Jim Leyden was waiting for word on the installation of his new backyard pool when he received a startling phone call.

In fact, while digging 8 feet beneath Leyden's Brighton, Tenn., backyard on June 30, contractors discovered the bones of a prehistoric mammal, possibly a trilophodon, part of the mastodon family. Mastodons are extinct relatives of today's elephants.

Leyden said he was shocked to learn that fossils, which one expert says could be anywhere from 30,000 years old to 2 million years old, were buried on his property.

David Mason of Affordable Pool Contractors unearthed the jawbone while installing a drain line for the Leydens' new pool....