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Gondwana Research 25 (2014) 999–1007

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Gondwana Research

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Small shelly fossils from the early Cambrian Yanjiahe Formation, Yichang, Hubei, China

Junfeng Guo a,b,⁎, Yong Li a, Guoxiang Li b a Key Laboratory of Western Mineral Resources and Geological Engineering, Ministry of Education, School of Earth Science and Resources, Chang'an University, Xi'an 710054, China b State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeobiology, Chinese Academy of Science, Nanjing 210008, China article info abstract

Article history: Abundant data have been acquired on the lower Cambrian small shelly fossils (SSFs) of the Yangtze platform Received 31 August 2012 during the last three decades, demonstrating that these fossils are an important piece of evidence for the Received in revised form 3 March 2013 Cambrian radiation and are useful biostratigraphic tools for correlating the lower Cambrian. Here we report Accepted 17 March 2013 SSF associations from the Yanjiahe Formation in the Three Gorges area, South China. The Yanjiahe Formation Available online 26 March 2013 is well exposed near the Yanjiahe village, and its 40-m-thick sequence can be subdivided on the basis of lithology into ﬁve stratigraphic intervals (beds). Small shelly fossils occur mainly in Beds 2 and 5, but abundant Keywords: – ﬁ Small shelly fossils SSFs were discovered in thin sections of siliceous phosphatic nodules from Bed 3 for the rst time. No skel- δ13 Terreneuvian etal fossils were discovered in the basal siliceous rock interval (Bed 1), but the negative Ccarb excursion Early Cambrian and the occurrence of the acritarch Micrhystridium regulare indicate that it belongs to the basal Cambrian. Yanjiahe Formation The SSF associations are somewhat similar to those of East Yunnan, and can be differentiated into three Yichang biozones (in ascending order): the Anabarites trisulcatus–Protohertzina anabarica assemblage zone (Bed 2), the Purella antiqua assemblage zone (Bed 3), and the Aldanella yanjiaheensis assemblage zone (Bed 5). The occurrence of A. yanjiaheensis in Bed 5 probably indicates that Bed 5 belongs to Cambrian Stage 2, but the Stage 2/Stage 1 boundary is uncertain since Bed 4 lacks fossils. SSF biostratigraphy indicates that the Yanjiahe Formation is pretrilobitic Meishucunian in age (equivalent to the Nemakit–Daldynian to Tommotian of Sibe- ria, Terreneuvian). Five SSF genera occur in Bed 2, more than six genera in Bed 3, and twenty-three genera in Bed 5. The stepwise increase in generic diversity through the Yanjiahe Formation is comparable with the global diversity increase through the Nemakit–Daldynian to early Tommotian interval. © 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction the biomineralization event, which is an important part of the Cambrian radiation. Many small shelly fossils represent the earliest record of some During Ediacaran to mid-Cambrian times (635–505 Ma), the Earth animal phyla, and some are of enigmatic zoological affinities and may underwent significant changes in paleogeography that include the be stem groups of metazoans. Therefore, SSFs are important for investi- rifting of a possible supercontinent and the formation of a second, gating the origin and early evolution of metazoans and for analyzing the slightly smaller supercontinent (Meert and Lieberman, 2004; Jiang et Cambrian radiation pattern (Li et al., 2007). al., 2011). The assembly of the Gondwana supercontinent during the Preceded by the soft-bodied Ediacaran fauna and by abundant late waning stages of the Proterozoic seems to have provided a tectonic Neoproterozoic algae, and succeeded by the oldest trilobites from the backdrop for the biological, climatological, tectonic and geochemical base of Cambrian Stage 3, a wealth of skeletal fossils (so-called small changes leading to, and including, the explosive Cambrian radiation shelly fossils) assignable to various animal phyla are distributed (Meert and Lieberman, 2008). Evidence for this radiation event includes worldwide in pre-trilobitic Cambrian sedimentary rocks. These the enigmatic (but widespread) Ediacaran fauna, trace fossils, small biomineralized or skeletonized animals essentially differ from the shelly fossils (SSFs) and soft-bodied fossils found in the Chengjiang, older soft-bodied metazoans, and the appearance of these various Burgess Shale and Sirius Passett Lagerstätten (Conway Morris et al., skeletal fossils represents a fundamental and profound biological in- 1987; McCall, 2006; Shu, 2008). The abrupt appearance of diversified novation. This pre-trilobitic Cambrian interval in geological history skeletal fossils during the early Cambrian has also been envisioned as was referred to as the Meishucunian age in South China (equivalent to the Terreneuvian epoch in the new Cambrian stratigraphical scheme), since the SSFs are particularly abundant and were well doc- ⁎ Corresponding author at: Key Laboratory of Western Mineral Resources and Geological umented in the succession of the Meishucun section, Jinning, Yunnan, Engineering of Ministry of Education, School of Earth Science and Resources, Chang'an Uni- versity, Xi'an 710054, China. Tel.: +86 13379261208; fax: +86 02982339267. South China (Qian, 1977, 1978; Jiang, 1980; Qian et al., 2001; Zhu et E-mail address: [email protected] (J. Guo). al., 2001), which had been taken as one of the candidate sections for

