Age and depositional environment of the Xiaheyan insect fauna, embedded in marine black shales (Early Pennsylvanian, China)

Abstract

The Early Pennsylvanian Xiaheyan entomofauna (Ningxia, China) is among the earliest assemblages of winged insects known so far, and therefore provides an essential input on deciphering the early diversification of this group. Despite its evolutionary significance, both the age and depositional environment remained poorly constrained. Here, we present high-resolution documentation of litho- and biofacies, biostratigraphy and geochronology of the up to 100 m thick type section. Accordingly, the insect-bearing strata are of latest Bashkirian (latest Duckmantian) to middle Moscovian (Bolsovian) age. The sediments, lithostratigraphically assigned to the Yanghugou Fm., represent a regressional marine sequence formed at the southern margin of the Qilian Inland Sea. Intercalations of crevasse channels and splays that laterally interlock with marine black shales and bioclastic limestones point to deposition in an interdelta bay. Ground-touching waves provoked the erosion of muds at shallow depths leading to mass mortalities among mollusc communities due to rising anoxia, turbidity and water turbulence. In addition, bottom currents transported mud intraclasts towards the basin, where sinking insects that reached the bay via winds and surface currents were buried sub-contemporaneously with bivalves below the storm wave base. Observed differences in insect preservation and assemblage composition across the sequence are found to correlate with lithology and are best explained by distance from land, and the action of events inducing insect carcasses to sink, such as storms. The proposed taphonomic model represents a new fossilization pathway from land to sea and provides new directions for prospecting Paleozoic deposits in the search for insects.