Aldanella attleborensis (Mollusca) from Cambrian Stage 2 of the Three Gorges Area and Its Stratigraphic Implications

Simple Summary: Aldanella attleborensis, is a mollusk fossil found worldwide; its first appearance datum (FAD) has been suggested as one of the potential markers for defining the base of Cambrian Stage 2. However, A. attleborensis was uncommonly recovered in the Cambrian strata of South China, and if occurring, was usually described as Aldanella yanjiaheensis or an indeterminate species in the literature due to lack of detailed taxonomic work. That is an obvious obstacle to correlate and subdivide the Cambrian strata in South China and worldwide. Recently, numerous specimens of Aldanella were collected from Member 5 of the Yanjiahe Formation in the Three Gorges area, South China, which is the type locality of A. yanjiaheensis. Morphological and taxonomic study of these specimens supports that A. yanjiaheensis is a junior synonym of A. attleborensis. Shell microstructure and muscle scar imprints on its internal mold surface indicate that A. attleborensis may belong to the gastropods. The occurrence of A. attleborensis in Member 5 of the Yanjiahe Formation suggests that this stratigraphic unit shall be of Cambrian Stage 2 in age, and can be correlated with the Dahai Member of the Zhujiaqing Formation in Yunnan, uppermost Kuanchuanpu Formation in Shaanxi, and the uppermost Tianzhushan Member of the Dengying Formation in Hubei, South China. Meanwhile, using the FAD of A. attleborensis as a bio-marker, the upper Terreneuvian strata in South China, Siberia, Avalonia, and Estonia can be well correlated.
Abstract: Some small shelly fossils are important index fossils for global stratigraphic subdivisions and correlations of the Cambrian Terreneuvian. The first appearance datum (FAD) of the cosmopolitan mollusk Aldanella attleborensis has been suggested as one of the potential markers for defining the base of Cambrian Stage 2. Aldanella fossils were uncommon in South China, and if occurring, were often described as Aldanella yanjiaheensis, A. attleborensis, or indeterminate species in the literature, while A. yanjiaheensis was often taken as a junior synonym of A. attleborensis. Nevertheless, a detailed taxonomic revision of A. yanjiaheensis based on material from its type locality awaits to be made. In this study, we systematically re-evaluated A. yanjiaheensis based on the numerous specimens collected from the base of Member 5 of the Yanjiahe Formation in the Three Gorges area, western Hubei Province of South China. Detailed taxonomic comparison further substantiates that A. yanjiaheensis is a junior synonym of A. attleborensis, signifying its strong potential for a global correlation across paleocontinents. Morphological parameter analyses indicate that the length and width of shell tube of A. attleborensis shows allometric growth. The nearly cosmopolitan distribution and characteristic morphology of A. attleborensis indicate that it can play a significant role in the subdivision and correlation of Cambrian Stage 2. The co-occurrence of A. attleborensis and Watsonella crosbyi from the base of Member 5 of the Yanjiahe Formation corroborates that Member 5 belongs to Cambrian Stage 2.