(Reuters) - A tiny fish characterized by a disproportionately large head and previously unknown to scientists has been found in mountain rivers of Idaho and Montana in what biologists said on Thursday marked a rare discovery. The new aquatic species is a type of freshwater sculpin, a class of fish that dwell at the bottom of cold, swiftly flowing streams throughout North America and are known for their oversized head and shoulder structure. "The discovery of a new fish is something I never thought would happen in my career because it's very rare in the United States," said Michael Young,...