Description: Highlights • Fatty acids preserved in an Oligocene whale bone were analysed. • The fatty acid content of the fossil was in the permil range vs. a recent whale vertebra. • Ca. 80% of the n-C16 and n-C18 alkyl moieties were extractable, ca. 20% being bound to kerogen. • Endogenous fatty acids were largely of microbial origin (sulfate reducers, actinobacteria). Abstract The taphonomic and diagenetic processes by which organic substances are preserved in animal remains are not completely known and the originality of putative metazoan biomolecules in fossil samples is a matter of scientific discussion. Here we report on biomarker information preserved in a fossil whale bone from an Oligocene phosphatic limestone (El Cien Fm., Mexico), with a focus on fatty acyl compounds. Extracts were quantitatively analysed using gas chromatography–mass spectrometry (GC–MS) and, to identify macromolecular-linked remains, demineralised extraction residues were subjected to catalytic hydropyrolysis (HyPy). To better recognise potential authentic (i.e. animal-derived) lipids, the data from the ancient bone were compared with those obtained from (i) the adjacent host sediment of the fossil and (ii) a recent whale (Phocoena phocoena) vertebra. In addition, the spatial distribution of organic and inorganic species was observed at the μm level by imaging MS (time-of-flight-secondary ion mass spectrometry, ToF-SIMS). Our results revealed a rather even distribution of hydrocarbon-, O- and N-containing ions in the trabecular network of the ancient bone. A different, more patchy arrangement of organic compounds was evident in the former marrow cavities that were partly cemented by clotted micrites of putative microbial origin. The concentration of fatty acids (FAs) in the ancient bone was in the permil range of the amount extracted from the recent whale vertebra. Endogenous compounds, including monoenoic n-C16 and n-C18 as well as branched FAs, were identified in the fossil bone by comparison with the host sediment. Ca. 80% of the prevalent n-C16 and n-C18 moieties in the ancient bone were extractable as FAs, whereas ca. 20% were covalently bound in the non-saponifiable kerogen fraction. Ample pyrite precipitates, distinctive 10-methyl branched FAs and microbial microborings (“tunneling”) indicate that sulfate reducers and collagen-degrading actinomycetes were central players in the microbial decomposition of the bone. Similarities with reported microbial FA patterns suggest that the FAs in the fossil bone were largely contributed by these microbial “last eaters”. The results highlight some of the degradation and preservation mechanisms during marine FA diagenesis in the “natural laboratory” of bones, and therefore the processes that lead to either degradation, preservation, or introduction of these widespread biomolecules in the fossils of ancient marine animals.

Description: The degradation and preservation affecting the biomarker record of ancient metazoa are not fully understood. We report on a five month experiment on the fate of fatty acids (FAs) during the degradation of recent whale vertebrae (Phocoena phocoena). Whale bones were analysed for extractable FAs and macromolecularly bound n-acyl compounds. Fresh bone showed extractable FAs dominated by 16: 1 omega 7c, 16:0, 18:1 omega 9c and 18:0. Calculated degradation rate constant (k) values showed a rapid decrease in FA concentration, with k values higher for unsaturated than for saturated compounds (0.08/day for 18: 1 omega 9c, 0.05/day for 16:0). The appearance or increased abundance of distinctive methyl branched (e.g. i/ai-15:0 and -17:0, 10Me-16:0) and hydroxy FAs (e.g. 10OH-16:0 and 10OH-18:0) were observed, providing clear evidence for the microbial degradation of bone organic matter and an input of lipids from specialised bacteria. Catalytic hydropyrolysis (HyPy) of demineralised extraction residues released up to 0.13% of the total n-C-16 and n-C-18 moieties in the degraded bones. This revealed that only a small, yet sizeable, portion of bone-derived fatty acyl units was sequestered into (proto)kerogen during the earliest stages of degradation.