Fossils: A Story of the Rocks and Their Record of Prehistoric Life

by Harvey C. Markman

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 Folsom, New Mexico, Bison (Bison taylori)

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.

Early Man in North America

There is abundant evidence to indicate that the great elephants of Pleistocene time were hunted by primitive Americans whose only weapons were darts or spears tipped with points of stone. A skull and the lower jaws of several mammoths are shown here.

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.