Symplesiomorphy vs. Plesiomorphy — What's the Difference?

The possession by two or more organisms or taxa of a shared primitive or ancestral character not exclusive to or characteristic of them in particular; the shared character itself.

The state of being plesiomorphic; ancestral or primitive condition as compared to a later, derived character state. Also (as a count noun): an ancestral or plesiomorphic feature or trait.

(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