1000 J. Guo et al. / Gondwana Research 25 (2014) 999–1007 the GSSP of the Precambrian–Cambrian boundary (Luo et al., 1982, The purpose of the present paper is mainly to provide an overview 1984). Stratigraphic successions of the Meishucunian stage are well de- of SSFs from the early Cambrian Yanjiahe biota, and to study the SSF veloped and widely exposed on the Yangtze platform, and contain biozonation of the Yanjiahe Formation. abundant SSFs (Qian et al., 1999). Previous studies of the Meishucunian fossils in the eastern Yangtze Gorges area focused mainly on the SSFs of 2. Material and methods the early Cambrian Tianzhushan member of the uppermost Dengying Formationintheeasternprofile of the Huangling anticline before For phosphatic carbonate rock samples, the traditional method of 1982 (Qian, 1977, 1978; Qian et al., 1979; Yu, 1979; Chen and Zhang, acid maceration was used. The fossils were extracted from the car- 1980; Chen et al., 1981a,b). But the Tianzhushan member ranges from bonate matrix with 3–5% acetic acid, and several thousand SSFs several cm to 8 m in thickness, and represents a very incomplete record were handpicked from the undissolved rock residues. All specimens of the Meishucunian. Also, there is a distinct unconformity between the examined with a scanning electron microscope (SEM) were placed Tianzhushan member and the overlying Shuijingtuo Formation. Later, a on metal stubs using double-sided carbon tape, and were gold coated. continuous succession of the Meishucunian was found in the southern For the siliceous–phosphatic nodules, petrographic thin sections were profile of the Huangling anticline (Chen, 1984), and the 40-m-thick se- made to examine skeletal fossils under a polarized microscope, and quence was called the Yanjiahe Formation since it is well exposed near fossils were photographed with a digital camera. All specimens are the Yanjiahe village (Fig. 1). The Yanjiahe Formation is overlaid by black registered and deposited in the School of Earth Science and Re- shale of the early Cambrian Shuijingtuo Formation, and is conformably sources, Chang'an University, Xi'an, China. underlain by finely crystalline dolomite of the uppermost Dengying Formation. 3. Fossil locality and stratigraphy Previous work on the Yanjiahe Formation in the Three Gorges area focused on the SSFs, microscopic algae (Chen, 1984; Ding et al., All small shelly fossils under this paper were collected from the 1992a,b) and C and Sr isotopes (Ishikawa et al., 2008; Sawaki et al., Yanjiahe Formation at seven sections (Yanjiahe, Gunziao, Jijiapo, 2008; Jiang et al., 2012). Since 2005, a number of macroscopic fossils Renjiaping, Yangjiachong, Wushizu and Heziao (Fig. 6)) in the Three have been discovered from intercalated silty shales in the middle of Gorges area (Fig. 1). The Yanjiahe Formation outcrops mainly around the Yanjiahe Formation. Besides SSFs and spheroidal fossils (possibly the southern and western flanks of the Huangling anticline, as well as embryos), fossils recovered from the Yanjiahe Formation near the the core of the Changyang anticline, in the Yichang area. The Yanjiahe Yanjiahe village include cyanobacteria, acritarchs, macroalgae and Formation can be subdivided on the basis of lithology into five beds, macroscopic metazoans (Guo et al., 2008, 2009, 2010a,b, 2012). The and the SSFs studied herein were recovered from Beds 2, 3 and 5 of fossil assemblage of the Yanjiahe Formation is quite different from the seven sections. But the exposure of the strata in different sections the Meishucun fauna, and hence was called the Yanjiahe biota (Guo are different, the successions are particularly well developed and ex- et al., 2008). posed around the Yanjiahe village (Sandouping town) and the Heziao

