from The American Heritage® Dictionary of the English Language, 5th Edition.
- noun An evolved character or trait that is shared by some or all members of a phylogenetic group and their common ancestor but is not unique to that group. A segmented spinal column is a plesiomorphy of mammals, being present as well in other groups such as fishes, reptiles, amphibians, and birds.
from Wiktionary, Creative Commons Attribution/Share-Alike License.
- noun cladistics A character state that is present in both
outgroupsand in the ancestors
from The American Heritage® Dictionary of the English Language, 4th Edition
A characteristic present in both the outgroups and the ancestors is called a plesiomorphy (meaning "close form," as in close to the root ancestor; also called an ancestral state).
I believe the "third eye" is a plesiomorphy for amniotes, so, yes, it makes sense to include that.
Any character or taxon that is prior in the evolutionary tree is considered “primitive”, or more technically, a plesiomorphy, the English of which is “underived”.
The first Old World higher primates would be a plesiomorphy; German edition of his book entitled (in English) 'Phylo - it has no valency for sorting taxa within that group.