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Popular Science Monthly/Volume 86/March 1915/Extinct Faunas of the Mohave Desert, their Significance in a Study of the Origin and Evolution of Life in America

EXTINCT FAUNAS OF THE MOHAVE DESERT, THEIR SIGNIFICANCE IN A STUDY OF THE ORIGIN AND EVOLUTION OF LIFE IN AMERICA
By Professor JOHN C. MERRIAM

Introduction

IT is almost a rule that features of the natural world which have exerted an unusual influence in developing our emotional, poetic and religious natures, when brought within the range of scientific inquiry seem only more deeply to excite our wonder and respect. Thus, it has happened that the deserts of the world, having furnished the stimulus for some of our earliest poetic and religious literature, appear to the scientist of to-day as places in which nature meets us with unusual frankness, and where her wonders almost clamor to be understood.

In those fields of history covering the development or evolution of. the external form of the earth and of the life upon it, deserts have been very significant sources of information, and the so-called bad-land formations in the arid or semi-arid regions of western North America have been recognized as playing a very important part. As the widespread exposures of these formations have elsewhere in America proved veritable museums of wonderfully preserved remains, it has seemed worthy of remark that the extensive bad-lands in the Great Basin region of America have with few exceptions furnished almost nothing bearing on the history of life. The early geologic explorers in Nevada and California found little bearing on the paleontologic story of the area they examined. Later investigators in the bad-lands of these regions have generally failed to report determinable vertebrate remains, and the life record has until recently remained practically a closed book. It has been with much interest, therefore, that those concerned with the history of western North America, and with its bearing on the whole story of life growth or evolution, have seen coming to light with the past decade chapter after chapter of this missing record.

With the exception of the John Day region of eastern Oregon, which supplies an important geologic and paleontologic record, the largest part of our knowledge of the history of mammalian life west of the Wasatch is obtained in the heretofore unexplored deposits of the Mohave Desert. At the present time there are available from the Mohave at least three extinct mammalian faunas previously unknown, or only imperfectly known, in the Great Basin. The life record given us by these faunas, the evolutionary series to which they contribute, and the suggestions which they offer concerning the origin, evolution and world relationships of life in America, furnish very significant chapters in the history of the western side of the continent.

Nearly twenty years ago several very fragmentary specimens from the Mohave were forwarded to the writer by Dr. Stephen Bowers, the material having been obtained in part by John T. Reed. The earliest material from definitely known localities coming to the writer was received in the spring of 1911 from John E. Suman, then a student at the University of California. The collection consisted of a small quantity of loose bones and teeth obtained on the desert by H. S. Mourning. These specimens furnished the basis for the first study of the Upper Miocene fauna of the Mohave. In the following year C. L. Baker, a fellow in paleontology at the University of California, visited the localities reported by Mr. Mourning and secured a fine collection of mammal material from the Miocene near Barstow, and a small amount of material from the Pliocene at Ricardo. Other important collections were made later by Mr. Baker, Mr. Mourning, Mr. J. P. Buwalda and by many students in paleontology from the University of California. Following his work on the Mohave in 1913, Mr. Buwalda independently visited a locality in the eastern portion of this region, and obtained a most interesting collection of Pleistocene remains in a formation to which he has given the name Manix beds. This material gave us for the first time a representative group of vertebrates from the Pleistocene of the Great Basin.

The collections brought together at various times have opened to us a view of the mammalian life of the Mohave Desert in three periods: the Barstow fauna of Upper Miocene age, the Ricardo fauna of early Pliocene stage and the Manix fauna from the Pleistocene.

 

The Mohave Desert of To-Day

The Mohave Desert area of California has been generally recognized as one of the least attractive portions of the southwest. It has been described as a forbidding land of heat and thirst. The deception of its mirages is a current example of the lure of unreality, and its great stretches of sand and dust have appeared to function mainly as barriers to human progress. The history of exploration has seemed amply to justify current views concerning the desert, as year after year prospectors or explorers, deceived by distances or miscalculating the position of scattered water sources, have paid with their lives the penalty for inaccurate judgment.

