The Detroit News reports Eric Stamatin of Macomb County's Shelby Township and his cousin Andrew Gainariu of Troy found the bone near a stream while exploring Eric's backyard. Eric says it looked like a rock, but a hole made them think it was a bone.
A researcher at Cranbrook Institute of Science in Bloomfield Hills confirmed it was an axis bone from an extinct American mastodon, a relative of the elephant. Paleontologist John Zawiskie says the fossil is likely between 13,000 and 14,000 years old.
A search of the area didn't turn up more bones. Zawiskie says there are records of numerous mastodon-related finds in Michigan's Lower Peninsula.