In cladistics a character shared by a number of groups, but inherited from ancestors older than the last common ancestor; a shared primitive trait ( plesiomorphy) more
an ancestral character state shared by two or more taxa.
A plesiomorphy shared by two or more terminal taxa, only diagnostic of a paraphyletic group (cf. synapomorphy).
plesiomorphy shared by two or more species or clades; almost all plesiomorphies are symplesiomorphies.
Symplesiomorphy (meaning "shared old form") is the persistence of ancestral (primitive) traits in different clades.
A symplesiomorphy or symplesiomorphic character is in cladistics a trait which is shared (a symmorphy) between two or more taxa, but which is also shared with other taxa which have an earlier last common ancestor with the taxa under consideration. They are therefore not an indication that the taxa considered are more closely related to each other than to the more distant taxa, as all share the more primitive trait. A close phylogenetic relationship, that the taxa form a certain clade to the exclusion of certain other taxa, can only be shown by the discovery of synapomorphies: shared traits that have originated with the last common ancestor of the taxa considered, or at least in the branch, not including the taxa to be excluded, leading to it.