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Cambrian Chengjiang Biota, South China – Geochronological Constraint

Compelling fossil evidence for the rapid appearance of metazoan phyla in the history of the Earth (the Cambrian explosion), has been provided by the Chengjiang biota from the Cambrian of South China. The timing of the Chengjiang biota is, however, poorly constrained as a result of a lack of rock materials that can be dated within the Maotianshan Shale that has produced the fossils. In this study Yang et al. integrated secondary ion mass spectrometry (SIMS) and the chemical ablation isotope dilution thermal ionisation mass spectrometry (CA-ID-TIMS) U-Pb analyses of detrital zircons from the Maotianshan Shale in order to provide high precision geochronological constraint on the Chengjiang biota. An age peak of 520 Ma is yielded by the youngest group of SIMS U-Pb detrital zircon dates. They used 6 zircons from this group to further date by the CA-ID-TIMS U-Pb technique, though suggesting that they were not formed from a single zircon growth event. Thereby neither the age peak nor the weighted mean age defined by the youngest of the SIMS U-Pb dates could represent the maximum age of the Maotianshan Shale. Instead, the first robust maximum age of the Chengjiang Biota was provided by the youngest CA-ID-TIMS U-Pb date, 518.03 ± 0.69/0.71 Ma (2σ, analytical uncertainty/incorporates uncertainty of the U-Pb tracer calibration. It is indicated by this new geochronological constraint on the Chengjiang biota that the Cambria explosion attained its major phase about 518.03 ± 0.69/0.71 Ma, which demonstrates a protracted process (>22 Myr) for the Cambrian explosion.

The Chengjiang biota that was recovered from the Maotianshan Shale Member of the Yu’anshan Formation in South China, which was characterised by soft-bodied and weakly biomineralised fossils that are exceptionally well preserved, shows the major phase of the metazoan Cambrian explosion (e.g. Chen et al., 1996; Li et al., 2007; Shu, 2008; Zhang & Shu, 2014). This biota contains almost all of the extant animal phyla, including Chordata (Chen et al., 1999; Shu et al., 1999; Hou et al., 2017), and provides a unique window into the diversity of metazoans and ecosystems in the Early Cambrian (Zhao et al., 2010, 2012). According to Yang et al. although it has been studied for more than 30 years, there is no high precision geochronological constraint on the Chengjiang biota. Direct age constraint on the Chengjiang biota is not provided by the current Chronostratigraphic model (Peng et al., 2012), which ultimately hampers estimation of the rate of the process of the Cambrian explosion.

Several Rb-Sr, Ar-Ar and Pb-Pb analyses were carried out on whole-rock and illite in order to the age of deposition, as there was a lack of ash beds in the Yu’anshan Formation. These dating results ranged from about 560 to about 534 Ma, though there were large analytical errors (e.g. Chen et al., 2001; Chang et al., 2004). A determination has been made from an ash bed at the bottom of the underlying Shiyantou Formation by the use of a sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb age of 526.5 ± 1.1 Ma, which provided a lower limit for the age of the Chengjiang biota. In recent years a breakthrough was achieved by analysing secondary ion mass spectrometry (SIMS) U-Pb ages of detrital zircons that were recovered from the basal Shiyantou Formation to the Maotianshan Shale Member, and it was demonstrated by the results that the maximum age of the deposition of the Maotianshan Shale is about 520 Ma (Hofmann et al., 2016). When an external uncertainty of 1% (2SD) for the SIMS zircon U-Pb technique (e.g. Ireland & Williams, 2003) is taken into account, however, the age constraint on the Chengjiang biota needs to be further refined. In this study Yang et al. aimed to obtain a geochronological constraint of high precision on the Chengjiang biota by the use of an integral approach that includes SIMS and chemical abrasion-isotope dilution thermal ionisation mass spectrometry (CA-ID-TIMS) zircon U-Pb analytical method. It was demonstrated by their new results that the age of the Chengjiang biota is no earlier than 518.03 ± 0.69/0.71 Ma, and the Cambrian explosion may have extended over a period of more than 22 Myr.