In spite of seeming obstacles offered to one who would make its acquaintance, those who have come to know the Mohave seem always to cultivate the friendship. The prospector has cheerfully risked his life.

PSM V86 D251 Outline of the mohave region and the pacific coast.jpg

Outline Map Showing Relation of the Mohave Area to Other Pacific Coast and Great Basin Regions During the Later Geological Periods. Within the Mohave region the principal faunal localities are indicated as follows: B, Barstow fauna; R, Ricardo fauna; M, Manix fauna.

PSM V86 D252 The mohave desert near the fossil beds.jpg

Characteristic View of the Mohave Desert in the Vicinity of the Fossil Beds. (Photograph by C. L. Baker.)

not alone for the desire of gain, but because the fascination of the desert always increases. The traveler is inevitably deeply influenced by the uncertain magnitude of distance, by the silence, and the unusual forms and brilliance of the landscape by day and night. Once an acquaintance is formed, distrust and fear are replaced by reverence of the quiet strength of nature exhibited here in factors which are too large or too elusive to be fully comprehended.

The Mohave lies in the middle of the southern half of the state of California, the desert proper being situated in the angle where the Sierras turn west to meet the Coast Ranges. The western limits of the

PSM V86 D252 Mohave desert cresosote bushes and joshua trees.jpg

Characteristic View of the Mohave Desert; showing at this locality an unusual abundance of vegetation, consisting of creosote bushes and Joshua trees. (Photograph by C. L. Baker.)

area are sharply marked by the abruptly rising wall of the bordering mountain ranges. The limits to the east are not so clearly marked, being considered by some to reach the eastern border of the state; by others they are held to extend less than half the distance to the Colorado River.

The elevation of the desert floor ranges from 2,000 ft. approximately to 4,000 ft. above the sea, in sharp contrast to the basin of the Salton Sea, which extends below the level of the ocean a short distance to the south. The topography of the region is characterized by great stretches of open plains many miles in extent, over which scattered mountain peaks or ranges are distributed with little suggestion of order in their arrangement.

The total rainfall of the desert amounts only to a few inches per year. Living streams are rare, and travel in all of this region is necessarily limited by accessibility of the few localities at which potable water can be obtained. Rain falls largely in the middle of the winter season, and throughout the greater part of the year there is no precipitation. The water at times comes with a rush, flows off rapidly as floods, and sometimes causes considerable damage to artificial obstacles in the path of the current. With the exception of the Mohave River, which runs a thin superficial stream for a considerable portion of the year, there are very few points at which a supply of water can be obtained on the surface. Investigation has shown that artesian water is available over certain areas, and agriculturists have operated to some extent by irrigation with water obtained from wells.

The diminished rainfall, the unhindered influence of a brilliant sun and the moderate altitude have given to the Mohave a distinctly arid climate; and with the climate go all of the accompanying characteristics of life, of erosion and deposition, and of the peculiar land forms of an arid country.

The vegetation of the Mohave area is at the present time limited mainly to desert types, the contrast with the flora beyond the ridge immediately to the west being very marked. In crossing the Tehachapi Range from the Great Valley of California to the Mohave one finds the valleys of the western side thickly studded with oak, sycamore, and willow, and the hills are carpeted with grass. On the eastern slope the whole aspect of the vegetation changes suddenly, as if one were entering a foreign land. The yuccas and the creosote bush replace oak and grass, and the oddly outstretched arms of the Joshua trees seem everywhere raised up as if to attract attention. Plants of arboreal type are rare, and, excepting a few junipers, the yuccas furnish the only trees. Creosote bushes are generally present, but are sometimes sparingly represented. Perhaps to show that under adverse conditions nature means only to be just and not unkind, the spring and early summer find the

PSM V86 D254 The barstow miocene formation.jpg

Typical Exposure of the Fossil-bearing Barstow Miocene Formation North of Barstow. (Photograph by C. L. Baker.)

desert dotted here and there with patches of flowers of unusual beauty and fragrance, offering their charms as an antidote for the misery of thirst about them.

