Part 8

Other skeletons include the saber-tooth tiger, characterized by the long curved upper canine teeth which undoubtedly were used for stabbing and slashing, and the dire wolf, the smaller of the two which are facing the sloth. The artist’s reconstruction of this scene also shows the great vulture, Teratornis, which is the largest known bird of flight.]

Over-specialization in the production of tusks appears to have been the principal factor in the downfall of the mammoths. The large size of the animals and the difficulties of finding sufficient food to sustain life must have been a serious handicap at times, but their ability and inclination to travel over long distances enabled some of them to find tolerable living conditions until the end of the Glacial Period. They are now extinct and the nearest living relatives are the elephants, somewhat reduced in size of tusks and body but otherwise very similar.

There are many other tribes of mammals whose ancient history is partially known though broken by periods of time for which there is no fossil evidence. All have undergone changes in which various forms and degrees of specialization are featured; this general process is best revealed by the horses and elephant-like animals which have left a clearer record. For other groups the story would differ but little except as to names and specific details.

THE RANCHO LA BREA FOSSIL PITS

The La Brea tar pits, as they are often called, provide a remarkable record of Pleistocene life in southwestern North America. Scattered over an area of about thirty acres just off Wilshire Boulevard in Los Angeles, these bone deposits were known, as far back as 1875, to contain the remains of prehistoric animals. It was not until 1905, however, that their value was recognized by paleontologists. In that year the University of California began an investigation, and excavations were carried on at intervals by various institutions during the next ten years. A great deal of material was acquired by the Los Angeles Museum of History, Science, and Art, where many skeletons, skulls, and other interesting specimens have been placed on exhibition.

The pits have the form of small craters formed by the seeping of oil from the underlying rocks. The seeps appear to have been active during part of the Pleistocene period but apparently not at the beginning. The oil is rich in asphalt which has served as a preservative for the bones, and owing to its sticky properties has been an effective animal trap for thousands of years.

The fossil beds at present are of oil-soaked earth and sand. In past times there must have been a greater percentage of oil, often concealed by a layer of dust or pools of water. The large number of carnivorous animals found in the deposits suggests that they were attracted by the cries and struggles of creatures wandering carelessly into the asphalt and serving as live bait to keep the traps in continuous operation.

Animals found there include many species still living in the locality, some that have migrated to other territory, and a large number that have become extinct. Among the latter may be mentioned species that differ but slightly from living relatives, others that have left no descendants. Horses, bison, and wolves, though extinct species, were of relatively modern types. On the other hand the large sloths and saber-tooth cats seem rather out of place. True cats are represented by the mountain-lion, bob-cat, and a species of lion which is nearly one-fourth larger than any of the great cats of the Old World. A long-legged camel, with a height of approximately eight feet to the top of the head, was among the native animals of the district. Skunks, weasels, badgers, squirrels, rabbits, bear, deer, and antelope were more or less abundant.

The La Brea group exhibited by the Denver Museum of Natural History includes the following species: horse (_Equus occidentalis_), bison (_Bison antiquus_), wolf (_Aenocyon dirus_), saber tooth (_Smilodon californicus_), sloth (_Mylodon harlani_). Horses had entirely disappeared from the North American continent by the time the first white man arrived. _Equus occidentalis_ was one of the several species living during the Pleistocene period, this one apparently being restricted to California and perhaps adjacent states. _Bison antiquus_ was slightly larger than the plains bison of recent times and had it horns set at a characteristic different angle. The species was first described from Kentucky and appears to have had a wide distribution.

The wolves in this group are about the size of timber wolves, but have heavier skulls with less brain capacity, massive teeth especially adapted to biting and crushing large bones, and limbs of rather light construction. They probably assembled in packs where meat was abundant and, hunting in this fashion, were able to attack and overcome the larger ungulates and edentates. To most visitors the large ground sloth is the most interesting animal of the group. This edentate animal is shown at the edge of the pool with one foot stuck in the “tar.”

The edentates are a group of primitive animals with very simple teeth, if any. Teeth are usually lacking in the front part of the mouth, sometimes entirely absent, as among anteaters. Better known living representatives of the group are the tree sloths, armadillos, and anteaters of South America. Ground-sloths were prominent among South American mammals during much of Cenozoic time. During Pliocene and Miocene time there was a marked tendency to large size, and it was principally during these two periods that they appeared in the United States area.

_Mylodon_ was one of the larger North American ground-sloths. Its teeth, without the protective enamel which is present among higher mammals, are restricted to the cheek region, and have the form of simple pegs; instead of being specialized they stand close to the extreme of generalization. The construction of the entire skeleton is massive, suggesting great strength with slow movements. The hands are well developed, provided with stout claws, and must have served the creature well as protection against attacks by predatory neighbors. We have some idea as to what caused the extermination of the ground-sloths in this particular region, but the complete disappearance of such a large and widely distributed group at the close of the Pleistocene period is a mystery that may never be explained.

