Answer:
Fossils are the remnants of the ancient plants and animals that are buried millions of years ago. These fossils are commonly found in the sedimentary rocks and are formed as it undergoes the process of fossilization.
These fossils are usually formed when the organisms (including its hard parts) are covered with soft sediments and are prevented from the oxidizing environment. These rocks comprising the fossils were formed in the shallow marine environment, that best favors the formation of fossils.
Thus, the discovery of fossils of ancient whales that existed millions of years ago in the Mississippi demarcates that the Mississippi was under the sea level in the past when this organism died and got covered with the sediments. It indicates that these rocks were present under the shallow marine environment.