The same picture through different lenses: quantifying the effects of two preservation pathways on Green River Formation insects

Abstract

Insects in the fossil record are generally preserved in lacustrine shales or in amber. For those in lacustrine shales, preservation is usually via keroginization or mineralization. Given the extended period of microbial decay required to generate ions for mineralization, there is a predicted inherent bias toward lower preservation quality for this pathway by most taphonomic indices compared with keroginization. This study tests this hypothesis by comparing multiple measures of preservation quality between sites with similar sedimentology in the Eocene Green River Formation of Colorado. Here, insects are either mineralized in iron oxides (likely after pyrite) at the Paleoburn site or keroginized at the Anvil Points site.Generally, the prediction that keroginization preserves soft-bodied fossils with higher preservational quality than mineralization is affirmed, but with some caveats. Beetles, known for their robust cuticles, are proportionately more abundant at the Paleoburn site, but eight of the nine orders recorded are shared between sites. As predicted, insects show lower preservation fidelity at the Paleoburn site, but they also show higher degrees of disarticulation. This second bias should be acquired primarily during the biostratinomy stage, and not early diagenesis. Nonetheless, higher-energy biostratinomic conditions may be compatible with taphonomic conditions that promote mineralization over keroginization.Comparing the inherent taphonomic bias of different preservation pathways is often difficult, since fossil deposits may be preserved millions of years or thousands of kilometers apart. By studying two different preservation pathways of insects within the same formation, we can affirm that keroginization does indeed preserve recalcitrant organic matter with higher quality than pyritization or iron-oxide mineralization. Additionally, some guidelines can be proposed concerning the body parts and taxa that can be compared, and for what purpose, when contrasting mineralized and keroginized soft-bodied deposits.

Distinct preservational pathways of insects from the Crato Formation, Lower Cretaceous of the Araripe Basin, Brazil

“…Based on the argument above, our results do not match this prediction and diverged from those of Anderson and Smith (2017). In their study, there is a significant difference between ironmineralized and keroginized insects from the Green River Formation, where the preservation quality of mineralized insects was consistently lower than the kerogenized insects.…”

“…Degradation in the BSR zone is more efficient than degradation in the methanogenesis zone because BSR is more metabolically efficient than the degradation pathways available in the methanogenesis zone (Elsayed et al, 2015;Mao et al, 2015). Therefore, it is expected that a specimen preserved in the BSR zone will undergo more degradation than a specimen preserved in the methanogenesis zone (Anderson and Smith, 2017).…”

“…There is a possibility that specimens were initially preserved with finer details, but during later diagenesis that preservationfidelity has been lost. Insects affected by intense oxidation process can exhibit iron-oxide overgrowths, or even changes in articulation state (Anderson and Smith, 2017). In the case of the pyritized insects, many specimens initially mineralized have been affected by narrow fractures.…”

“…Both keroginization and pyritization are able to preserve recalcitrant components of soft-bodied material. However, pyritized fossils tend to undergo further degradation than kerogenized ones, given the greater amount of time in the BSR which is more metabolically efficient than the Methanogenesis Zone (Anderson and Smith, 2017). Surprisingly, pyritized insects from the Crato Formation are preserved with ultrastructural cuticular details (Barling et al, 2015;Os es et al, 2016).…”

The damselfly palaeofauna from the Eocene of Wyoming and Colorado, USA (Insecta, Odonata, Zygoptera)

“…The Fossil Lake deposits are dominated by packages of laminated micrites interlayered with kerogen. Although fossil fishes, for which the Green River Group is best known, occur in both kerogen‐rich and poorly laminated micrites, well‐preserved insects are found exclusively in kerogen‐rich micrites deposited in the lake's deeper water (Anderson & Smith, 2017). Rocks of the central lake basin have thinner limestone laminae, creating a relative increase in the waxy kerogen layers, which slows water penetration.…”