When you hear the term “living fossil,” one of the first examples you’re likely to think of is the coelacanth. Fossil coelacanths were first described over 160 years ago, and their fossil record spans the Mesozoic, even reaching back to the Devonian. That means coelacanths have been on Earth for more than twice as long as mammals, but there are no fossils known from later than the Cretaceous.