The saber-tooth cat, sometimes referred to as a tiger, was specialized as a meat eater though hardly as a hunting animal. In the La Brea region its principal food was probably the flesh of the sluggish ground-sloths. The size was equal to that of the African lion, with hind limbs slightly longer and the front legs more powerfully developed. The most remarkable characteristic is to be found in the development of the upper canine teeth and modifications of the skull which were necessary to enable the animal to use these teeth as weapons.

In order to make the “sabers” effective it was necessary to get the lower jaws out of the way, and this was provided for in an unusual type of hinge which enabled the mouth to open wider than is possible in the case of the less specialized carnivores. Judging by all the structural features of the skeleton, _Smilodon_ could not have lived well on small animals, for it was not equipped to capture that kind of prey. It is evident that large mammals were preferred, and that the method of attack was to spring upon the victim and cling there with the powerfully developed fore limbs until the kill was completed by stabbing into a vulnerable spot. That the position of the large sabers near the front of the mouth interfered with normal feeding, is a reasonable conclusion. There are also anatomical features which lead to the belief that this carnivore was a blood sucker, perhaps more than it was meat-eater.

If most of these conclusions are correct we have here another case of over-specialization and a possible explanation of the extinction of two species. Such evidence as we have is far from conclusive, for there is no proof that Rancho La Brea was the last stand of either the saber-tooth or the ground-sloth. Both races were widely distributed and their living conditions could not have been exactly duplicated in other localities. It has been suggested, however, that _Smilodon_ ate the last of _Mylodon_, and starved soon afterward because it had become unable to partake of other foods. The conjecture is offered for what it is worth, together with the facts on which the story has been based.

The geological record for Pleistocene time is not as complete as one might imagine. Numerous localities have produced representative fossils but the yield is rarely large enough to solve many of the riddles which are constantly arising as investigation proceeds. Aside from those areas which bordered the retreating ice cap and where living conditions were far from favorable, the sedimentary deposits of this period are not continuous over large areas. Many Pleistocene fossils are found in stream channel beds which are always subject to removal by subsequent floods.

Isolated patches of fossil-bearing sediments frequently record the migration of animals in unmistakable terms, but the details of the wanderings and the conditions encountered in the newly established habitats are often left in doubt. To correlate the facts revealed at one locality with findings at other places and, if possible, to date all prehistoric events with a greater degree of accuracy are among the major tasks of current investigations.

THE AGE OF MAN

The Pleistocene or “Ice Age,” and the Recent period in which we are living at the present moment are not sharply separated by any event readily recognized or dated, and the two combined are of very short duration as compared with other periods more clearly established by the passing of centuries. Together they comprise the Age of Man as commonly recognized, with about a million years representing the Pleistocene period, some ten to twenty thousand years the Recent. When geologists of the nineteenth century suggested that the coming of man should be regarded as the beginning of a new era, the name Psychozoic was proposed, and to some extent this term has been applied to the present period. More in keeping with other period names is Holocene, meaning _entirely recent_. Common usage, however, applies the simple term Recent to this unfinished chapter which is also without a clear-cut beginning.

Zoologically, man is merely one of the creatures that arrived in the course of time, along with other mammals. Just when he arrived and how he looked at the time of his coming cannot be determined from a study of fossils. Perhaps it is of no importance. There is nothing to indicate his existence before the Cenozoic, no completely satisfactory proof of existence before the Pleistocene period. As with other inhabitants of the earth, it is probable that he became prominent only after a great deal of competition with other creatures which kept his ancestors submerged for thousands of years. The Ice Age, with its check upon the progress of competing animals, undoubtedly gave him an advantage. His superior mentality enabled him to overcome adversity by methods not available to other mammals; his inventive and mechanical genius must have been greatly strengthened by his experience during this interval.

At about this point, where prehistory begins to merge into history, the geologist and paleontologist must let other interpreters carry on. Archeologists and anthropologists take up the work, and through their efforts many details have been added to our knowledge of the human race. The study of biology, which is the science of life, has provided an instructive viewpoint that enables us to see ourselves against the vast background built up by investigations into the nature of the earth and its ancient inhabitants. This science deals with living creatures as _organisms_—plants and animals so organized as to be capable of existence only in an environment which provides exact life requirements.

The Age of Man has been variously characterized as an age of soul, of higher intelligence, of culture, and finally, of civilization, freedom and democracy. The “crowning glory” of the organic world is pictured in history as a creature who has busied himself for thousands of years with the building up and tearing down of civilizations. Prehistory reveals this habit as something unique in the human character, for there is no other organism that has specialized so persistently in the creation of its own environment, no other that has had the combined power and talent to produce so much change.

