characteristics of these groups of hoofed mammals are the number of toes
and how the animal's weight is borne on the toes. (Their teeth are also
distinctive.) Artiodactyls have either two or four toes, and their weight
is borne along an axis that passes between the third and fourth digits.
For those artiodactyls with two toes, such as today's swine and deer, the
first, second, and fifth digits have been lost or remain only as vestiges.
Perissodactyls have one or three toes, although a few fossil species retained
four toes on their forefeet. Nevertheless, their weight is borne on an
axis passing through the third toe. Even today's horses have vestigial
side toes, and rarely they are born with three toes.
Trends in Cenozoic
evolution of the present-day horse (Equus)
A number of horse genera existed throughout the Cenozoic that evolved differently. For example, some horses were browsers rather than grazers and never developed high-crowned chewing teeth and retained three toes. Some of the major trends are listed below:
1. Size increase.
2. Legs and feet become longer, an adaptation for running.
3. Lateral toes reduced to vestiges. Only the third toe remains functional in Equus.
4. Straightening and stiffening of the back.
5. Incisor teeth become wider.
6. Molarization of premolars yielded a continuous row of teeth for grinding vegetation.
7. The chewing teeth, molars and premolars, become high-crowned and cement-covered for grinding abrasive grasses.
8. Chewing surfaces of premolars and molars become more complex---also an adaptation for grinding abrasive grasses.
9. Front part of skull and lower jaw become deeper to accommodate high-crowned premolars and molars.
10. Face in front of eye becomes longer to accommodate high-crowned teeth.
11. Larger, more complex brain.