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The present study focuses on a group of macroinvertebrates that have played an important role in past and present lentic ecosystems: the Ephemeroptera. The study analyses the diversity, relative abundance, taphonomic alterations and fossilisation patterns of the Las Hoyas nymphs. The objective is to investigate the factors that may contribute to the presence of soft tissues in fossils with poor corporeal preservation. In accordance with this, only three morphotypes of the Leptophlebiidae family and one of the Euthyplociidae family have been characterised. However, the ultrastructural details of digestive and respiratory tissues have been documented, including the digestive canal, the shape of the gills, tracheoles, chloride cells, and importantly, in situ bacteria in cytokinetic stages located at the hindgut. The nymphs were fossilised in a variety of ways, including carbonaceous films, mineralised casts, and impressions. The nymphs that have been fossilised as composite moulds (Type III) and carbon films (Type II) tend to be larger in size. However, their bodies are rather well-preserved and show less soft tissue remains than those of the nymphs that have been preserved as impressions (Type I) and mineralised phosphorus casts (Type IV). These latter nymphs are smaller in size, and their bodies are poorly preserved. However, they show a greater variety of soft tissue details. This bias suggests an interesting link to environmental factors differentiating watered and flooding from microbial mat binding conditions. • Mayflies preserve a variety of soft tissues in Cretaceous limestones. • Nymphs contain bacteria fossilized in situ in the hindgut. • Endogenous bacteria induced phosphorus precipitation in inner tissues. [ABSTRACT FROM AUTHOR]

The insect fossils recorded in the lithographic limestones of the Crato Formation (Aptian of the Araripe Basin, Brazil) have a high degree of morphological fidelity of external and internal anatomical features, including cuticular remains, muscles and organs. The main factor for this exquisite preservation is considered to be the influence of microbial mats in the fossilization process. Petrographic and scanning electron microscope analysis with coupled x-ray spectroscopy (SEM/EDS) showed direct and indirect microbial features associated with the microfabric of the orthopteran insect fossils. We propose a four-steps model for insect fossil preservation, with the capture, protection, creation of the microbial sarcophagus, and mineralization of the organic remains mediated by microbial mats. The confluence of morphological and taphonomic data indicates possible climatic variations on the Aptian of the Araripe Basin, with the exquisitely preserved insect fossils associated with favorable environmental conditions for the installation of microbial mats on the lacustrine substrate. • The insect fossils from the Crato Lagerstätte are associated with direct and indirect microbial features. • We propose a four-steps preservational model mediated by microbial mats. • Insect morphological and taphonomic data can provide climatic inferences during the Aptian of the Araripe Basin. [ABSTRACT FROM AUTHOR]

The paleoentomofauna recorded in the Crato Formation laminated limestones (Araripe Basin, Aptian) represents one of the most important Konservat-Lagerstätten deposits globally documented. Among the insects, the high morphological fidelity, abundance and diversity of the Grylloidea superfamily allows a more refined study of the external and internal cuticular morpho-anatomical features and preserved soft tissues. Scanning electron microscope analysis with coupled energy dispersion x-ray spectroscopy (SEM/EDS) show that the carcasses of the grylloids are predominantly replaced by three distinct mineral phases with occurrence of features hardly preserved in the fossil record such as the ommatidia's polygonal facets of compound eyes, cuticular layers differentiation (epi and procuticle), internal anatomical microstructures of digestive tract, ovarian fragments, eggs, and visceral and locomotor muscle tissues. The identification of significant microbial evidence associated with the cricket's carcasses allow us to interpret that the main factor responsible for the exceptional cricket's preservation is the influence of microbial mats in the fossilization process. Image 1 • We analyzed 178 specimens of fossil crickets from the Lagerstätten Crato Formation (Aptian), Araripe Basin, Brazil. • Morpho-anatomical features of the compound eyes, cuticle, digestive tract, muscles, ovaries and eggs were preserved. • The carcasses are predominantly replaced by three distinct mineral phases. • The main factor responsible for the exceptional preservation of these insects is the influence of microbial mats. [ABSTRACT FROM AUTHOR]