megatherian mammal vs megatherian

megatherian mammal and megatherian both are nouns.

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

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

Other synonyms of megatherian mammal include megatheriid.

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