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?

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 : )

 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.

as general terms,  indian territory,  the indian territories, or  indian country  describe an evolving land area set aside by the  united states government  for the relocation of  native americans  who held  aboriginal title  to their land. in general, the tribes ceded land they occupied in exchange for  land grants  in 1803. the concept of an indian territory was an outcome of the 18th- and 19th-century policy of  indian removal. after the  civil war(1861–1865), the policy of the government was one of  assimilation.