Watsonella crosbyi from the lower Cambrian (Terreneuvian, Stage 2) Yanjiahe Formation in Three Gorges Area, South China

Abundant specimens of Watsonella crosbyi are here documented for the first time from the Yanjiahe Formation in Three Gorges area, South China. Specimens were collected from siliceous-phosphatic, intraclastic limestones at the base of Bed 5 in the measured Yanjiahe section, indicating a Cambrian Stage 2 position for Bed 5. The widely used, regional Aldanella yanjiaheensis assemblage zone in Three Gorges area is revised herein as the Watsonella crosbyi zone, allowing for greater accuracy and utility when correlating on a global scale. The examination of microstructures of W. crosbyi further confirms that its shell consists of two layers: an outer prismatic layer and an inner lamello-fibrillar layer with a stepwise texture. A pair of muscle attachment sites below the apex provide new soft part information about Watsonella, confirming that Watsonella is an untorted helcionelloid (mollusc) with endogastrically coiled shell. The stratigraphic range of W. crosbyi in the Yanjiahe Formation correlates with the ranges of the taxon elsewhere in South China as well as in Siberia, Mongolia, Avalonia, Australia and France. Its nearly cosmopolitan distribution and its occurrence across a range of facies and palaeolatitudes reinforce the notion that the FAD of W. crosbyi represents the best candidate for defining the base of Cambrian Stage 2. (C) 2020 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights reserved.

Automated summary

The study documents abundant specimens of Watsonella crosbyi for the first time in the Yanjiahe Formation in the Three Gorges area of South China, proposing it as a key index fossil for the Cambrian Stage 2. The microstructural examination of W. crosbyi confirmed its shell composition and provided new information about its soft body structure, supporting its classification as an untorted helcionelloid. The nearly cosmopolitan distribution and occurrence of W. crosbyi across various facies and paleolatitudes suggest its significance in defining the base of Cambrian Stage 2 on a global scale.