This tiny fossil is a tooth of a very small shark called Lissodus (also known as Lonchidion). Sharks like Lissodus have been found in rocks ranging in age from about 350 million to 65 million years old from around the world. Interestingly, these fossils are found in both marine and nonmarine rocks, which indicates that these tiny sharks lived in both fresh water and saltwater. This particular fossil is from Triassic rocks called the Sloan Canyon Formation (206 million years old) in northeastern New Mexico that are known to represent the deposits of a freshwater lake. This is also among the smallest Lissodus teeth ever found. Scientists hypothesize that Lissodus specialized in eating small, shelled organisms such as tiny clams and crustaceans.