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PeerJ. 2016 Dec 21;4:e2756. doi: 10.7717/peerj.2756. eCollection 2016.

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

Author information

1
Programa de Pós-graduação em Geoquímica e Geotectônica, Institute of Geosciences, Universidade de São Paulo , São Paulo , Brazil.
2
Institute of Geosciences, Universidade de São Paulo , São Paulo , Brazil.
3
Programa de Pós-Graduação em Ecologia e Recursos Naturais, Universidade Federal de São Carlos , São Carlos , São Paulo , Brazil.
4
Institute of Geosciences, Universidade Federal do Pará , Belém , Brazil.
5
Institute of Physics, Universidade de São Paulo , São Paulo , Brazil.
6
Department of Fundamental Chemistry/Institute of Chemistry, Universidade de São Paulo , São Paulo , Brazil.
7
Brazilian Synchrotron Light Laboratory , Campinas , Brazil.
8
Department of Physics, Centro Universitário FEI , São Bernardo do Campo , Brazil.
9
Laboratory of Technological Plasmas, Universidade Estadual Paulista , Sorocaba , Brazil.
10
Department of Biology, Universidade Federal de São Carlos , Sorocaba , 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.

KEYWORDS:

Araripe Basin; Crato Member; Early Cretaceous; Exceptional preservation; Insects; Paleometry; Pyrite; Taphonomy

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