Taphonomy, deposition and pedogenesis in the Upper Triassic dinosaur beds of Trossingen

Abstract

The Upper Norian (Late Triassic) red-beds of the Knollenmergel (Trossingen-Formation) are famous for their richness in prosauropod dinosaurs. Among the most productive localities, the Obere Muhle (Rutschete) at Trossingen (SW Germany) stands out for the large number of skeletons found in the early 20th century and the wide stratigraphic range of finds within a 12-m sequence of monotonous floodplain mudstones. New excavations at the original site have been carried out since 2007, aimed at securing taphonomic, sedimentological and palaeopedological data on the formation of the deposit. Based on the findings of the present and earlier excavations, the following conclusions can be drawn. (1) The Obere Muhle yields at least two different types of vertebrate lagerstaetten: (1) miring deposits, preserving numerous articulated skeletons in “life-like” postures and all stages of subsequent decomposition and (2) more regular sheet flood deposits, producing only a few skeletons (e.g. turtles), which were disarticulated into body parts and single elements, or in bonebeds. (2) The deposits were laid down in numerous successive cycles of sedimentation, interrupted by pedogenic phases. A broad range of pedologic features is recognised, leading to the interpretation of the deposits as paleo-vertisols and aridisols. This indicates a long depositional history for the whole section, interrupted by arid phases in which few vertebrates were preserved and life was probably less rich. (3) The occurrence of vertebrate skeletons correlates with more humid periods and traces of intense vegetation. (4) Weathering played a significant role in some beds of the Trossingen section. The absence of smaller tetrapods could be caused by intense weathering.