New Discovery of Bradoriids from the Lower Cambrian Shuijingtuo Formation in the Eastern Three Gorges Area, South China

The early Cambrian complex ecosystem was dominated by lophotrochozoans and ecdysozoans, with arthropods constituting the most prominent animal group during the Cambrian Explosion. Bradoriids, an extinct lineage of small bivalved arthropods, first appeared slightly earlier than the oldest known trilobites, in the early Cambrian Abadiella Trilobite Biozone (Series 2, Stage 3) of both South China and South Australia. They subsequently achieved a global distribution and reached particularly high diversity and abundance in the early Cambrian of South China. Here we systematically describe two bradoriid species from the upper Shuijingtuo Formation in western Hubei: Houlongdongella disulcata Lee, 1975 and Houlongdongella elevata Cui, 1989. This study represents the first record of both species in the eastern Three Gorges Area, Hubei Province, thereby extending their known geographic ranges. Biostratigraphic correlation based on the occurrence of H. disulcata and H. elevata, alongside with previously reported trilobites and small shelly fossils, indicates that the middle–upper Shuijingtuo Formation in western Hubei corresponds to the late Chiungchussuan to Tsanglangpuan ages (upper Stage 3–Stage 4). Therefore, bradoriids would be a useful proxy for resolving biostratigraphic issues of regional or global correlation within the Cambrian.