The living mammalian fauna of the Mohave comprises thirty-five species, of which twenty-one are rodents. The Ungulata are re]:)resented only by the pronghorn antelope and the desert big-horn. The Carnivora include the desert coyote, the Mohave Desert kit fox, the California raccoon, a spotted skunk, a striped skunk, the northwest cougar, and the desert wildcat. The rodent fauna includes thirteen genera. The species are mainly characteristic desert forms. Of the living mammals only a few genera are known also in the older faunas of

PSM V86 D254 Folded and faulted strata north of barstow.jpg

Folded and Faulted Strata Representing a Portion of the Section Containing Fossil Remains North of Barstow. (Photograph by C. L. Baker.)

the Mohave. Some of the existing types, as the bighorn, are immigrants from the Old World, and arrived very late in the history of this region. Others, as the pronghorn, are evidently of American origin.

 

Geologic Occurrence and Age of the Mohave Fossil Beds

The Miocene and Pliocene faunas in the Mohave area occur in an accumulation of strata amounting to not less than 7,000 to 8,000 ft. in thickness. The beds consist in large part of volcanic materials which are interstratified with clay strata, shales, and desert conglomerates. The origin of the immense quantities of ashes piled up in these formations is as yet unknown. They were probably derived from volcanoes and other channels for extrusion of lavas and ash in or near the Mohave area. In a few strata abundant remains of fresh-water mollusks indicate deposition of these beds in fresh-water ponds or lakes. At other levels the skeletons of large desert tortoises and numerous remains of land mammals now characteristic of flat open country suggest accumulation upon dry land.

Mr. Baker considered that the Miocene and Pliocene deposits of the Mohave were formed mainly under physical conditions similar to those operating in the desert at the present time. As nearly as the writer can judge, the climate conditions in the Mohave area through the period in which the mammal beds were being laid down, were those of a semi-arid region somewhat more humid than the Mohave of to-day, and the climate corresponded approximately to that now obtaining in the southern end of the Great Valley of California.

Sections of the older formations containing fossils in the Mohave area are most satisfactorily shown in great thicknesses of strata exposed in the hills north of the town of Barstow, and in excellent exposures at Ricardo between the eastern foot of the Sierras and the El Paso Range. At both localities exposures extending for many miles give unusual opportunity to examine the structure of the formations, and bring to view the strata containing mammalian remains. As shown in the accompanying photographs, the formations at these localities are sculptured by erosion into most fantastic shapes, like those of the famous bad-land forms of the western Great Plains region. In the intricate gullies and caverns of these exposures there is found a most fascinating field, in which to hunt for the big game of the Mohave of ancient times.

The oldest fossil-bearing beds of the Mohave area rest upon a basement consisting in part of granite and metamorphosed or altered rocks of pre-Tertiary age. They may also rest upon extruded igneous rocks, presumably at least as old as Lower Miocene.

The oldest known strata containing vertebrate fossils in the Mohave area are found in the Upper Miocene near Barstow. Leaves stated to be of Eocene age were collected by H. W. Fairbanks at Black Mountain

PSM V86 D256 Exposure of the ricardo pliocene near ricardo.jpg

Typical Exposure of the Ricardo Pliocene near Ricardo. (Photographed by C. L. Baker.)

in the El Paso Range on the western border of the desert, hut no additional material representing this stage has as yet been found.

