THE CHEMISTRY OF FOSSIL COLOUR: THE ROLE OF CUTICULAR LIPIDS IN FOSSILISATION OF INSECT STRUCTURAL COLOURS

Structural coloration underpins communication strategies in modern insects, but its evolution is poorly understood. We resolve this by conducting heating experiments, employing elevated pressures upto 500 bars and temperatures upto 260 degrees C, to simulate the effects of burial on structurally colored cuticles of modern beetles. Such experiments have shown that the color generated by multilayer reflectors in cuticles change due to alteration of the refractive index and periodicity of the cuticle layers.The underlying reaction mechanism, however, remained unclear. (1) The timing of the diagenetic reactions generating the lipid-protein complex relates to color change; chitin does not quantitatively reduce during the experiments but protein does, independant of autolytic/microbial decay, (2) in situ polymerisation and/or esterification reactions do not occur at these temperatures to bind the chitin to lipids macomolecularly, (3) the chemistry of the epicuticular lipids drastically change during the experiments producing N-bearing compounds from proteins, both at 200 and 260 degrees (4) the amount of protein and related amino acid decreases at higher temperatures, reacting with lipids as the amount of chitin relmains constant, but there is increased aromatization in the experiments (5) whether preservation of epicuticular lipids (and thus of visible colour) correlates with preservation of other key biomolecules, e.g. chitin in fossils remains to be seen. Further, traces of specific lipid-protein compounds, where they occur in fossil insects that are preserved in a black colour, may be diagnostic of originally structurally coloured cuticles, but this hypothesis has not been tested. Resolution of these issues requires a deeper understanding of the chemical taphonomy of structurally coloured fossil cuticles and will illuminate the diagenetic transformation of cuticular lipids and arthropod cuticle as a whole.