Geological background and sampling

A deposit in the Maotianshan Shale Member of the Yu’anshan Formation in     Yunnan Province, South China, contains the Chengjiang biota that dates to the Early Cambrian. The Chengjiang biota has been discovered in many localities in eastern Yunnan Province, as well as the fossils that are the best-preserved and quarries are found in the Chengjiang-Haikou-Anning areas (Zhao et al., 2010). The Chengjiang biota were first discovered in the Xiaolantian section that has been studied is about 6 km to the east of the Chengjiang County town, and about 3 km from the Maotianshan section.

The section from the Early Cambrian of the Xiaolantian section had been deposited in a shallow marine setting that includes, from the bottom upwards, the Zhujiaqing, Shiyantou and Yu’anshan formations. There are 3 members in the eastern Yunnan Province that comprise the Zhujiaqing Formation: from the bottom upwards, the Daibu, Zhongyicun and Dahai members. The Daibu member of the Xiaolantian section is predominantly composed of cherty dolostone. The Zhongyicun member is 37 m thick and is composed of dolomitic phosphate in which there is interlayered phosphate. The Dahai Member is not present in this section. The Zhujiaqing Formation is overlain unconformably by the Shiyantou Formation, consisting of siltstone, which is about 52 m thick. The 170 m thick Yu’anshan Formation, which consists of 3 members: the lower Black Shale Member, the middle Maotianshan Shale Member, and the upper Siltstone Member (Zhao et al., 2012). It is indicated by regional stratigraphic correlation that the Anabarites trisulcatus-Protohertzina anabarica Assemblage Zone and Paragloborilus subglosus-Purell squamulosa Assemblage Zone occur in the Daibu-lower middle Zhongyicun members and the upper Zhongyicun Member,  respectively (Yang et al., 2014, 2016b). The Sinosachites flabelliformis-Tannuolina zhangwentangi Assemblage Zone is present in the upper part of the Shiyantou Formation, extending to the basal Yu’anshan formations, the Parabadiella Zone is located in the lower Yu’anshan Formation and the Eoredlichia-Wutingaspis Zone occurs in the overlying Maotianshan Shale Member (Zhu Yang et al., 2001; Steiner Yang et al., 2007; Yang et al., 2014, 2016b). There are 2 SIMS zircon U-Pb ages that may have been dated from ash beds in the middle Zhongyicun Member and at the base of the Shiyantou Formation in the Meishucun section in the same area, respectively (Compston et al., 2008; Zhu et al., 2009).

There were 2 samples recovered from the lower part of the Maotianshan Shale Member in the Xiaolantian section (24^o40’53”N, 102⁰58’50”E) for SIMS and CA-ID-TIM S zircon U-Pb dating. Sample 14CJ-2 is a fine-grained siltstone and sample 14CJ-3is a mudstone from about 60 m above 14CJ-2.

Geochronological constraint at the major phase of the Cambrian explosion

The unprecedented, unique event when nearly all metazoan phyla first appeared in the fossil records over a relatively short time span during the Ediacaran-Cambrian transition, and which led to an ecosystem that was dominated by metazoans accompanied by widespread biomineralisation, and increases in size and morphological diversity among metazoan phyla is delineated by the Cambrian explosion (Zhang & Shu, 2014). The first major stage of the Cambrian explosion is marked by the diversification of coelomates that dug deep, complex burrows, which corresponds to the base of the Cambrian System (Landing et al., 2013). It has been demonstrated by molecular clocks, however, that the common ancestor of all metazoans originated earlier than about 800 Ma, and the bilaterians diversified at least 100 million years earlier than the Cambrian (e.g. dos Reis et al., 2015). According to Yang et al. in the perspective of fossil records, such as Treptichnus, skeletal fossils of Cambrian type such as Anabarites and Cambrotubulus, and Kimberella, the earliest triploblastic bilaterian, also made their first appearance earlier than the Cambrian (Jensen et al., 2000; Gehling et al., 2001; Macdonald et al., 2014; Zhu et al., 2017). A deep root for the Cambrian explosion is implied for the Cambrian explosion of metazoans by molecular clock studies, as well as fossil records (Zhu et al., 2017).

