Systematic and Ecological aspects of fossil mayflies from the Crato Formation, Araripe Basin (Lower Cretaceous, Northeast Brasil)

Abstract

Among the groups of insects, mayflies stand out in the Crato Formation, Araripe Basin. A newly discovered adult individual from the family Oligoneuriidae is here described as new species, genus and new subfamily. Also, a previously described “oligoneuriid” specimen from the same unit is here reviewed, redescribed and considered as belonging to the family Hexagenitidae. The new genus bridges an important gap between an oligoneuriid ancestor and the extant Chromarcys. Furthermore, demonstrate that the divergence between Oligoneuriidae clades probably occurred in South America. Additionally, we described recently collected fossil specimens of extinct family Hexagenitidae. Two new genera and species are proposed, and one specimen is described in detail and identified as Protoligoneuria limai, providing new information on intraspecific variability and allowed us to emend the taxon’s diagnosis. Another specimen described is considered incertae sedis, and presents an interesting taxonomic challenge, as it presents plesiomorphic features on the forewings. Moreover, we comment on the taxonomy of Cratohexagenites minor and consider it possibly synonymous with Protoligoneuria limai. As a supplementary material, we also present a checklist of all valid mayfly species from the Cretaceous, providing new morphological information about the Crato Formation mayfly fauna, specially the hexagenitids. Finally, we analyze a mayfly mass mortality collected at controlled excavations, the first of their kind for the Crato Formation. All mayflies found belong to the Hexagenitidae, whose morphology demonstrates that the nymphs occurred in quiet waters. The nymphs were smaller than usual, suggesting a shallow water column, with excellent preservation and no preferential orientation in the samples, suggesting lack of transport. Thus, we conclude that the Crato Formation lake experienced a seasonal high evaporation caused by the hot climate tending to aridity.