A Placozoan Affinity for Dickinsonia and the Evolution of Late Precambrian Metazoan Feeding Modes

The Ediacaran taxon Dickinsonia has endured a controversial taxonomic history, and most previous attempts to place it within the tree of Life have relied on largely superficial characters (e.g., “segmentation”, mesenteries). However, all that can be said unequivocally about Dickinsonia, using both the body and trace fossil record, is that: 1) it had a distinct anterior-posterior and dorsal-ventral axis; 2) it had the ability to move; and 3) it digested the ubiquitous Ediacaran microbial mats with its entire lower surface and thus fed extracellularly (i.e., saprobically) with a ventral sole. These three characteristics in toto preclude an affinity within crown-group Eumetazoa or within any living sponge lineage, but Dickinsonia could lie between “Porifera” and Eumetazoa on the animal tree. Indeed, we believe it can be most readily interpreted as having affinities with the Placozoa, a small phylum whose modern representatives feed saprobically on bacterial or algal biofilms. Our hypothesis predicts then that placozoans are the sister group of the eumetazoans (here Cnidaria + Bilateria) rather then nested either within eumetazoans or within cnidarians as most molecular analyses have previously indicated. To test this hypothesis, we performed a phylogenetic analysis of a concatenated set of seven nuclear housekeeping genes that has proven accurate in other areas of the metazoan tree. We consistently and with high precision find Placozoa to be above a paraphyletic “Porifera” as the sister taxa to Eumetazoa (Cnidaria + Bilateria), consistent with the hypothesis that Dickinsonia is allied with the Placozoa, most likely as a stem-group member given its size and unique morphology with respect to all living placozoans. Our data strongly suggest that all three major modes of metazoan feeding, microphagy (sponges), saprobic (placozoan) and macrophagy (eumetazoan), were in place by the end of the Ediacaran, and that no novel non-parasitic trophic modes have arisen since.