To the whole series of older or Tertiary sediments of the Mohave area, O. H. Hershey has given the name Rosamond series. Mr. Baker has shown that the series is divisible into a number of quite distinct divisions. Some of these may represent quite widely separated periods. Evidence which the writer obtains from a study of the faunas indicates that the deposits north of Barstow containing a Miocene fauna, may represent a formation quite distinct from that at Ricardo containing a Pliocene fauna. The term Barstow formation is used for the beds containing the Upper Miocene fauna. The lower portion

PSM V86 D256 Ricardo pliocene showing badland structure.jpg

Exposures of Ricardo Pliocene, Showing Characteristic Bad-land Structure in Outcrops near Ricardo. (Photograph by C. L. Baker.)

of the Barstow section may be considerably older than Upper Miocene and may constitute a distinct formation. The name Ricardo formation is used for the strata with a Pliocene fauna at Ricardo.

The youngest fossil beds of the Mohave region appear in a small basin about 20 to 40 miles east of Barstow. The deposits cover an area about 35 miles in length and represent accumulation in a small body of fresh-water, to which Mr. Buwalda has given the name Manix Lake. The deposits consist of clays and sands aggregating about 75 feet in thickness. Their accumulation was initiated by the raising of a barrier across the Mohave River drainage, causing the ponding of the river which formed Manix Lake. The lake disappeared and deposition ceased when the river cut through the barrier across its path.

Remains of extinct vertebrates are found over a wide area in the deposits of the Mohave region. They are not abundant in many places, and one may search long for even a fragment of a bone or tooth. In a few localities fragmentary specimens were found scattered over the ground in considerable numbers, but connected parts of skeletons are rare. At several points where bones were found well exposed, and in their original position in the rock, they seemed to be scattered and disconnected, showing that the parts of skeletons were generally widely separated and broken or weathered before final burial. The process of entombment was probably similar to that in operation on the desert at the present time, where bones of horses and cattle are pulled apart by coyotes, scattered by rain-wash, and in a large measure rotted away before any portion of the animal is permanently covered over.

The collections obtained include several thousand specimens, mostly teeth and portions of limb-bones. In a few cases, good jaws and parts of skulls were secured, but unlike the occurrence in many of the formations in the west, these beds seem almost never to contain complete skeletons.

In the Miocene beds of Barstow vertebrate remains are found almost exclusively in the uppermost zone. In the Ricardo Pliocene fossil remains were found in several parts of the section, but the best representation of the fauna appears near the middle and toward the top of the formation.

Although only a few localities have been found at which even small collections of mammalian bones can be obtained in the area of the Mohave region examined, it is evident that deposits representing the formations in which bones occur are very widely spread over this area, and future exploration may be expected to add greatly to the information now available.

The formations containing mammalian faunas in the Mohave area, and their approximate relations to the recognized geological scale are as follows:

PSM V86 D258 A basin in the ricardo pliocene.jpg

A Basin in the Ricardo Pliocene near Ricardo. (Photograph by C. L. Baker.)

Geological Periods Mammal Beds of the
Mohave Area
Recognized Formations of other Western
Areas
Pleistocene Manix Beds Rancho La Brea, California
Pliocene Upper
Middle
Lower Ricardo Beds Thousand Creek, Nevada
Rattlesnake, Oregon
Miocene Upper Barstow Beds Cedar Mountain, Nevada
Middle Mascall, Ore. and Virginia Valley, Nev.
Lower Columbia Lava of Oregon

PSM V86 D258 The most important fossil locality in the ricardo formation.jpg

Ricardo Pliocene at Ricardo. This is one of the most important fossil localities in the Ricardo formation. (Photograph by C. L. Baker.)
 

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 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 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

PSM V86 D260 Manix pleistocene lake beds of the mohave river.jpg

Characteristic Exposure of the Manix Pleistocene Lake-beds on the North Bank of the Mohave River near Field Station in the Manix Basin. (Photograph by J. P. Buwalda.)

mammal group in the whole history of our fauna. It included creatures resembling on the one hand the pigs and on the other hand the camels and deer. There are also rare remains of a large antelope or deer of the genus Dromomeryx. A small deer-antelope, Merycodus, a dainty creature with teeth like an antelope and horns like a deer, is represented at several localities by abundant fragments of teeth, limb-bones and

PSM V86 D260 Badland structure in the manix basin.jpg

Bad-land Structure in the Manix Basin. (Photograph by J. P. Buwalda.)

antlers. A large four-tusked mastodon is known by numerous fragments and occasional complete bones or large pieces of tusk.

