We want to understand the evolution of vertebrates, particular the nature of the developmental changes that have permitted the more significant evolutionary transitions that has permitted the group to be very successful. Taxons in the Linnean system are useful for our understanding. An important set of ancient genes, shared with many invertebrates, are the Hox genes that encode the AP organization of the body. A second model of MA's Bauplan comes from comparing the larva of one of the most primitive extant vertebrates, the ammocoetes, with the vertebrate sister group, Amphioxus. Then we move along to the second part of our course.
Some vertebrates appear to depend on pristine habitats such as ancient forests.
We now can consider the archetypal Bauplan as MA's body plan, a craniate, bilaterally symmetrical segmented animal. The cranium is a vertebrate synapomorphy, seemingly derived from the anterior end of MA's ancestors. Ideas about the details of invertebrate phylogenies are not generally agreed to, but the general pattern shown in PJH Fig. 2-2 is a reasonable current idealization. A middle layer of mesoderm evolved in the triploblasts, bilaterality, and a coelom as a split in the mesoderm and permitting the internal organs to be separated from the body wall.