Answer: Evidence of the environment of the past in the area where the rock was found.
A fossil is a trace or remain of the living being. A fossil is formed when the living being: plant or animal bury under the sedimentary layers. The softer tissues of plants and animals remains are decomposed by the activities of microorganisms, but the harder components like shells, bones, skeletons, leaves, seeds, woody plants remains unchanged. These remains get covered by different layers of sediments. These sediments attain aggregation into hard rocks. Therefore, fossils get preserved in the rocks. The analysis of different layers of sediments and rocks is known as striagraphy. This gives indication about the relative age of the fossil with respect to the rock in which it was found. In course of evolution, the plant and animal species have undergone many adaptations with respect to time. Therefore, rocks bearing fossil gives evidence of primitive living organisms and changes occurred in them with respect to time. A large piece of rock contains a thick-shelled clam fossil is the evidence of the environment of the past in the area where the rock was found.