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Unique Lowermost Cambrian Small Shelly Fossils (Ssfs) from Eastern Yunnan, South China: Implications for the Early Diversification History of Animals

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Unique Lowermost Cambrian Small Shelly Fossils (Ssfs) from Eastern Yunnan, South China: Implications for the Early Diversification History of Animals Available online at www.sciencedirect.com ScienceDirect Palaeoworld 30 (2021) 199–207 www.elsevier.com/locate/palwor Unique lowermost Cambrian small shelly fossils (SSFs) from eastern Yunnan, South China: implications for the early diversification history of animals Sena Kono a,⇑, Yukio Isozaki a, Tomohiko Sato b, Moriaki Yasuhara c,d, Wei Liu e, Xing-Liang Zhang e a Department of Earth Science and Astronomy, The University of Tokyo, 3-8-1-Komaba, Meguro, Tokyo 153-8902, Japan b Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152-8550, Japan c Division of Ecology & Biodiversity, School of Biological Sciences, and Swire Institute of Marine Science, University of Hong Kong, Hong Kong SAR, China d State Key Laboratory of Marine Pollution, City University of Hong Kong, Hong Kong SAR, China e Shaanxi Key Laboratory of Earth Life and Environment, Department of Geology, Northwest University, Xi’an 710069, China Received 8 December 2020; received in revised form 24 January 2021; accepted 2 February 2021 Available online 16 February 2021 Abstract With a particular focus on the earliest Cambrian diversification of small shelly fossils (SSFs), stratigraphic analysis was conducted on the lower Cambrian Zhongyicun Member at the Hongjiachong section in the Chengjiang area, Yunnan, China. From ca. 3-m-thick bedded phosphorites (Unit A) in the lower part of the member, we recovered unique SSFs. This interval with 4 SSF-bearing horizons is characterized by the dominance in blade-shaped SSFs, including Halkieria spp., Brushenodus prionodes, Sinosachites delicatus, and Pteronus sp., and the absence of typical tube/cap-shape SSFs common in the Fortunian. This interval tentatively named ‘‘Halkieria- dominant interval” is stratigraphically positioned ca. 3 m below the previously known level of the oldest mollusks in the continuous bedded phosphorite sequence between the Anabarites trisulcatus-Protohertzina anabarica Assemblage Zone and the overlying Paraglo- borilus subglobosa-Purella squamulosa Assemblage Zone of the Fortunian (the earliest Cambrian). The ‘‘Halkieria-dominant interval” yields some SSFs likely affiliated with ostracods in view of size and morphology, suggesting that arthropod body fossils appeared in the Fortunian, considerably earlier than previously believed. Ó 2021 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights reserved. Keywords: small shelly fossil (SSF); Lower Cambrian; Fortunian; phosphorite; Chengjiang; South China 1. Introduction latest Ediacaran and the early Cambrian (e.g., Shu et al., 2014), the word ‘‘explosion” was emphasized, mainly The rapid appearance of almost all modern animal owing to the two well-known Lagersta¨tten, i.e., the phyla at the beginning of the Paleozoic has often been Chengjiang-Qingjiang fauna in South China (Cambrian described as the ‘‘Cambrian explosion”, which represents Stage 3; e.g., Zhang and Hou, 1985; Hou et al., 1995; the most significant event of animal diversification. Shu et al., 1999; Hu, 2005) and the Burgess Shale fauna Although the first appearances of animal phyla in fact in Canada (Wuliuan; e.g., Walcott, 1911; Conway Morris occurred during an interval of ca. 30 Myrs spanning the et al., 1987; Caron et al., 2013)(Fig. 1). As to the earliest Cambrian, in contrast to various ichnofossils, macroscopic ⇑ Corresponding author. soft-bodied fossils were scarcely recovered to date except E-mail address: [email protected] (S. Kono). for a worm-like Vittatusivermis annularis (Zhang et al., https://doi.org/10.1016/j.palwor.2021.02.001 1871-174X/Ó 2021 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights reserved. 200 S. Kono et al. / Palaeoworld 30 (2021) 199–207 Parag. subglobosus-Purel. squamulosa AZ (Li et al., 2007). The former assemblage zone is composed of simple, tube-shaped SSFs probably of chaetognaths, whereas the latter is dominated by cap-shaped SSFs of mollusks. This major transition in SSF composition indeed marks the ear- liest diversification of Cambrian animals, which occurred much earlier than the occurrence of trilobite (arthropod) body fossils and other impressive fossils from Lagersta¨tten in Stage 3. The occurrence of the oldest trilobite skeletons marks the base of Stage 3 (Zhang et al., 2017b), whereas that of the oldest trace fossil of trilobite Rushophycus is from Fortunian, much lower than the body fossils (Fig. 1). In this regard, the nature and pattern of the first SSF diversification has drawn attention of many stratigra- phers and paleontologists to the transitional interval between the first and second SSF assemblage zones in var- ious sections in Yunnan (Sato et al., 2014; Yang et al., 2014). In order to document the pattern of the earliest Cam- brian SSF diversification, we sampled the lower part of the Zhongyicun Member at the Hongjiachong section in the Chengjiang area, Yunnan, South China (Fig. 2), and obtained unique SSFs from a stratigraphic gap between the typical first and second SSF assemblages within a single section of bedded phosphorites, considered to be the first such discovery, likely representing the first rapid diversifi- cation of various animal taxa during the earliest Cambrian. This article preliminarily reports the occurrence of the Fig. 1. Standard stratigraphy of Cambrian System (Peng et al., 2020) with unique SSF assemblage from eastern Yunnan that may representative Lower Cambrian fossil horizons and the diversification be significant for the earliest Cambrian SSF stratigraphy. pattern of SSFs (Steiner et al., 2007). Note that SSFs are the most significant fossil group for the lowermost Cambrian interval below the well-known Lagersta¨tten of the Chengjiang, Qinjiang, and Burgess Shale faunas. 