Horses colonized South America about 2.5 million years ago after a land bridge emerged connecting it to North America. These early colonizers belonged to the hippidion genus, a group that became extinct in North America a few hundred thousand years after they entered South America. The hippidion horses lived in South America until the end of the Pleistocene ~10,000 years BP. Horses from the equus genus arrived in South America about 1 million years ago and also survived there and in North America until the end of the Pleistocene. Hippidion horses were anatomically similar to North American pliohippus horses, a primitive line common during the late Miocene from 14 million years BP-6 million years BP.
Paleontologists long considered the hippidion horses to be a different evolutionary branch from the equus horses because of their distinctly different nasal bones. Hippidion horses had longer nasal bones that were domed, and their “nasoincisual notches” were deeper. The long domed nose may have given them an advantage in dry dusty environments. In 2008 a study of hippidion DNA suggested they were more closely related to the equus genus than paleontologists thought. But in 2015 another study of hippidion DNA determined the hippidions diverged from the equus branch of horses about 6 million years ago. The data from the latter study is more consistent with the anatomical evidence.
Artist’s depiction of hippidion. Note the broad nose.
During the late Pleistocene there were 3 species of horses in the hippidion genus–Hippidion soldiasi, H. principale, and H. devilliei. The build of H. devilliei suggests it was an “high altitude specialist.” The earlier extinction of hippidions in North America is puzzling. Competition with equus was not likely a factor because the 2 genera co-existed in South America for about 1 million years. Hippidions did disappear from North America when Ice Ages became more severe and climate became drier, but they were probably well adapted to dry environments. I have no explanation.
I was surprised to learn how large hippidions were. They could reach a weight of 2200 pounds–about the size of the largest breed of domesticated horse, the Clydesdale. I didn’t realize some horses could weigh over a ton.
Some hippidions were about the size of the Clydesdale horse, one of the largest breeds, weighing over a ton. All extant species of horses and donkeys belong to the equus genus.
References:
Orlando, Ludovic; et. al.
“Ancient DNA Clarifies the Evolutionary History of American Late Pleistocene Equids”
Journal of Molecular Evolution May 2008
Sarkission, Clio; et. al.
“Mitochondrial Genome Reveal the Extinct Hippidion was an Outgroup to all Living Equids”
Biology Letters March 2015
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