A multi-spectroscopic investigation into the diagenesis of avian polyene pigments: simulated maturation, chemical pathways and palaeontological implications

Bright plumage colours in birds such as yellow, orange, red, and green are predominantly produced by polyene-pigments (carotenoids and psittacofulvins). Carotenoids evolved independently in various passerine groups and may have been present in early archosaurs, while psittacofulvins are unique to parrots and evolved only once. In the context of animal integuments, while melanin has been well-studied due to its wide distribution and high fossilization potential, polyene preservation remains less understood. To test their preservation potential, we conducted sediment-encased pressure-temperature maturation experiments on modern feathers. Feather samples containing carotenoids and psittacofulvins were compacted in clay and subjected to 250 bars of pressure at temperatures from 40-200°C, simulating geological diagenesis. Using UV-Vis-NIR, Raman, and FTIR spectroscopy we tracked the pigments’ chemical transformations. Our results show carotenoids are extremely unstable, rapidly degrading and acting as pro-oxidants that accelerate the decay of the feather's protein matrix. In contrast, psittacofulvins demonstrate greater resilience, predictably transforming into a semi-stable polycyclic aromatic geopolymer, or ‘psittacofulvin-char’, at higher temperatures. This establishes a clear taphonomic hierarchy (Melanin, Psittacofulvins, Carotenoids, and Corneous β Proteins) that explains the fossil record’s melanin bias. We propose a framework for a narrow polyene preservation window, identifying ‘psittacofulvin-char’ as a potential geochemical fossil biomarker.