What Killed Off The Mastadons?

Mastodons roamed North America within human history.  What, in the absence of SUVs and coal-fired power plants, killed them off?

Mastodons -- elephant-like beasts that lumbered across North America more than 10,000 years ago -- are long extinct, but apparently it wasn't tooth decay that did them in.

A 9-year-old Michigan boy stumbled across something -- literally -- that, it turns out, is a mastodon tooth.

Something killed them all off.  I wonder what it could have been.

Climate changes.  It just does.  Sucks to be a mastodon.