noun, plural: synapomorphies
It is a recent traits shared by two directly related groups of organisms like cats and dogs -snakes and lizards which primary showed in most modern shared ancestor.
Excellent examples of this are the lacks of legs in snakes, which belong to class Tetrapoda. A character state that is a synapomorphy for a clade, but for ancestry of this clade is plesiomorphy where it happens to changed in some lineages as underlying synapomorphy
Another case are the exceptionally tailored hind wings originate in all families of winged flies dipteran halteres wherein no other group of insects holding a similar makeup which convergently evolved halteres in place of fore wings.
Word Origin: syn = “with”, “together with” + apo = “away from” + morph = “shape”
• synapomorphic ( adjective)