Hippidion

Life restoration of H. principale from 1913

Hippidion has been considered a descendant of pliohippines,[1] horses that migrated into the South American continent around 2.5 million years ago.[1][2] Early analysis of the ancient DNA of Hippidion and other New World Pleistocene horses supported the novel hypothesis that Hippidion was actually a member of the living genus Equus, with a particularly close relationship to the wild horse, Equus ferus.[1][2] However this was later shown to be incorrect, with more complete sequences finding Hippidion as an outgroup to all living equines[3] and less closely related to living horses than the "New World stilt legged horse", Haringtonhippus francisci.[4]

Hippidion is traditionally thought to have 3 species, H. principale, H. saldiasi and H. devillei, however, in the DNA analysis, the single sampled H. principale specimen was found to be nested with H. saldiasi, with H. devillei found to be clearly genetically distinct.[3]

Hippidion and other South American horses became extinct approximately 8,000 years ago.[5] Specific archaeological recovery at the Cueva del Milodon site in Patagonian Chile demonstrates that Hippidion saldiasi existed in that vicinity in the era of 10,000 to 12,000 years before present.[6] Horses did not reappear in South America until the 16th century, as a result of introduction by humans.

Skeleton in Natural History Museum, London

It stood approximately 1.4 metres (4.6 ft) (also 13.2 hh) high at the shoulders and resembled a donkey. The skull of Hippidion is noted for its nasal bone, which projects forward from the skull.[7]

Restoration of Pleistocene South America

A study found that Late Pleistocene specimens of Hippidion had lower δ¹³C values than those of specimens of Amerihippus, indicating a preference for C3 woodland and wooded open habitats.[7]

Remains of Hippidion saldiasi have been recovered in locations such as the Piedra Museo site, Santa Cruz, Argentina[8] and Cueva del Milodon, Chile.[9] The significance of such archaeological recovery is amplified by the association with hunting of these animals by prehistoric man at possible Pre-Clovis horizons.

Fossils of Hippidion have been found in:[10][7]

Pleistocene

• Tarija, Ñuapua and Ulloma Formations, Bolivia
• Jandaíra Formation, Brazil
• Chíu-Chíu Formation and Cueva del Milodón, Chile
• Sopas and Dolores Formations, Uruguay
• Taima-Taima, Venezuela

Hippidion became extinct alongside the other South American equines at the end of the Late Pleistocene, between 15,000 and 10,000 years ago as part of the Quaternary extinction event, which resulted in the extinction of most large animals in both North and South America. Climactic modelling suggests that the preferred habitat for species of Hippidion declined after the Holocene transition, but the decline is not enough to explain the extinction.[11]