Where’s the Glass? Biomarkers, Molecular Clocks and MicroRNAs Suggest a Near 200-Million-Year Missing Precambrian Fossil Record of Demosponges  

The earliest evidence for animal life comes from the fossil record of 24-isopropylcholestane, a sterane found in Cryogenian deposits, and whose precursors are found in modern demosponges, but not choanoflagellates, calcisponges, hexactinellids or eumetazoans.   However, many modern demosponges are also characterized by the presence of siliceous spicules, and there are no convincing fossil spicules until the Lower Cambrian.  This temporal disparity highlights a problem with our understanding of the Precambrian fossil record – either these supposed demosponge-specific biomarkers were derived from the sterols of some other organism and are simply retained in modern demosponges, or there is a significant gap in the demosponge spicule fossil record.   To resolve this issue, we must establish the phylogenetic placement of another sponge group, the hexactinellids, whose spicules are thought to be homologous to the spicules of demosponges, and who also make their first appearance near the Precambrian/Cambrian boundary.  Using two independent data sets – traditional molecular phylogenetic analyses and the presence or absence of specific microRNA genes – we show that demosponges are monophyletic, and that hexactinellids are the sister group of demosponges.   Thus, spicules must have evolved before the last common ancestor of all living demosponges.  A molecular divergence estimate places the origin of this last common ancestor well within the Cryogenian, consistent with the biomarker record, and strongly suggests that there was a massive failure to preserve siliceous spicules during the Precambrian.