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.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.In such sequences of layers in different geographic locations, the same, or similar, fossil floras or faunas occur in the identical order.
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.
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.
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.
In still other cases, circulating acid solutions may dissolve the original shell but leave a cavity corresponding to it, and circulating calcareous or siliceous solutions may then deposit a new matrix in the cavity, thus creating a new impression of the original shell.