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

Abstract

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.