A Late-Ediacaran Crown-group Sponge Animal

Sponges are the most basal metazoan phylum and may have played important roles in modulating the redox architecture of Neoproterozoic oceans. Although molecular clocks predict that sponges diverged in the Neoproterozoic era, their fossils have not been unequivocally demonstrated before the Cambrian period, possibly because Precambrian sponges were aspiculate and non-biomineralized. Here we describe a late-Ediacaran fossil, Helicolocellus cantori gen. et sp. nov., from the Dengying Formation (around 551-539 million years ago) of South China. This fossil is reconstructed as a large, stemmed benthic organism with a goblet-shaped body more than 0.4 m in height, with a body wall consisting of at least three orders of nested grids defined by quadrate fields, resembling a Cantor dust fractal pattern. The resulting lattice is interpreted as an organic skeleton comprising orthogonally arranged cruciform elements, architecturally similar to some hexactinellid sponges, although the latter are built with biomineralized spicules. A Bayesian phylogenetic analysis resolves H. cantori as a crown-group sponge related to the Hexactinellida. H. cantori confirms that sponges diverged and existed in the Precambrian as non-biomineralizing animals with an organic skeleton. Considering that siliceous biomineralization may have evolved independently among sponge classes, we question the validity of biomineralized spicules as a necessary criterion for the identification of Precambrian sponge fossils.