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.

Fig_S1Experimental setup. A, schematic showing experimental pathways used. B, detailed schematic showing potential natural scenarios corresponding to each experimental pathway (indicated by numerals). DDW, distilled-deionized water.Fig_S2SEM images of feather barbules showing the paucity of obvious decay bacteria in feathers decayed in pH 6 and 12.2. A–B, pathway 1. C–D, pathway 2. A, decay at pH 12.2; D, decay at pH 6. Scale bars in A, C = 10 µm, B, D = 5 µm.Table S1Maximum dimensions of moulds and intact melanosomes, measured from transverse sections.Table S2Abundance of melanosome moulds in transverse sections of oxidized feather barbules.Table S3AHPO analysis of Gallus gallus feathers.