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The single living species within this family - Antilocapra americana
- is endemic to western North America; indeed, the entire fossil record of
this family is restricted to the Nearctic. The first representatives
of this family, the genus Merycodus (Merycodontinae), arose in
the early Miocene. This family was once much larger: thirteen now-extinct
genera are known, and at least four species of pronghorn coexisted in central
Mexico during the Pleistocene.
The modern pronghorn is one of the fastest-running mammals. At full
speed, a pronghorn can reach 85-95 kilometers per hour, and can maintain
a speed of 65 kilometers per hour over at least 10 kilometers. Adult
pronghorns can easily outrun any potential predator; their incredible speed
is thought to be the result of an evolutionary arms race with the now extinct
North American cheetah (Felix trumani).
The "horns" of the pronghorn are of special interest. Each horn is
comprised of a slender, laterally-flattened blade of bone which grows from
the frontal bones of the skull, forming a permanent core. As in the
Giraffidae, skin covers the bony cores, but
in the pronghorn it develops into a keratinous sheath which is shed and regrown
on an annual basis. Unlike the horns of the family
Bovidae, the horn sheaths of the pronghorn are
branched, each sheath possessing a forward pointing tine (hence the name
pronghorn). The horns of males are well developed; in females, they
are either small, misshapen, or absent.
The orbits (eye sockets) are prominent and sit high on the skull; there is
never an antorbital pit. The feet have only two digits; no dewclaws
are present. A gall bladder is present. The teeth are hypsodont,
and the dental formula is I 0/3, C 0/1, P 3/3, M 3/3 x 2 = 32. |
