Deciphering the preservation of fossil insects: a case study from the Crato Member, Early Cretaceous of Brazil

Abstract

Exceptionally well-preserved three-dimensional insects with fine details and even labile tissues are ubiquitous in the Crato Member Konservat Lagerstätte (northeastern Brazil). Here we investigate the preservational pathways which yielded such specimens. We employed high resolution techniques (EDXRF, SR-SXS, SEM, EDS, micro Raman, and PIXE) to understand their fossilisation on mineralogical and geochemical grounds. Pseudomorphs of framboidal pyrite, the dominant fossil microfabric, display size variation when comparing cuticle with inner areas or soft tissues, which we interpret as the result of the balance between ion diffusion rates and nucleation rates of pyrite through the originally decaying carcasses. Furthermore, the mineral fabrics are associated with structures that can be the remains of extracellular polymeric substances (EPS). Geochemical data also point to a concentration of Fe, Zn, and Cu in the fossils in comparison to the embedding rock. Therefore, we consider that biofilms of sulphate reducing bacteria (SRB) had a central role in insect decay and mineralisation. Therefore, we shed light on exceptional preservation of fossils by pyritisation in a Cretaceous limestone lacustrine palaeoenvironment.

Throwing light on an uncommon preservation of Blattodea from the Crato Formation (Araripe Basin, Cretaceous), Brazil

“…Almost all of paleoentomofauna from Araripe Basin occurs within the laminated limestones of Crato Formation. The majority of the fossils have a brown tone due to iron oxides/hydroxides (hematite, goethite, limonite), which replaces the original organic matter, as observed by Menon & Martill (2007), Delgado et al (2014), Barling et al (2015) and Osés et al (2016). Wang et al (2012) also documented similar preservation features in Juhol biota insects, northeastern China.…”

“…The fossils are relatively abundant and diverse, including trace fossils, invertebrates, vertebrates, plants, and palynomorphs (Viana & Neumann, 2000). Due to the exceptionally preserved biota (Davis & Martill, 1999;Fielding et al, 2005;Heads et al, 2005;Menon & Martill, 2007;Silva et al, 2013;Delgado et al, 2014;Barling et al, 2015;Osés et al, 2016Osés et al, , 2017 the Crato Formation is considered a Konservat-Lagerstätte. The Crato Formation represents one of the largest Cretaceous insect deposits in the world, considering both taxonomic diversity and number of individuals (Grimaldi, 1990).…”

Cretaceous gnetalean yields first preserved plant gum

“…We can speculate that there are at least two factors involved. Firstly, the nature of the microbially-mediated taphonomy and diagenesis in the Crato palaeo-lake setting has been shown to be critical in the preservation of labile structures in animals from the Crato Formation Konservat-Lagerstätte [14][15][16][17] . Secondly, perhaps only in part, the coriaceous nature of the fossil leaves played a role.…”

The resin transfer technique: An application to insect fossils in laminated limestones of the Crato Formation (Lower Cretaceous) of north-east Brazil

“…The Crato Formation is a world famous Konservat-Lagerstätte that yields a diverse flora and fauna, and has been studied extensively (Martill et al, 2007 and the references therein). Importantly, these fossils are preserved primarily as goethite (FeO(OH)), apatite (Ca 5 (PO 4 ) 3 (OH,F,Cl)), and pyrite (FeS 2 ) replacements, whereas the surrounding matrix is 99% re-crystallised calcium carbonate (CaCO 3 ) of varying porosity (Barling et al, 2015;Osés et al, 2016). Rarely, galena, sphalerite and barite also occur as part of the diagenetic mineral assemblage (Martill et al, 2007).…”

“…However, many Crato Formation insect fossils preserve labile internal tissues (e.g. flight muscles) in apatite (Barling et al, 2015;Osés et al, 2016), which readily dissolves in hydrochloric acid. Consequently, the use of HCl may damage the fossils and it must be used with caution.…”