Astrohippus is an extinct member of the 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...

 tribe

Tribe (biology)

In biology, a tribe is a taxonomic rank between family and genus. It is sometimes subdivided into subtribes.Some examples include the tribes: Canini, Acalypheae, Hominini, Bombini, and Antidesmeae.-See also:* Biological classification* Rank...

 Equini, the same tribe that contains the only living equid genus, 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...

. Fossil remains have been found in the central United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, 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...

, and the Mexican

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...

 states of Chihuahua, Jalisco

Jalisco

Jalisco officially Estado Libre y Soberano de Jalisco is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is located in Western Mexico and divided in 125 municipalities and its capital city is Guadalajara.It is one of the more important states...

, and Guanajuato

Guanajuato

Guanajuato officially Estado Libre y Soberano de Guanajuato is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 46 municipalities and its capital city is Guanajuato....

. The fossils date in time from the oldest dating from the Barstovian

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 , typically set from 16,300,000 to 13,600,000 years BP, a period of . It is usually considered to overlap the Langhian and Serravallian...

 in the 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...

 to the youngest dating in the Hemphillian

Hemphillian

The Hemphillian North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology , typically set from 10,300,000 to 4,900,000 years BP, a period of . It is usually considered to overlap the Tortonian epoch of the Late...

 faunal stage

Faunal stage

In chronostratigraphy, a stage is a succession of rock strata laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convention have the same name, and the same boundaries.Rock...

 of the early 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...

.

Based on study of the tooth morphology it is considered unlikely that Astrohippus could be an ancestor of 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...

s, with the most likely ancestor of Astrohippus being 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 species A. ansae was originally described by W.D. Matthew and R.A. Stirton based on several cheek teeth found in the Coffee Ranch quarry, Hemphill County, Texas

Texas

Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

. This quarry has produced the remains of six other equid genera including 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...

and 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...

. A. stockii was described from the Yepomera locality in Chihuahua, Mexico, by J.F. Lance in 1950 as Pliohippus stockii. The species was moved to Astrohippus five years later by Quinn. The species A. albidens was named by O. Mooser in 1965 from Mexico, the species was subsequently moved to Dinohippus in 1988 by T. S. Kelly and E. B. Lander. In 1998 A. stockii and A. albidens were combined by T. S. Kelly, with the older A. stockii name being retained and A. albidens becoming a junior synonym.

Both species of Astrohippus have been reported to occur in the Ocote Local Fauna of Guanajuato, Mexico by O. Mooser in 1973.