elasmobranch vs chondrichthian

elasmobranch and chondrichthian both are nouns.

Word	Noun	Adjective	Verb	Adverb
elasmobranch	Yes	No	No	No
chondrichthian	Yes	No	No	No

As nouns, chondrichthian is a hypernym of elasmobranch; that is, chondrichthian is a word with a broader meaning than elasmobranch:

elasmobranch: any of numerous fishes of the class Chondrichthyes characterized by a cartilaginous skeleton and placoid scales: sharks, rays, skates
chondrichthian: fishes in which the skeleton may be calcified but not ossified

Other hypernyms of elasmobranch include cartilaginous fish.

elasmobranch (noun)	chondrichthian (noun)
any of numerous fishes of the class Chondrichthyes characterized by a cartilaginous skeleton and placoid scales: sharks, rays, skates	fishes in which the skeleton may be calcified but not ossified