''Parahippus'' ("near to

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 ...

"), is an extinct

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 ...

, a relative of modern horses,

ass Ass most commonly refers to: * Buttocks (in informal American English) * Donkey or ass, ''Equus africanus asinus'' **any other member of the subgenus ''Asinus'' Ass or ASS may also refer to: Art and entertainment * Ass (album), ''Ass'' (albu ...

es, and

zebra Zebras (, ) (subgenus ''Hippotigris'') are African equines with distinctive black-and-white striped coats. There are three living species: Grévy's zebra (''Equus grevyi''), the plains zebra (''E. quagga''), and the mountain zebra (''E. ...

s. It lived from 24 to 17 million years ago, 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 ...

epoch. It was very similar to ''

Miohippus ''Miohippus'' (meaning "small horse") is an extinct genus of horse existing longer than most Equidae. It lived in what is now North America from 32 to 25 million years ago, during the late Eocene to late Oligocene. According to the Florida Museu ...

'', but slightly larger, at around tall, at the

withers Withers are the ridge between the shoulder blades of an animal, typically a quadruped. In many species, this ridge is the tallest point of the body. In horses and dogs, it is the standard place to measure the animal's height. In contrast, catt ...

. Their fossils have been found in North America, primarily in the Great Plains region and

Florida Florida ( ; ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders the Gulf of Mexico to the west, Alabama to the northwest, Georgia (U.S. state), Georgia to the north, the Atlantic ...

Taxonomy

The following fossil species are known: * ''P. cognatus'' Leidy, 1858 (

type species In International_Code_of_Zoological_Nomenclature, 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 spe ...

) - mid-

(

Hemingfordian The Hemingfordian on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 20,600,000 to 16,300,000 years BP. It is usually considered to overlap the lates ...

Barstovian The Barstovian North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 16,300,000 to 13,600,000 years BP, a period of . It is usua ...

) of Nebraska ( Loup Fork Formation) & South Dakota ( Batesland Formation), US * ''P. coloradoensis'' Gidley, 1907 - early-mid Miocene (

Arikareean The Arikareean North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 30,600,000 to 20,800,000 years BP, a period of . It is usuall ...

to Barstovian) of South Dakota ( Rosebud Formation), Colorado ( Pawnee Creek Formation), & Oregon ( Butte Creek Formation) * '' P. leonensis'' Sellards, 1916 - early to mid-Miocene (Arikareean to Hemingfordian) of Florida ( Torreya Formation), Delaware (

Calvert Formation The Calvert Formation is a Formation (geology), geologic formation in Maryland, Virginia, and Delaware. It preserves fossils dating back to the early to middle Miocene, Miocene epoch of the Neogene Period (geology), period. It is one of the three ...

), Texas (

Oakville Formation The Oakville Formation is a geologic formation in Texas. It preserves fossils dating back to the Neogene period. See also * List of fossiliferous stratigraphic units in Texas * Paleontology in Texas Paleontology in Texas refers to paleontol ...

), Colorado ( Browns Park Formation), & Oregon ( John Day Formation) * ''P. pawniensis'' Gidley, 1907 - early-mid Miocene (Arikareean to Hemingfordian) of Colorado (Pawnee Creek Formation), Wyoming, South Dakota (Rosebud Formation), Nevada, California ( Vaqueros Formation) & Oregon (John Day Formation) The former species ''P. tyleri'' Loomis, 1908 is now placed in '' Desmatippus''.

Description

''Parahippus'' was larger than ''Miohippus'', with longer legs and face. The bones in the legs were fused and this, along with muscle development, allowed ''Parahippus'' to move with forward-and-back strides. Flexible leg rotation was eliminated, so that the animal was better adapted to fast forward running on open ground without moving from side to side. Most importantly, ''Parahippus'' was able to stand on its middle toe, instead of walking on pads, which gave it the ability to run faster; its weight was supported by ligaments under the

fetlock Fetlock is the common name in horses, large animals, and sometimes dogs for the metacarpophalangeal and metatarsophalangeal joints (MCPJ and MTPJ). Although it somewhat resembles the human ankle in appearance, the joint is homologous to the ...

to the big central toe. Since leafy food had become scarce, these animals were forced to subsist on the newly evolved

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 ...

es that were by now taking over the plains, and their teeth adapted accordingly. The extra molar crest that was variable in ''Miohippus'' became permanent in ''Parahippus''. The molars developed high crowns and a hard covering for grinding the grass, which was typically covered with high-silica dust and sand.

References

Horse Evolution

Anchitheriinae Miocene horses Miocene Perissodactyla Transitional fossils White River Fauna Prehistoric placental genera Miocene genus extinctions Miocene mammals of North America Fossil taxa described in 1858 Taxa named by Joseph Leidy {{horse-stub