synapsid
noun
syn·ap·sid sə-ˈnap-səd 
: any of a subclass (Synapsida) of terrestrial vertebrates (such as the pelycosaurs and therapsids) having a single pair of lateral temporal skull openings
Note: Traditional classifications typically consider synapsids to be mammal-like, extinct reptiles ancestral to mammals. Cladistic classifications also consider these extinct forms ancestral to mammals but do not categorize them as reptiles.
synapsid adjective
Example Sentences
Recent Examples on the Web At the end of the Permian, there was a sort of bottleneck that caused only smaller synapsids to survive into the Triassic. 
John Wenz, Popular Mechanics, 22 Nov. 2018 Four-limbed vertebrates were becoming more diverse and among them were primitive amphibians, early reptiles and synapsids, the group that would one day include mammals. Fox News, 23 Jan. 2011
John Wenz, Popular Mechanics, 22 Nov. 2018 Four-limbed vertebrates were becoming more diverse and among them were primitive amphibians, early reptiles and synapsids, the group that would one day include mammals. Fox News, 23 Jan. 2011 Word History
Etymology
New Latin Synapsida, from Greek syn- + apsid-, apsis arch, vault — more at apsis
First Known Use
1940, in the meaning defined above