinct genus of three-toed, medium-sized equine belonging to the extinct tribe Hipparionini, which lived about 10-5 million years ago. While the genus formerly included most hipparionines, the genus is now more narrowly defi ...

s. These were very successful and split into four

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

and at least 16

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

, including small and large grazers and browsers with large and elaborate facial fossae. The second was a group of smaller horses, known as protohippines, which included ''

'' and '' Calippus''. The last was a line of "true equines" in which the side toes were smaller than those of other proto-horses. In later genera, these were lost altogether as a result of the development of side ligaments that helped stabilize the middle toe during running.

References

External links

Photo of a ''Merychippus'' tooth.A classification scheme from The Paleobiology Database

{{Taxonbar, from=Q584323 Miocene horses Transitional fossils Miocene mammals of North America Prehistoric placental genera Miocene genus first appearances Fossil taxa described in 1856 Equinae