The striking feature of life development in the Cenozoic era is the
great progress and expansion over the earth of the mammalian races. The
division of the era into periods, however, was based largely on a study
of fossil mollusks. In the Paris basin of France, it was noticed by the
geologists of a century ago that the youngest of the sedimentary beds
contained the greatest number of recent or still living species.
Successively downward into the older beds the percentage of recent
species decreased until there were practically no living species
represented in the oldest rocks of the series. From the percentage of
recent forms among prehistoric ones it was proposed that the following
division be made: Eocene, meaning _dawn of the recent_; Miocene, meaning
less recent; and Pliocene, meaning _more recent_. Sometime later it was
suggested that another period be added, and to this was given the name
Pleistocene, meaning _most recent_. In 1854, the older Miocene
formations were segregated and referred to a newly provided Oligocene
period, this name meaning _little of the recent_.
Early geologists grouped the rocks in three great divisions, applying
the names Primary, Secondary, and Tertiary. To these was added
afterwards the name Quaternary, which applied to the youngest formations
of the earth. Only two of these terms remain in common use at present:
it is a frequent practice to refer to the combined Eocene, Oligocene,
Miocene and Pliocene periods as the “Tertiary” division of Cenozoic
time; to the Pleistocene and Recent periods as the “Quaternary”
division. The geology of some remote future may be clearer with regard
to the full significance of this subdivision of the Age of Mammals into
two parts. It may be that a great era was concluded at the end of
Pliocene time as others have been concluded, by the usual earth
disturbances and climatic changes and by the decline of animals once
prominent in the faunas of the world. Events of such character have
registered their occurrence but may eventually prove to have been a
series of minor events not comparable with the revolutionary changes
that terminated other great time divisions. The favored practice of
including ourselves and our times in the Cenozoic is based on a trend of
opinion which holds that no great era has been ended since the Age of
Reptiles was concluded.
[Illustration: Skulls of the clumsy, six-horned uintathere and the
early, hornless titanothere form part of this Eocene display. In the
mural these animals and the little “three-toed” Eohippus, smallest
of horses, are pictured with a contemporary turtle and crocodile.]
The oldest of Eocene rocks show a great variety of mammals and a strange
assortment of forms far in advance of the Mesozoic record. Ancestries
and successive stages of development have been only partially worked
out, though details have been better preserved for some of the groups
than for others. Some oddly shaped creatures such as the uintatheres
reached their full development in a relatively short time and passed out
of the picture before the end of the period. These animals, represented
in our collection by _Uintacolotherium_, acquired large bodies and many
horns, but a peculiar tooth equipment fitted them for a special diet
which apparently failed to be supplied in sufficient abundance at a
critical time.
On the other hand, we find in this period the ancestors of more
successful groups, some of which continue on into modern times. Only a
few of these histories can be traced in a brief sketch, but in a general
way it may be stated that the successful races had modest beginnings and
that they developed very slowly into what they are now, by a process of
adjusting themselves, or by becoming better adapted to new or previously
unused conditions in their respective environments. In their early
stages the various types had much in common; they were generalized,
rather than specialized for any particular kind of existence. What they
were fitted for is best indicated by their teeth and feet, though other
structural features frequently contribute valuable information. The rise
of mentality is indicated by skull capacities and the increasing
development of the upper lobes of the brain, as revealed by casts taken
from the interior of skulls.
The creodonts were the earliest and most primitive of the flesh-eating
mammals or carnivores. Many of them were small of body and brain, and
equipped with teeth that indicate a mixed or largely insectivorous diet,
or possibly the habit of feeding on carrion. Although there was
considerable variation among them, and some tendency toward
specialization, there was little to suggest the coming of more
progressive groups such as the cats and dogs, with teeth perfected for
the tearing and cutting of flesh, and feet especially fitted for the
life of hunters. The ancestry of the cats cannot be traced farther back
than the Oligocene but it probably connects somewhere prior to that time
with the creodonts.
_Cynodictis_, an Oligocene carnivore slightly under two feet in length
is commonly regarded as a primitive dog, but its characters are so
generalized that it probably differs but little from the ancestors of
many other carnivores. The skeleton of this animal suggests a slender
and flexible body like that of the weasel, with somewhat shortened limbs
and a long tail. It lived in forested regions and was probably more or
less of a tree dweller. The more advanced carnivores required longer
legs, better adapted for running and overtaking the prey, which is the
dog’s way of hunting, or for stalking and springing upon the quarry,
which is the method of the cat.
[Illustration: Moropus (_Moropus cooki_)
Though its teeth clearly indicate a diet of plant material, this
strange animal had claws on its toes, like the carnivores. It is
probable that the claws were used in digging for roots, as indicated
by the artist.]
