Mass mortality events of autochthonous faunas in a Lower Cretaceous Gondwanan Lagerstätte

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.

Figure 1

Locality map. Antônio Finelon Mine, Nova Olinda municipality, Ceará State, Brazil. Outcrops of the Crato Formation and of the Araripe Basin are also indicated.

Figure 2

Excavation profile of an outcrop of the Crato Formation. Antônio Finelon Mine, Nova Olinda municipality, Ceará State, Brazil. On the right, the section excavated of 3.10 m in depth, at level C6, evidencing the lithostratigraphic position of the fossil assemblage and levels with fossil accumulation.

Figure 3

Photograph of the controlled excavations at Antônio Finelon Mine. Nova Olinda municipality, Ceará State, Brazil.

Figure 4

Halite crystals. Halite crystals recovered from layer 288 cm. White arrows point to crystals. Scale bar 20 mm.

Figure 5

Preservation of larvae from layer 285 cm. Larvae of Protoligoneuria limai recovered from layer 285 cm, evidencing the excellent preservation of specimens. Scale bar 5 mm.

Figure 6

Gonorynchiform fish Dastilbe. (A) Dastilbe specimen recovered at level 205 cm. Scale bar: 25 mm; (B) One of the smallest Dastilbe specimens recovered at level 285 cm. Scale bar: 5 mm; (C) A layer with several Dastilbe specimens (inside the blue circles) with preferential orientation. The values written next to the fossils refer to the azimuth.

Figure 7

Brachyphyllum sp. and Pseudofrenelopsis sp. (A) Brachyphyllum sp. recovered from the controlled excavation; (B) Pseudofrenelopsis sp. recovered from the controlled excavation.