symplesiomorphy
/sɪmˈpliːzɪə(ʊ)mɔːfi/Taxonomy
noun
The possession by two or more organisms or taxa of a shared primitive or ancestral character not exclusive to or characteristic of them in particular; the shared character itself.
Origin
1950s; earliest use found in Systematic Zoology. From sym- + plesio- + -morphy.