Every kid who ever read a Tarzan book (or comic book) knows more about the "Elephant Burial Ground" than is known by all the people who ever actually looked for it. I suspect the same holds true for the "Mammoth Burial Ground."
So far as I have reason to suspect, the Mastodons were not members of an organized religion that required a "dying trip to OomGala" for their salvation. (Had they been so inclined, there would probably be at least a few members of some resurection of their "sect" still practicing today - although I'm not that familiar with the smaller religions for places like France...)
Being herbivores, we would expect them to be somewhat migratory - much like the historically recent large herds of elephants or bison. The best one might expect would be where a natural "disaster" both confined and killed a single herd.
Buffalo (bison) graveyards resulting from the (known but rare) Native American hunting practice of stampeding whole herds off a cliff seldom left bones of more than a few hundred animals, and it is unlikely that the mastodon ever "herded" in groups as large as the bison. In a herd of more than a few hundred, the ones in the middle would starve because the "front ranks" ate everything.
Wearing my physicist/mechanic's hat, I would have to venture the suggestion that the "much better" sound from fossil/antique material in pegs and bridges probably comes more from good installation than from the material. If you buy workmanship "worthy" of the outrageous expense of these parts, you'll get a better sound - even if you use more conventional (and more appropriate?) materials.