ornithischian dinosaur vs ornithischian

ornithischian dinosaur and ornithischian both are nouns.

Word	Noun	Adjective	Verb	Adverb
ornithischian dinosaur	Yes	No	No	No
ornithischian	Yes	No	No	No

As nouns, ornithischian dinosaur and ornithischian are synonyms defined as:

ornithischian dinosaur and ornithischian: herbivorous dinosaur with a pelvis like that of a bird

ornithischian dinosaur (noun)	ornithischian (noun)
herbivorous dinosaur with a pelvis like that of a bird	herbivorous dinosaur with a pelvis like that of a bird