Fig. 1. Locality and stratigraphy of the early Cambrian Yanjiahe Formation, Yichang, Hubei, China. (a) Sketch map of the People's Republic of China, showing the position of the collecting locality in Hubei province; (b) simpliﬁed geological sketch map of the Three Gorges area, Hubei province, South China, showing the outcrops of Cambrian strata; (c) detailed geological sketch map of the Yanjiahe area, showing the outcrops of the Yanjiahe Formation; and (d) stratigraphic sequence of lower Cambrian strata in the Gunziao section, Three Gorges area, indicating the horizons where SSFs were collected. Author's personal copy

J. Guo et al. / Gondwana Research 25 (2014) 999–1007 1001 village (Changyang county) (Fig. 1,(a)–(c)). In the Gunziao section near i.e., in eastern Yunnan (Jiang, 1980; Luo et al., 1982, 1984), western the Yanjiahe village, for instance, the Yanjiahe Formation is 40.3 m thick Sichuan (Yin et al., 1980) and western Guizhou (Qian and Yin, 1984). and ﬁve beds are continuously exposed (Fig. 1, (d)). Here the Yanjiahe Since 2005, we have made many extensive fossil collections in the Formation rests conformably on the underlying Baimatuo member Three Gorges area, and recovered a variety of abundant fossils from the (dolostone) of the Dengying Formation, but there is a minor disconfor- lower Cambrian Yanjiahe Formation. The SSFs were found in siliceous– mity between the Yanjiahe Formation and the overlying Shuijingtuo phosphatic intraclastic dolostone or limestone in the lower part Formation (Ding et al., 1992a; Guo et al., 2008, 2009). Bed 1 is (Bed 2) and top part (Bed 5) of the Yanjiahe Formation (Fig. 1(d)) about 12.3 m thick and consists of siliceous rocks intercalated with through laboratory maceration of the samples with acetic acids. sandy dolostone and shale. The siliceous rocks contain the acritarch Most importantly, abundant SSFs were examined for the ﬁrst time Micrhystridium regulare (Ding et al., 1992a). The 2.6-m-thick Bed 2 in thin sections of siliceous–phosphatic nodules, which are sporadically consists of siliceous–phosphatic intraclastic dolostones containing abun- and horizontally distributed within dolostones and shales of Bed 3 dant small shelly fossils such as Conotheca subcurvata, Anabarites trisulcatus (Fig. 1(d)). On the basis of these discoveries, the stratigraphic distribu- and Protohertzina anabarica. Bed 3 is about 18.5 m thick and consists of tion of small shelly fossils in the Yanjiahe Formation (Fig. 6)canbe carbonaceous limestone interbedded with silty shale containing macro- re-studied with reference to recent progress on SSF taxonomy (e.g. fossils (algae and enigmatic metazoans) (Guo et al., 2009, 2010a, 2012). Qian et al., 1999; Parkhaev and Demidenko, 2010; Kouchinsky et al., Bed 3 contains abundant siliceous–phosphatic nodules, and abundant 2012). Based on these fossil distributions, three SSF assemblage SSFs were found in thin sections of these nodules. The 4.7-m-thick zones are recognized here within the Yanjiahe Formation, namely, Bed 4 consists of carbonaceous limestone that is non-fossiliferous. Bed in ascending order, the A. trisulcatus–P. anabarica assemblage zone, 5 is about 2.2 m thick and consists of siliceous–phosphatic intraclastic the P. antiqua assemblage zone and the Aldanella yanjiaheensis as- limestone containing diverse SSFs. semblage zone.

