GEOLOGY

Detailed explanation-1: -When we find the same kinds of fossils in rocks from different places, we know that the rocks are the same age. Scientists look for ancestors and descendants through geologic time.

Detailed explanation-2: -Since the rock formation contains both types of fossils the ago of the rock formation must be in the overlapping date range of 415 to 420 million years. Studying the layers of rock or strata can also be useful. Layers of rock are deposited sequentially.

Detailed explanation-3: -Thus, in any sequence of layered rocks, a given bed must be older than any bed on top of it. This Law of Superposition is fundamental to the interpretation of Earth history, because at any one location it indicates the relative ages of rock layers and the fossils in them.