2017a). Instead, body fossil records for this interval are totally dominated by small shelly fossils (SSFs), which con- tain mineralized body parts (shell, scale, spine etc.) of a large variety of animal phyla, such as porifera, cnidarian, chaetognath, mollusk, and brachiopod. South China is one of the most SSF-producing domains as well as Siberia, where 4 early Cambrian SSF assem- blages (A) and 5 relevant assemblage zones (AZ) were established; i.e., 1) Anabarites trisulcatus-Protohertzina anabarica AZ, 2) Paragloborilus subglobosus-Purella squa- mulosa AZ, 3) Watosonella crosbyi AZ, 4) poorly fossilifer- ous interzone, and 5) Sinosachites flabeliformis-Tannuolina zhangwentangi AZ, in ascending order (Steiner et al., 2007). It is noteworthy that the first diversification of Cam- brian animals was not recorded by soft-bodied fossils from Lagersta¨tten but by various SSFs composed of diverse clades from the Fortunian (538.3–529 Ma) (e.g., Steiner et al., 2007; Sato et al., 2014; Shu et al., 2014; Yang et al., 2014). These SSFs recorded not only an extremely rapid increase in number of genera but also a significant Fig. 2. Distribution of Cambrian strata in eastern Yunnan and localities diversification in morphology; from 19 genera of the A. of representative sections for Lower Cambrian stratigraphy (compiled trisulcatus-Protoh. anabarica AZ to 140 genera of the after Luo et al., 1984, and Zhu et al., 2003). S. Kono et al. / Palaeoworld 30 (2021) 199–207 201 2. Stratigraphic framework of the Hongjiachong section The lower Cambrian marine strata in the southwest Yangtze Platform demonstrate a great variety in deposi- tional facies from shallow- to deep-water settings (Fig. 2), i.e., platform interior in the northwest, slope/deep basin in the north-central, and a transition zone between them. The extensive occurrence of thick phosphorite in the lower- most Cambrian is the most unique feature, known as one of the largest phosphorite deposits of the world (Shen and Schildlowski, 2000; Shields et al., 2000). The stratigra- phy of the Ediacaran to Lower Cambrian beds has been studied for many years mainly in eastern Yunnan, in par- ticular, in the Jinning and Chengjiang areas (Luo et al., 1982; Qian and Bengtson, 1989). Lowest Cambrian strata in eastern Yunnan are called the Zhujiaqing Formation, which is subdivided into the follow- ing three members: Daibu, Zhongyicun, and Dahai mem- bers in ascending order (Zhu et al., 2001). The Zhongyicun Member comprises the unique phosphorite beds. A new SSF assemblage was unearthed from the Hongji- achong section (Sato et al., 2014) in the Chengjiang area to the northeast of Fuxian Lake (Figs. 2, 3), located next to the Maotianshan section, the discovery site of the Cheng- jiang fauna (Zhang and Hou, 1985), and also adjacent to the Xiaolantian section (Kono et al., 2021), another local- ity yielding well-preserved Anomalocaris (Hou et al., 1995, 2004; Hu, 2005). According to Luo et al. (1984) and Sato et al. (2014), the Hongjiachong section exposes ca. 120 m thick uppermost Fig. 3. Outcrop of the Hongjiachong section in the Chengjiang area, Ediacaran (Dengying Formation) and Lower Cambrian Yunnan. This section displays a continuous sequence of the Zhongyicun Member, mostly composed of well-bedded phosphorite. strata; i.e., the Daibu, Zhongyicun, and Dahai members Fig. 4. Close up view of the uppermost Unit A and lowermost Unit B of the Zhongyicun Member at the Hongjiachong section (Fig. 3) and sample horizons. Note that the lithostratigraphic boundary between Unit A and Unit B (red line). The 4 horizons marked by yellow stars yielded significant SSFs (Figs. 6, 7). 202 S. Kono et al. / Palaeoworld 30 (2021) 199–207 of the Zhujiaqing Formation. The Daibu Member, ca. 45 m thick, is composed of bedded dolomite and siliceous mudstone, whereas the overlying Zhongyicun Member, ca. 40 m thick, comprises bedded phosphorite and dolo- mite. This member is overlain by 2 m thick dolomite of the Dahai Member. The following 5 lithologic units were identified in Zhongyicun Member at this section (Fig. 3); i.e., Unit A (22 m thick, bedded dark gray phosphorite), Unit B (2.5 m thick, phosphorite/limestone alteration), Unit C (4.5 m thick, alteration of phosphorite and limestone alter- ation with ca. 20 cm thick sandstone at the base), Unit D (8 m thick, thin bedded phosphorite), and Unit E (3 m thick, thin bedded dark gray phosphorite) in ascending order (Sato et al., 2014; Figs.
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