symplesiomorphy The possession of a character state that is primitive (plesiomorphic) and shared between two or more taxa. Shared possession of a symplesiomorph character state is not evidence that the taxa in question are related.

symplesiomorphy An ancestral trait that is shared by two or more modern groups. Because of their ancient origin, symplesiomorphies are not usually helpful in assessing more recent evolutionary relationships within a larger group. For example, the simple leaves of some modern flowering plants are probably inherited from simple-leaved ancestors. But this does not mean that all simple-leaved modern plants are closely related. See plesiomorphy.