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Fossils of hard and soft parts that are too small to be observed by the naked eye are called microfossils.
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.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.Fossil collection as performed by paleontologists, geologists, and other scientists typically involves a rigorous excavation and documentation process.Unearthing the specimen from the rock is often painstaking work that includes labeling each part of the specimen and cataloging the location of each part within the rock.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.
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 some places, such as the Grand Canyon in northern Arizona, one can observe a great thickness of nearly horizontal strata representing the deposition of sediment on the seafloor over many hundreds of millions of years.
It is often apparent that each layer in such a sequence contains fossils that are distinct from those of the layers that are above and below it.
For example, they serve to indicate the stratigraphic position of coal seams.