The most diverse representatives of the biotas of the Cambrian that are younger than 520 Ma, such as the Chengjiang biota, are the arthropods (Li et al., 2007; Zhao et al., 2010). A clade is formed in cladistic analyses of the Arthropoda, the trilobites (Wills et al., 1994). The first occurrence of a trilobite is, therefore, a key event in the biostratigraphy of the Cambrian, marking the most dramatic event in the modernisation of ecological communities in the Cambrian explosion; i.e., the onset of the major phase of the Cambrian  explosion (Zhang et al., 2014, 2017). It has also been considered for a long time that first appearance of the trilobites was the primary indicator to define the base of Cambrian Series 2 and Stage 3 (for reviews see (Peng et al., 2012; Zhang et al., 2017). The first appearances of trilobites are endemic and diachronous on separate palaeocontinents (Landing et al., 2013; Zhang et al., 2017). There are scarce geochronological constraints on the first appearance of trilobites. A zircon U-Pb age of 520.93 ± 0.14 Ma from the upper part of the Lie de vin Formation in Morocco (Maloof et al., 2010) provides a maximum geochronological constraint on the first determinable trilobite in this region (Geyer & Landing, 2006). A zircon U-Pb age of 519.30 ± 0.23 Ma from the Caerfai Bay Shales Formation in Wales (Harvey et al., 2011), roughly constrains the first determinable trilobites in Avalonia, which includes the olenelloid Callavia broeggeri, which are relatively late among the early trilobites (Landing et al., 2013). The occurrence of the first trilobites is, collectively, possibly bracketed between 520.93 ± 0.14 Ma and 519.30 ± 519.30 ± 0.23 Ma.

It is indicated by generic diversity of South China that the most significant boost of metazoan diversity in the Middle Cambrian Age 3, due to the exceptional Chengjiang biota (Li et al., 2007), which is consistent with the global biodiversity data (Na & Kiessling, 2015). The Chengjiang biota provides a unique insight to the major phase of the Cambrian explosion, as it contains 228 species in more than 18 phyla of animals and exhibits the establishment of the modern style of complex marine community (Li et al., 2007; Zhao et al., 2010; Zhang & Shu, 2014). It is implied by the maximum age of the Chengjiang biota being constrained at 518.03 ± 0.69/0.71 Ma, that the Cambrian explosion was a protracted evolutionary process (Erwin et al., 2007; Shu et al., 2014; Zhang & Shu, 2014) that took more than 22 Myr. The global first appearance of the trilobites, which is possibly bracketed between 520.93 ± 0.14 Ma and 519.30 ± 0.23 Ma, marked the onset of the major phase of the Cambrian explosion. A rapid episode of metazoan diversification over a relatively short interval of time is represented by the major phase of the Cambrian explosion. This phase was followed by a decline in diversity in Cambrian Age 4, which extended further through the Cambrian (Li et al., 2007; Na & Kiessling, 2015).

Conclusions

Yang et al. carried out integrated SIMS and CA-ID-TMS U-Pb dating on detrital zircons from the Maotianshan Shale Member, which yielded the Chengjiang biota in South China. A SIMS-ID-UP-Pb age peak at about 520 Ma is defined by the youngest detrital zircon. CA-ID-TMS U-Pb dates for these zircons are scattered, the youngest concordant one being at 518.03 ± 0.69/0.71 Ma, which provides a maximum age for the Chengjiang biota. It is indicated by the new geochronological constraint on the Chengjiang biota that the Cambrian explosion was a protracted evolutionary process, as well as a rapid episode of metazoan diversification that occurred at about 518.03 ± 0.69/0.71 Ma.

Sources & Further reading

Yang, C., et al. (2018). Geochronological constraint on the Cambrian Chengjiang Biota, South China.