Fossil Biomarkers and Biosignatures Preserved in Coprolites Reveal Carnivorous Diets in the Carboniferous Mazon Creek Ecosystem

SIMPLE SUMMARY: Coprolites (fossilised faeces) can preserve important dietary information through geological time, offering insights into extinct animal diets. When digestion of dietary items leaves no unambiguous morphology to reconstruct the food spectrum of a coprolite producer, preserved biomolecular information can offer unique perspectives into the individual dietary composition and trophic relationships in ancient ecosystems. In this study we combine a uniquely diverse array of chemical techniques to demonstrate that biomarkers and macromolecular biosignatures from Carboniferous coprolites can reveal the dietary spectrum and trophic position of their extinct producers: an overwhelming abundance of cholesteroids, biomarkers of animal cholesterol, and an animal-affinity of the preserved macromolecular phase revealed by the statistical analysis of in situ Raman spectra, indicate a likely carnivorous diet for the coprolite producer. The presence of intact primary metabolites, such as sterols, and informative fossilization products of biopolymers, demonstrates the significance of siderite (iron carbonate) concretions in the exceptional preservation of biomolecular information in deep time, facilitated by the rapid encapsulation and remineralisation of organic matter within days to months. ABSTRACT: The reconstruction of ancient trophic networks is pivotal to our understanding of ecosystem function and change through time. However, inferring dietary relationships in enigmatic ecosystems dominated by organisms without modern analogues, such as the Carboniferous Mazon Creek fauna, has previously been considered challenging: preserved coprolites often do not retain sufficient morphology to identify the dietary composition. Here, we analysed n = 3 Mazon Creek coprolites in concretions for dietary signals in preserved biomarkers, stable carbon isotope data, and macromolecular composition. Cholesteroids, metazoan markers of cholesterol, show an increased abundance in the sampled coprolites (86 to 99% of the total steranes) compared to the surrounding sediment, indicating an endogenous nature of preserved organics. Presence of unaltered 5α-cholestan-3β-ol and coprostanol underline the exceptional molecular preservation of the coprolites, and reveal a carnivorous diet for the coprolite producer. Statistical analyses of in situ Raman spectra targeting coprolite carbonaceous remains support a metazoan affinity of the digested fossil remains, and suggest a high trophic level for the coprolite producer. These currently oldest, intact dietary stanols, combined with exquisitely preserved macromolecular biosignatures in Carboniferous fossils offer a novel source of trophic information. Molecular and biosignature preservation is facilitated by rapid sedimentary encapsulation of the coprolites within days to months after egestion.