Coelacanths are sarcopterygian fish in the order Actinistia. There are more than one hundred identified species, nearly all of which are extinct; the first coelacanths in the fossil record date to roughly 409 million years ago, during the early Devonian period. At one point, scientists believed that coelacanths had become extinct during the Cretaceous-Paleogene mass extinction event, but two surviving species have since been identified: the West Indian coelacanth (Latimeria chalumnae) and Indonesian coelacanth (Latimeria menadoensis). Both are sluggish and inhabit deep waters, traits that helped them survive the extinction. These fish are commonly called “living fossils” because they closely resemble some of their fossil ancestors, but the coelacanths as a whole were once far more diverse than they are today.