After the notochord, what is the other most ancient feature in the anatomy of a vertebrate animal?

Question

Thanks for the responses so far. Also, the answer that I've been leaning towards is the presence of a cranium or braincase. I'm just nervous about the question because my Comp Vert Anat prof is very strict in terms of classifications, terminology, etc and he is very anti-helping students.

Answer 1

Actually, there are several organisms with a notochord but wihout a vertebral column.
The other characteristics are probably those of the chordates: a dorsal, hollow nerve cord, pharyngeal gill slits and a post anal tail. However, there are two sub groups of non-vertebrate chordates (the urochordata and the cephalochordata) that also show these characteristics.
Within the Vertebrata, the existence of a jaw (except in the case of the jawless fish) is probably an ancestral characteristic; another may be the existence of a cranium.

Answer 2

vertebral column..... what say???