Mastodon vertebrae...
Two boys named Eric and Andrew from Detroit, Michigan were building a dam in the creek that flows through their backyard when Eric saw a strange-looking rock sticking up from the ground. Supposing it might have been a bone, the family called in a paleontologist John Zawiskie at the Cranbrook Institute of Science in Bloomfield Hills who later authenticated the bone as a vertebrae from a mastodon some 13,000 years old.