The Neoproterozoic era (1,000–542 Myr ago) was an era of climatic extremes and biological evolutionary developments culminating in the emergence of animals (Metazoa) and new ecosystems1. Here we show that abundant sedimentary 24-isopropylcholestanes, the hydrocarbon remains of C30 sterols produced by marine demosponges, record the presence of Metazoa in the geological record before the end of the Marinoan glaciation (∼635 Myr ago). These sterane biomarkers are abundant in all formations of the Huqf Supergroup, South Oman Salt Basin, and, based on a new high-precision geochronology2, constitute a continuous 100-Myr-long chemical fossil record of demosponges through the terminal Neoproterozoic and into the Early Cambrian epoch. The demosponge steranes occur in strata that underlie the Marinoan cap carbonate (>635 Myr ago). They currently represent the oldest evidence for animals in the fossil record, and are evidence for animals pre-dating the termination of the Marinoan glaciation. This suggests that shallow shelf waters in some late Cryogenian ocean basins (>635 Myr ago) contained dissolved oxygen in concentrations sufficient to support basal metazoan life at least 100 Myr before the rapid diversification of bilaterians during the Cambrian explosion. Biomarker analysis has yet to reveal any convincing evidence for ancient sponges pre-dating the first globally extensive Neoproterozoic glacial episode (the Sturtian, ∼713 Myr ago in Oman2).
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Funding support for this work came from Petroleum Development Oman (PDO), the NASA Exobiology Program, the NSF EAR Program, the Agouron Institute and the NASA Astrobiology Institute. We thank PDO for access to sample materials and Z. Rawahi and P. Taylor, in particular, for their input. C. Colonero, R. Kayser and A. Lewis provided laboratory assistance, including the maintenance of mass spectrometers at MIT.
Author Contributions G.D.L. interpreted the data and wrote the manuscript with input from R.E.S, D.A.F, A.S.B and E.G. G.D.L., E.G., C.S. and A.E.K. acquired the Huqf biomarker data working in the research group of R.E.S. A.S.B. and M.B. screened extant demosponges for their sterol contents. C.E.S. and W.M. made facilities available for HyPy experiments on kerogens and trained C.S. to use the equipment. J.P.G. provided a robust stratigraphic framework for the Huqf Supergroup in the SOSB and with D.A.F. identified key sedimentary rock samples to use in this investigation. S.A.B and D.J.C. measured important U–Pb ages for ash beds and detritral zircons through the stratigraphy to constrain the age range and distribution of our demosponge biomarkers.
This file contains Supplementary Data, Supplementary Figure 1 with Legend, Supplementary Tables 1-4, Supplementary Results, a Supplementary Discussion and Supplementary References