4.1. A. trisulcatus–P. anabarica assemblage zone (Zone I) 4. SSF associations of the Yanjiahe Formation The first occurrence of SSFs in the Yanjiahe Formation is in the Chen (1984) first recovered abundant SSFs from the siliceous– siliceous–phosphatic intraclastic dolostone of Bed 2. SSFs present phosphatic intraclastic carbonates in both the lower and upper parts in this interval include A. trisulcatus, P. anabarica, C. subcurvata, of the Yanjiahe Formation, and made a brief systematic description Hyolithellus tenuis(?) and Spinulitheca billingsi. These fossils belong of these fossils. Meanwhile, two SSF assemblage zones were recognized to the A. trisulcatus–P. anabarica assemblage zone (Fig. 2), which is sim- within the Yanjiahe Formation: the lower Circotheca (a misidentification ilar to that of East Yunnan (Steiner et al., 2007) and is early Meishucunian of Conotheca)–Anabarites–Protohertzina assemblage zone, and the upper in age. The early Meishucunian zone in South China was first Scenella–Aldanella–Maidipingoconus assemblage zone. These two SSF as- recognized as the Circotheca (a misidentification of Conotheca)– semblage zones correlate approximately with the Conotheca–Anabarites– Tiksitheca–Anabarites–Protohertzina zone by Qian (1978),butwas Paragloborilus assemblage zone of the Tianzhushan member, and they then named the A. trisulcatus–P. anabarica zone by Qian et al. provide good biostratigraphical tools for correlations of the Yanjiahe (1996). Steiner et al. (2007) re-definedthisassemblagezoneby Formation with other SSF-bearing sections on the Yangtze platform, the co-occurrence of the species Protohertzina unguliformis, P. anabarica

Fig. 2. Small shelly fossils in the Anabarites trisulcatus–Protohertzina anabarica assemblage zone. (a)–(c) Hyolithellus tenuis(?), GZA-Yjh-4-2/244, 245, 246; (d)–(f) Protohertzina unguliformis, GZA-Yjh-4-1/249, 253, 250; (g)–(h) P. anabarica, GZA-Yjh-4-1/57, 63; (i)–(j) A. trisulcatus, GZA-Yjh-4-2/20, 33; and (k)–(l) Conotheca subcurvata, GZA-Yjh-4-2/35, 22. All specimens from siliceous–phosphatic intraclastic dolostone of Bed 2 of the Yanjiahe Formation, Yichang, China. Scale bars represent 1 mm in (a), and (c)–(d); 500 μmin (b), and (k)–(l); 400 μm in (e), and (i)–(j); and 300 μm in (f)–(h). Author's personal copy

1002 J. Guo et al. / Gondwana Research 25 (2014) 999–1007 and A. trisulcatus (Fig. 2(d)–(f), (g)–(h), and (i)–(j)). Chen (1984) intro- correlations of the basal Cambrian, as it has been recognized in Kazakhstan duced the Circotheca (a misidentiﬁcation of Conotheca)–Anabarites– and Iran (Hamdi et al., 1989), Siberia (Khomentovsky and Karlova, 1993) Protohertzina assemblage zone for the lower SSF-bearing bed of and Mongolia (Khomentovsky and Gibsher, 1996). the Yanjiahe Formation, while Qian et al. (1999) introduced the P. anabarica–S. billingsi assemblage zone. Here we utilize the 4.2. Purella antiqua assemblage zone (Zone II) A. trisulcatus–P. anabarica assemblage zone since Bed 2 contains both of the widespread nominal species in South China and use of this zone This assemblage zone is erected here on the basis of discoveries promotes regional biostratigraphic correlations. of SSFs from thin sections of nodules in Bed 3 of the Yanjiahe Forma- This biozone has been recognized over a wide area on the Yangtze tion. Siliceous–phosphatic nodules (3–15 cm in diameter) are abun- platform, such as in eastern Yunnan (lower part of the Zhongyicun dant within dolostones and shales of Bed 3. Although it is difﬁcult to member), western Sichuan (lower part of the Maidiping Formation), recover SSFs from these nodules through traditional acid maceration southwestern Shaanxi (lower part of the Kuanchuanpu Formation) methods, a variety of SSF cross sections (Fig. 3(a)–(n)) were found in thin and northern Guizhou (lower part of the Gezhongwu Formation) sections of these nodules with various preservations. As the images in (Qian, 1989). It has great potential for international stratigraphic thin sections represent essentially random sections (cross, longitudinal

