1. synapsid - Noun
2. synapsid - Adjective
synapsid (comparative more synapsid, superlative most synapsid)
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.
synapsid (plural synapsids)
Any animal (including all mammals) of the class Synapsida.
Ureotelism appears to be the ancestral condition among primitive amniotes, and it is retained by mammals, which likely inherited ureotelism from their synapsid and therapsid ancestors. Source: Internet
Archaeothyris was a very early synapsid and the oldest known. Source: Internet