megathere vs megatherian

megathere and megatherian both are nouns.

Word	Noun	Adjective	Verb	Adverb
megathere	Yes	No	No	No
megatherian	Yes	No	No	No

As nouns, megatherian is a hypernym of megathere; that is, megatherian is a word with a broader meaning than megathere:

megathere: gigantic extinct terrestrial sloth-like mammal of the Pliocene and Pleistocene in America
megatherian: a large extinct ground sloth

Other hypernyms of megathere include megatherian mammal, megatheriid.

megathere (noun)	megatherian (noun)
gigantic extinct terrestrial sloth-like mammal of the Pliocene and Pleistocene in America	a large extinct ground sloth