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The hard parts of organisms that become buried in sediment may be subject to a variety of other changes during their conversion to solid rock, however.
Solutions may fill the interstices, or pores, of the shell or bone with calcium carbonate or other mineral salts and thus fossilize the remains, in a process known as permineralization.
By contrast, the soft parts of animals or plants are very rarely preserved.
The embedding of insects in amber (a process called resin fossilization) and the preservation of the carcasses of Pleistocene mammoths in ice are rare but striking examples of the fossil preservation of soft tissues.
A shell or bone that is buried quickly after deposition may retain these organic tissues, though they become petrified (converted to a stony substance) over time.
Unaltered hard parts, such as the shells of clams or brachiopods, are relatively common in sedimentary rocks, some of great age.
In other cases there may be a total replacement of the original skeletal material by other mineral matter, a process known as mineralization, or replacement.
Some fossils are completely devoid of plant and animal parts but show evidence of an organism’s activities.For example, they serve to indicate the stratigraphic position of coal seams.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.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.