Next to the horses the most numerous of the hoofed animals are the camels. They are known by at least three types. One is a small form of the genus Procamelus. A second and very large type probably belongs to the genus Pliauchenia. A third form with very large long limbs, a larger animal than the living camel, is possibly to be referred to the genus Alticamelus. Other genera may be present in the collection.

Of the remaining fauna, the rodents are represented by rabbits. The carnivores are known by at least eight species, including three large cats, at least one of which is a sabre-tooth with the greatly developed upper canine teeth. Two others may belong to the true cats, represented by the modern puma and wild cat, without the saber-like upper teeth. The dogs include one small form similar to the fox. A second type, Tephrocyon, one of the most characteristic animals of this horizon, is a form considered by many to be possibly the ancestor of the modern dogs and wolves. The most abundant creatures of the dog group are found in one or two representatives of the genus Aelurodon, very large, very heavy-jawed animals, much larger than any modern wolves, and even greatly exceeding the extinct dire wolf, now so well known by abundant skeletons from the asphalt deposits of Rancho La Brea. These animals were evidently not rare. They probably lived off the herds of large ungulates, sometimes bringing down a live animal, sometimes robbing the smaller wolves and the big cats of their prey. Their unusually massive jaws and teeth seem built to serve as bone crushers, and there can be little doubt that* the general state of dismemberment and destruction of all skeletons, and the absence of satisfactory paleontologic materials in the Barstow formation, is due in large part to the destruction of these scavengers.

Birds are, known in the Upper Miocene beds by a few fragments representing an owl. Reptiles are represented by numerous fragments, and several nearly perfect skeletons of a large tortoise resembling in many respects the living desert tortoises of the Mohave.

The fauna of the Upper Miocene is as a whole that of an open country affording fairly abundant grass and herbage, and evidently better watered than the Mohave Desert of the present day. The numerous remains of grazing horses of the Merychippus type, the presence of mastodons, oreodons, of many deer-antelope, a considerable variety of camels, and a wild pig all indicate that grass and other nutritious vegetation must have been more abundant than at present. The relatively small representation of oreodons, and of browsing horses like Hypohippus, and the presence of large tortoises are possibly to be correlated with open semi-arid character of the country.

That small bodies of water were present at times in this area is shown by the presence of many fresh-water molluscan remains at certain horizons.

The fauna of the Barstow beds represents a stage in the evolution of Tertiary mammalian faunas previously not distinctly recognized in the Great Basin Province. It seems clearly later than the Middle Miocene life stage well known in the Mascall beds of Oregon and in the Virgin Valley beds of northern Nevada. The fauna is markedly older than the Rattlesnake Pliocene of Oregon and the Thousand Creek Pliocene of Nevada, representing the next described stages following the Middle Miocene in the Great Basin. The fauna of the Barstow beds has few if any species in common with that of the Ricardo formation, and is of a distinctly older type. Its nearest relationships are with the fauna of the Cedar Mountain region of southwestern Nevada, from which it possibly differs somewhat in stage.

 

The Second Fauna, the Ricardo Pliocene

The number of species represented in the Ricardo fauna is approximately equal to that found in the Barstow Miocene and the groups of animals represented are in general of the same type. Comparisons between these two faunas or life stages can therefore be made with some degree of satisfaction. Coupled with the fact that the Mohave and Ricardo faunas comprise an approximately equal representation of similar groups, it is a matter of interest to note the almost complete difference between the species represented in the two, and that with one or two possible exceptions the species of the Ricardo stage represent more specialized or more progressive stages of evolution than the corresponding types seen in the Barstow fauna.