Fig. 3. Small shelly fossils in the Purella antiqua assemblage zone. (a) Lophotheca sp., Yjh-nodules/6; (b) P. antiqua, Yjh-nodules(thin slice number Yb-106); (c1)–(c3) A. trisulcatus,

Yjh-nodules(thin slice number Yb-40, Yjh-30, Yjh-30); (d1)–(d3) tubular microfossils, cross section, Yjh-nodules(thin slice number Yjh-30, Yjh-59A, Yjh-59A); (e1)–(e2) Conotheca sp., Yjh-nodules/96-1, 96-2; (f1)–(k3) tubular microfossils., longitudinal section; (f1)–(f2), Yjh-nodules(thin slice number Yb-64, Yb-67); (g1)–(g2), Yjh-nodules(thin slice number

Yb-129, Yjh-59A); (h1)–(h4), Yjh-nodules(thin slice number Yb-2, Yb-13, Yb-33, Yb-34); (i), Yjh-nodules(thin slice number Yb-38); (j1)–(j3), Yjh-nodules(thin slice number Yb-84,

Yb-57, Yb-107); (k1)–(k3), Yjh-nodules(thin slice number Yb-14, Yb-104,Yb-125); (n), Yjh-nodules(thin slice number Yb-98); (l1)–(l2) P. unguliformis, Yjh-nodules(thin slice number Yb-34, Yb-5); and (m1)–(m2) P. anabarica, Yjh-nodules(thin slice number Yb-5, Yb-116). All specimens from thin sections of siliceous–phosphatic nodules within Bed 3 of the

Yanjiahe Formation, Yichang, China. Scale bars represent 670 μm in (a); 300 μm in (b), (c2), (f2)–(g1), (h3), (j1)–(k2), (l2), and (m2)–(n); 200 μmin(c1), (d1), (h1)–(h2), (i), and

(l1); 500 μmin(d2), (e2)–(f1), (g2), (k3), and (m1); and 1 mm in (e1). Author's personal copy

J. Guo et al. / Gondwana Research 25 (2014) 999–1007 1003 or oblique) of skeletal fossils, variation in shape may have limited taxo- Paragloborilus–Siphogonuchites zone by Luo et al. (1980). Steiner et al. nomic value, and many of them are difficult to identify precisely. Never- (2007) re-defined this low middle Meishucunian zone in order to ad- theless, some of them with unique features can still be tentatively dress problems with the definition of the former S. triangularis– identified to genus or even species level. SSFs from Bed 3 include Paragloborilus subglobosus zone (Qian et al., 1996). The P. antiqua P. antiqua, Lophotheca sp., A. trisulcatus, Conotheca sp., P. unguliformis, assemblage zone in the Yanjiahe Formation is correlatable with the P. anabarica and abundant tubular microfossils. We here define this second SSF assemblage zone on the Yangtze platform since P. antiqua assemblage zone by the occurrence of the cap-shaped fossil P. antiqua is the most distinct taxon for defining the second biozone, though (=Emeiconus antiqua)(Qian et al., 1999). S. triangularis has not been recovered or identified from Bed 3 of the On the Yangtze platform, especially in eastern Yunnan, the second Yanjiahe Formation. Bed 3 can be correlated with the upper Maidiping SSF biozone is the Siphogonuchites triangularis–Purella squamulosa as- Formation in western Sichuan, the upper Zhongyicun member in east- semblage zone (Steiner et al., 2007), which was first introduced as the ern Yunnan, and the upper Gezhongwu member in northern Guizhou

