A unique notostracan trace fossil assemblage from the Upper Triassic Chinle Formation (northeastern Utah, USA) and its paleoecological and paleoenvironmental implications

The Upper Triassic (Norian) Chinle Formation of the southwest USA is world-renowned for its diverse fossil assemblages. Here we describe a new trace fossil assemblage from the formation in Dinosaur National Monument (northeastern Utah), and discuss its paleoenvironmental implications. The trace fossils comprise freshwater bilobate resting and feeding traces (Isopodichnus-Rusophycus-morphotype and Isopodichnus-Cruziana-morphotype respectively) as well as walking traces (Acripes), probably of notostracan crustaceans, in a previously unknown subparallel alignment. Extant notostracans are known to live in ephemeral bodies of freshwater with a low water level and temporary desiccation. However, associated sedimentological features such as prod marks and groove casts indicate that the traces were produced under the influence of a low velocity, unidirectional current which is typical for sheet-flows - an interpretation independently corroborated by further sedimentological data. Thus, the trace fossil assemblage supports the presence of rheotaxis in notostracans, a thus far little-known behavior in these animals. Based on the inferred ecology of the trace maker, the sedimentological features associated with the traces, and the sedimentology of the surrounding rocks, the traces were most likely formed in an ephemeral part of a fluvial system dominated by sheet-flow events. This interpretation supports the results of previous studies suggesting that the upper Chinle Formation at Dinosaur National Monument area is characterized by ephemeral low flow regime sheet-flows deposited on mudflats.