From Greek πλησίος 'close, near' and μορφή 'form'.
In cladistics, as theorised by Willi Hennig (cf. Schmitt 2013), a character or a character state may be plesiomorphic (ancestral or primitive) or apomorphic (derived). The polarisation of characters (the determination of the direction of character change), which is at the core of the phylogenetic method, is comparable to the concept of "error of copying" in the Lachmann's method. A plesiomorphic character state is equivalent to a primary reading.
– Kitching, Ian J., Peter L. Forey, Christopher J. Humphries, and David M. Williams. 1998. Cladistics: The Theory and Practice of Parsimony Analysis. 2nd ed. Oxford: Oxford University Press.
– Schmitt, Michael. 2013. From Taxonomy to Phylogenetics: Life and Work of Willi Hennig. Leiden: Brill.
In other languages
DE: plesiomorph (adj.), Plesiomorphismus (noun)
FR: plésiomorphique (adj.), plesiomorphisme (noun)
IT: plesiomorfo (adj.), plesiomorfismo (noun), plesiomorfia (noun)