Mass mortality events of autochthonous faunas in a Lower Cretaceous Gondwanan Lagerst\xe4tte

Arianny P Storari,Taissa Rodrigues,Renan A M Bantim,Antonio A F Saraiva,Flaviana J Lima

doi:10.1038/s41598-021-85953-5

Arianny P Storari, Taissa Rodrigues + Show 3 more

Open Access

https://doi.org/10.1038/s41598-021-85953-5

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Abstract

Mass mortality events are unusual in the Crato Formation. Although mayflies’ accumulations have been previously reported from that unit, they lacked crucial stratigraphic data. Here we provide the first taphonomic analysis of a mayfly mass mortality event, from a layer 285 cm from the top of the Formation, with 40 larvae, and an overview of the general biological community structure of a three meters deep excavated profile. The only other autochthonous taxon observed in the mayfly mortality layer was the gonorynchiform fish Dastilbe. The larvae and fishes were smaller than usual in the layer 285 cm, suggesting that they lived in a shallow water column. Their excellent preservation and a lack of preferential orientation in the samples suggest an absence of significant transport. All mayflies belong to the Hexagenitidae, whose larvae lived in quiet waters. We also recovered allochthonous taxa in that layer indicative of drier weather conditions. Adjacent layers presented crystals and pseudomorphs of halite, suggesting drought and high salinity. In other layers, Dastilbe juveniles were often found in mass mortality events, associated with a richer biota. Our findings support the hypothesis that the Crato Formation’s palaeolake probably experienced seasonal high evaporation, caused by the hot climate tending to aridity, affecting the few autochthonous fauna that managed to live in this setting.

Highlights

The Crato Formation is a lithostratigraphic unit well-known by its fossiliferous laminated limestones
Menon and M artill[8] alleged there was no clear evidence for mass mortality events in the Crato Formation, Martins-Neto and Gallego[15] stated that many of the taxa that depended on freshwater to live and/or reproduce probably suffered mass mortality events, that could have been caused by a periodic increase of H 2S16
A layer collected at 285 cm from the top of the Formation, belonging to top-level carbonate C6 sensu Neumann and Cabrera[19] and composed of yellowish limestone, presented evidence of at least one mass mortality event, with 40 mayflies’ larvae recovered over 5.0 × 2.0 m2

Summary

Introduction

The Crato Formation (northeastern Brazil) is a lithostratigraphic unit well-known by its fossiliferous laminated limestones. It represents a paleoenvironment composed of a lacustrine complex approximately 100 km × 50 km in total area 1,2, with freshwater constituting the superficial and marginal portions of the lakes[1,2,3]. The aquatic insect fossils (e.g. Ephemeroptera) of this deposit represent both autochthonous and allochthonous taxa, as some may have been transported from the lotic to the lentic regions[8]. Mass mortality events are unusual in the Crato Formation, but assemblages of mayflies’ larvae found in its yellowish limestones have been previously reported as representing such episodes[9,13]. We provide the first taphonomic analysis of specimens collected from controlled excavations, the first of their kind for that unit

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