Taxon-specific redox conditions control fossilisation pathways

Abstract

The preservation of fossils in the rock record depends on complex redox processes. Redox conditions around different decaying organisms have rarely been monitored in the context of experimental taphonomy. Here, microsensors were used to measure redox changes around decomposing carcasses of various taxa, including shrimp, snail, starfish, and planarian. Our results show that different decaying taxa lead to various post-mortem environmental redox conditions. Large carcasses tend to reach reducing conditions more rapidly than smaller ones. However, size does not explain all observed patterns, as environmental redox conditions are also influenced by the nature of the organic material. For instance, taxa with higher proteins-to-lipids and (proteins + carbohydrates)-to-lipids ratios tend to achieve reducing conditions more rapidly than others. The generation of distinct redox environments around different taxa originally put under the same original environmental conditions suggests that various fossilisation patterns of macrofossils and molecules can co-occur within a single sedimentary layer.

Fig. 1. Oxidation-reduction potential (ORP) values (mV) over time for four different animals.

The biochemical reactions corresponding to different redox values: nitrification (+100 to +350 mV), carbonaceous biochemical oxygen demand (cBOD) (+50 to +250 mV), denitrification (−50 to +50 mV), sulphate reduction (−250 to −50 mV), biological phosphorus release (−250 to −100 mV), and methanogenesis (−400 to −175 mV). The shaped points represent individual measurements of redox potential for each animal and control, and the lines follow the average redox potential of each animal and control at each time point. Source data is provided as a Source Data file.

Fig. 2. Normalized and body-mass corrected oxidation-reduction potential (ORP) values.

A Normalized ORP values over time for the four different taxa and controls. B Redox potential corrected for animal mass: average normalized ORP values of controls subtracted from the normalized ORP values of animals, and divided by the average weight of the animals. Low values reflect a notable drop in ORP values when correcting for the mass of the organism, while zero values reflect no drop in ORP values after mass correction. In other words, showing the lowest values in (B) planarians leads to the biggest drop in ORP values followed by snails, starfish, and shrimps of the same size. C Redox potential corrected for organic matter mass: average normalized ORP values of controls subtracted from the normalized ORP values of animals, and divided by the average weight of the organic matter by subtracting the mass of biomineralized matter from the mass of animal. The shaped points in (A–C) represent individual measurements and the lines are following the average of each animal at each time point. D Proportions of proteins, carbohydrates and lipids in the organic matter of shrimp (Neocaridina davidi), starfish (Asterina sp.), snail (Stenophysa marmorata) and planarian (Procotyla sp.). E Mass-corrected (MC) average redox potential for each animal, and ratios between the components of organic matter: Carbohydrates/Lipids (Carb/Lip), Proteins/Lipids (Prot/Lip), (Proteins + Carbohydrates)/Lipids (Prot + Carb)/Lip). F Pearson correlation coefficients between the mass-corrected (MC) average redox potential for each animal and the ratios between the components of organic matter. The blue colour represents a positive correlation, while the red colour represents a negative correlation (anti-correlation). The larger and darker the circles, the higher the correlation coefficient.