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

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 to 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/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.

化石记录中的昆虫通常保存在湖相页岩（lacustrine shales）或琥珀（amber）中。对于湖相页岩中的昆虫标本，其保存通常通过干酪根化（keroginization）或矿化作用（mineralization）实现。由于矿化作用需经历较长的微生物降解过程以产生矿化所需离子，多数埋藏学指标（taphonomic indices）均显示，相较于干酪根化途径，矿化作用的保存质量先天偏低，该结论已被学界提前预测。本研究针对该假说展开验证，通过对比美国科罗拉多州始新世绿河组（Eocene Green River Formation）内沉积学背景相似的两处化石遗址的多项保存质量评价指标完成测试。其中，帕莱伯恩遗址（Paleoburn site）的昆虫化石以氧化铁矿化（大概率形成于黄铁矿（pyrite）之后）为主要保存方式，而安维尔波因特遗址（Anvil Points site）的昆虫化石则以干酪根化方式保存。总体而言，"干酪根化较矿化作用能以更高质量保存软体化石（soft-bodied fossils）"的预测得到了证实，但存在部分限制条件。以外骨骼坚硬著称的甲虫（beetles）在帕莱伯恩遗址中的占比更高，但两处遗址共记录的9个昆虫目中，有8个目在两地均有分布。正如预测，帕莱伯恩遗址的昆虫化石保存保真度（preservation fidelity）更低，同时化石解体程度也更高。该次生偏差主要产生于生物沉积阶段（biostratinomy stage），而非早期成岩作用（early diagenesis）。尽管如此，高能生物沉积环境或许与偏好矿化作用而非干酪根化的埋藏学条件相兼容。由于不同化石矿床的形成时间可相隔数百万年，空间分布也可相距数千公里，对比不同保存途径的先天埋藏学偏差往往极具挑战性。通过对同一地层内两种不同昆虫保存途径的研究，我们证实干酪根化确实能比黄铁矿化（pyritization）/氧化铁矿化作用更好地保存难降解有机质（recalcitrant organic matter）。此外，针对对比矿化与干酪根化软体化石沉积的研究，我们可提出若干关于可对比类群、躯体部位及研究目的的指导原则。