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have a diverse fossil record although there are only 6 modern genera of
horses, tapirs, and rhinos. Perissodactyls
are easily recognized by their odd number of toes, unlike the artiodactyls
which possess an even number of toes.
The primitive arctocyonids evolved into
the condylarth family Phenacodontidae
that included Early Eocene forms like Loxolophus which had a short diastema
behind the first premolar and Karagalax, a tapiroid. In Phenacodus, digits
I and V are reduced on both hands and feet.
was closer to horse ancestry.
was the ancestor of tapirs, including
basal species of tapir such as Heptodon, Helates (Eocene), and Prototapir (Oligocene) (
The skull of a modern tapir is depicted below.
of the Eocene was ancestral to the various groups
of rhinos. They were common and diverse
in the Northern Hemisphere during the Oligocene and Miocene but became extinct
Fossils of wooly rhinos indicate that the flattened horns were often worn in a manner which suggests that they could be used to clear snow (Kurten, 1988).
The brontotheres were an extinct group of rhinos that had reached dog-sized species by the Oligocene and rhino-sized species by the Miocene.
Moropus was horse sized.
|The Chalcotheroids had very large claws which they protected by walking on their knuckles.|