Researchers have identified three species of horse that inhabited North America’s western interior from around 50,000 years ago until their extinction on the continent around 10,000 years ago.
Horses were a dominant component of North American Pleistocene land mammal communities and their remains are well represented in the fossil record.
Christina Barrón-Ortiz and her colleagues noted that, despite the abundant material available for study, there was still considerable disagreement over the number of species of Equus that inhabited the different regions of the continent.
The study team set out to investigate cheek teeth and the mitochondrial DNA – that’s DNA passed down maternal lines – from late Pleistocene Equus specimens in the hope of clarifying the species that lived in the North American interior before the end-Pleistocene extinction.
Their study concentrated on fossil material retrieved from five geographic regions arranged in a north-south transect along the western interior, from the High Arctic of Canada’s Yukon Territory to northeastern Mexico.
A total of 1454 cheek teeth were measured in the study. Attempts were made to obtain mitochondrial DNA from 50 of these late-Pleistocene specimens, and was successful in 22 cases.
Based on their analysis, reported in the open-access journal PLOS ONE, the study team found evidence of the presence of a caballine horses, Equus ferus, and a non-caballine horses, E. conversidens, in different localities across most of the western interior.
A second non-caballine species, E. cedralensis, was found in southern localities, based exclusively on the examination of cheek teeth.
The teeth identified as being from E. conversidens yielded ancient mitochondrial DNA which showed the species fell within the New World stilt-legged grouping of horses recognized in previous studies.
The authors found different enamel patterns on the grinding surfaces of the E. ferus specimens from Bluefish Caves, in the northern Yukon, compared to the other geographic regions.
The researchers said they could not be certain whether this represented an adaption due to local ecological factors and/or a certain degree of geographic and genetic isolation of these Arctic populations.
E. ferus appeared to have been widely spread through much of the western interior, from Cedral, in Mexico, up through Alberta, and as far as the Bluefish Caves in the northern Yukon.
E. conversidens samples indicated a range from northeastern Mexico, up through the American Southwest and into Alberta.
E. cedralensis appears to have been restricted during the late Pleistocene to the southern latitudes of the western interior, with remains identified from Cedral, Mexico, and the American Southwest, at sites in northern Chihuahua, Mexico.
The study team noted that more than 40 species of Equus have been named from the Pleistocene of North America. Several researchers have attempted to revise the taxonomy of this group, but no consensus has been reached.
“The taxonomy of North American Equus is highly confused and its resolution is beyond the scope of this study,” they noted.
The study team comprised Barrón-Ortiz , Antonia Rodrigues, Jessica Theodor, Brian Kooyman, Dongya Yang, and Camilla Speller. They are variously affiliated with three Canadian institutions − the Royal Alberta Museum, the University of Calgary and Simon Fraser University − and the University of York, in England.
Barrón-Ortiz CI, Rodrigues AT, Theodor JM, Kooyman BP, Yang DY, Speller CF (2017) Cheek tooth morphology and ancient mitochondrial DNA of late Pleistocene horses from the western interior of North America: Implications for the taxonomy of North American Late Pleistocene Equus. PLoS ONE 12(8): e0183045. https://doi.org/10.1371/journal.pone.0183045