Chemical and Physical Analysis of Melanin in Complex Biological Matrices

Preservation of soft tissue in the fossil record is rare because microbial decomposition occurs rapidly post-mortem. Tissues that do survive often polymerize into long chains of hydrocarbons. Biomolecules like melanin that are highly cross-linked exhibit the greatest resistance to alteration. Two forms of melanin are present in animals ¿ eumelanin and pheomelanin. Recent reports relied on morphological evidence to confirm the presence of melanin in the fossil record; this is insufficient because many microbodies and minerals adopt similar morphologies. Here, chemical signatures used to characterize melanin in modern organisms were used to study fossil pigmentation. Analysis of two Jurassic cephalopod ink sacs from southern England provided the first conclusive evidence for eumelanin in the fossil record. The fossil eumelanin was chemically and morphologically identical to that of modern ink. Comparison of fossil ink sacs from three deposits with different maturation histories showed that the chemistry of eumelanin alters at the onset of the oil window regardless of the age of the specimen. The decrease in melanin preservation was accompanied by an increase in kerogen. While characterizing the fossils, we found that the eumelanin preserves intact protein. Amino acid profiles of the fossil inks were determined with an amino acid analyzer and compared to modern ink.