FOSSILIZATION
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sand or mud must be neither too soft nor too hard to take the form of the foot and retain its shape when the foot is withdrawn.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.
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.
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.