Trichoplax adhaerens is more simply organized than any other living metazoan. This tiny marine animal looks like a irregular “hairy plate” (“tricho plax”) with a simple upper and lower epithelium and some loose cells in between. After its original description by F.E. Schulze 1883, it attracted particular attention as a potential candidate representing the basic and ancestral state of metazoan organization. The lack of any kind of symmetry, organs, nerve cells, muscle cells, basal lamina and extracellular matrix originally left (...) little doubt about the basal position of T. adhaerens. Nevertheless, the interest of zoologists and evolutionary biologists suddenly vanished for more than half a century when Trichoplax was claimed to be an aberrant hydrozoan planula larva. Recently, Trichoplax has been rediscovered as a key species for unraveling early metazoan evolution. For example, research on regulatory genes and whole genome sequencing promise insights into the genetics underlying the origin and development of basal metazoan phyla. Trichoplax offers unique potential for understanding the minimal requirements of metazoan animal organization. BioEssays 27:1294–1302, 2005. © 2005 Wiley Periodicals, Inc. (shrink)

The placozoan Trichoplax adhaerens is a tiny hairy plate and more simply organized than any other living metazoan. After its original description by F.E. Schulze in 1883, it attracted attention as a potential model for the ancestral state of metazoan organization, the “Urmetazoon”. Trichoplax lacks any kind of symmetry, organs, nerve cells, muscle cells, basal lamina, and extracellular matrix. Furthermore, the placozoan genome is the smallest (not secondarily reduced) genome of all metazoan genomes. It harbors a remarkably rich (...) diversity of genes and has been considered the best living surrogate for a metazoan ancestor genome. The phylum Placozoa presently harbors three formally described species, while several dozen “cryptic” species are yet awaiting their description. The phylogenetic position of placozoans has recently become a contested arena for modern phylogenetic analyses and view‐driven claims. Trichoplax offers unique prospects for understanding the minimal requirements of metazoan animal organization and their corresponding malfunctions. (shrink)

The placozoan Trichoplax adhaerens has been bridging gaps between research disciplines like no other animal. As outlined in part 1, placozoans have been subject of hot evolutionary debates and placozoans have challenged some fundamental evolutionary concepts. Here in part 2 we discuss the exceptional genetics of the phylum Placozoa and point out some challenging model system applications for the best known species, Trichoplax adhaerens.

Paulin and Cahill‐Lane explore the origins of event processing and event prediction in animal evolution. They propose that the evolutionary benefit of being able to predict and thus to quickly react to anticipated events may have triggered the evolution of the earliest nervous systems.