The Perissodactyla (or Mesaxonia) have feet whose axis of support runs through the middle toe, the third in the original series.
Most perissodactyls are three-toed, but some early ones still had four toes on the hind feet, and modern horses are one-toed.
The paleotheres and titanotheres are grouped with the horses into a suborder Hippomorpha, leaving the rhinoceroses and tapirs
in a second group (Ceratomorpha) and the clawed chalicotheres in a third group (Ancylopoda).
The horse family has been most extensively studied, and shows an early adaptive radiation of
three-toed browsing horses, a transition of one lineage (Merychippus) to grasslands
and to high-crowned (hypsodont) teeth, a second adaptive radiation of grazing horses,
a third and smaller adaptive radiation of horses in South America, and a transition of
one lineage from three toes to one. Most of this evolution occured in the Americas, but several offshoots
migrated to Eurasia, where horses of the genus Equus were tamed while they were hunted to extinction
in their American homeland.
- Order Perissodactyla:
- Perissodactyl origins
- Perissodactyl classification
- Suborder Hippomorpha:
- Horses
- Paleotheres
- Titanotheres
- Suborder Ancylopoda (chalicotheres)
- Suborder Ceratomorpha:
Illustrations
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