Historically placozoans have been placed at the base of the metazoan tree of life because of their simple morphology. From a morphological point of view their bauplan is by far the closest surrogate of the hypothetical Urmetazoon . Early molecular phylogenetic studies based on small and large ribosomal subunits (18S and 28S) have placed Trichoplax at all different. Recent studies with very large datasets also failed to resolve the phylogenetic position. Nonetheless, a very basal position seems to become corroborated.
We follow a comprehensive, total evidence approach and use nuclear as well as mitochondrial protein coding and ribosomal genes, molecular morphology (secondary structure) and morphological and developmental characters to address the issue of relationships near the base of the Metazoa. Our analyses show an early evolutionary split into two sister clades: the ‘bilateria’ (or triploblasts) and the ‘non-bilaterian’ (or diploblasts). The later comprises the basal metazoan Placozoa, Porifera, Cnidaria and Ctenophora, with plaozoans being basal within this clade [2,3; cf. 4]
legend: Maximum Likelihood phylogenetic tree of metazoan relationships using a concatenated sata matrix. Here Placozoa group basal within the diploblasts (like in many other studies) but diploblasts and triploblasts occur as sister groups, a hypothesis that is controversially discussed. See Schierwater et al. 2009 .
1. Schierwater B, Eitel M, Osigus HJ, von der Chevallerie K, Bergmann T, et al. (2010) Trichoplax and Placozoa: one of the crucial keys to understanding metazoan evolution. In: DeSalle R, Schierwater B, editors. Key transitions in animal evolution: CRC Press. pp. 289-326.
2. Schierwater B, Eitel M, Jakob W, Osigus HJ, Hadrys H, et al. (2009) Concatenated Analysis Sheds Light on Early Metazoan Evolution and Fuels a Modern "Urmetazoon" Hypothesis. Plos Biology 7: 36-44.
3. Schierwater B, Kolokotronis SO, Eitel M, DeSalle R (2009) The Diploblast-Bilateria sister hypothesis: Parallel evolution of a nervous systems may have been a simple step. Communicative & Integrative Biology 2: 1-3.
4. Dellaporta SL, Xu A, Sagasser S, Jakob W, Moreno MA, et al. (2006) Mitochondrial genome of Trichoplax adhaerens supports Placozoa as the basal lower metazoan phylum. Proceedings of the National Academy of Sciences USA 103: 8751-8756.
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