Fig. 4. Small shelly fossils in the Aldanella yanjiaheensis assemblage zone. (a) A. yanjiaheensis, Yjh-003/5; (b)–(d) P. antiqua, RJP-Yjh(V/C)-01/66, 5, 6; (e) Purella lepidites, RJP-Yjh(V/C)-01/7; (f)–(g) Scenella jijiapoensis, Yjh-003/1, 6; (h)–(j) Siphogonuchites triangularis, RJP-Yjh(V/C)-01/141, 85, 15; (k) Yunnanodus sp., RJP-Yjh(V/C)-01/17; (l) Xianfengella ovata, Yjh-003/15; (m) Xianfengella prima, YJC-Yjh/238; (n) Igorella sp., YJC-Yjh/239; (o)–(p) A. trisulcatus, RJP-Yjh(V/C)-01/3, 8-2; (q)–(r) Anabarites cf. trisulcatus, RJP-Yjh(V/C)-01/4, 74; (s)–(t) Anabarites tianzhushanensis, RJP-Yjh(V/C)-01/117, 10; and (u)–(v) C. subcurvata, RJP-Yjh(V/C)-01/16, 81. All specimens from siliceous–phosphatic intraclastic limestone of Bed 5 of the Yanjiahe Forma- tion, Yichang, China. Scale bars represent 500 μmin(a),(c),(g)–(i), (l), (n)–(q), and (u)–(v); 300 μm in (b), (e), and (k); 400 μmin(d),(j),and(r)–(t); and 1 mm in (f), and (m). Author's personal copy

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(Qian et al., 1999). The P. antiqua assemblage zone can also be correlat- described by Chen (1984). Our more recent work yielded a large number ed with the same nominal zone in the upper Nemakit–Daldynian of of additional specimens from Bed 5. Taxa (Fig. 4(a)–(v); Fig. 5(a1)–(q2)) Siberia (Khomentovsky and Karlova, 1993). recovered from this interval include A. yanjiaheensis, A. trisulcatus, A. gracilis, A.cf.trisulcatus, A. tianzhushanensis, Ceratoconus bucerus, C. 4.3. A. yanjiaheensis assemblage zone (Zone III) subcurvata, C. obesa, C. nana, C. transulcata, C.sp.,P. antiqua, P. lepidites, Hertzina guizhouensis(?), H. tenuis(?), H. decorus (according to Chen, The siliceous–phosphatic limestone of Bed 5 of the Yanjiahe Forma- 1984), Lophotheca costellata (according to Chen, 1984), Igorella emeiensis tion is very rich in small shelly fossils, which were ﬁrst discovered and (=Maidipingoconus maidipingensis), Igorella sp., Microcornus parvulus(?),

Fig. 5. Small shelly fossils in the A. yanjiaheensis assemblage zone. (a1)–(a3), and (e) C. subcurvata, Yjh-003/59-2, 60, 50, 33-3; (b1)–(b2) Conotheca obesa, Yjh-003/27, RJP-Yjh(V/C)-01/24;

(c1)–(c2) Conotheca nana, Yjh-003/30, 59-1; (d1)–(d2) Conotheca transulcata, RJP-Yjh(V/C)-01/136, 18; (f1)–(f2) Conotheca sp., RJP-Yjh(V/C)-01/44, 73; (g1)–(g2) Hertzina guizhouensis(?),

RJP-Yjh(V/C)-01/93, 96; (h) H. tenuis(?), GZA-Yjh(V/C)-01/202; (i1)–(i2) Gen. et sp. uncertain, GZA-Yjh(V/C)-01/211, 210; (j) Microcornus parvulus(?), WSZ-Yjh/167; (k1)–(k2)

Paleosulcachites biformis(?), RJP-Yjh(V/C)-01/12, WSZ-Yjh/51; (l1)–(l3) Cupittheca mira, RJP-Yjh(V/C)-01/39, 25, Yjh-001/83; (m1)–(m2) P. unguliformis, RJP-Yjh(V/C)-01/83, 97;

