Shared Ancestral Character

∞ generated and posted on 2016.12.24 ∞

Homology that evolved prior to the establishment of a specific taxon and therefore not uniquely defining that taxon.

A shared character that is to a specific taxon is a shared ancestral character. In comparison with a shared derived character, it is the difference broadly between the character arising more recently (shared derived, that is, both within and defining a taxon) or instead less recently (shared ancestral).

The four characters that define , for example – ; ; ; and (or ) – are seen in all members of phylum Chordata. As a consequence, they are ancestral to the , again for example, so don't define the mammals as a clade.

Similarly, neither keratinized skin nor the amniotic egg were "invented" by mammals (but instead by more amniotes). These all therefore represent shared ancestral characters (rather than shared derived characters).