More than anything else, the prehistoric record is a lesson in adaptation, which in its broadest sense means fitness for life under particular conditions, and always subject to organic law. Man’s efforts to bring about an adjustment between himself and his civilization have centered largely on the method of forcing himself into the mold that happens to be present, one pattern today, another tomorrow. No creature of the past has had to adapt itself to anything so radically new or so thoroughly revolutionary. The vital problem now is whether this man-made environment will prove helpful or disastrous.

Though one of its names is “culture,” it has grown sporadically and unevenly, with little evidence of the cultivation that is implied and required. Parts have been expanded to extraordinary proportions while others equally essential have been retarded in their growth. A more intelligent handling of this environment factor seems to be possible, and the present mania for “organization” may become tempered with an awakening consciousness of organic requirements where organism and environment are involved. Once we grasp the idea that “culture” results from man’s effort to improve his living, by putting into his environment something that was not there before—then, surely, this history of a billion years of living, and as many “ways of life,” should teach us something we ought to know as we go into an all-out endeavor to teach a whole world how to obtain a one-and-only way.

We may stand at the beginning of an era for which an appropriate name has not yet been suggested. Civilization, on the other hand, may provide only a minor epoch to be added in some remote time to the story of fossils.

SUPPLEMENTARY READING

The literature pertaining to fossils is widely scattered and usually too technical for the layman. It is better to use the resources of the nearest library than to feel that a specified list of books is necessary.

Any textbook on geology, zoology, or botany will provide helpful information. Most books of this type will be found interesting and readable if used to solve definite problems suggested by the student’s immediate curiosity. Very few can be read from beginning to end without a great deal of effort and discouragement.

The following have been prominent among the books consulted by the author:

_Textbook of Geology_; by Pirsson and Schuchert. This work has undergone several revisions and currently appears in two volumes: _Physical Geology_ by Longwell, Knopf, and Flint; _Historical Geology_ by C. O. Dunbar. Published by John Wiley & Sons. (Historical geology covers the entire range of prehistoric life—plant, invertebrate, and vertebrate.)

_Historical Geology_ (The Geologic History of North America); by Russell C. Hussey. Published by McGraw-Hill. Concise, interesting, and informative.

_Geology and Natural Resources of Colorado_; by R. D. George. Published by the University of Colorado. Contains an excellent summary of the historical geology and sedimentary formations of Colorado.

_Vertebrate Paleontology_; by Alfred Sherwood Romer. Published by the University of Chicago Press. This is one of the most comprehensive and up-to-date treatments of the subject for students desiring to go beyond the elementary stage.

_A History of Land Mammals in the Western Hemisphere_; by William Berryman Scott. Published by The Macmillan Company. This well-known account of living and extinct mammals is one of the favorites among students.

_The Age of Mammals_; by Henry Fairfield Osborn. A classic in this field of literature, but for advanced reading. The book is now out of print.

_The Dinosaur Book_; by Edwin H. Colbert. Published by the American Museum of Natural History, New York. An illustrated story of amphibian and reptilian evolution.

_Down to Earth_; by Carey Croneis and William C. Krumbein. Published by the University of Chicago Press. An excellent popularization of the earth sciences—geology and paleontology.

_Lexicon of Geologic Names of the United States_; compiled by M. Grace Wilmarth. Bulletin 896 (in two parts) of the United States Geological Survey. A rich source of information concerning the age, character, and distribution of geologic formations, with numerous references to fossil-bearing beds.

_Bibliography of North American Geology_ (including paleontology); various bulletins of the United States Geological Survey. Where library facilities provide access to the technical literature of museums, universities, and scientific societies, this is a valuable aid in locating publications dealing with original work in paleontology. Bulletins 746 and 747 cover the years between 1785 and 1918; Bul. 823 (1918-1928); Bul. 937 (1929-1939); Bul. 938 (1940-1941); Bul. 949 (1942-1943); Bul. 952 (1944-1945); Bul. 958 (1940-1947); Bul. 968 (1948); Bul 977 (1949). Preparation is a continuous process with recent bulletins appearing at one or two year intervals.

_Ancient Man in North America and Prehistoric Indians of the Southwest_; by H. M. Wormington. Published by Denver Museum of Natural History, City Park, Denver 6, Colorado. Both volumes contain authentic and up-to-date accounts of early American cultures.

MAPS

_Geologic Maps._ United States Geological Survey: map of the United States (1932); map of Colorado (1935). Geologic maps of a few other states are available; information regarding these may be obtained from state universities or state geological surveys.

Note: Bulletins of the U.S.G.S. are purchasable from the Superintendent of Documents, Washington, D. C. Maps are sold by the Director of the Geological Survey, Washington, D. C.

Transcriber’s Notes

—Silently corrected a few typos

—Restored one accidental omission in the Table of Illustrations

—Retained publication information from the printed edition: this eBook is public-domain in the country of publication.

—In the text versions only, text in italics is delimited by _underscores_.