Page 308 - Introduction to Paleobiology and The Fossil Record

THE BASAL METAZOANS: SPONGES AND CORALS 295

Box 11.10 Colonies: the source of the fi rst 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, inte-
grated 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 falsifi ed.

sponge grade

clonal sponge grade modular sponge

choanoflagellate colony

colonial cnidarian
(two branches)
choanoflagellate individualized colony
(bilaterian)
Figure 11.38 A possible origin for bilaterians in the colonies? The process involves the
development of multicellularity, followed by multifunctional modules (short arrows) and fi nally a
shift in their functional morphology within the cnidarians and the bilaterians. (From Dewel
2000.)