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Integrative and Comparative Biology Advance Access published online on June 1, 2007
Integrative and Comparative Biology, doi:10.1093/icb/icm041
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© The Author 2007. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: journals.permissions@oxfordjournals.org.

Cnidarian milestones in metazoan evolution

Ferdinando Boero1,*, Bernd Schierwater{dagger} and Stefano Piraino1,*
*Dipartimento di Scienze e Tecnologie Biologiche ed Ambientali DISTEBA, Università del Salento, I-73100 Lecce;
{dagger}ITZ, Ecology & Evolution, TiHo Hannover, Bünteweg 17d, D-30559 Hannover; {dagger}American Museum of Natural History, New York, Division of Invertebrate Zoology, 79th Street at Central Park West, New York, NY 10024, USA

Correspondence: 1E-mail: boero{at}unile.it

Cnidarians display most of the characters considered as milestones of metazoan evolution. Whereas a tissue-level organization was probably already present in the multicellular common ancestor of all animals, the Urmetazoa, the emergence of important animal features such as bilateral symmetry, triploblasty, a polarized nervous system, sense organs (eyes, statocysts), and a (chitinous or calcium-based) continuous skeleton can be traced back before the divergence between cnidarians and bilaterians. Modularity and metamery might be also regarded as two faces of the same medal, likely involving conserved molecular mechanisms ruling animal body architectures through regional specification of iterated units. Available evidence indicates that the common ancestor of cnidarians and bilaterians, the UrEumetazoa, was a surprisingly complex animal with nerve cell differentiation. We suggest that paedomorphic events in descendants of this ancestor led to the array of diversity seen in the main extant animal phyla. The use of molecular analyses and identifying the genetic determinants of anatomical organizations can provide an integrative test of hypotheses of homologies and independent evidence of the evolutionary relationships among extant taxa.

From the symposium "Key Transitions in Animal Evolution" presented at the annual meeting of the Society for the Integrative and Comparative Biology, January 3–7, 2007, in Phoenix, Arizona.


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