Bilaterian animal body plan origins are not solely about adult forms. Most animals have larvae with body plans, ontogenies and ecologies distinct from adults. There are two primary hypotheses for larval origins.
The first hypothesis suggests that the first animals were small pelagic forms similar to modern larvae, with adult bilaterian body plans evolved subsequently.
The second hypothesis suggests that adult bilaterian body plans evolved first and that larval body plans arose by interpolation of features into direct-developing ontogenies.
The two hypotheses have different consequences for understanding parsimony in evolution of larvae and of developmental genetic mechanisms. If primitive metazoans were like modern larvae and distinct adult forms evolved independently, there should be little commonality of patterning genes among adult body plans.
However, sharing of patterning genes is observed. If larvae arose by co-option of adult bilaterian-expressed genes into independently evolved larval forms, larvae may show morphological convergence, but with distinct patterning genes, and this is observed.
Thus, comparativestudies of gene expression support independent origins of larval features.
Precambrian and Cambrianembryonic fossils are also consistent with direct development of the adult as being primitive, with planktonic larvae arising during the Cambrian.
Larvae have continued to co-opt genes and evolve new features, allowing study of developmental evolution.
Origins of the other metazoan body plans: the evolution of larval forms.
Philos Trans R Soc Lond B Biol Sci. 2008 Apr 27;363(1496):1473-9. doi: 10.1098/rstb.2007.2237. Review.