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In other words, the fossil species must have had a short temporal range.
On the practical side, an index fossil should be distinctive in appearance so as to prevent misidentification, and it should be cosmopolitan both as to geography and as to rock type.
Correlating two separated outcrops means establishing that they share certain characteristics indicative of contemporary formation.
Suppose, too, that all the layers contain a good representation of the animal life existing at the time of deposition.
Furthermore, useful fossils are either rare or totally absent in rocks from Precambrian time, which constitutes more than 87 percent of Earth history.
Precambrian rocks must therefore be correlated by means of precise isotopic dating.
In addition, its fossilized population should be sufficiently abundant for discovery to be highly probable.
Such an array of attributes represents an ideal, and much stratigraphic geology is rendered difficult because of departure of the natural fossil assemblage from this ideal.
Presumably if all the world’s outcrops were integrated, sediments representing all of geologic time would be available for examination.