Exceptional preservation in mayfly nymphs (Insecta: Ephemeroptera) from the Early Cretaceous of the Las Hoyas fossil site

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.