Watsonella crosbyi from the lower Cambrian (Terreneuvian, Stage 2) Yanjiahe Formation in Three Gorges Area, South China
Introduction
The Cambrian Period witnessed one of the most profound phases of evolutionary change in life history, as most of the major animal phyla suddenly emerged and diversified around the globe. This remarkable radiation was closely coupled with the development of complex ecological interactions, infaunalization, niche exploitation and the rise of predatory life modes amongst early bilateral animals. Our understanding of the emergence and evolution of the early Cambrian animal communities however is hampered by an incomplete chronostratigraphy and the absence of a high resolution Cambrian (especially Terreneuvian) timescale (Zhu et al., 2001, 2007, 2019; Babcock et al., 2005, 2014; Betts et al., 2018). A reliable and accurate time scale for the traditional ‘Lower Cambrian’ is pivotal for understanding the timing and sequence of biological and geological events surrounding the Cambrian radiation (Yang et al., 2014; Steiner and Yang, 2017; Betts et al., 2018; Zhu et al., 2019).
At present, GSSPs of the Fortunian, Wuliuan, Drumian, Guzhangian, Paibian and Jiangshanian stages have all been ratified. The remaining main task for the International Subcommission on Cambrian Stratigraphy is to focus on choosing criteria and selecting GSSP candidate sections for defining the bases of stages 2, 3, 4 and 10, respectively (see Zhu et al., 2019).
The definition of the base of Cambrian Stage 2 (Terreneuvian Series) has been a source of much conjecture, especially regarding the reliability and biostratigraphic utility of particular Small Shelly Fossil (SSF) taxa. Nevertheless, recent studies have continued to refine the stratigraphic ranges of key taxa, demonstrating that some SSFs can be an invaluable tool for relative dating and correlation (Qian, 1999; Steiner et al., 2004, 2007; Devaere et al., 2013; Yang et al., 2014; Betts et al., 2016, 2017, 2018). During the Fortunian–Stage 2 interval, microscopic molluscs were a particularly prevalent component of SSF assemblages (Betts et al., 2016, 2017, 2018; Jacquet et al., 2017, 2019). Some of them have a worldwide distribution and therefore show great potential for global correlation (Gubanov, 1998, 2001; Gubanov et al., 1999), although not without some limitations (see Betts et al., 2016, 2017, 2018; Jacquet et al., 2017, 2019). The FAD of the cosmopolitan mollusc taxon Watsonella crosbyi has been suggested by many authors as a potential marker to define the base of Cambrian Stage 2 (Zhu et al., 2006, 2008; Li et al., 2011; Devaere et al., 2013; Jacquet et al., 2017; Kouchinsky et al., 2017). Often co-occurring with W. crosbyi is another widely distributed mollusc taxon, Aldanella attleborensis (Shaler and Foerste, 1888), and its FAD has also been proposed as a marker for the base of Cambrian Stage 2 (Rozanov et al., 2008; Parkhaev et al., 2011, 2012). In addition to these molluscs, the FAD of Skiagia ornata in the upper Skiagia ornata–Fimbrioglomerella membranacea acritarch Zone has also been proposed as a potential marker for the base of Cambrian Stage 2 (Moczydłowska, 1991; Moczydłowska and Zang, 2006; Moczydłowska and Yin, 2012). But Landing et al. (2013) argued that early Cambrian fossils are characterized by diachronous FADs, suggesting that utilizing the FAD of a fossil to define the base of a global chronostratigraphic unit during this time is unreliable.
Landing and Geyer (2012) suggested that the subdivision of Terreneuvian should be based entirely on carbon isotopic markers. They proposed a ‘Laolinian Stage’ based on the ZHUCE δ13C positive peak (named by Zhu et al., 2006). However, Steiner et al. (2013), Yang et al. (2014) and Steiner and Yang (2017) argued that defining the GSSP based solely on δ13C values was subjective and ambiguous. Betts et al. (2018) stressed that the integration of multiple proxies was needed for robust global correlation of lower Cambrian successions and this idea was recently heeded by Zhu et al. (2019), who combined biostratigraphic and chemostratigraphic data to propose the ‘Xiaotanian Stage’. The proposed ‘Xiaotanian Stage’ is defined by the FAD of W. crosbyi as the primary marker and the onset of the ZHUCE positive δ13C excursion as a secondary marker to define the base of Cambrian Stage 2. Although universal agreement has not been reached, the widespread distribution of W. crosbyi has resulted in the taxon being repeatedly cited in biostratigraphical discussions, and the species is seen as one of the best candidates for defining the base of Cambrian Stage 2 (Steiner et al., 2007; Li et al., 2011; Peng and Babcock, 2011; Devaere et al., 2013; Landing et al., 2013; Jacquet et al., 2017; Kouchinsky et al., 2017; Betts et al., 2018; Zhu et al., 2019).
