How fossils are formed

How fossils come to be
Any evidence of a once-living or­ganism, including invisible sub­stances recognizable only by chemical methods, can be termed a fossil. Even tracks and tunnels can fossilize if they become filled with sand or mud that differs from the surrounding material. However, most animal fossils are hard parts. These include bones, teeth, shells, and exoskeletons (outside skeletons, such as those of crabs and insects). Likewise, in the case of plants, only the most resistant parts fossilize.
A knowledge of the process of fossilization helps explain why the fossil record is always incom­plete. Such an understanding also explains why we have better fos­sil evidence of some organisms than of others. Optimal condi­tions for fossilization include pro­tection from mechanical and chemical disturbances. Burial in mud, silt, or sand beneath quiet water is ideal. The fine sediments seal the remains from oxygen that would otherwise support decay and spontaneous chemical disin­tegration. In quiet water there is no mechanical churning that might break up the body. If more and more sediments accumulate, the deeply buried sediments may turn to rock that will encase the remains and protect the fossil. Therefore organisms that fall into lake-bottom muds have a good chance of becoming fossils.
On the other hand, the re­mains of animals that die in a warm, moist forest are likely to be torn apart and consumed by insects and other scavengers. If bones should be trampled into the earth intact, acid soil waters will soon dissolve bone minerals. Since most primates live in warm, moist forests, it isn't surprising that primate fossils are rare com­pared with those of clams, for ex­ample. Clams nearly always leave their shells in the accumulating sediments of lakes or ocean shores.
But just because an organism fossilizes, there is no guarantee-­nor is it even probable-that the fossil will end up in a museum. The longer a fossil remains in the earth, the deeper it is likely to be buried and the more apt it is to be deformed by heat, pressure, or the folding of surrounding rock. Furthermore, people find only those fossils that are revealed through erosion or by mining and other rock-moving operations. Most fossils lie buried where hu­man eyes will never see them. And when rare fossils are exposed by erosion, they often go unrecognized and unpreserved.
From fossils to theories
The fruitful study of primate foss­ils requires teams of scientists with many skills. After the location f a fossil find is marked precisely, geologists examine the rock structure and determine the age of the fossil or of the layer in which it y. The site is then carefully excavated and precise records are made of the location of any other objects. All animal or plant fossils must be identified, for they may provide dues about the habitat. Bits of garbage can reveal which animals were hunted or bear marks showin­g the use of tools or fire. Discovery of domestic varieties of plants or animals can establish the exist­ence of agriculture. In addition to fossils, humans leave artifacts, ob­jects they have modified intentionally. Stone tools are the most com­on artifacts at older sites. Such tools are classified as both fossils and artifacts.
Discovering and excavating a fossil site is only the beginning. Interpretation of primate fossils de­mands additional work. The as­sembly of fossil fragments requires extensive knowledge and experiecne. Minute details can provide cues concerning the proper placem­ent of a fragment within the skeleton. The shape of a bone can die ate posture or suggest patterns f movement. Even when it is pos­ible to reconstruct a nearly com­Jete skull or skeleton, much work remains to be done. Specimens must be examined and compared repeatedly to determine their sig­nificance. Despite careful work, conclusions are tentative and must be reconsidered as further data be­ome available.