Fossil fish provide evidence of geomelanin preservation with implications on the visual accuracy of an extinct fish species

This is the authors' abstract. We don't add key points for this paper.

Eumelanin is a ubiquitous type of pigment, standing present in all major life branches. Chemically, it consists of 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) units bonded with varied functional groups. This biochrome is involved in many different roles, such as free radical scavenging, microbial inhibition, etc. Eumelanin is produced by organelles called melanosomes, which are found throughout the animals’ body. In the fish’s eyes, this pigment mainly plays a protective role against UV radiation damage and waterborne insults. The previous detection of melanosomes in the eyes of fossil teleosts already provided evidence for the palaeobiology and palaeoecology of extinct fish lineages. Nonetheless, the presence of these organelles remains to be detected in exceptionally preserved fossils from Brazil. Here, we report the microscopic and chemical investigation of fossil melanin from the eyes of the Cretaceous fish Dastilbe crandalli. Results show that the eye has a circular shape with non-recalcitrant dark brown tissues at its rims, exhibiting densely packed, solid, subspherical micrometric granules rich in carbon and with vibrational spectra of eumelanin. Geothermic calculations of the Raman spectra indicate that melanin is not much thermally altered. This result is consistent with other proposals for the maximum temperature for this unit, raising the possibility of its use to estimate the thermal alteration of geomelanins. Besides that, these results also indicate that Dastilbe fish possibly had a limited visual capability or lived in the shallow but shadowed (by aquatic plants) portions of the palaeolake.