Footprints need little explanation other than a consideration of the
factors which make it possible for them to be preserved. The sand or mud
must be neither too soft nor too hard to take the form of the foot and
retain its shape when the foot is withdrawn. Then in some manner the
impression must be protected while the rock-making process goes on. When
such protection is obtained it is usually in the form of more mud and
sand, deposited over the surface which received the impression. At a
later time the covering may be separated from the lower part of the
deposit, which serves as a mold, and if the separation be accomplished
successfully a natural cast of the foot will be obtained as well as the
mold in which it was produced. Since conditions for perfect work are not
always present in a laboratory of this kind, it is not surprising that
fossil footprints are very rare considering the number and variety of
tracks left by wandering animals.
Impressions of leaves are explained in much the same way except that the
leaf remains under its protective covering until it decays. Similar
impressions may be obtained from the bodies of delicate invertebrate
animals but they are seldom preserved because of the softness of the
tissues. The smaller fishes provide much better material for the
production of fossils according to this method. While the fish is being
flattened by the weight of surrounding sediments, scales, fins, and soft
bones retain their positions and provide the necessary resistance to
leave an impression of the body form when the flesh is gone.
[Illustration: Insect Fossils (enlarged)
Fine specimens of this type are obtained from an old lake bed at
Florissant, Colorado.]
The larger and more spectacular fossils, such as skeletons, skulls, and
detached bones are nearly always of the replacement type. Replacement of
plant and animal substances by mineral matter is a slow process and in
younger fossils the change is rarely completed, some of the original
material being present in a partially altered condition or not modified
at all. Since air does not often carry the necessary materials and
provide other essential conditions, replacement may be regarded as
something which happens underground or in water. It is perhaps best
explained in connection with limestones, because calcite or “lime” is
frequently the replacing substance although other minerals, especially
quartz, may serve the purpose.
Besides converting bony or woody objects into rock substance, mineral
replacements may assist in the production and preservation of fossils in
another manner. It often results in the filing of cavities with some
rock-making substance which retards destruction through crushing or
other injury. In many cases, so-called fossil shells are not shells at
all; instead, they are merely a stony filling which was once surrounded
by shell substance. In other instances the original shell remains as it
was during the life of its former occupant, preservation of the shell
being due largely to the substitution of a mineral filler for the soft
animal tissues once present.
Limestone comes into existence through a more elaborate process than
that which produces sandstone and shales. It is one of the three types
of common rocks, known collectively as the sedimentaries, in which
fossils are found. It differs from sandstones and shales, however, in
that much of its substance has been dissolved in water instead of being
transported in the form of finely ground rock particles. Lime occurs in
many varieties of rock which are exposed to the wear and tear of the
elements throughout the world. Slowly but more or less continuously it
is taken from this source by ground and surface waters coming in contact
with it. Particularly active is carbonated water, moving underground
through pores and crevices.
This underground circulation of mineral matter in a dissolved condition
explains the occurrence of fossils in land areas which have not
necessarily been submerged during any great length of time, for it is
well known that plant and animal remains are not invariably washed into
lakes or seas, and that all sedimentary deposits have not been built up
in large bodies of water. Here we are dealing with what is known as the
continental type of sedimentation and such fossils as dinosaurs,
mastodons, three-toed horses, and other former inhabitants of land
areas.
In order to become properly fossilized, certain conditions are
absolutely necessary, and only a small percentage of the once-living
multitude secures the required treatment. There must be present, soon
after death, some protection from the activities of the carnivorous
birds and beasts that would separate and scatter the parts of a carcass,
also from the smaller gnawing animals that would continue the
destruction, and finally from wind, sun, rain, frost, and bacterial and
chemical activities which in the course of only a few years would remove
everything but possibly a few scraps of tooth enamel, which is the
hardest of animal tissues.
A slight covering of earth substance in any form serves to check the
disintegration, and this may be acquired in several ways. Animals that
perish in bogs or quicksands are soon covered over; in many localities
wind-blown dust and sand do the work; and flooded river valleys provide
an abundance of mud for the necessary burial of others. Even
underground, the decay of soft tissues is too rapid to permit of
replacement by mineral substance in a manner that would reproduce form
and texture. Skin and flesh are almost invariably lost, although in a
few instances the thick scaly hides of dinosaurs are known to have
produced natural molds and casts by the method explained in connection
with footprints and other impressions.
[Illustration: The skeleton of this prehistoric American rhinoceros
is mounted in a “half shell” which was modeled over the bones to
show the form of the living animal. The artist’s reconstruction
appears in the painting above the fossil exhibit.]
With regard to the more durable tissues found in the teeth, bones, and
shells of animals, or the woody parts of plants, the case is different.
These parts become firmly imbedded in the ground, but moisture still has
access, and it begins to work immediately; for all water moving
underground finds soluble substances which it picks up and carries with
it wherever it goes, and much of the load consists of mineral matter
which may be unloaded again when the necessary conditions are found.
Mineral-laden waters will drop one kind of substance to take up another
which dissolves more readily, and this happens sooner or later when a
buried bone or log is encountered. Complications of various sorts enter
into the process, but the final outcome frequently is a complete change
from one chemical composition to another which is more enduring, the
transformation being brought about so gradually and thoroughly that in
many fossils the inner structure of the original tissue is as accurately
reproduced as the fine detail of surface features.
Converted into stone, however, the result is still far from permanent.
While yet underground the fossil is subjected to distortion and breakage
due to earth movements which bend and dislocate the rock deposits. What
causes these upheavals and depressions of the earth’s surface remains
the subject of much discussion, but that they have occurred on a large
scale and continue to occur is clearly evident. At higher altitudes the
surface rocks and fossils are exposed to a larger variety of destructive
activities than at lower levels where protective coverings are more
likely to be provided and retained. Once stripped of that protection
there is little chance for a fossil to survive. Beyond a doubt there are
many thousands of tons of prehistoric remains damaged or destroyed each
year, by weather and stream erosion.