The calico scallop is an index fossil. It is well documented to have lived 1.8 million years ago. Therefore, the approximate age of the rock in which this fossil is found is

An index fossil helps estimate the age of the rock layer in which it is found because it lived during a known time period. Since the calico scallop lived about 1.8 million years ago, the rock is approximately 1.8 million years old. The correct choice is 1.8 million years.

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