There were over a dozen species of the genus Mammuthus, 3 species were pygmy versions that were evolved separately by convergence They were located in areas temporally and geographically isolated (each species was on an island system that was originally connected to a mainland). The pygmy species were from varied times , separated by at least 100K years. The most recent pygmy mamoth to roam the grasslands was a teeny guy from the Wrangle Islands. C14 had dated the average date of death at about 2000 y BP +/- 187 yr.
THey had several tens of specimens and the dates vary within an error bar of about 250 years (with 2 outliers of over 500 years).
These guys lived during early civilization building . The Channel Island Mammoths (In the California straits) went extinct much earlier , and the Cyprus Pigmyies earlier still.
The fact that convergence occurs in similr environments in widely different locations asserts the common mechanisms in evolution, just like convergence of dissimilar plants continents apart ( new world cactus v African succulents) different families, similar appearances in species.
I believe that Lewis and Clark were told to look for mammoths because Jefferson had a collection of bones from peat bogs in New ENgland and Ny and Pa. He thought that , if these dead mammoths could be found in bone assemblages in peat bogs (he had no idea about the ages of the bogs), he could be assured that Mammoths still existed in the West. (He was wrong). Any Indian tales about "Chasing mammoths " back into the woods, may have been a tale from the social history of the Mandan since they had symbols of mammoths on their bark "scrip". No evidence of mammoths as contemporary with modern AMerinds was ever shown. However, its strongly evidenced that Paleo Indians enjoyed mammoth meat during the first couple thousand years of populating this hemisphere. THere are several glyphs on "Newspaper Rock" that could be mammoths.