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 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.

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（mineralization）作用实现。由于矿化作用需要经历较长的微生物降解过程以产生成矿所需离子，相较于干酪根化，多数埋藏学（taphonomic）指标均显示该途径的保存质量先天偏低，这一现象已被学界预测。本研究针对科罗拉多州始新世格林河组（Eocene Green River Formation）中沉积学特征相似的两处遗址，通过对比多种保存质量评价指标，对该假设进行了验证。其中，古燃烧点遗址（Paleoburn site）的昆虫以氧化铁（iron oxides，大概率源自黄铁矿（pyrite））矿化保存为主，而铁砧点遗址（Anvil Points site）的昆虫则以干酪根化保存为主。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）”这一预测得到了证实，但存在若干限定条件。外骨骼（cuticles）较为坚固的鞘翅目昆虫在古燃烧点遗址中占比更高，但两处遗址共记录了9个昆虫目中的8个。正如预测，古燃烧点遗址的昆虫化石保存保真度更低，同时解体程度（disarticulation）更高。该二次偏差主要产生于尸积阶段（biostratinomy），而非早期成岩作用（diagenesis）。不过，高能的尸积环境可能与优先促进矿化而非干酪根化的埋藏条件相兼容。由于不同化石沉积点往往相隔数百万年或数千公里，对比不同保存途径的先天埋藏学偏差通常颇具难度。本研究通过对同一地层中两种不同昆虫保存途径的分析，证实干酪根化确实较黄铁矿化（pyritization）或氧化铁矿化，能以更高质量留存难降解有机质（recalcitrant organic matter）。此外，针对矿化与干酪根化软体化石的对比研究，我们还可提出若干关于可对比类群、躯体部位及研究目的的指导性原则。