Data from: Fossilization of melanosomes via sulfurization

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.化石黑色素颗粒（melanosomes，黑色素体）是推断动物体色演化历程及其功能的重要研究资源。然而，学界对黑色素及黑色素体的埋藏学机制仍未完全阐明，其中负责黑色素体保存的化学过程尚未得到系统探究。这导致化石黑色素体中含硫化合物的成因难以厘清，进而影响基于潜在富硫褐黑素体（phaeomelanosomes）对化石原始体色的解读。本研究依托西班牙东北部中新世晚期利夫罗斯生物群（Late Miocene Libros biota）中保存的、经黑色素检出证实为黑色素体的化石微结构，采用热解气相色谱-质谱联用法（Py-GCMS）、傅里叶变换红外光谱（FTIR）及飞行时间二次离子质谱（ToF-SIMS）对其保存模式展开分析。研究结果显示，化石组织与宿主沉积物中均富集有机硫化合物与未硫化脂肪酸甲酯；化石组织中的化学特征与褐黑素的保存模式并不相符。本研究结果表明，化石黑色素体的保存过程涉及硫的成岩掺入，即硫化作用（天然硫化）及其他聚合反应。此前已有多项研究在多种无脊椎动物、脊椎动物化石及不同沉积环境中保存的黑色素体中发现有机硫化合物或高浓度硫元素，这提示硫化作用介导的保存模式可能广泛存在。未来针对富硫化石黑色素体的研究，需对宿主沉积物的地球化学特征开展检测，以验证硫化作用的存在证据，从而严格约束对潜在褐黑素体的解读，进而准确推断化石的原始体表颜色。