Taphonomic experiments resolve controls on the preservation of melanosomes and keratinous tissues in feathers

Fossils are a key source of data on the evolution of feather structure and function through deep time, but their ability to resolve macroevolutionary questions is compromised by an incomplete understanding of their taphonomy. Critically, the relative preservation potential of two key feather components, melanosomes and keratinous tissue, is not fully resolved. Recent studies suggesting that melanosomes are preferentially preserved conflict with observations that melanosomes preserve in fossil feathers as external moulds in an organic matrix. To date, there is no model to explain the latter mode of melanosome preservation. We addressed these issues by degrading feathers in systematic taphonomic experiments incorporating decay, maturation and oxidation in isolation and combination. Our results reveal that the production of mouldic melanosomes requires interactions with an oxidant and is most likely to occur prior to substantial maturation. This constrains the taphonomic conditions under which melanosomes are likely to be fossilized. Critically, our experiments also confirm that keratinous feather structures have a higher preservation potential than melanosomes under a range of diagenetic conditions, supporting hitherto controversial hypotheses that fossil feathers can retain degraded keratinous structures.

化石是揭示地质历史时期羽毛结构与功能演化的关键数据来源，但由于对埋藏学（taphonomy）的认知不足，限制了其解答宏演化（macroevolution）相关问题的能力。尤为关键的是，羽毛两大核心组分——黑素体（melanosomes）与角质组织（keratinous tissue）的相对保存潜力尚未完全明确。近期有研究提出黑素体更易被优先保存，但该结论与观测结果相悖：化石羽毛中的黑素体是以有机基质中的外部铸型形式得以保存的。迄今为止，尚无模型可解释这类黑素体的保存模式。我们通过系统性埋藏学实验，分别或组合设置腐烂、成熟与氧化条件对羽毛进行降解处理，以解决上述问题。实验结果表明，铸型黑素体（mouldic melanosomes）的形成需要氧化剂的参与，且最可能发生于显著成熟阶段之前，这一结论限定了黑素体得以石化的埋藏学条件。尤为重要的是，本实验还证实，在多种成岩（diagenetic）条件下，羽毛角质结构的保存潜力均高于黑素体，这为此前颇具争议的“化石羽毛可保留降解后的角质结构”的假说提供了支撑。