synapsid
nounDefinition of synapsid
: 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.
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