synapomorphy

noun

1. A derived trait that is shared by two or more taxa of shared ancestry.

   “Another supposed synapomorphy of Rhamporhynchidae + Pterodactyloidea presented by Unwin (1995) is the subequal length of metacarpals I–III. In all non-pterodactyloid specimens where this region is known metacarpals I–III are about the same length. Therefore this feature cannot be used as a synapomorphy of Rhamporhynchidae + Pterodactyloidea.”

   “A “synapomorphy” for a group then amounts to a state change in a character along the corresponding branch; some synapomorphies may be ambiguous when optimization is ambiguous and multiple MPRs exist.”