Taphonomy of the pygocephalomorphan crustaceans of the Permian, Irati Formation, Paraná Basin, Brazil

Abstract

The Permian Irati Formation, Paraná Basin, Brazil, includes a succession of black shales and mudstones with interbedded carbonate layers, generated under marine conditions. Worldwide known by its famous mesosaur fossillagerstätten, this unit also harbors bedding planes where pygocephalomorphan (Liocaris, Paulocaris and Pygaspis) remains are also abundant. New field observations and a review of literature data indicate that at least, three main types of preservation are shown: 1) isolated, disperse, articulate and/or partially articulated or disarticulated, very compressed skeletons in black shales. The articulated specimens are autochthonous elements; their preservation may have been favoured by quite waters and oxygen deficient conditions during shale deposition; 2) dense accumulations of remains, as micro-coquines on the top of a 3 meters thick dolomitic bank or interbeded within a succession of black shales above the dolomitic limestones. These are made of 50cm thick concentrations of millimetric layers of tiny fragments of crustacean shells, mainly parts of carapaces and abdominal somites. These fragments are normally preserved as concave-up, nested and stacked remains, with no clear signs of abrasion. The high degree of fragmentation point out to background episodes of high energy, but with limited exposure in the sediment water interface. The final deposition seems to be tied to event episodes (storms); 3) thin pavements of disarticulated, but non fragmented carapaces, in a concave-down posture. The carapaces have bimodal orientation, and size frequency histograms are polymodal. These observations indicate that the remains were reoriented by bidirectional tractional currents, prior to the final burial, but without sorting or winnowing. Since some pavements are lying directly above the microcoquines, these are amalgamated, sedimentologic concentrations. Finally, the dense accumulations of crustaceans in some bedding planes may suggest episodes of seasonal mass mortality, which the causes are still speculative.