Figure 7-4 A phylogenetic tree of animal phyla based on stages of embryonic development. The most significant innovations are the appearence of (1) ectoderm, (2) endoderm, (3) mesoderm, (4) pseudocoelom, (5) coelom, (6) protostome-deuterostome dichotomy, and (7) dorsal chord.

The traditional explanations

So far, three types of hypotheses have been made on the conservation of body plans.
(1) The environmental explanation
No new body plan has been invented after the Cambrian explosion because all ecological niches were already occupied.