Scientists recently recognized a new species and genera of extinct Pleistocene horse from fossil specimens already in museums. Some of these specimens were collected over 100 years ago and were wrongly assumed to represent previously known species or genera. During the Pleistocene there were 3 lineages of horses in the Americas–the caballine horses, the New World stilt-legged horses, and the hippidion horses. The caballine horses belong to the Equus genus which includes all living species of horses, donkeys, and zebras. The species of caballine horses that lived in North and South America likely included the predecessor of the modern day domesticated horse. It was probably the same species. The New World stilt-legged horses so anatomically resembled Asiatic wild asses and donkeys that paleontologists mistakenly thought they were closely related. In recent years paleontologists began to reject this assumed affinity, and the genetic study cited in this blog entry supports their re-assessment. The hippidion horses were robust species restricted to South America. A new genetic study determined the New World stilt-legged horses, previously classified as belonging to the equus genus, were different enough to deserve their own genus. Scientists gave this species the scientific name Haringtonhippus francisi. The species was named after the renowned Canadian paleontologist, Richard Harington. The type specimen anatomically described in the paper was originally discovered in Wharton County, Texas.
Artists’s representation of Haringtonhippus francisi. The coat color is the artist’s fanciful guess.
The genetic evidence suggests haringtonhippus horses diverged from equus horses between 4-5 million years ago. The hippidion horses diverged from the equus/haringtonhippus genera between 5-7 million years ago. Convergent evolution explains why haringtonhippus horses anatomically resembled Old World asses. Both evolved long slender limbs as an adaptation to arid environments.
Fossil remains of Haringtonhippus francisi have been found in east Texas, eastern Mexico, Kansas, Nevada, California, the Yukon, and Alaska. Stilt-legged horse fossils are known from sites thought to be 3 million years old, and they occurred until as recently as 12,000 years ago about the time man became prevalent on the continent.
If scientists are able to extract DNA from even more ancient extinct genera of horses, they may be able to straighten out horse evolution. Many biology textbooks use the fossil record of horses and their ancestors as an example of evolution, but these family trees are based on anatomical analyses that can be misleading. DNA evidence would produce more reliable family trees.
Reference:
Heinztman, P; et. al.
“A New Genus of Horse from Pleistocene North America”
Genomics and Evolutionary Biology Nov. 2017
Tags: Haringtonhippus francisi
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