A geochemical model for the preservation of insects in the Crato Formation (Lower Cretaceous) of Brazil

The Crato Formation Lagerstätte of NE Brazil is well known for the exceptional preservation of its fossil assemblage. A wide variety of plants and animals are preserved with a high degree of fidelity, revealing structures of labile tissues at the micron scale, or even smaller. Chemical analyses and descriptions of replacement fabrics observed in insect fossils from the formation are here re-evaluated. Two mineral phases with three distinct fabrics are identified replacing soft tissues: 1) nano-crystalline high-fidelity pyrite replacements of insect epicuticle (pseudomorphed in goethite); 2) pyrite framboids and pseudoframboids obliterating original insect morphology (pseudomorphed in goethite); 3) globular replacements and encrustations of internal labile tissues in calcium phosphate. A revised taphonomic and geochemical model is presented that accounts for both the high fidelity of preservation and the micro-fabrics observed. In addition, we consider aspects of the depositional environment, including substrate consistency, water depth and light penetration. We conclude that the water depth was likely in excess of 50 m, but within the photic zone, and the sediment was bound by stratified microbial communities.