Cambrian petalonamid Stromatoveris phylogenetically links Ediacaran biota to later animals

Abstract

Macro-organisms of the Ediacaran period (635–541 Ma) were large and morphologically complex, with some living in aphotic habitats, presenting the possibility that they were early animals. However, ‘bizarre’ Ediacaran morphologies and mouldic preservation have frustrated comparison to later taxa. Consequently, both the positions of Ediacaran biota in the tree of life and the origins of the Metazoa have remained disputed. Here we provide phylogenetic evidence to identify Ediacaran macro-biota as animals, based on 206 new fossils of Stromatoveris psygmoglena from the lower Cambrian Chengjiang Lagerstätte. Exceptionally preserved soft-tissue anatomy shows that Stromatoveris was a soft-bodied, radially symmetric animal with multiple, sub-branched petaloids and a differentiated holdfast. Photo-referenced morphological character analysis enables phylogenetic reconstruction of a monophyletic clade designated Petalonamae, that unites Stromatoveris with iconic Ediacaran genera (Rangea, Pteridinium, Ernietta, Swartpuntia, Arborea, Pambikalbae and Dickinsonia) and is placed as sister-group to the Eumetazoa. Therefore, based on phylogenetic bracketing within the Metazoa, the Ediacaran petalonamids are established as animals. From these findings, it follows that petalonamids remained an important component of Cambrian marine ecosystems and that the metazoan radiation can be dated to a minimum age of between 558 and 571 myr.