Symplesiomorph

Part of speech: noun

Definitions

1. A trait or characteristic shared by two or more taxa inherited from their most recent common ancestor, typically considered primitive or ancestral within a larger group
2. An ancestral feature found in multiple related groups that originated in a distant common ancestor and is retained across those groups
3. A shared primitive character state present in various organisms due to inheritance from an early ancestor, contrasting with more recently evolved traits

Etymology: Willi Hennig, the German entomologist who founded cladistics, coined this term as part of a vocabulary for deciding which shared traits reveal kinship. Its irony lies in the prefix. "Syn" means "together," yet a symplesiomorphy is a shared trait that tells you almost nothing about who is related to whom. The word combines Greek roots: "syn" (together), "plesios" (near), and "morphe" (form). It names an ancient character state that several organisms share because each inherited it from a common ancestor long ago. Hennig's own term "plesiomorphy" refers to the older, ancestral state itself, and the "sym-" form marks the case where that old state appears in several lineages. Lacking feathers is a good example. Lizards, mice, and people all lack them, but that shared absence says nothing about how closely any two are related, because feathers were never present in their common ancestor. Hennig's method required researchers to set such traits aside and rely on shared derived features instead.