The "Flat Animal": Is It a Cambrian Ancestor?

Placozoans ("flat animals") are free-living marine invertebrates with a very simple, blob-like structure. Possessing no organs, their relationship to other animal phyla is a matter of debate. The only known examples are lumped into a single species, Trichoplax adhaerens. Since they do not appear in the fossil record, their origins can only be inferred from genomics and comparative anatomy. The results have been troublesome for evolutionary theory.

The most complete examination of cell types o...