And then there was one: Did a single species of horse roam South America?

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A statue in Osorno, Chile, of an extinct American horse. Photo: Unic CC BY-SA 3.0 via Wikimedia Commons

Horses once ranged across much of South America, but how many species were there?

Five species of Equus were traditionally recognized on this continent, although recently they have been narrowed down into three, Equus neogeus, E. insulatus, and E. andium.

Now, two researchers propose that there was only one.

Helena Machado and Leonardo Avilla, with the Laboratory of Mastozoology at the Federal University of the State of Rio de Janeiro in Brazil, say the evidence points to E. neogeus being the only species.

The Equus family tree in South America may have fallen victim to a historical tendency of scientists to split species, they say. It began in the second half of the 19th century, and several poorly identified species were named as a result.

Most of them were maintained until the second half of the 20th century, when a more stable picture emerged, along with a smaller family tree. Size was assumed to be the best (and even only) basis for identifying South American Equus species within this group, and this consensus held until only recently.

The pair’s work backs a previous study which suggested the means by which the three species were identified was not taxonomically valid, with the implication there was just a single species in South America.

The pair, for their paper published in the journal Frontiers in Ecology and Evolution, examined the evidence of horses in South America and updated the fossil distribution of Equus.

Dental analyses were carried out, and the results revealed a clear overlap among the currently recognized species.

Characters of the fossilized remains of lower legs were also reanalyzed with greater sampling, and the results once again confirmed that a single species was present in South America.

There are fossil records of the horse throughout almost the entire continent, they reported, except for regions in the Amazonian rain forest, at latitudes south of 40° South, and at altitudes above 3000 metres.

Horses occupied highlands and lowlands, although mostly at altitudes of less than 500 metres.

When they evaluated the new data in relation to geography, it revealed variations that were gradual and continuous. Such body changes were most likely related to topography, rather than being a different species.

The pair said most of the traditional taxonomy involving South American horses was founded on poor samples and superficial conclusions, which mainly used differences in size to characterize the various supposed species.

This neglected the fact that body variations can reflect factors other than being a different species. These can include differences between the sexes, a range of factors that might affect an animal while growing up, individual/population differences, and adaptations driven by exposure to different environments, such as differences between lowlands and highlands.

“However, the taxonomic zeitgeist of the mid-nineteenth to mid-twentieth century favored splitting, which generated many poorly diagnosed species and resulted in a distorted view of ancient diversity. South American native Equus taxonomy conformed to this pattern.”

The Diversity of South American Equus: Did Size Really Matter?
Helena Machado and Leonardo Avilla
Front. Ecol. Evol., 03 July 2019 | https://doi.org/10.3389/fevo.2019.00235

The study, published under a Creative Commons License, can be read here

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