Sterane biomarkers preserved in ancient sedimentary rocks hold promise for tracking the diversification and ecological expansion of eukaryotes. The earliest proposed animal biomarkers from demosponges (Demospongiae) are recorded in a sequence around 100 Myr long of Neoproterozoic–Cambrian marine sedimentary strata from the Huqf Supergroup, South Oman Salt Basin. This C30 sterane biomarker, informally known as 24-isopropylcholestane (24-ipc), possesses the same carbon skeleton as sterols found in some modern-day demosponges. However, this evidence is controversial because 24-ipc is not exclusive to demosponges since 24-ipc sterols are found in trace amounts in some pelagophyte algae. Here, we report a new fossil sterane biomarker that co-occurs with 24-ipc in a suite of late Neoproterozoic–Cambrian sedimentary rocks and oils, which possesses a rare hydrocarbon skeleton that is uniquely found within extant demosponge taxa. This sterane is informally designated as 26-methylstigmastane (26-mes), reflecting the very unusual methylation at the terminus of the steroid side chain. It is the first animal-specific sterane marker detected in the geological record that can be unambiguously linked to precursor sterols only reported from extant demosponges. These new findings strongly suggest that demosponges, and hence multicellular animals, were prominent in some late Neoproterozoic marine environments at least extending back to the Cryogenian period.
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The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information files.
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Funding support for this work came from the NASA Astrobiology Institute teams Alternative Earths (NNA15BB03A) and Foundations of Complex Life (NNA13AA90A), NASA Exobiology program (grant number 80NSSC18K1085), NSF Frontiers in Earth System Dynamics programme (grant number 1338810), Agouron Institute, and European Union’s Horizon 2020 research and innovation programme through the SponGES project (grant agreement number 679849). Many thanks go to the sponge collectors, including T. Pérez, H. Tore Rapp, A. Plotkin, J.-S. Hong, Y. M. Huang, S. Rohde, S. Nichols, J. V. Lopez, B. Calcinai, G. Gatti, B. Ciperling, J. Boavida, J.-P. Fonseca, L. Magro and F. Azzini. The fieldwork on the Mohns and Knipovich Ridges aboard the RV G. O. Sars in 2014 was supported by the Research Council of Norway through the Centre for Geobiology, UiB (contract number 179560). A Vazella pourtalesii specimen was provided by E. Kenchington through funding from the Marine Conservation Target fund of the Department of Fisheries and Oceans Canada. Other specimens were provided by E. Kenchington as Canadian lead for the NEREIDA (NAFO Potential Vulnerable Marine Ecosystems-Impacts of Deep-sea Fisheries) project led by Spain and Canada. We thank K. Ubhayasekera (Department of Chemistry, Uppsala University) for GC-MS analyses, and S. Rajendran and T. Aljazar (Department of Medicinal Chemistry, Uppsala University) for help with the isolation of sponge sterols. We thank Petroleum Development Oman for Neoproterozoic–Cambrian rock samples from the South Oman Salt Basin for ancient biomarker analysis. We are grateful to D. Rocher for GC-QQQ-MS analysis and GeoMark Research (Houston, TX) for providing oil samples from Eastern Siberia.