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.
[Illustration: 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.
[Illustration: 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.