Article: Questioning the evidence of organic compounds called sponge biomarkers
Jonathon B. Antcliffe
Elevated concentrations of an organic compound, 24-isopropylcholestane, found in the Precambrian Huqf Supergroup of Oman may provide the oldest known sponge ‘fossil’. This evidence is of critical importance for a properly balanced understanding of the origin of animals. Several different pelagophyte (Class Pelagophyceae part of the Stramenopiles within the Chromaveolata) algae are also capable of producing these exact compounds, and may similarly have done so in deep time. Modern marine algae are also reported to produce structural isomers that are compositionally identical to the sponge marker; they do this in copious quantities. Further, 24-isopropylcholestane can be produced by diagenetic alteration of compounds produced in large quantities by algae. It is also possible that contamination by petroleum derived lubricating oil used when coring while extracting these compounds from subsurface layers, has affected the data. All extinct organisms that may have produced this compounds are unavailable for analysis by the modern organic chemist and cannot be eliminated from the list of possible producers of the sponge marker. There are also significant uncertainties regarding the dating of the strata from which these ancient compounds are found. Although the compounds are widely reported as c. 751 Ma, they are younger than 645 Ma. It seems more likely that these compounds represent algal biochemical evolution at a time when algal burial occurred in great quantity with well known coeval algal fossils but no sponge fossils. The macroalgal biomass may have declined during the agronomic revolution at the base of the Cambrian Period owing to processing by metazoans, accounting for the comparative scarcity of these sponge markers in Phanerozoic sediments, after which time sponge spicules and body fossils become evident.