Bacteria or melanosomes? A geochemical analysis of micro-bodies on a tadpole from the Oligocene Enspel Formation of Germany

Abstract

Many exceptionally preserved fossils have long been thought the product of preservation by bacterial autolithification, based largely upon the presence of, micronsized, spherical or elongate bodies on their surface. This has recently been challenged by studies of similar fossils which cite morphological and geochemical evidence that these structures could be fossilized melanosomes, melanin-containing organelles. We geochemically analysed a tadpole from the Oligocene Enspel Formation, Germany, which displays such spherical bodies on its surface. Pyrolysis gas chromatography mass spectroscopy (Py-GCMS) and Fourier transform infrared spectrometry (FTIR) indicate that the organic remains of the tadpole are original and are not the result of external contamination, shown by the different chemical compositions of the fossil and its enclosing matrix. Py-GCMS also demonstrates the presence of bacterial and plant biomarkers in the matrix but not the tadpole, suggesting that the spherical bodies are unlikely to be bacterial, and also that such fossils do not develop their dark colour from incorporating plant material, as has been suggested. X-ray absorption spectroscopy (XAS) shows high levels of organically bound Zn(II) in the fossilized soft tissue, a metal known to chelate both eu-and pheomelanin.The zinc in the tadpole shows greater similarity to that bound in pheomelanized extant samples than to that in eumelanized ones. Though further geochemical analysis of both pure pheomelanin and bacterial samples is required to completely exclude a bacterial origin, these results are in line with a pheomelanic origin for the spherical bodies on the tadpole.

Chemical, experimental, and morphological evidence for diagenetically altered melanin in exceptionally preserved fossils

“…The fossil Carboniferous fish shows a similar trend in our PCA plot, also suggesting a mixed melanin composition. Another study used pyrolysis GC-MS to study a fossil tadpole from Enspel (38) and could not conclusively present evidence for melanin due to lack of comparisons to extant, fossil, or artificially matured melanin samples and to previous studies on fossil eumelanin using the same experimentation (25,27). However, we show here that, in our comparative framework, the Enspel melanosomes do retain melanin residues, contrary to earlier studies suggesting a microbial origin (39).…”

“…It could be surmised that these reactions mainly affect marine fossils due to the higher sulfate content of marine settings. We note, however, that feathers and hair from the freshwater locality, Messel oil shale, are locally enriched in pyrite framboids, which suggests availability of reactive hydrogen sulfide, as confirmed by other studies (38). A more pressing concern is that the data from the TOF-SIMS analyses used in melanin characterization (7,8) are based on assemblages of fragment ions from larger molecules.…”

Fossilization of melanosomes via sulfurization

“…) but can be most plausibly interpreted as fossil melanosomes based on details of their arrangement, location, precise context within soft tissues, and in particular their geochemistry (for further discussion see Barden et al . ; Moyer et al . ).…”

“…Indeed, such aliphatic compounds are present in melanosome‐bearing soft tissues from various fossils (Barden et al . , ; Glass et al . ), suggesting that polymerization processes are also involved in the preservation of melanin and melanosomes.…”

“…) or are associated with elevated levels of sulfur (Barden et al . , ; Lindgren et al . ; Pinheiro et al .…”