Bone infills reveal the origin of the Middle Triassic Miedary vertebrate assemblage

Taphonomic data is an important source of information related to fossil assemblages. Fossilised bones, characteristics of their preservation, and their sedimentary infill can serve as a basis for unravelling the origin of fossil bonebeds. Assessing the taphonomic data thereby allows for deciphering the genesis of multitaxic mass accumulations of elements of organisms from different habitats. Here we provide scenarios explaining the origin of the Middle Triassic Miedary vertebrate assemblage from Poland. In this locality remains of both freshwater and marine animals are found, but their sympatric co-occurrence in life could not be previously confirmed. We used a suite of analytical methods on a broad sample of thin sections made from bones of reptiles, amphibians and fishes, to assess the diversity of their taphonomic characteristics. This allowed us to propose a set of taphonomic trajectories that resulted in specific preservation characteristics, which can be extrapolated for other sites. The remains of the long-necked archosauromorph Tanystropheus do not differ in their preservation from those of aquatic taxa, supporting its interpretation as a water-dwelling animal. Our results demonstrate that in the Middle Triassic some environments combined faunistic elements from principally different habitats, underlining the complexity of these ecosystems.