Any trace of life from the past can be called a fossil. This includes the molecular products of life as well as worm tracks in the mud of some bygone time. It also includes various kinds of hard parts, such as shells, bones, or teeth. It can include the impressions left by soft parts, which may be flattened and distorted by the pressures of the overlying mud that turned to rock or replaced by chemicals, which have no direct connec-tion to living systems, but nonetheless are a record of fife in the past. Perhaps the most extraordinary fossils are the living things that were frozen during an Ice Age. A mammoth, which is an extinct relative of the elephant, has been found which was trapped in the frozen Alaskan tundra some milllions of years ago and preserved there. Discovered and recovered some years ago, it provides us with an extraordinary fossil.
Fossils are obviously formed in a variety of ways, and therefore, there is no one answer to our first question, How are fossils formed? Fossil formation is a chance event. It depends on living things, or any of their many products, being in a place where they are relatively undisturbed. Or, if they are disturbed, where the resulting changes still retain the record of the past. The examples of petrified wood show this dramatically. Here the original wood has been completely replaced by various chemicals from the environment. These latter were selectively deposited in various parts, even down to cellular details of the wood. Thus, despite replacement of the original living parts, a marvelously detailed record of that life is retained.
More typically, the fossilization of an animal, such as one of the ancestors of present-day horses, occurs along the following lines. The organism in question, say, for example, the grazing form called Merychippus that lived about 20 million years ago, died on the grassland where it lived. In a relatively short time the scavengers of that time--giant hyenas and vultures, and a whole array of other forms including beetles and flies and their larvae--would have consumed and dismembered the carcass of Merychippus. Such a fate would have precluded any fossil formation and was by all odds the most common situation. Another, less likely possibility was that a Merychippus was drowned while crossing a river. If the body sank and was covered by silt, the possibility of a fossil resulting is more likely. However, if the river dried up--perhaps our grazer was simply caught in a flash-flood--before the drowned animal was well buried, scavengers could have destroyed the carcass.