This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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adj. Pertaining to the class Synapsida, of animals which have an opening low in the skull roof behind each eye, leaving a bony arch beneath each.

n. Any animal (including all mammals) of the class Synapsida.

Same as synapsidan.

n. extinct reptile having a single pair of lateral temporal openings in the skull

"The term "synapsid" is increasingly used cladistically, including mammals -- thus, synapsids are not extinct at all."

reptile    reptilian   

therapsid    ictodosaur    pelycosaur    edaphosaurus    protomammal