type of plesiomorphy. shared ancestral homology. shared close form. found in an ancestor of a group but not all of the members of the group have the trait (relatively uninformative) cannot be used to determine members of a clade without ambiguity because symplesiomorphic traits can indicate different groups. ex. hair is a shared ancestral trait (symplesiomorphy) but would not indicate the close relationship of marsupials and placental mammmals.