Fig. 25. Sketches of [Pleistocene] (a) mastodon tooth (×⅙) and (b) mammoth tooth (×⅙).
During the [Pleistocene], mammoths were widespread over the United States, and their remains are abundant in many stream deposits of this age. Proboscidean bones have been reported from Pleistocene rocks in many parts of Texas, where they are commonly found in sand and gravel pits.
Order Perissodactyla.—
The perissodactyls, or odd-toed animals, are mammals in which the central toe on each limb is greatly enlarged. Modern representatives include the horses, rhinoceroses, and tapirs. Extinct members of the Perissodactyla include the titanotheres, chalicotheres, and baluchitheres, all of which grew to tremendous size and took on many unusual body forms.
HORSES.—
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.
Fig. 26. Typical [Pliocene] horse tooth. Top view (a) and [lateral] view (b) of molar tooth (×½).