Investigating the Putative Diagenetic Origins for Neoproterozoic Sponge Steranes

Abstract

Living organisms often leave molecular traces in the fossil record and these can provide a lens to understand the history of life on Earth, even when body fossils are not present. One widely discussed molecular fossil, useful for unraveling the record of animal life, is 24-isopropylcholestane (24-ipc). 7KLV µELRPDUNHU¶ LV the C30 hydrocarbon derivative of 24-isopropylidene cholesterol, an uncommon marine sterol that is abundant in some species of demosponges. The hydrocarbon 24-ipc has been identified in many late Neoproterozoic sedimentary rocks and oils and notably earlier than the first uncontroversial sponge fossil spicules. The sponge biomarker hypothesis argues that this Neoproterozoic 24-ipc comes from ancient demosponges, which would have implications for the timing of the evolution of animals as a whole. Despite the sponge biomarker hypothesis having considerable support in terms of the secular distribution patterns of 24-ipc and related molecules, combined with the limited biological distributions of its biological precursor, it has seen numerous challenges. In one example, Bobrovskiy et al. (2021) suggest that the sponge steranes found in the rock record have a diagenetic origin via methyl migrations from and to C29 algal sterols. Here, we examine the suggested diagenetic production of 24-ipc from algal sterols based on geologically plausible diagenesis experiments in order to further understand the preservation pathways of algal sterols. We find that, while 24-ipc can be produced diagenetically from C29 algal sterols, it is only produced sporadically and in minute quantities, making it an unlikely source for the Neoproterozoic sponge steranes.