5. Why are so many fossil land - dwelling mammals found in the western United States, but so few in the eastern part of the country?

Answer: The correct answer is : During the Cretaceous period a sea covered many states of the midwestern United States, such as Nebraska, Kansas, Utha, Colorado, Wyoming and Montana. Where there is mud due to the waters that run to the sand, there are also bones and buried teeth. It is easier to find fossils in dry environments (sand, desert). On the other hand, where there are trees or shrubs, it is very difficult to detect them, as in the eastern part of the United States that it is difficult to find fossils due to the thick foliage found.

It likely has to do with continental movements after Pangaea and climate of that time. I could be wrong because I don't have a lot of context but that is my best answer : )