EXTINCT FAUNAS OF THE MOHAVE DESERT 255 



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 \yhich are known only by frag- 

 mentary 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 j\Ie?-ychippvs. 



Of the horse there are at least four species represented. Merychip- 

 pus 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 de- 

 pressions 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 ProtoMppus, the next or later stage in the evolutionary series of 

 the horse. An exceedingly rare form related to Merychippus is repre- 

 sented by a few large teeth which may possibly belong to a representa- 

 tive of the genus Pliohippus, a larger animal somewhat like the modem 

 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 Eicardo 

 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 feed- 

 ing 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 Paraliippus, also of a brows- 

 ing rather tlian of a grazing type. It may be repeated that Hypohippus, 

 Parahippus and Protohippus are collectively Ivnown 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 



