Organisms vary from microscopic soft-bodied protists to large creatures with massive skeletons. Organisms are constructed of an enormous variety of substances, all of them under selection for the utility of the substance during life. Hardly any substances were selected for their properties after death, though there are a few examples. Post-mortem changes to bodies and their traces are often rather unexpected.
Clearly there is a spectrum of probability of preservation, directly correlated with the anatomy and structure of the organism. Something like 98-99% of species known from fossils had hard parts when they were alive. Even hard parts vary in their resistance to physical and chemical destruction, so the relationship between skeletons and their potential for fossilization is not simple. Hard part materials are often characteristic of a particular group. There are two main ways of making hard parts:
Other things being equal, the inorganic substances are more likely to be soluble, and therefore are most easily destroyed by inorganic or organic solution; the organic substances are more likely to be insoluble, but vulnerable to destruction by oxidation. After burial, therefore, the fate of the fossil may depend on the chemical condition of the surrounding sediment-and this may or may not reflect the conditions in which the organisms were living. Organisms with organically structured hard parts that die in or are carried into sediments that lack oxygen are protected from oxidation. They are often very well preserved, and the best examples of such preservation are called Lagerstatten. Examples are the soft-bodied animals of the Cambrian Burgess Shale, the fossils from the Devonian Hunsruckschiefer and the Jurassic Posidonienschiefer of Germany, and the animals of the Messel Oil Shale of Germany and the La Brea tar pits of California. These same conditions may act to dissolve inorganic skeletons.
An organism's way of life may affect its preservation. Rodents are small and have fragile bones, but they also have very strong teeth and they often live in burrows. After death, then, rodent teeth are fossilized more often than one would expect. Even rodents that die away from their burrows may be food for owls and other predators, and their teeth are again so resistant that they may be preserved in accumulations of owl pellets. Burrowing bivalves are preserved more readily than those that live on the surface.
Biases Produced by The Geologist
But do you crawl over the rock using your eyes only? Or do you collect a huge block and work through it systematically back in the lab? Do you sieve through fine-grained sediment to collect the tiny fossils, or do you go for the big flashy ones? (Are you collecting dinosaurs or fossil mammals from these Jurassic rocks?)
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Page last revised February 5, 2000.