A new U/Pb date for the basal Meishucun section and implications for the age of the Cambrian explosion

The Early Cambrian of southern China has long been recognized to record the spectacular transition from microscopic small shelly fossils to a fauna characterized by large, diverse higher bilaterians during part of the Early Cambrian evolutionary explosion. Understanding the timing and rate of this evolutionary transition has been aided through the integration of U/Pb geochronology into Cambrian fossil biozones, which has established strong tie-points between platforms. The Meishucun section (Yunnan, China) is one of the units that best preserves what could be considered the prelude to the Cambrian explosion. Several attempts at dating volcanic horizons within this section have resulted in U/Pb zircon dates that range from 538 to 525 Ma, making correlations to sections further afield problematic. This new high-precision U/Pb date of 533 Ma for Meishucun's Bed 5 is in broad agreement but considerably more precise than previous U/Pb ages. This indicates that the low diversity fauna of the Anabarites trisulcatus--Protohertzina anabarica zone persisted for no less than the first 9 m.y. of the Cambrian in China, and sets an additional lower bracket age for the emergence of higher bilaterians (e.g. trilobites, soft-bodied fauna).