Gene expression patterns can be used as characters to assign evolutionary homology. The understandings about Bauplan on which we have been focusing have come from comparing gene expression in embryos. New ideas include why birds have more vertebrae in their necks than do mammals, why snakes have no body region corresponding to a neck, and possibly no forelimb, what part of the body of the vertebrate sister group corresponds to the vertebrate head, and what amphioxus cells correspond to vertebrate neural crest cells. These are important ideas. We continue today with the same theme, pushing understanding of vertebrate body patterning further back during evolution.
Some vertebrates appear to depend on pristine habitats such as ancient forests.
The adult anatomies of primitive extant vertebrates, the lamprey and the hagfish. The adult anatomy of amphioxus, the vertebrate sister group. We can 'close the book' now about MA's Bauplan, because we have covered all of the kinds of available evidence. We now can consider the archetypal Bauplan as MA s body plan, a craniate, bilaterally symmetrical segmented animal. These plesiomorphic features, shared with the outgroup, are down deeper in MA's ancestry.