Abstract Fossil melanin granules (melanosomes) are an important resource for inferring the evolutionary history of colour and its functions in animals. The taphonomy of melanin and melanosomes, however, is incompletely understood. In particular, the chemical processes responsible for melanosome preservation have not been investigated. As a result, the origins of sulfur‐bearing compounds in fossil melanosomes are difficult to resolve. This has implications for interpretations of original colour in fossils based on potential sulfur‐rich phaeomelanosomes. Here we use pyrolysis gas chromatography mass spectrometry (Py‐GCMS), fourier transform infrared spectroscopy (FTIR) and time of flight secondary ion mass spectrometry (ToF‐SIMS) to assess the mode of preservation of fossil microstructures, confirmed as melanosomes based on the presence of melanin, preserved in frogs from the Late Miocene Libros biota (NE Spain). Our results reveal a high abundance of organosulfur compounds and non‐sulfurized fatty acid methyl esters in both the fossil tissues and host sediment; chemical signatures in the fossil tissues are inconsistent with preservation of phaeomelanin. Our results reflect preservation via the diagenetic incorporation of sulfur, i.e. sulfurization (natural vulcanization), and other polymerization processes. Organosulfur compounds and/or elevated concentrations of sulfur have been reported from melanosomes preserved in various invertebrate and vertebrate fossils and depositional settings, suggesting that preservation through sulfurization is likely to be widespread. Future studies of sulfur‐rich fossil melanosomes require that the geochemistry of the host sediment is tested for evidence of sulfurization in order to constrain interpretations of potential phaeomelanosomes and thus of original integumentary colour in fossils.

中文翻译：

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通过硫化使黑素体僵化

摘要 化石黑色素颗粒（黑素体）是推断动物颜色进化史及其功能的重要资源。然而，黑色素和黑素体的埋藏学尚不完全清楚。特别是，尚未研究负责黑素体保存的化学过程。因此，化石黑素体中含硫化合物的来源难以解析。这对基于潜在的富含硫的褐黑素体对化石原始颜色的解释具有重要意义。在这里，我们使用热解气相色谱质谱 (Py-GCMS)、傅立叶变换红外光谱 (FTIR) 和飞行时间二次离子质谱 (ToF-SIMS) 来评估化石微结构的保存模式，基于黑色素的存在，保存在来自晚中新世Libros生物群（西班牙东北部）的青蛙中。我们的结果揭示了化石组织和寄主沉积物中大量的有机硫化合物和非硫化脂肪酸甲酯；化石组织中的化学特征与褐黑素的保存不一致。我们的结果反映了通过硫的成岩结合，即硫化（自然硫化）和其他聚合过程进行的保存。据报道，保存在各种无脊椎动物和脊椎动物化石和沉积环境中的黑素体中含有有机硫化合物和/或硫浓度升高，这表明通过硫化进行保存可能很普遍。