Bernd Schierwater1,2,3*, Michael Eitel1, Wolfgang Jakob1, Hans-Jürgen Osigus1, Heike Hadrys1,
Stephen L. Dellaporta2, Sergios-Orestis Kolokotronis3, Rob DeSalle3 (2009)

Concatenated Analysis Sheds Light on Early Metazoan Evolution and Fuels a Modern
‘‘Urmetazoon’’ Hypothesis 

 PLoS Biology | www.plosbiology.org January 2009 | Volume 7 | Issue 1 | e1000020

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1 ITZ, Ecology and Evolution, Tiera¨ rztliche Hochschule Hannover, Hannover, Germany,
2 Department of Molecular, Cellular and Developmental Biology, Yale University, New Haven, Connecticut,
   United States of America,
3 Sackler Institute for Comparative Genomics and Division of Invertebrate Zoology, American Museum of
   Natural History, New York, New York, United States of America

Abstract

 For more than a century, the origin of metazoan animals has been debated. One aspect of this debate has been
centered on what the hypothetical ‘‘urmetazoon’’ bauplan might have been. The morphologically most simply
organized metazoan animal, the placozoan Trichoplax adhaerens, resembles an intriguing model for one of several
‘‘urmetazoon’’ hypotheses: the placula hypothesis. Clear support for a basal position of Placozoa would aid in resolving
several key issues of metazoan-specific inventions (including, for example, head–foot axis, symmetry, and coelom) and
would determine a root for unraveling their evolution. Unfortunately, the phylogenetic relationships at the base of
Metazoa have been controversial because of conflicting phylogenetic scenarios generated while addressing the
question. Here, we analyze the sum of morphological evidence, the secondary structure of mitochondrial ribosomal
genes, and molecular sequence data from mitochondrial and nuclear genes that amass over 9,400 phylogenetically
informative characters from 24 to 73 taxa. Together with mitochondrial DNA genome structure and sequence analyses
and Hox-like gene expression patterns, these data (1) provide evidence that Placozoa are basal relative to all other
diploblast phyla and (2) spark a modernized ‘‘urmetazoon’’ hypothesis.
Citation: Schierwater B, Eitel M, Jakob W, Osigus H-J, Hadrys H, et al. (2009) Concatenated analysis sheds light on
early metazoan evolution and fuels a modern ‘Urmetazoon’’ hypothesis.

PLoS Biol 7(1): e1000020. doi:10.1371/journal.pbio.1000020