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Hippidion; Temporal range: Late Pliocene-Holocene (Uquian-Lujanian); ~2.5–0.011 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
	H. principale skeleton
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Perissodactyla
Family:	Equidae
Subfamily:	Equinae
Tribe:	Equini
Genus:	†Hippidion; Owen, 1869
Species
	H. principale (Lund, 1846) (type); H. saldiasi Roth, 1899; H. devillei (Gervais, 1855);
Synonyms
	Hipphaplous Ameghino 1882; Hipphaplus Ameghino 1882; Onohippidion Filhol 1888; Onohippidium Moreno 1891; Parahipparion Ameghino 1904;

Hippidion (meaning little horse) is an extinct genus of equine that lived in South America from the Late Pliocene to the end of the Late Pleistocene (Lujanian), between 2.5 million and 11,000 years ago. They were one of two lineages of equines native to South America during the Pleistocene epoch, alongside Equus (Amerhippus) neogeus.

Taxonomy

Evolution

Although early ancient DNA analysis studies suggested a close relationship with the wild horse, Equus ferus,^[1]^[2] 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 equines than the North American "New World stilt legged horse", Haringtonhippus francisci. Cladogram shown below:^[4]

Equini

†Hippidion

†Haringtonhippus

Genus Equus

Equus ferus (wild horse)

†Equus ovodovi

	Subgenus Asinus (asses)

	Subgenus Hippotigris (zebras)

Hippidion is part of a distinct lineage of equines belonging to the tribe Equini that are suggested to have diverged from the ancestors of living equines of the genus Equus at least 6 million years ago.^[4] The earliest members of the Hippidion lineage are known from the Late Miocene of North America.^[5]^[6] Hippidion migrated into the South American continent at the Pliocene-Pleistocene transition, around 2.5 million years ago as part of the Great American Biotic Interchange^[1]^[2] with the hippidiform lineage becoming extinct in North America during the Early Pleistocene.^[5] It is disputed as to whether Hippidion is an exclusively South American genus or whether remains from North America are attributable to it.^[7]^[6]

Hippidion is traditionally thought to have 3 species, H. principale, H. saldiasi and H. devillei, however, in a 2015 DNA analysis, the single sampled H. principale specimen was found to be nested with H. saldiasi, while H. devillei was found to be clearly genetically distinct.^[3]

Description

Specimens of Hippidion saldiasi have been estimated to weigh in the range of 252.7–326.4 kilograms (557–720 lb), while specimens of H. devillei have been estimated to weigh in the region of 227.3–366.5 kilograms (501–808 lb). H. principale was somewhat larger, with an estimated mass range of 252.7–468.6 kilograms (557–1,033 lb).^[8] The skull of Hippidion is noted for its very long nasal notch separating the elongate nasal bone from the rest of the skull.^[9] This structure is suggested to correspond to the presence of a prehensile upper lip.^[10] The teeth morphology is more similar to that of Pliohippus than to Equus. The limb bones are short and stocky (robust).^[11]

Paleobiology

Isotopic evidence from δ¹³C suggests that Hippidion consumed plants of either C₃ or mixed C₃ and C₄ type carbon fixation.^[12] The δ¹³C values are consistent with the occupation of woodland and wooded grassland habitats.^[13] H. principale is suggested to have been a mixed feeder (both browsing and grazing), as opposed to the grazing diet of the contemporary Equus neogeus.^[14] Hippidion has been suggested to have probably lived in herds like living equines.^[15]

Distribution

Fossils of Hippidion have been found across South America. Hippidion principale is primarily known from the Pampas grasslands of Argentina, but its range may have extended to Peru and Ecuador. Hippidion devillei is also known from Argentinan Pampas, but is also found in the Peruvian central Andes, with remains possibly found in Venezuela. Hippidion saldiasi was native to the Southern Cone, including Patagonia and the southern Andes.^[8]

Extinction

Hippidion became extinct alongside the other South American equines at the end of the Late Pleistocene, between 12,000 and 10,000 years Before Present (BP) as part of the Late Pleistocene megafauna extinctions, which resulted in the extinction of most large animals in both North and South America. Climatic 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.^[16] At the Arroyo Seco 2 site in the Argentinan Pampas (14,782–11,142 cal yr BP), and Piedra Museo site in Santa Cruz Province, Patagonia (two separate layers dating to 12,463–10,457 cal yr BP and 15,517–12,352 cal yr BP), as well as the similarly aged Cueva del Medio site in southern Chile remains of Hippidion are associated with human artifacts, including Fishtail projectile points, and bear cut marks, indicating that they were hunted by recently arrived humans, which may have played a role in their extinction.^[17]^[18]