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plesiomorphy (ancestral trait) An evolutionary trait that is homologous within a particular group of organisms but is not unique to members of that group (compare apomorphy) and therefore cannot be used as a diagnostic or defining character for the group. For example, vertebrae are found in zebras, cheetahs, and orang-utans, but the common ancestor in which this trait first evolved is so distant that the trait is shared by many other animals. Therefore, possession of vertebrae sheds no light on the phylogenetic relations of these three species. See also symplesiomorphy.