A 9-year-old fossil hunter from Maryland stumbled across a once-in-a-lifetime find while scouting for treasures on the Calvert County coastline on the Chesapeake Bay: a huge tooth from a 15 million-year-old megalodon. On Dec. 25, Molly Sampson, a fourth grader from Southern Maryland was out fossil hunting with her big sister, Natalie. Molly was wrapped in…