A taphonomic model for the Mesosauride assemblage of the Irati Formation (Paraná Basin, Brazil)

The Irati Formation (Permian, Paraná Basin, Brazil) is made up of a succession of black bituminous and nonbituminous shales and mudstones with interbedded carbonate layers, which record deposition in marine environments. Mesosaur remains are abundant at several sites in this formation, where they occur preserved in centimetre- thick beds and displaying various degrees of skeletal disarticulation. This paper seeks to establish a model to explain the processes that generated the mesosaurian taphocoenosis in the Irati Formation. Three types o successions containing mesosaur remains were analysed: 1) Carbonate turbidite facies in the State of São Paulo; 2) Carbonate tempestite facies in Rio Grande do Sul; 3) Black shale facies in the State of Paraná. Three taphonomic classes are established: Class I) articulated skeletons; Class II) partially articulated skeletons; Class III) isolated bones (including Class IIIA with complete isolated bones and Class IIIB with fragmented isolated bones). Based upon the sedimentological and taphonomic evidence, a taphofacies model is proposed. The hypothesis here advanced is that all accumulations of mesosaur remains resulted from catastrophic mortality caused by storm events. The basic assumption is that these reptiles died and were buried through the action of successive storms that occurred in the Irati Sea. The taphonomic model points to repeated episodic events of burial, erosion, reworking and redeposition of skeletons and suggests significant time averaging.