Darriwilian faunas and biofacies from the Stairway Sandstone, Amadeus Basin, central Australia

During the Mid Ordovician (Darriwilian) marine sand- and siltstones were deposited in an epicontinental sea penetrating profoundly into the Australian continent. The Middle Darriwilian Stairway Sandstone has, for the first time, been sampled stratigraphically to track marine benthic biodiversity in this shallow water regime in Eastern Gondwana. The fauna is dominated by richly diverse bivalves, with the remaining fauna comprising trilobites, brachiopods, rostroconchs, gastropods, cephalopods, monoplacophorans and rare bryozoans and possible sponges. Trace fossils are abundant and have recently been documented. Bivalves dominate regardless of subfacies type, varying from siltstone, calc-arenite, quartzite to dolomitic calc-sandstone. Brachiopods and trilobites, represented in great numbers in the remaining part of the fauna, typically occur in the silt dominated intervals. Filter feeders including bivalves, brachiopods and rostroconchs constitute a large proportion of the fauna. The Stairway Sandstone was deposited in near shore environment, with relatively high energy conditions. The filter feeders have adapted to these relatively hostile palaeoenvironmental conditions, whereas many other filter feeder communities tend to prefer deeper- and calmer water conditions. The macrofossils from the Stairway Sandstone display a high degree of endemism at species level (at least 75%). The rare species, in common with faunas from other nearby basins, are generally cosmopolitan taxa. New species of trilobites and brachiopods have now been described from the material.