The diagram below shows four columns that represent rocks from different geographic regions. Each layer within a column represents a brief period of time. Which of the four fossils would be the most useful as a reference to determine the age of newly discovered fossils? Why?

(A) fossil is the most useful.

It serves as an index fossil. The seashell is a great index fossil because it only appears in the sandstone layer in multiple locations in the same layer of time.

Explanation:

Index fossils are fossils used to define and identify geologic periods. Index fossils must have a short vertical range, wide geographic distribution and rapid evolutionary trends.

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