Two 13-year-old friends found a mastodon mandible and tusk estimated to be between 50,000 and 150,000 years old while exploring a creek bed this summer.The discovery was announced by the Denver Museum of Nature & Science Saturday. The fossils were donated to the museum, which is studying them.Museum spokeswoman Laura Holtman said Jake Carstensen and Tyler Kelley found the mandible -- a jaw bone -- on June 1, while exploring a stream near Ken-Caryl Ranch's community pool after a heavy rain."The boys could tell right away they had found the jaw of some large animal because it contained a huge tooth. They brought their discovery home to determine what they had found," Holtman said.The boys turned to the Internet to identify the bone and found it matched a mastodon mandible. Curator of Archeology Dr. Steve Holen confirmed the discovery, Holtman said."Mastodon fossils are extremely rare in Colorado," Holtman said. A tooth fragment was found in northeast Pueblo in 1875 and a full molar was found in Golden sometime around 1924, she said.Holen went back to the creek bed with the boys to look for more bones in early August. That's when they found the intact tusk in a stream bank submerged in water.With the blessing of the land owners, Holen, the boys and several volunteers used pumps to divert water from the stream around the tusk and dug it out."After digging carefully for several hours, the five-foot segment of tusk was unearthed, wrapped carefully in paper towels and plaster, and removed from the stream bed," Holtman said.The team didn't find any other remains although Holen said it is possible the rest of the mastodon is buried nearby.Volunteers cleaned the fossils and will work to preserve them. The museum has not yet decided if the fossils will go on public display.