Noun
symplesiomorphy (plural symplesiomorphies)
(cladistics) An ancestral trait shared by two or more taxa.
Coordinate term: synapomorphy
However, unlike shared innovations (or synapomorphies ), the common retention of archaisms (or symplesiomorphy ) is not considered conclusive evidence of a period of common isolated development. Source: Internet
When two or more taxa that are not nested within each other share a plesiomorphy, it is a symplesiomorphy (from syn-, "together"). Source: Internet