As in the Mohave stage, we find the Ricardo collections consisting mainly of horses and camels, the horses furnishing the most important and most characteristic forms thus far known.

The Ricardo horses are of at least three types, of which the most common includes one or more species of the genus Hipparion. These are large, three-toed forms with the side-toes reduced and the grinding teeth large. They resemble to some extent one of the small species of the Barstow Miocene, but are much larger; the side-toes are more reduced; and the teeth were longer-crowned, heavier, and of more complicated structure. The Ricardo Hipparion differs from most of the species referred to this genus in America, and belongs to the true Hipparion type, which J. W. Gidley considers as characteristic of the Old World, in contrast to a New World form, Neohipparion. Many of the teeth of the Ricardo species are practically indistinguishable from these of Hipparion richthofeni, a species abundantly represented in the early Pliocene or late Miocene of China. It has generally been assumed that the Old World horses of the Hipparion type are descended from North American stock. No types from which Hipparion might presumably be immediately derived by evolution are known in the Old World formations of the period just anterior to that in which Hipparion first appears, whereas in North America stages of evolution leading toward Hipparion are found in formations representing the period preceding the birth of this genus. So far as the writer's observations have been carried, an evolutionary sequence leading to the genus, Hipparion is nowhere more clearly suggested than in the relation of the Hipparion of Ricardo to the Hipparion-like Merychippus of the Barstow Miocene. It seems not improbable that the Old World Hipparion is derived from a West-American form near the Barstow Merychippus.

Living in the same region with the Hipparion in Ricardo time were at least two other types of horses of an advanced stage referred to the genus Pliohippus. The animals of these species were nearly as large as the smaller forms of the modem domestic horse. Their teeth were long crowned and well adapted to grazing as in existing forms, but their feet still bore small side-toes somewhat as in Merychippus of the Barstow. The pattern of their teeth is quite unlike that of the Hipparion and considerable differences separate them in skeletal structure. They presumably occupied a different niche in the organization of the fauna, but what it was is not entirely clear.

In the Ricardo fauna, as at Barstow, we find a rare oreodon, the last representative of this important family known west of the Wasatch. The Ricardo type follows the rule in being more specialized than that in the Barstow Upper Miocene. Little deer-antelope much like those of Barstow are also known by the last representatives in the Great Basin. Rodents are rare. The mastodon group is still represented by animals with four tusks, a pair being present in the lower jaw as well as one in the upper jaw.

Of the camels there are several species known from Ricardo. They represent genera similar to those in the Barstow Miocene, but are generally of larger type, and are presumably in a large part specifically different. Carnivores are relatively abundant. Large heavy-headed ælurodons like those of Barstow are present, but possibly all belong to new species. With these are other forms of the same group, but larger and stronger. There is a marten of a new species. Of the cats, one is a saber-tooth of a rare type somewhat similar to a species known in India. One specimen, belonging to a gigantic animal of the Felis or true cat type, was at least as large as a male African lion of the present day. Another specimen is from a smaller cat possibly like a puma.

Large tortoises are known in the Ricardo, as at Barstow. At least one form seems to differ in its character from the Barstow species.

In the table on page 262 a comparison of the Ricardo and Barstow faunas would show almost complete specific separation of the life stages. This difference extends in a considerable measure to groups of the rank of genera; as in the case of the horses, in which Hipparion replaces Merychippus. As has been noted above, in nearly all cases in which it has been possible to make a satisfactory comparison of animals in similar groups, the Ricardo types are seen to be more specialized or more progressive. In the Carnivora the common Tephrocyon of the Mohave seems to have disappeared. A single specimen shows some resemblance to that genus, but is not comparable to any Barstow species. The heavy-jawed ælurodons, which are the characteristic canids of the Ricardo fauna, seem to be mainly, if not entirely, distinct, and are generally more specialized than those from the Barstow beds.

