Placozoa – no longer a phylum of one

Abstract More than a century ago, the simplest of all metazoans was discovered and described as Trichoplax adhaerens [1]. These tiny, flattened animals lack symmetry, mouth, gut, nervous system, and extra-cellular matrix and constitute the apparently monotypic phylum Placozoa. Placozoans diverged early in metazoan history [2–7], making them important organisms for evolutionary research [2,3,8]. Placozoans can be found in warm, shallow, marine environments around the world [9] and all observed individuals fit the general morphological description of T. adhaerens . Our analyses, however, show that the phylum Placozoa is significantly more diverse than previously thought.