faunal succession is basically stating that the more complex something is, the younger it is. So the bigger the fossil is the younger it is.

The Law of Fossil Succession states that fossil organisms originate, coexist, and disappear from the geologic record in a definite sequential order.

William Smith

Faunal succession

A geologist is studying layers of rock He finds a fossil with an imprint of a shelled animal According to the law of faunal and floral succession what kind of fossil would he most likely find next is a fossil of fishes.