Wiktionary
coelacanths
n. (plural of coelacanth English)
Usage examples of "coelacanths".
The coelacanths were a large group of 'fish' (actually, although they are called fish they are more closely related to us than they are to trout and herrings) that flourished more than 250 million years ago and apparently died out at about the same time as the dinosaurs.
It is conceivable that coelacanths stopped evolving because they stopped mutating — perhaps because they were protected from cosmic rays at the bottom of the sea!