The 7 day-old larva of a zebrafish, which we study in this week's lab, exhibits all of these features. We consider the 3-lobed brain to be a vertebrate synapomorphy. Some folks call our subphylum Craniata, rather than Vertebrata, emphasizing that vertebrates have a cranium containing a 3-lobed brain. A goal of this week's lab is to discover other vertebrate characters. We propose that all vertebrates are related to one another and not to other sorts of creatures by their ancestry.
All vertebrates derive from a common ancestor.
Hence the similarities among vertebrates, by the MA hypothesis, are due to inheritance. Shared-derived vertebrate characters, vertebrate synapomorphies, arose as MA evolved, way back when. And MA passed those characters along to her descendants. We, as modern vertebrates, inherited those characters from her. This is an interesting idea, a deep one.