coelacanth vs crossopterygian

coelacanth and crossopterygian both are nouns.

Word	Noun	Adjective	Verb	Adverb
coelacanth	Yes	No	No	No
crossopterygian	Yes	No	No	No

As nouns, crossopterygian is a hypernym of coelacanth; that is, crossopterygian is a word with a broader meaning than coelacanth:

coelacanth: fish thought to have been extinct since the Cretaceous period but found in 1938 off the coast of Africa
crossopterygian: any fish of the order Crossopterygii; most known only in fossil form

Other hypernyms of coelacanth include lobe-finned fish, lobefin.

coelacanth (noun)	crossopterygian (noun)
fish thought to have been extinct since the Cretaceous period but found in 1938 off the coast of Africa	any fish of the order Crossopterygii; most known only in fossil form