The Oldest Known Mammal Fauna of the Mohave, the Upper Miocene of Barstow
The fauna of the oldest mammal-bearing beds of the Mohave area includes about thirty species, many of which are known only by fragmentary material. The larger part of the collection consists of the remains of horses and camels. The bones of horses, accompanied by those of other animals, are sufficiently abundant at one horizon to mark a zone or layer which can be traced for a number of miles, and is known as the Merychippus zone, from the most common fossil, a little three-toed horse of the genus Merychippus.
Of the horse there are at least four species represented. Merychippus is the most abundant form and includes two or three types. They were mainly animals about as large as small colts of the modern horse. They possessed one large middle toe and two small, scarcely-functional side toes on each foot. Their heads were long and had peculiar depressions on the sides of the face. The back-teeth were long, and as they were worn off from the top, they grew up from the root, as in the modern horse. These animals were of a distinctly open country or plains type, and evidently supported themselves by grazing or grass feeding, rather than by browsing from brush as do the deer. One of the larger species of Merychippus is almost indistinguishable from the genus Protohippus, the next or later stage in the evolutionary series of the horse. An exceedingly rare form related to Merychippus is represented by a few large teeth which may possibly belong to a representative of the genus Pliohippus, a larger animal somewhat like the modern horse. One of the most common Merychippus species is a small form approaching in its characters the genus Hipparion, the characteristic horse of the following Ricardo or early Pliocene epoch. The Ricardo Hipparions are possibly descendants of this small Barstow horse.
Two rare horses found in the Barstow fauna, like the earliest forms of the horse group, have back-teeth with short crowns not adapted for grazing. One belonging in the genus Hypohippus was a large three-toed animal, in which the side-toes are much larger than in Merychippus. The teeth are those of a browsing, not of a grazing animal. The feeding habits of this horse must have differed very considerably from those of Merychippus, and it probably occupied a somewhat different range. The other rare form represents a species of Parahippus, also of a browsing rather than of a grazing type. It may be repeated that Hypohippus, Parahippus and Protohippus are collectively known only by a very small number of specimens. The grazing Merychippus is the common and characteristic animal of the fauna.
Associated with the horses are rare remains of a primitive wild pig or peccary. There is also a rare oreodon, one of the late representatives of a large family, which is perhaps the most characteristic American
