megatherian mammal vs megatheriid

megatherian mammal and megatheriid both are nouns.

Word	Noun	Adjective	Verb	Adverb
megatherian mammal	Yes	No	No	No
megatheriid	Yes	No	No	No

As nouns, megatherian mammal and megatheriid are synonyms defined as:

Other synonyms of megatherian mammal include megatherian.

megatherian mammal (noun)	megatheriid (noun)
a large extinct ground sloth	a large extinct ground sloth