Watch Fmovies. Coelacanth (SEEL-uh-kanth): a large fish with limblike fins, armored scales and a tail that no other living fish possesses. This prehistoric fish was thought to have died out with the dinosaurs in the great extinction. It has laid hidden deep in our oceans undisturbed and undetected for 70 million years—a true living fossil. The coelacanth is thought to be related to the creatures that grew legs, breathed air and came ashore nearly 400 million years ago.