Late Triassic fossil insects from the Laohugou Formation of northern China (Qinhuangdao, Hebei Province)

Triassic insect fossils from China are very limited. Here we report on numerous insect fossils discovered in the Upper Triassic Laohugou Formation at Heishanyao, Qinhuangdao, Hebei Province. These fossils are represented mainly by forewings of cockroaches and coleopteran elytra. The insect assemblage is most similar to that from the Upper Triassic Momonoki Formation of Japan. Fossils of the Laohugou Formation have been poorly known, so our discovery of new fossil insects bear significance for understanding the biota and sedimentary environment of this formation. The Mesozoic stratigraphic division in the Liujiang Basin has been controversial, and the usage of stratigraphic units has been inconsistent. Here we refine the stratigraphic sequence, from the bottom to the top, of the Upper Triassic Laohugou Formation, the Lower-Middle Jurassic Yaopo Formation, the Upper-Middle Jurassic Longmen Formation, the Upper Jurassic Tiaojishan Formation, and the Lower Cretaceous Zhangjiakou Formation. The Laohugou Formation is scattered in various basins in western Liaoning and northern Hebei, with the lower part mostly characterized by thick layers of complex conglomerates, suggesting a regional tectonic uplift. There is a sedimentary discontinuity between the Laohugou Formation and the Yaopo/Beipiao Formation, reflecting the uplifting activities during the late Late Triassic-early Early Jurassic in eastern China. The Laohugou Formation is overlaying on various ancient strata, representing the first regional unconformity of the northeastern margin of the North China Craton.