Chemical and mineral comparison of fossil insect cuticles from Crato Konservat Lagerstätte, Lower Cretaceous of Brazil

Abstract

The Crato Formation palaeoentomofauna from the Lower Cretaceous (Aptian) of northeast Brazil is extremely well preserved. Crato insects are often complete with abdomen, thorax, head, legs, wings articulated and fragile cuticle details observed at the macro and micro scale. The Crato Formation stands out for the high diversity of fossil insects with at least 386 described species, so far. We investigate the preservation pathways through SEM–EDS and Raman spectroscopy, which give fundamental insights regarding the understanding of this complex theme. Our study compared cuticle soft-tissue preservation of Ensifera in different layers of the Crato limestone. The results of our analyses confirmed that the anaerobic bacterial respiration processes influenced the labile-tissues preservation. Ensifera fossils display preservational stages ranging from kerogenization to pyritization. Kerogenization represents the partial or complete chemical transformation of organic material into aliphatic and cyclic hydrocarbons in situ during mesodiagenesis, while pyritization occurs during the decaying of carcass in the early diagenesis. Here, we followed the previous hypothesis of fish tissue preservation that these processes are governed principally by variations in the positioning and residence time of the carcasses in different microbial zones within the sediment column. Besides early/mesodiagenetic modifications, oxidation processes during recent weathering led to some mineral transformations that played a key role in the preservation of Ensifera.