Although W. crosbyi has been widely recovered from eastern Yunnan, it was not discovered from the Yanjiahe Formation in Three Gorges area before. Herein we document W. crosbyi for the first time from the Yanjiahe Formation. The purposes of this study are to describe features of W. crosbyi, to establish its stratigraphic range in a measured section, and to correlate the sequence in a regional context by combining biostratigraphic and chemostratigraphic data. In addition, its shell microstructure and potential muscle attachment sites are assessed. The Yanjiahe Formation in Three Gorges area represents one of the most important stratigraphic sequences in South China that bridges the Fortunian–Stage 2 boundary interval (Ishikawa et al., 2008, 2013; Jiang et al., 2012; Guo et al., 2014; Topper et al., 2019 and herein). As such, this work will improve the stratigraphic correlation of the Fortunian–Stage 2 interval in South China and provide further evidence to support the utility of W. crosbyi as a GSSP candidate marker for defining the base of Cambrian Stage 2.
Section snippets
Geological setting and stratigraphy
All W. crosbyi specimens documented herein were collected from the Yanjiahe Formation in the Yanjiahe section, Yichang, Hubei Province, China (Figs. 1, 2). The Yanjiahe Formation crops out mainly around the southern and western flanks of the Huangling anticline (Fig. 1A) where it is particularly well-exposed around Yanjiahe village in the Yichang area (Fig. 1B).
The Yanjiahe Formation rests disconformably on the underlying dolomitic Baimatuo Member of the Dengying Formation (Ediacaran), and is
Material and methods
The specimens described herein were recovered from Bed 5 of the Yanjiahe Formation from the Yanjiahe section. Watsonella crosbyi and A. attleborensis first occur at the base of Bed 5. All specimens are registered and deposited in the School of Earth Science and Resources, Chang'an University (CU), Xi'an, China.
Phosphatic limestone samples were digested in 10% acetic acid for 4‒5 days and the insoluble residues were passed through sieves (80‒120 μm). Several hundred specimens of W. crosbyi were
Systematic palaeontology
Phylum Mollusca Cuvier, 1797
Class, Order and Family uncertain
Genus Watsonella Grabau, 1900
1900 Watsonella ‒ Grabau, p. 631.
2013 Watsonella Grabau ‒ Devaere et al., p. 36 (and synonymy list therein).
2017 Watsonella Grabau ‒ Jacquet et al., p. 1098.
2017 Watsonella Grabau ‒ Kouchinsky et al., p. 343.
Type species: Watsonella crosbyi Grabau, 1900; Stage 2, Terreneuvian, Cambrian, Massachusetts, USA; by original designation.
Species included: The type species only.
Diagnosis: See Li et al. (2011, p.
Shell microstructures of Watsonella crosbyi
Early diagenetic phosphate fillings of small shells have the potential to replicate very fine structures (Runnegar and Pojeta, 1985). Therefore, the morphology and microstructure of the inner shell surface could be determined, even if the original skeleton has been recrystallized or removed (Kouchinsky, 1999). Indeed, most Cambrian molluscs were preserved as secondary phosphatized shells or molds that can preserve imprints of shell microstructure (Runnegar, 1985; Bengtson et al., 1990;
Conclusions
Watsonella crosbyi, a characteristic taxon of Cambrian Stage 2, has for the first time been recovered from the Yanjiahe Formation in Three Gorges area of Hubei Province, China. Specimens exhibit at least three distinct types of microstructure: polygonal impressions, lamello-fibrillar fabric and stepwise texture. The polygonal structure is interpreted as replicas of the prismatic shell layer. The lamello-fibrillar microstructure is representative of variably oriented fibers. The stepwise texture
Acknowledgements
We are grateful to Sarah Jacquet and Mao-Yan Zhu for constructive comments and suggestions. We thank Heyo Van Iten (Hanover College, Indiana), Timothy P. Topper (Swedish Museum of Natural History) and Yue-Han Ma (Chang'an University) for their suggestions and for improving the usage of the English language in this work. This work is supported by the Natural Science Foundation of China (Nos. 41890844, 41890840, 41472015, 41772010, 41720104002, 41672009, 41621003), Key Scientific and
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