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

• Published in: Palaeobiodiversity and palaeoenvironments 2015-03, Vol.95 (1), p.33-45
• Main Authors: Barden, Holly E., Bergmann, Uwe, Edwards, Nicholas P., Egerton, Victoria M., Manning, Phillip L., Perry, Sarah, van Veelen, Arjen, Wogelius, Roy A., van Dongen, Bart E.
• Format: Article
• Language: English
• Subjects: Animal Systematics/Taxonomy/Biogeography; Biodiversity; Earth and Environmental Science; Earth Sciences; Freshwater & Marine Ecology; Original Paper; Paleontology; Plant Systematics/Taxonomy/Biogeography
• ISSN: 1867-1594; 1867-1608
• DOI: 10.1007/s12549-014-0177-5

Many exceptionally preserved fossils have long been thought the product of preservation by bacterial autolithification, based largely upon the presence of, micron-sized, 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.