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Related to Synapsida: Diapsids
Synapsida(in the taxonomy of reptiles), a subclass fossil form, having a single inferior temporal FOSSA on each side of the skull. These are the Theromorphs from which mammals probably evolved. The term is sometimes extended to include the mammals.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005