Hippidion

''Hippidion'' (meaning ''little horse'') is an extinct genus of equine that lived in South America from the Late Pliocene (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,'' this was later shown to be incorrect, with more complete sequences finding ''Hippidion'' as an outgroup to all living equines and less closely related to living equines than the North American "New World stilt legged horse", '' Haringtonhippus francisci.'' Cladogram shown below:

''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. The earliest members of the ''Hippidion'' lineage are known from the Late Miocene of North America''.''MacFadden, B. J., and M. Skinner. 1979. Diversification and biogeography of the one-toed horses Onohippidium and Hippidion. Yale Peabody Museum of Natural History Postilla 175:1–9. ''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 with the hippidiform lineage becoming extinct in North America during the Early Pleistocene. It is disputed as to whether ''Hippidion'' is an exclusively South American genus or whether remains from North America are attributable to it.

''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.

Description

Specimens of ''Hippidion saldiasi'' have been estimated to weigh in the range of , while specimens of ''H. devillei'' have been estimated to weigh in the region of . ''H. principale'' was somewhat larger, with an estimated mass range of . The skull of ''Hippidion'' is noted for its very long nasal notch separating the elongate nasal bone from the rest of the skull. This structure is suggested to correspond to the presence of a prehensile upper lip. The teeth morphology is more similar to that of '' Pliohippus'' than to '' Equus''. The limb bones are short and stocky (robust).

Paleobiology

Isotopic evidence from ''δ¹³C'' suggests that ''Hippidion'' consumed plants of either C₃ or mixed C₃ and C₄ type carbon fixation. The ''δ¹³C'' values are consistent with the occupation of woodland and wooded grassland habitats. ''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.'' ''Hippidion'' has been suggested to have probably lived in herds like living equines.

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.

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. 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.

References

{{Taxonbar, from=Q133193

Pliocene horses
Pleistocene horses
Pliocene first appearances
Pleistocene genus extinctions
Prehistoric placental genera
Pleistocene mammals of South America
Pliocene mammals of South America
Uquian
Ensenadan
Lujanian
Pleistocene Argentina
Fossils of Argentina
Pleistocene Bolivia
Fossils of Bolivia
Pleistocene Brazil
Fossils of Brazil
Pleistocene Chile
Fossils of Chile
Sopas Formation
Dolores Formation, Uruguay
Fossils of Uruguay
Pleistocene Venezuela
Fossils of Venezuela
Fossil taxa described in 1869
Equini