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.
Highlights
Preserved biotas have been recorded since the Precambrian (e.g., Chen et al, 2014)
Whilst previous studies have considered the preservation of these organisms (Heimhofer & Martill, 2007; Menon & Martill, 2007; Delgado et al, 2014; Barling et al, 2015), microtextural and geochemical analyses have not been performed, nor has a detailed taphonomic model been proposed Based on imaging, geochemical, and mineralogical analyses, this paper presents data that supports the central role of microorganisms in the fossilisation of the Crato Member insects
scanning electron microscopy (SEM) analysis revealed that fossil exoskeletons (Fig. 2) are preserved by sub-spherical to spherical closely-packed grains, with diameters mainly in the range of 5–10 μm (Fig. 3A), which are formed by anhedral to euhedral nanocrystals (Figs. 3B–3D)
Summary
Preserved biotas have been recorded since the Precambrian (e.g., Chen et al, 2014). The high preservational fidelity of insects from the Crato Member (Santana Formation, northeastern Brazil) defines it as a taphonomic window for an Early Cretaceous ecosystem (Soares et al, 2013) Due to this kind of unique record, we know that the evolutionary history of the insects was characterised by major radiation and extinction events in the Cretaceous (Nicholson, Mayhew & Ross, 2015), when the diversification of social insects (Jarzembowski & Ross, 1996; Engel, Grimaldi & Krishna, 2007) and the radiation of flowering plants (Lidgard & Crane, 1988) took place. The latter has impacted insect evolution thereafter (Jarzembowski & Ross, 1996; Labandeira, 2014)
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