synapsid vs pelycosaur

synapsid and pelycosaur both are nouns.

Word	Noun	Adjective	Verb	Adverb
synapsid	Yes	No	No	No
pelycosaur	Yes	No	No	No

As nouns, pelycosaur is a hyponym of synapsid; that is, pelycosaur is a word with a more specific, narrower meaning than synapsid:

synapsid: extinct reptile having a single pair of lateral temporal openings in the skull
pelycosaur: large primitive reptile having a tall spinal sail; of the Permian or late Paleozoic in Europe and North America

Other hyponyms of synapsid include protomammal, therapsid, ictodosaur, edaphosaurus, dimetrodon.

synapsid (noun)	pelycosaur (noun)
extinct reptile having a single pair of lateral temporal openings in the skull	large primitive reptile having a tall spinal sail; of the Permian or late Paleozoic in Europe and North America