Read it once, a long time ago, that they might have been ridden - or used in building. Most scholars I've read think North American prehistory was so highly unsophisticated, it would have never happened.
I do tend to concur however, with Jeff Goldblum's character that - these animals had their time on Earth - and they're gone. Bringing them back is for amusement - they can't adapt.
I can however get on board with trying to save animals that are near extinction, especially those HUNTED to extinction - they didn't die off because of natural causes, unless you think of humans killing them for sport counts.