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Related to plesiomorphy: Cladistic analysis, Plesiomorphic
n. pl. ple·si·o·mor·phies
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.