(n1)–(n2) P. anabarica, RJP-Yjh(V/C)-01/32, WSZ-Yjh/50; (o) Rushtonia minuta, RJP-Yjh(V/C)-01/8-1; (p1)–(p2) Turcutheca crasseocochlia, GZA-Yjh(V/C)-01/233, 237; and (q1)–(q2) Turcutheca lubrica, RJP-Yjh(V/C)-01/98, 108. All specimens from siliceous–phosphatic intraclastic limestone of Bed 5 of the Yanjiahe Formation, Yichang, China. Scale bars represent

500 μmin(a1)–(a3), (b1), (c1)–(c2), (d1), (e), (f1)–(g2), (i1)–(k1), (l1), (l3)–(o), (p2), and (q2); 300 μmin(b2), and (d2); 400 μmin(k2), (l2), (p1), and (q1); and 1 mm in (h). Author's personal copy

J. Guo et al. / Gondwana Research 25 (2014) 999–1007 1005

Obtusoconus paucicostatus (according to Chen, 1984), Paleosulcachites The third SSF biozone in eastern Yunnan is the Watsonella crosbyi as- biformis(?), Cupittheca mira, P. unguliformis, P. anabarica, Rushtonia semblage zone (Steiner et al., 2007). This upper middle Meishucunian minuta, Sacciconus yanjiaheensis (according to Chen, 1984), Scenella zone was first introduced as the Heraultipegma yunnanensis zone by jijiapoensis (Parkhaev and Demidenko (2010) considered it a hyolith Qian et al. (1996),butitwasnotdefined in detail. Since H. yunnanensis operculum), S. triangularis, Tiksitheca korobovi (according to Chen, is a junior synonym of W. crosbyi (Steiner et al., 2007; Li et al., 2011), 1984), Turcutheca crasseocochlia, T. lubrica, Yunnanodus sp., Xianfengella Steiner et al. (2007) re-described the zone as the W. crosbyi assemblage ovate and X. prima. Chen (1984) defined this interval as the Lophotheca– zone, which is defined by the co-occurrence of W. crosbyi, Aldanella Aldanella–Maidipingoconus assemblage zone. But Lophotheca first oc- yanjiahensis and Oelandiella korobkovi. Since there is no occurrence curs in the second zone and Maidipingoconus is an incorrect identifica- of W. crosbyi or O. korobkovi in the Yanjiahe Formation, A. yanjiahensis tion, hence we re-define the assemblage zone by the occurrence of is taken here as the nominal taxon for defining the biozone. The the species A. yanjiaheensis, which is an important taxon for corre- A. yanjiahensis assemblage zone in the Yanjiahe Formation is equivalent lating the lower Tommotian and for defining the base of Cambrian to the W. crosbyi assemblage zone in eastern Yunnan, indicating that the Stage 2 (Li et al., 2011). Jiang et al. (2012) put this SSF biozone in the upper Yanjiahe Formation can be approximately correlated with the basal Shuijingtuo Formation, but this interpretation is inconsistent with Dahai member in eastern Yunnan. But since Bed 4 is unfossiliferous, it is fossil distributions although the lowermost interval (below the FAD uncertain whether the base of the Dahai member should be correlated of trilobites) of the Shuijingtuo Formation may be latest Meishucunian withthebaseofBed5orwithahorizoninBed4.BothW. crosbyi and in age. Aldanella attleborensis (possibly a senior synonym of A. yanjiaheensis)

Fig. 6. The distributions of small shelly fossils through the Yanjiahe Formation in seven sections, Yichang, China. Author's personal copy

