Dickinsonia - Affinity

Affinity

Dickinsonia is generally regarded as a member of the Vendazoa — a group of organisms that thrived just before most of the modern multicellular animal phyla appeared in the fossil record. Other vendazoa such as Yorgia and Marywadea somewhat resemble Dickinsonia, and may be related.

The affinities of Dickinsonia are uncertain. It has been variously interpreted as a jellyfish, coral, polychaete worm, turbellarian, mushroom, xenophyophoran protist, sea anemone, lichen, and even a close ancestor of the chordates.

Gregory Retallack has attempted to use the mode of decay of the fossil as a clue to its affinity. He originally proposed that all of the Ediacara biota were lichens based on their post-burial compaction, but faced strong criticism. His revised opinion reiterates the fact that the decay mode of the organisms is most similar to that of leaves, fungi or lichens, and not at all like soft-bodied animals, which clot and distort as they wilt and decay.

However, it is possible that Dickinsonia falls into a group of organisms that went extinct before the Cambrian. Its construction is loosely similar to other Ediacaran organisms, and the similarity of their architecture suggests that dickinsoniamorphs may belong in a clade with Charnia and other rangeomorphs.

There is a strong argument that the organism is more derived than a sponge, but less so than a eumetazoan. The organism could clearly move, evidenced by its association with trackways which could only have been produced by feeding. However, it lacks any convincing evidence for a mouth, anus or gut, and appears to have fed by absorption on its bottom surface. The placozoans are simple animals which feed with their soles and are phylogenetically between sponges and eumetazoa; this suggests that Dickinsonia may have been a stem-group placozoan, or somewhere more crownwards than sponges on the eumetazoan stem.