The fauna of the Ricardo beds is widely different from that of the Middle Miocene west of the Wasatch, and is distinctly more advanced in the stage of progress or evolution. It is quite different from the Lower Pliocene of Thousand Creek of Northern Nevada, and seems less advanced. It differs so far as known from the Rattlesnake Lower Pliocene of Oregon, and is possibly somewhat older.

The beds in which the Ricardo fauna occurs were evidently deposited on plains lying at the eastern base of a Pliocene Sierra range rising to a height of several thousand feet above the level of the Great Basin region. The elevation of the Mohave area as a whole was probably not greater than at present, and may have been somewhat less. The Ricardo deposits are probably in part land-laid and in part water-laid. The volcanic material which they contain may at times have accumulated rapidly, but seems in general to have been deposited so slowly that the region was nearly continuously habitable.

The Ricardo fauna consists largely of forms that would naturally prefer to inhabit plains areas, or might thrive in partly open, level regions at least as well as in other environment. Hipparion, Pliohippus, the camels, and Merycodus would find this a favorable habitat. The carnivores associated with them would not necessarily find the surroundings unfavorable, provided sufficient cover were available. The mastodons and oreodons might inhabit the plains or frequent the border of the mountain area to the west. There are no elements in the Ricardo fauna which are necessarily considered as representatives of a forest or mountain assemblage washed or carried out on the plains.

The Ricardo fauna suggests climatic conditions permitting the development of vegetation suitable for grazing animals. This indicates a somewhat heavier growth of grass than is found in the Mohave at the present time. There is nothing in the constitution of the fauna to suggest conditions radically different from those obtaining in this region to-day, but the presumption is in favor of less extreme aridity than is now known on the western border of the desert. The conditions obtaining here in Ricardo time were probably more nearly like the present environment in the southern portion of the Great Valley of California.

 

The Latest Extinct Fauna of the Mohave, the Manix Pleistocene

The fragmentary remains obtained by Mr. Buwalda from the deposits of Manix Lake include only scattered bones and teeth with a few shells of snails and clams. The collection includes the bones of two horses of the genus Equus. One is a large species evidently closely related to the existing horses. The other is a much smaller form, but evidently of the same genus. There are two camels; one near the size of the dromedary, the other much smaller. The larger camel was probably near or incidental with the large Camelops known by splendid specimens from Rancho La Brea. The other species is unlike any Pleistocene camel described from the west. There are bones of a proboscidean, probably an elephant. A large antelope, probably like the pronghorn is known by a single bone. Two birds like existing species are found in this fauna. The molluscs are fresh-water species closely related to living forms.

As fragmentary as is the material from the beds of Manix Lake, it represents the first assemblage of mammalian species of Pleistocene age from a definitely known horizon in the Mohave region. It is, in fact, the most important collection made at any single locality in the Pleistocene of the Great Basin. It gives for the first time a grouping of the most important mammalian forms living together in this region at any particular stage in the Pleistocene.

Taken alone these fragmentary specimens might never tell more than a very short story, but the wonderful Pleistocene collection obtained at Rancho La Brea just across the range to the west will ultimately furnish comparative material adequate to make possible a definite determination of the animal represented by every bone found in the Manix beds.

The Manix fauna is entirely distinct from that of the Ricardo. The horses are of the latest and most advanced genus, that is the modern Equus, which includes most of the living representatives of the horse group. The larger camels seem to represent the last genus known in North America. The relationships of the smaller camel are as yet uncertain. If the antelope is near the pronghorn, as seems probable, it is also of the latest known type.

It is perhaps unnecessary to state that the Manix fauna differs from that of the present day in the inclusion of camels and a proboscidean. When it is better known, this fauna will probably be found to contain few if any modern species.

Comparative Table Showing Known History of the Mammalian Faunas in the Mohave Area.

