Synchrotron x‐ray fluorescence analysis reveals diagenetic alteration of fossil melanosome trace metal chemistry

Abstract

A key feature of the pigment melanin is its high binding affinity for trace metal ions. In modern vertebrates trace metals associated with melanosomes, melanin‐rich organelles, can show tissue‐specific and taxon‐specific distribution patterns. Such signals preserve in fossil melanosomes, informing on the anatomy and phylogenetic affinities of fossil vertebrates. Fossil and modern melanosomes, however, often differ in trace metal chemistry; in particular, melanosomes from fossil vertebrate eyes are depleted in Zn and enriched in Cu relative to their extant counterparts. Whether these chemical differences are biological or taphonomic in origin is unknown, limiting our ability to use melanosome trace metal chemistry to test palaeobiological hypotheses. Here, we use maturation experiments on eye melanosomes from extant vertebrates and synchrotron rapid scan‐x‐ray fluorescence analysis to show that thermal maturation can dramatically alter melanosome trace element chemistry. In particular, maturation of melanosomes in Cu‐rich solutions results in significant depletion of Zn, probably due to low pH and competition effects with Cu. These results confirm fossil melanosome chemistry is susceptible to alteration due to variations in local chemical conditions during diagenesis. Maturation experiments can provide essential data on melanosome chemical taphonomy required for accurate interpretations of preserved chemical signatures in fossils.

Synchrotron-X-ray fluorescence provides elemental signatures for diverse coloration mechanisms in fossil insects

“…Certain elements in fossil insects may be derived, at least in part, from diagenetic processes, but may nonetheless be considered biomarkers for certain pigments where such elements preferentially associate with the pigment molecule during diagenesis. For instance, Cu preferentially associates with melanin-rich cuticle during diagenesis 44,45 and S is known to associate with melanin during thermal maturation 46 ), Based on the distribution of certain elements in extant taxa, particularly Cu, Zn and Ca, the data reported in this study suggests an originally melanin-rich composition for dark-toned cuticle regions of fossil Coleoptera, Hemiptera and Hymenoptera. In contrast, enrichment of light-toned cuticle regions in fossils in S, Ti and Fe in fossil Coleoptera, Hemiptera and Neuroptera and K and Ca in fossil Hymenoptera suggest a stronger sedimentary signal or an original composition in non-melanin pigments, such as carotenoids and flavonoids.…”

Maturation experiments reveal bias in the chemistry of fossil melanosomes

“…Skin melanosomes are enriched in Ca and Zn; liver melanosomes are enriched in Fe and Cu (Rossi et al, 2019). Experiments (Ito et al, 2013) on Sepia (cuttlefish) melanin reveal that maturation alters eumelanin and its associated metal inventory; experiments on vertebrate eye melanosomes reveal diagenetic alteration of Cu:Zn ratios (Rogers et al, 2020). No comparable studies exist for other vertebrate tissues.…”

Forged soft tissues revealed in the oldest fossil reptile from the early Permian of the Alps

“…Organically preserved soft tissues can reveal biological information about ancient animals (Vinther 2015;Gabbott et al 2016;McNamara et al 2016a;Spindler et al 2018;Manning et al 2019;Rossi et al 2022;Slater et al 2023), including phylogenetic affinities (Clements et al 2016;Miyashita et al 2019;Rogers et al 2019), and provide important insights into the taphonomic processes that occurred during fossilization. They are thus important to better constrain the diagenetic history of a fossil (Ma et al 2015;McNamara et al 2016b;Rossi et al 2020;Rogers et al 2021).…”