Seminole --- A 16-year-old high school student stumbled upon what archaeologists say could be the biggest fossil find in Pinellas County in nearly a century.

A shiny black rock caught Sierra Sarti-Sweeney's eye as she was taking pictures last month in Boca Ciega Millennium Park.

'I looked down and saw a huge bone that could not be a rock. Most of it was exposed, but we dug and found that it was bigger and bigger. I thought, 'Oh my gosh, what are these? Are they people bones?' '' she said.

The jaw and tooth weigh 65 pounds and are about a yard long. Sarti-Sweeney took the bones home and, after some online research with her older brother, determined the football-size rock was actually the tooth of a long-extinct mammoth.

Paleontology and archaeology experts have confirmed the find, and recent digging at the site has turned up teeth and bones from a second mammoth, giant sloths, camels, turtles with shells up to six feet long, saber-toothed cats and giant armadillos the size of Volkswagen Beetles.

Scientists believe the remains are between 10,000 and 100,000 years old.

''It's possible that it's an old river valley, [and] the animals got caught in the muck or the river washed all these animals down into one place at one time,'' he said. ''We can get a better handle on it by analyzing the soil,'' said Richard Estabrook, director of USF's Florida Public Archaeology Network.