Data from: Fossilization of melanosomes via sulfurization

Fossil melanin granules (melanosomes) are an important resource for
inferring the evolutionary history of colour and its functions in animals.
The taphonomy of melanin and melanosomes, however, is incompletely
understood. In particular, the chemical processes responsible for
melanosome preservation have not been investigated. As a result, the
origins of sulfur-bearing compounds in fossil melanosomes are difficult to
resolve. This has implications for interpretations of original colour in
fossils based on potential sulfur-rich phaeomelanosomes. Here we use
pyrolysis gas chromatography mass spectrometry (Py-GCMS), fourier
transform infrared spectroscopy (FTIR) and time of flight secondary ion
mass spectrometry (ToF-SIMS) to assess the mode of preservation of fossil
microstructures, confirmed as melanosomes based on the presence of
melanin, preserved in frogs from the Late Miocene Libros biota (NE Spain).
Our results reveal a high abundance of organosulfur compounds and
non-sulfurized fatty acid methyl esters in both the fossil tissues and
host sediment; chemical signatures in the fossil tissues are inconsistent
with preservation of phaeomelanin. Our results reflect preservation via
the diagenetic incorporation of sulfur, i.e. sulfurization (natural
vulcanization), and other polymerization processes. Organosulfur compounds
and/or elevated concentrations of sulfur have been reported from
melanosomes preserved in various invertebrate and vertebrate fossils and
depositional settings, suggesting that preservation through sulfurization
is likely to be widespread. Future studies of sulfur-rich fossil
melanosomes require that the geochemistry of the host sediment is tested
for evidence of sulfurization in order to constrain interpretations of
potential phaeomelanosomes and thus of original integumentary colour in
fossils.