In phylogenetics, a plesiomorphy, symplesiomorphy or symplesiomorphic character is an ancestral character or trait state shared by two or more taxa. A plesiomorphy refers to the ancestral trait state, usually in reference to a derived trait state. A symplesiomorphic trait is also shared with other taxa that have an earlier last common ancestor with the taxa under consideration. They are therefore not an indication that the taxa be considered more closely related to each other than to the more distant taxa, as all share the more ancestral character state. The term symplesiomorphy was first introduced in 1950 by German entomologist Willi Hennig.

In phylogenetics, a plesiomorphy, symplesiomorphy or symplesiomorphic character is an ancestral character or trait state shared by two or more taxa. A plesiomorphy refers to the ancestral trait state, usually in reference to a derived trait state. A symplesiomorphic trait is also shared with other taxa that have an earlier last common ancestor with the taxa under consideration. They are therefore not an indication that the taxa be considered more closely related to each other than to the more distant taxa, as all share the more ancestral character state. The term symplesiomorphy was first introduced in 1950 by German entomologist Willi Hennig.