tuatara vs diapsid

tuatara and diapsid both are nouns.

Word	Noun	Adjective	Verb	Adverb
tuatara	Yes	No	No	No
diapsid	Yes	No	No	No

As nouns, diapsid is a hypernym of tuatara; that is, diapsid is a word with a broader meaning than tuatara:

tuatara: only extant member of the order Rhynchocephalia of large spiny lizard-like diapsid reptiles of coastal islands off New Zealand
diapsid: reptile having a pair of openings in the skull behind each eye

Other hypernyms of tuatara include diapsid reptile.

tuatara (noun)	diapsid (noun)
only extant member of the order Rhynchocephalia of large spiny lizard-like diapsid reptiles of coastal islands off New Zealand	reptile having a pair of openings in the skull behind each eye