The Diversity of South American Equus: Did Size Really Matter?

Abstract

The extant horse genus Equus originated in North America, probably during the late Miocene/early Pliocene, and its entrance into South America was possibly related to one of the last four stages of the Great American Biotic Interchange. Nonetheless, the genus had a broad geographic distribution throughout South America. Five species of Equus were traditionally recognized on this continent, although recently they have been synonymized into three: Equus neogeus, E. insulatus, and E. andium. However, the diagnoses of the three species are still unclear and a recent study showed that the characters previously used are not taxonomically valid, with the implication that in South America, Equus was represented by a single species, E. neogeus. This contribution is intended to update current knowledge on the diversity of South American Equus. Accordingly, a synthesis of prior knowledge as well as an update on the fossil distribution of Equus is presented here. Dental analyses were carried out, and the results revealed a clear overlap among the currently recognized species. Characters of the autopodia were also reanalyzed with greater sampling, and the results once again confirmed that a single species was present in South America. The update of fossil occurrences increased knowledge of the geographical distribution of the genus in South America. There are records throughout almost the entire continent, except for regions in the Amazonian rain forest, at latitudes south of 40 • S, and at altitudes above 3,000 m. Furthermore, evaluating the new data with reference to geography revealed that E. neogeus formed a smooth cline, in which variation was gradual and continuous. It is also inferred that observed phenotypic variation is probably related to topography.

Evolution of the Family Equidae, Subfamily Equinae, in North, Central and South America, Eurasia and Africa during the Plio-Pleistocene

“…Two genera, Equus and Hippidion , inhabited the South American continent with records from the Late Pliocene to the Late Pleistocene [ 98 , 99 , 100 ]. Hippidion is an endemic genus of South American horses characterized mostly by the retraction of the nasal notch, a particular tooth morphology (considered more primitive than Equus and comparable with Pliohippus) and the robustness and shortness of its limb bones [ 98 ].…”

“…The genus is, at present, represented by three species: Hippidion saldiasi , Hippidion devillei and Hippidion principale [ 98 ]. On the other hand, the South American Equus is represented by a single species, E. neogeus , with caballine affinities and metapodial variation corresponding to an intraspecific characteristic representing a smooth cline [ 9 , 99 , 100 ].…”

“…Equus neogeus , Lund, 1840 [ 101 ] (MTIII, 866 Zoologisk Museum). This is a Middle–Late Pleistocene species (Ensenandan and Lujanian SALMA) and the only representative of the genus in the South American continent [ 9 , 98 , 99 , 100 ]. Most records are from the Late Pleistocene, but its earliest appearance is recorded in the Middle Pleistocene in Tarija, Bolivia, dated at approximately 1.0–0.8 Ma [ 102 , 103 ].…”

“…Most records are from the Late Pleistocene, but its earliest appearance is recorded in the Middle Pleistocene in Tarija, Bolivia, dated at approximately 1.0–0.8 Ma [ 102 , 103 ]. It has a wide geographic range distribution, encompassing all of South America, except for the Amazon basin and latitudes below 40° [ 100 ]. The species probably became extinct sometime during the Late Pleistocene–Holocene transition as suggested by the youngest direct radiocarbon date of 11,700 BP (Río Quequén Salado, Argentina [ 104 ]).…”

Ice Age megafauna rock art in the Colombian Amazon?

“…There are other fossil occurrences of Equus known from localities in northwestern South America (e.g. Colombia, Ecuador, Peru, Venezuela; see [ 63 ]), but many lack a refined chronology.…”