What Significance do Index Fossils Have?

What Significance do Index Fossils Have?

December 27, 2005


Index fossils are used to define geological periods. These fossils can be defined as "commonly found, widely distributed fossils that are limited in time span." If one finds an index fossil in a given layer, then one has bounds on the age of the layer. Using index fossils, the geological periods are defined. These are intervals of time that are believed to have lasted for tens of millions of years, in most cases. The geological periods make up the "geological column" which lists the periods in sequence. The question arises whether the "geological column" is real or results from our attempt to make sense of the fossil distribution.

Because the geological periods always, or almost always, appear in the proper sequence in the fossil record, though some periods may be missing in places, it is argued that the geological column is real and represents the order in which various life forms existed in the past. However, this argument is circular. The geological periods are defined using index fossils, and the index fossils are chosen so that the resulting geological periods occur in order.

There is indeed an approximate order in which fossils appear in the fossil record, but I do not believe that it is as rigorous as the diagrams in textbook would indicate. The life forms that existed in the past were buried in an approximate order, but it can vary from place to place.

The significance of this for the creation-evolution controversy is that creationists sometimes attempt to construct flood models to produce a rigid sequence of fossils as displayed in textbook diagrams of the geological column. It is not necessary for the flood models to reproduce this sequence so exactly. Any flood model that puts the fossils in an approximate order would probably by chance create some fossils of limited distribution that could then be considered as index fossils and used to define a geological column with properties such as the current column possesses.

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