The Fad of Watsonella Crosbyi—A Potential Fossil Standard for Defining the Cambrian Stage 2 GSSP

Small shelly fossils (SSFs) have been one of the most important biostratigraphic tools for correlation and subdivision of the pre-trilobitic Cambrian (Terreneuvian) strata in both regional and global aspects. Some SSF taxa exhibit a worldwide distribution and enable a biostratigraphic correlation of the lower Cambrian sequences between different blocks. SSFs were recommended as GSSP index fossils for defining the Precambrian-Cambrian boundary, although the PC-C boundary GSSP was later chosen in Newfoundland with the first appearance datum (FAD) of the trace fossil Trichophycus pedum as the point marker (Landing, 1994). But utility of the FAD of Trichophycus pedum as the PC-C boundary marker is questionable and not practical in carbonate lithofacies, such as in South China, Treptichnus pedum was only reported from the Meishucun section in eastern Yunnan, occurring within the Anabarites trisulcatus – Protohertzina anabarica Zone (Zhu et al., 2001).

Among SSFs, the micromollusc Watsonella crosbyi is one of the most widely occurring small skeletal fossils, and it has been recovered from South China, Siberia, Mongolia, America, France, South Australia, etc. It mainly occurs in the late Terreneuvian (late Meishucunian in South China, Tommotian in the Siberian Platform, late Placentian in Newfoundland) and has been taken as a nominal fossil for biozonation in these areas. The wide geographic distribution may be related with its pelagic larvae stage. The wide occurrence in both carbonate and siliciclastic environments indicates that Watsonella crosbyi was an important fossil for both regional and global correlation of the pre-trilobitic strata. Meanwhile, this fossil is very characteristic in morphology, and easily identifiable. Therefore, the FAD (approximately 533 to 529 Ma in age) of Watsonella crosbyi is suggested herein as a candidate GSSP marker for defining the Fortunian-Stage 2 boundary. Using this biomarker, the sequences in South China, Mongolia, Siberia and Newfoundland could be well correlated. The occurrences of this species in France and Mongolia had been considered in the Atdabanian, but Gubanov (2002) thought their occurrences in these two regions could be correlated with the Tommotian. The scanty occurrence of Watsonella crosbyi in South Australia is an exception: it occurred in the Adtabanian to Botomian interval (Parkhaev, 2001), though it was reported from older strata by Daily (1976) but has never been properly examined.

As a potential GSSP candidate marker, the FAD of Watsonella crosbyi could be calibrated with other fossils. The FADs of Anabarella plana and Purella spp. are below this marker, while the FADs of Lapworthella spp. are usually above it (except in Mongolia). The FAD of Aldanella approximates to or is a little above this marker. Thus, the FAD of Aldanella attleborensis could also be a potential biomarker for defining the base of the Cambrian Stage 2 since this species is especially widely distributed in the Tommotian over the Siberian Platform. But in South China, the occurrence of the fossil (called Aldanella yanjiahensis) (Steiner et al., 2007) is not widespread and its FAD is higher than that of W. crosbyi in eastern Yunnan (Li and Xiao, 2004). In Mongolia, the occurrence of Aldanella is questionable. The FAD of Aldanella attleborensis in Siberia is mainly confined to the base of the Pestrotsvet Formation, but the boundary between the Pestrotsvet and the underlying Ust-Yudoma formations may represent a hiatus (Gubanov, 2002). Thus, it is difficult to evaluate whether the FAD represents the lowest occurrence there. Meanwhile, a detailed taxonomic revision of Aldanella is also needed before Aldanella attleborensis could be considered as a GSSP index fossil for Cambrian Stage 2, since it probably has been called different specific names in different regions. Besides the SSFs, acritarchs are useful microfossils for the Cambrian biostratigraphy. The FAD of Skiagia plexus as a global recognizable level is a little higher than that of Watsonella crosbyi (Moczydlowska and Zang, 2006), and it can be complementary for making the base of the Cambrian Stage 2.

The FAD of Watsonella crosbyi could be calibrated with chemostratigraphic data. In northeastern Yunnan, South China, the FAD of W. crosbyi is near the base of the Dahai Member of the Zhujiaqing Formation (Li and Xiao, 2004), and it is below the major positive ?¹³C excursion [dubbed ZHUCE by Zhu et al. (2007)] in the Terreneuvian.