Published project results to date can be considered under the themes of two major classes of biomolecules: melanins and keratins. More specifically, our research on melanins has used two primary approaches, i.e. the analysis of fossils and the use of taphonomic experiments to inform the interpretation of fossil data. Our experimental approaches have yielded important new insights into chemical signatures for ancient melanins. One major study focussed on the detection of melanin using AHPO-HPLC, widely considered a diagnostic technique for melanin. We performed taphonomic experiments on feathers at elevated temperatures in order to investigate how the molecular signature of melanin is altered during thermal maturation, a common process during fossil burial. Our results showed progressive alteration of feather melanins with increasing thermal maturation, and allowed us to identify diagnostic signatures for thermally altered eumelanin and phaeomelanin, via modified absolute and relative amounts of melanin monomers. Our data also confirmed the potential for non-melanin compounds such as proteins to generate the same monomers, albeit in different ratios to the monomers in thermally altered melanins. Application of these results to fossils allowed us to determine that signatures for eumelanins in Cretaceous and Miocene fossils were authentic, but that authentic signatures for phaeomelanins survive only in the Miocene fossils analysed and not in the much older Cretaceous fossils. These data suggest a stronger temporal control on the preservation of phaeomelanin in the fossil record, but these results require testing with a larger dataset. Our data on fossil phaeomelanin represent the first evidence of molecular preservation of phaeomelanin in the fossil record.
The second major finding from our taphonomic experimental programme relates to the applicability of Raman spectroscopy to the identification of chemical signatures for melanin in thermally altered fossils. Thermally altered geomaterials are generally considered uninformative as the Raman spectrum shows only two broad bands – the D- and G-bands. Our experiments on melanotic and non-melanotic tissues and cellular materials from a broad range of bacterial, algal, fungal, higher plant, arthropod and vertebrate species revealed that the Raman spectrum of all of these biomaterials retains a diagnostic signature despite thermal maturation. These signatures are evident following analysis of Raman spectra using peak deconvolution and statistical analysis of extracted peak parameters. This approach clearly discriminates melanised from non-melanised biomaterials and preliminary data suggest that the approach can discriminate between melanins from different source taxa, i.e. that thermally matured melanins retain a phylogenetic fingerprint. This approach is an exciting new method to identify evidence of melanin in older, thermally matured, fossils.
Our melanin research also includes novel data on other fossils. We identified skin patterning in a fossil moonfish from the Eocene of Bolca; the style of patterning – a series of spots – differs from the stripes of moonfish today and indicates a shift in moonfish ecology since the Eocene.
Another major fossil discovery relates to both melanin and feathers. We discovered branched feathers in a pterosaur from the Cretaceous of Brazil, suggesting that feathers evolved in the common avemetatarsalian ancestor of dinosaurs and pterosaurs in the Early Triassic. Moreover, our analyses of the pterosaur feathers revealed the partitioning of melanosomes with different geometries in feathers of different types. This demonstrates that the pterosaur feathers were being used to generate colour patterns, in turn suggesting that coloration may have been an early functional adaptation of feathers.
Finally, our experimental programme has yielded a major advance in our understanding of the fossil record of keratin proteins. Our taphonomic experiments on feathers, and analysis of the altered feathers using FTIR and XANES, showed that during thermal maturation, feather beta corneous proteins transform to alpha helix structures. This explains the alpha helix signal in some fossil feathers as a simple taphonomic artefact. The experimental data also validate our new chemical data from fossil feathers, which demonstrates that even feathers from the Cretaceous can retain secondary and tertiary protein structure, supporting previous claims of protein preservation in Mesozoic fossils.
