Symplesiomorphy vs. Plesiomorphy: What's the Difference?
Symplesiomorphy and Plesiomorphy Definitions
(cladistics) An ancestral trait shared by two or more taxa.
An evolved character or trait that is shared by some or all members of a phylogenetic group and their common ancestor but is not unique to that group. A segmented spinal column is a plesiomorphy of mammals, being present as well in other groups such as fishes, reptiles, amphibians, and birds.
(cladistics) A character state that is present in both outgroups and in the ancestors