Old Red Sandstone facies

Box 11.10 Colonies: the source of the first bilaterians?

Perhaps colonial organisms in the Late Precambrian had a deep significance for animal evolution. Is it possible that the complex bilateralians we see today originated within a colonial structure prior to the Cambrian explosion? Ruth Dewel (Appalachian State University, Boone) has developed a model involving the individuation of colony modules. Colonial organisms tend to develop greater degrees of integration and internal specialization through time as they begin to function as superor-ganisms. In this model an organism with bilaterian features, i.e. bilateral symmetry, with three body regions and epithelium-lined body compartments, can apparently break away from a complex, integrated cnidarian colony to form something like a pennatulacean octocoral that may have formed the stem group to both the cnidarians and bilateralians (Dewel 2000). A pathway from sponge to cnidarian to bilateralian body plans in her model is plausible (Fig. 11.38). Pure fantasy? Why then are outgroups to the early bilaterians large and simple whereas the bilaterians, themselves, are small and complex? It is an interesting hypothesis; but such hypotheses are there to be rigorously tested and falsified.

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