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Encyclopedic overview
Synapsida is a clade of amniotes with one opening on the sides of the skull. The synapsids' sister clade is the sauropsids. They appeared in the Late Carboniferous, the same time as the first sauropsids.
Subtaxon Therapsida include mammals.
Excerpted from Wikipedia’s “Category:Synapsida” article, available under the CC BY-SA 4.0 licence.