Becoming a fossil is no easy adventure. In the more than two billion years of life on earth, an incalculable number of organisms have lived and died. If all had been preserved, our earth would have become nothing but a mass of fossils. Fortunately, most organisms have returned to the earth from which they came and left no fossil litter behind.
A creature destined to become a fossil usually is one that possesses hard parts, such as a shell, horny armor like a crab, or bones that will resist the abrasive effect of water and wind and the appetites of bacteria. As has been said, "You have to be hard and tough to get into the fossil record." Besides being tough the creature must come to rest in a place where it stands a good chance of being buried before it decays or disintegrates. If it is not buried deeply and quickly, aerobic bacteria will reduce it to rubble; or water, given enough time, will dissolve it.
For this reason, fossils of some kinds of organisms are rarer than others. Butterflies, for example, are common in nature, but their soft bodies and fragile wings leave few epitaphs in nature's graveyard. The soft tissues of jellyfish likewise stand little chance of becoming fossils even though their marine environment is far more favorable for this purpose then land.
The ideal place to become a fossil is at the bottom of a quiet sea or lake where the prospective fossil is safe from damage and where it is covered rapidly with fine sediment. Clay is excellent. The sediment protects the tissues and helps to exclude predators and solvent water. If the water is poisoned with dissolved chemicals that will keep predators away, so much the better for the future fossil's chances.
Consequently, fossils are most commonly found in fine-grained sedimentary rocks, such as shale derived from the compressed clay and silt of an ancient sea or lake bottom, or in limestone formed in warm sea water by chemical precipitation and the constant accumulation of carbonate shells of living organisms. Wave currents strong enough to wash in sand and gravel are also strong enough to sweep away or damage most shells and skeletons; hence only the toughest fragments of fossils are generally present in sandstones and conglomerates.
Fossils, then, are not only fairly rare as compared with the plenitude of life through the long history of the earth, but they also give a distorted view of it, because of nature's favoritism to certain types of organisms. Furthermore, comparatively few of earth's fossil resources have been tapped. It has been said that all the fossils available to science represent the variety of life of the past about as accurately as one mosquito represents the enormous variety of insect life today. Reconstructing the past from fossils is like trying to recreate the Parthenon from a basket of marble fragments. Here a piece of column, there a tile from the roof, here a limb of a statue —how did they once fit together?
But imperfect though the fossil record may be, it is the definitive record written in the rocks, and it is written in many ways. Fossils can be divided into half a dozen categories of preservation. Most specimens found by the average collector will fall into two or three categories, but some acquaintance with the others is also his legitimate concern. These divisions, in order of progressive change, include fossils preserved by freezing, drying, original preservation, petrifaction, and carbonization, and those preserved as casts and molds.
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