One of the first perissodactyls was _Hyracotherium_ (also called
_Eohippus_), which is the earliest known horse (Pl. 47). This small
animal, whose remains have been found in Big Bend National Park, was
about 1 foot high and his teeth indicate a diet of soft food. Following
the first horse, there is a long series of fossil horses which provide
much valuable information on the history of this important group of
animals.
The record of the development of the horse is well represented in Texas,
and the bones and teeth of fossil horses are common in certain parts of
the State. Fossils of this type have been reported from the Tertiary of
the Trans-Pecos, Gulf Coastal Plain, and High Plains regions of Texas,
and the teeth of Pleistocene horses have been found in sand and gravel
pits in many parts of the State. Horse teeth (fig. 26) are particularly
useful fossils as they may be accurately identified and used to
determine the age of the rocks in which they are found.
[Illustration: Fig. 26. Typical Pliocene horse tooth. Top view (a) and
lateral view (b) of molar tooth (×½).]