Most major groups of invertebrate animals have a calcareous skeleton or shell (e.g., corals, mollusks, brachiopods, bryozoans).
Other forms have shells of calcium phosphate (which also occurs in the bones of vertebrates), or silicon dioxide.
Many other fossils, however, are collected by hobbyists and commercial entities.
Often such specimens are not carefully documented or excavated, resulting in a loss of data from the site and risking potential damage to the specimen.
Such traces of organisms, which are appropriately known as “trace fossils,” include tracks or trails, preserved waste products, and borings.
The great majority of fossils are preserved in a water environment because land remains are more easily destroyed.
Leaves, stems, and other vegetable matter may be preserved through the process of carbonization, where such parts are flattened between two layers of rock.
The chemical reduction of the part produces a carbon film that occurs on one layer of rock, while an impression of that part occurs on the other layer of the rock.
In recent years, geologists have been able to study the subsurface stratigraphy of oil and natural gas deposits by analyzing microfossils obtained from core samples of deep borings.Fossils also provide the geologist a quick and easy way of assigning a relative age to the strata in which they occur.The precision with which this may be done in any particular case depends on the nature and abundance of the fauna: some fossil groups were deposited during much longer time intervals than others.Anaerobic conditions at the bottom of the seas or other bodies of water are especially favourable for preserving fine details, since no bottom faunas, except for anaerobic bacteria, are present to destroy the remains.In general, for an organism to be preserved two conditions must be met: rapid burial to retard decomposition and to prevent the ravaging of scavengers; and possession of hard parts capable of being fossilized.