Data from: Low fossilization potential of keratin protein revealed by experimental taphonomy

Recent studies have suggested the presence of keratin in fossils dating back to the Mesozoic. However, ultrastructural studies revealing exposed melanosomes in many fossil keratinous tissues suggest that keratin should rarely, if ever, be preserved. In this study, keratin's stability through diagenesis was tested using microbial decay and maturation experiments on various keratinous structures. The residues were analysed using pyrolysis-gas chromatography-mass spectrometry and compared to unpublished feather and hair fossils and published fresh and fossil melanin from squid ink. Results show that highly matured feathers (200–250°C/250 bars/24 h) become a volatile-rich, thick fluid with semi-distinct pyrolysis compounds from those observed in less degraded keratins (i.e. fresh, decayed, moderately matured, and decayed and moderately matured) suggesting hydrolysis of peptide bonds and potential degradation of free amino acids. Neither melanization nor keratin (secondary) structure (e.g. ⍺- vs β-keratin) produced different pyrograms; melanin pyrolysates are largely a subset of those from proteins, and proteins have characteristic pyrolysates. Analyses of fossil fur and feather found a lack of amides, succinimide and piperazines (present even in highly matured keratin) and showed pyrolysis compounds more similar to fossil and fresh melanin than to non-matured or matured keratin. Although the highly matured fluid was not water soluble at room temperature, it readily dissolved at elevated temperatures easily attained during diagenesis, meaning it could leach away from the fossil. Future interpretations of fossils must consider that calcium phosphate and pigments are the only components of keratinous structures known to survive fossilization in mature sediments.

近期已有研究表明，中生代（Mesozoic）化石中存在角蛋白（keratin）。然而，针对众多化石角质组织中暴露黑素体（melanosome）的超微结构研究显示，角蛋白即便能被保存下来，也极为罕见。本研究通过对多种角质结构开展微生物降解与成熟化实验，测试了角蛋白在成岩作用（diagenesis）过程中的稳定性。研究采用热解-气相色谱-质谱联用法（pyrolysis-gas chromatography-mass spectrometry）对反应残留物进行分析，并将结果与未公开的羽毛、毛发化石，以及已发表的乌贼墨中新鲜黑素与化石黑素进行对比。结果显示，高度成熟化的羽毛（200–250℃/250巴（bars）/24小时）会转变为富含挥发物的黏稠流体，其热解产物与降解程度较低的角蛋白（即新鲜、已降解、中度成熟化，以及已降解且中度成熟化的角蛋白）的热解产物存在半显著差异，这提示肽键发生水解，游离氨基酸也可能遭到降解。黑素化过程与角蛋白的二级结构（如α-角蛋白（α-keratin）与β-角蛋白（β-keratin））均不会产生不同的热解色谱图；黑素的热解产物在很大程度上属于蛋白质热解产物的子集，而蛋白质本身具有特征性的热解产物。对化石毛皮与羽毛的分析显示，其中不存在酰胺类、琥珀酰亚胺与哌嗪类物质（这类物质即便在高度成熟化的角蛋白中也存在），且其热解产物与化石及新鲜黑素的相似程度，远高于未成熟化或已成熟化的角蛋白。尽管该高度成熟化的流体在室温下不溶于水，但在成岩作用过程中极易达到的高温条件下可快速溶解，这意味着它可能会从化石中淋滤流失。未来对化石的解读必须考虑到：在成熟沉积物中，能够在化石形成过程中得以保留的角质结构组分仅为磷酸钙（calcium phosphate）与色素。