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first occur near the base of the Tommotian in Siberia, indicating that Bed 5 (2). Based on extensive fossil collections, three SSF biozones are rec- of the Yanjiahe Formation could be correlated with the lower Tommotian. ognized within the Yanjiahe Formation. Listed in ascending order, these are the A. trisulcatus–P. anabarica assemblage zone, 5. Discussion the P. antiqua assemblage zone and the A. yanjiaheensis assemblage zone. The first and third biozones are modified from Chen's Since neither skeletal nor trace fossils have been found in the basal biozonation (Chen, 1984), while the second one is new and is part (Bed 1) of the Yanjiahe Formation, the age of this interval is some- based on SSFs discovered in siliceous–phosphatic nodules from 13 what uncertain. However, the strong negative δ Ccarb excursioninthe Bed 3. These three biozones can be well correlated to those of basal Yanjiahe Formation (Jiang et al., 2012) implies that Bed 1 belongs SSF assemblage zones in eastern Yunnan (Steiner et al., 2007). to the lowermost Cambrian (Maloof et al., 2010). Additionally, the occur- (3). The occurrence of A. yanjiaheensis in Bed 5 indicates that Bed 5 rence of the earliest Cambrian acritarch M. regulare in Bed 1 (Ding et al., probably belongs to Cambrian Stage 2, but the position of the 1992a) indicates that it belongs to the basal Cambrian. Small shelly fos- Stage 2/Stage 1 boundary is uncertain since Bed 4 lacks fossils. sils were recovered mainly from Beds 2, 3 and 5 of the Yanjiahe Forma- (4). The stepwise increase in generic diversity through the Yanjiahe tion, and correspondingly three SSF assemblage zones are recognized, Formation is consistent with the early–mid Meishucunian diversi- namely (in ascending order) the A. trisulcatus–P. anabarica zone (Bed ty pattern on the Yangtze platform (Li et al., 2007), and is compa- 2), the P. antiqua zone (Bed 3), and the A. yanjiaheensis zone (Bed 5). rable with the global diversity increase through the Nemakit– These three SSF biozones can be well correlated with the SSF biozones Daldynian to early Tommotian interval. The pre-trilobitic faunal in eastern Yunnan (Steiner et al., 2007), indicating that the Yanjiahe diversity increase largely resulted from the diversification of SSFs. Formation is early to middle Meishucunian in age. The fourth SSF Sinosachites flabelliformis–Tannuolina zhangwentangi assemblage zone in eastern Yunnan is not recognized in the Three Gorges area. Around Acknowledgments the Yanjiahe village, the first occurrence of trilobite is usually several me- ters above the base of the Shuijingtuo Formation, indicating that the low- This research is funded by the National Basic Research Program of ermost interval (below the FAD of trilobites) of the Shuijingtuo China (grant no. 40902007), the Chinese Academy of Sciences (grant Formation may be latest Meishucunian in age. Thus, the minor disconfor- no. KZCX2-EW-115), the Ministry of Science and Technology of China mity between the Yanjiahe and Shuijingtuo formations is also late (grant no. 2013CB837100 and no. 2013CB835002), the China Postdoc- Meishucunian in age. Based on bio- and chemo-stratigraphic evidence, toral Science Foundation (grant no. 20100471020), the Doctoral Fund the Meishucunian has been roughly correlated with the Nemakit– of Ministry of Education of China (grant no. 20100205110003), and the Daldynian and the Tommotian of the Siberia platform (Brasier et al., Special Fund for Basic Scientific Research of Central Colleges, Chang'an 1990; Zhang et al., 1997; Qian et al., 2001; Zhu et al., 2001; Steiner et University, China (grant no. CHD2011TD005, no. CHD2011TD020, and al., 2007; Maloof et al., 2010; Kouchinsky et al., 2012). Hence, the no. CHD2010JC074). We thank Heyo Van Iten (Hanover College, Indiana) Yanjiahe Formation can be correlated with the Nemakit–Daldynian and for his suggestions and for improving the usage of the English language the lower Tommotian, and this biostratigraphic correlation is approxi- in this work. Three anonymous reviewers critically read the manuscript mately consistent with chemo-stratigraphy (C and Sr isotopes) data and gave many useful comments and suggestions that helped to improve (Sawaki et al., 2008; Jiang et al., 2012). On the Yangtze platform, the the text. Yanjiahe Formation can be correlated with the following lithological units: the Zhongyicun and Dahai members in eastern Yunnan, the Maidiping Formation in southwestern Sichuan, and the Kuanchuanpu References

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