A prominent group of mammals today is that known as the ungulates, or
hoofed animals, which includes the horses, cattle, deer, swine,
rhinoceroses, tapirs, and other types both living and extinct. Their
probable ancestors were the condylarths, primitive ungulates of the
Eocene period. One of these, known as _Phenacodus_, serves well to
illustrate the general character of the early hoofed mammals. It was
about five and one-half feet long, rather large for its time, with long
tail and short limbs, low elongated skull and small brain, very similar
in many respects to the creodonts or ancestral carnivores. The teeth,
however, were partially of the grinding type so essential to the welfare
of plant feeders.
The condylarths were five-toed animals and evidently provided with small
hoofs, but the more progressive ungulates soon lost one or more of the
toes, and a division of the group into odd-toed and even-toed branches
became firmly established. Consequently, the families of ungulates
having one, three, or five toes are classed together as being closely
related to one another, and those having two or four toes are segregated
in a second lot. The odd-toed clan, known as perissodactyls, included
such animals as the horses, rhinoceroses, tapirs, and titanotheres, each
of these types being placed in a separate family. The even-toed clan has
been treated in a similar way and named the artiodactyls. In this
division are such families as swine, cattle, deer, camels, oreodonts,
and others. The odd-toed group dominated among the larger animals of
North America for a time but has been completely replaced by the
even-toed division which is still flourishing, although some of the
older families have become extinct.
Among the exhibits of the Denver Museum of Natural History may be seen
complete skeletons of extinct horses, rhinoceroses, titanotheres, and
chalicotheres representing the perissodactyls. _Moropus_ was one of the
chalicotheres, an exceptional family which never became very prominent
although it had a prolonged history and persisted in Europe and Asia
after its extinction in North America. The family is grouped with the
ungulates because of many similarities found in the molar teeth, skulls,
and other parts of the skeletons, but the toes were provided with claws
instead of hoofs. The use of these claws is somewhat of a puzzle:
possibly for defense against carnivorous enemies, for dragging down
branches in order to obtain food, or for digging roots which may have
been an important part of the diet.
Titanotheres are represented in our collection by the skeletons of the
large, horned type which was the last of the race and destined to
extinction by the middle of the Oligocene period. Smaller hornless
varieties of Eocene time are illustrated by skulls. This family of
ungulates had an unprogressive dental equipment, and a small brain in a
flattened skull. The molar teeth readily distinguish the group from
other ungulates and enable us to trace the relationship between earlier
and later varieties. These teeth were of a type which is soon destroyed
by wear, and it is evident that the animals survived only so long as
their environment provided them with an abundance of soft vegetation.
[Illustration: Titanotheres of Oligocene Time
The name of these animals refers to the large size though they were
greatly exceeded in bulk by the mastodons and mammoths of later
periods. Ancestral titanotheres, dating back to the Eocene, were
hornless animals of much smaller size. These splendid specimens were
obtained in Weld County, Colorado.]
The large assortment of rhinoceros material provides an idea of the
great abundance and variety of forms in this family which was once
prominent in North America but no longer among the inhabitants of that
continent. Some of the mounted skeletons have been restored on one side
to show how these animals appeared in the flesh.
Of the even-toed ungulates there are also several types illustrated by
complete skeletons. _Merycochoerus_, the subject of one of our mounted
groups, represents the oreodonts, a large family of mammals whose
history begins with the Upper Eocene and ends in the Lower Pliocene. The
oreodonts were small animals, rather pig-like in form and quite common
in the western plains region shortly after the time of the titanotheres.
Ancient swine are represented in our exhibits by two mounted skeletons
which were obtained from northeastern Colorado, where the bones were
found associated with rhinoceros and titanothere remains. Some of these
animals were of very large proportions, and the entire family is
commonly known as the “giant pigs.”
Camels and closely related forms were quite abundant in North America
from early Oligocene to comparatively recent time. Numerous types were
developed during the course of their history, some small and delicately
formed, others tall and clumsy and much like the giraffe in structure.
Parts of many of these creatures have been found but the only completely
prepared skeletons in our collection are of the little gazelle-camel,
_Stenomylus_, from Lower Miocene deposits in northwestern Nebraska.
Pleistocene bisons are represented by several complete skeletons and
numerous skulls and horncores, some of the species showing an extreme
development in the length of horns. With two of the bison skeletons are
shown prehistoric weapon points, found with the bones and indicating
that these animals were hunted by primitive men at some time near the
close of the Ice Age. The artifacts first discovered near Folsom, New
Mexico, by field workers of our Museum, have become known to
archeologists as Folsom points.