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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.Those fossils slated for removal from the rock are slowly and carefully excavated using techniques designed to prevent or minimize damage to the specimen.Such fossils often become part of museum or university collections.
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.
Ten insect orders are known as fossils, mostly of Late Carboniferous…
Only a small fraction of ancient organisms are preserved as fossils, and usually only organisms that have a solid and resistant skeleton are readily preserved.
In such sequences of layers in different geographic locations, the same, or similar, fossil floras or faunas occur in the identical order.
By comparing overlapping sequences, it is possible to build up a continuous record of faunas and floras that have progressively more in common with present-day life forms as the top of the sequence is approached.Most major groups of invertebrate animals have a calcareous skeleton or shell (e.g., corals, mollusks, brachiopods, bryozoans).