PSM V86 D266 History of the mammalian fauna in the mohave area.png

 

At the present time we are not in a position to state definitely the exact position or relationship of the Manix fauna with relation to other Pleistocene life in the west. The problem of the Pleistocene in this region is complicated and large, and the many elements still require much study before their interrelations can be determined. The Rancho La Brea fauna seems to contain elements similar to those of Manix, whether it is older or younger is not yet entirely clear.

The significance of the Manix fauna in relation to its environment is unfortunately not large. The presence of camels, horses, and antelopes indicates a climate somewhat more humid than that in this region at the present time, and such was the suggestion furnished by Mr. Buwalda's work on the physical history of the Manix Lake basin.

 

Significance of the Faunal Succession in the Mohave

The physical history of the Mohave area, in the time that has passed since the accumulation of the oldest formation containing a mammalian fauna in this basin, is only a small part of the long and complicated geologic story of the region; but the changes that have occurred since the earliest of these records of life were completed take on stupendous proportions when measured against human standards of stability. Since the deposition of the oldest beds of the Barstow section, not less than 8,000 ft. of known sediments have been laid down in this region, and there are evidences of long periods from which the only record that we have is of erosion instead of deposition. The strata of both the Barstow and Ricardo sections have been subjected to extreme movements of the earth's crust in folding and faulting or breaking. They have also been extensively eroded or worn down, and the strata now exposed can be considered only as remnants of the original mass. In terms of accumulation and erosion of deposits, judged by the best estimates that we can make, the lapse of time since burial of the oldest mammal remains in this region must be very long.

Physical changes of great significance in the history of this region, and of the life in it, are also noted in variations in the nature of the bordering mountain ranges. At present the Mohave owes its distinctive characters in large measure to separation from the Pacific coast by high ridges to the west. Throughout a large part of the known life history of this region, a barrier seems to have existed between the Mohave area and the Pacific coast province. The height of the separating wall has presumably varied much, being relatively small in Miocene time, and probably reaching its maximum since the Ricardo Pliocene. Variation in height of the barrier depended on the balance between erosion constantly wearing it down, and on the magnitude of crustal movements concerned in the making of the mountain chains. To some extent variation in physical conditions in the Mohave has therefore been related to stages in the life of our great ranges. The latest period in the history of the mountains is the stage in which the peaks and valleys were modeled to their present form through gradual wearing down by ice, water and chemical decay. The clearly visible evidences of this last epoch mark for us a period longer than the full span of human history. In the story of the mountains, the earlier stages standing in relation to the history of life on the Mohave are observed only through study of a complicated geologic problem, but the measure of these early stages in time is far longer than that of the latest epoch.

The Barstow, Ricardo and Manix faunas present three stages in the life history of the Mohave area within the extent of a long period marked by many great physical changes. The records of these faunas are incomplete, and should be considered only as imperfect pages from a volume that has passed through fire, flood, earthquake and decay incident to the passage of almost limitless time. As fragmentary and unsatisfactory as the story is, it opens to us a wide vision of previously unknown life history in this region; it offers significant evidence regarding the origin, evolution and migration of important mammal groups; it furnishes information concerning the climatic history of the Mohave; and it contributes largely to our knowledge of the chronology of great crustal movements in western North America. If this were the only record known in the world, from it alone we could gather evidence that the life of the earth is very old, that this life has completely changed from time to time, and that in each successive fauna there was a nearer approach to the life types now in existence. We might not be able from the Mohave story to demonstrate the fact of evolution, as the fragments are small, and represent periods so widely separated that the suggestion of continuity is indistinct. Taken in connection with the great volume of records now available from other regions of the world, the Mohave story serves in a modest way to fill gaps in the previously known history; and in its close relationship to faunas remotely separated from it geographically, it illustrates the faunal unity of the world as a whole when the broader outlines of evolution are followed through long periods.

The story of the Mohave read alone cannot do less than impress one with the magnitude of faunal changes and with their apparent definite trend toward the life of to-day. Related to other records, it becomes a part of the great world-scheme of life growth or evolution leading up through the ages to the present living world of which we are a part.