An article published in the journal “Paleontology” describes a comparative study of various organisms of the so-called Ediacara fauna with the one called Stromatoveris psygmoglena. Jennifer F. Hoyal Cuthill of the Tokyo Institute of Technology and the British University of Cambridge and Jian Han of the Northwest University in Xi’an, China, analyzed more than 200 fossils of specimens of this organism that lived about 518 million years ago. A comparison with other organisms that lived later suggests that they were all animals that formed their own group, which was called Petalonamae.
The first eight specimens of Stromatoveris psygmoglena were discovered in 2006 in the Chengjiang fossil deposit, in southern China. It’s one of the sites where soft tissues of organisms dating back to the Cambrian period, the one after the Ediacaran, were preserved. This happens rarely and generally bones, shells and other hard parts fossilize but in some places there were special conditions that allowed to soft tissues to conserve long enough to fossilize.
During subsequent excavations at the Chengjiang site, many more specimens of Stromatoveris psygmoglena were found for a total of over 200. At least for now, specimens of this organism have been found only in that deposit. From the beginning, they were classified as animals despite having a structure with a shape curiously similar to a leaf for a total height of about 10 centimeters.
For this new animal there was still the problem of classification, which in many cases of species from the Cambrian period is a complex problem. It’s even more so for the species of the Ediacaran period, commonly called Ediacara fauna, which appear so different from the later ones, so much so that someone proposed that at least some species belonged to a different kingdom in the tree of life that became completely extinct.
Some researchers had already noted similarities between the fossils of Stromatoveris psygmoglena and those of species belonging to the genus Charnia, which lived in the Ediacaran. In this new research, Jennifer F. Hoyal Cuthill and Jian Han tried to conduct a phylogenetic analysis to compare them with fossils of other genera from the Ediacaran. The analysis also included nine other types of animals and single-cell organisms.
According to the analysis, seven genera of organisms that lived in the Ediacaran share a series of anatomical features with Stromatoveris psygmoglena: Rangea, Pteridinium, Ernietta, Swartpuntia, Arborea, Pambikalbae and Dickinsonia. For this reason the researchers believe that all these organisms formed a single large group they named Petalonamae. If they really are all related, this means that animals emerged at least 571 million years ago, like the oldest organisms of the new group, and that they were diversifying well before the Cambrian explosion.
The animals of the Petalonamae group may have formed a separate phylum related to ctenophores, the ones commonly known as comb jellies, or a group within the ctenophore phylum. This new reconstruction offers some possibilities but at least for the moment it’s impossible to provide certainties. Surely there will be new discussions to assess the nature of at least some species of the Ediacara fauna and the origin of the animals.