Lower Cambrian Small Shelly Fossils of northern Sichuanand southern Shaanxi (China), and their biostratigraphic importance
Small Shelly Fossils du Cambrien inférieur du nord du Sichuanet du sud du Shaanxi (Chine) et leur importance biostratigraphique
Michael Steiner a,*, Guoxiang Li b, Yi Qian b, Maoyan Zhu b
a Technische Universität Berlin, Sekr. ACK 14, Ackerstrasse 71-76, 13355 Berlin, Germanyb Nanjing Institute of Geology and Palaeontology, Academia Sinica, 210008 Nanjing, China
Au cours des vingt dernières années des informations complètes ont été obtenues sur la distribution des Small Shelly Fossils (SSF) dans lessédiments peu profonds de la bordure ouest de la plate-forme du Yangtze (partie est de la province du Yunnan et partie centrale de la provincedu Sichuan). Cependant, on connaît peu de choses sur la distribution stratigraphique et spatiale des SSF sur la marge nord de la plate-forme duYangtze. Nous décrivons ici des associations de SSF provenant de Kuanchuanpu (Ningqiang County, sud de la province de Shaanxi), Shatanet Xinli (ces deux localités étant dans le Xixiang County, sud de la province de Shaanxi), région SE Shaanxi – NW Hubei. L’épaisseur desstrates à SSF du Cambrien inférieur décroît fortement d’ouest en est dans cette région et le hiatus stratigraphique augmente. Un hiatus à la basedu Cambrien est largement distribué sur la marge nord de la plate-forme du Yangtze. Au même moment on note la formation de brèchessynsédimentaires dans des environnements de large plate-forme peu profonde. Une mobilisation tectonique peut-être liée à l’assemblage du
Keywords: Small Shelly Fossils; Biostratigraphy; Early Cambrian; China; Yangtze Platform; Embryos
Mots clés : Small Shelly Fossils ; Biostratigraphie ; Cambrien inférieur ; Chine ; Plate-forme du Yangtze ; Embryons
1. Introduction
Small Shelly Fossils (SSF) are abundant in Early Cam-brian shallow water carbonates of theYangtze Platform. Theyare of great importance for the biostratigraphic subdivisionand correlation of the pre-trilobitic sequences, since thestratigraphically important archaeocyathids are widely lack-ing on the Yangtze Platform and trace fossils also vary ac-cording to different facies. The importance of the SSFs forthe biostratigraphy of the Lower Cambrian was partly ques-tioned (Landing, 1994) because of a common provincialismand restriction to shallow water facies, a long stratigraphicalrange of critical taxa, and the fact that the affiliation of themorphospecies (part-based taxa) to genuine biological spe-cies is, to a large extent, undetermined, and an artificialdiversity has been developed by taxonomic over-splitting(Bengtson, 1985). Simply constructed sclerites of differentorganisms may show homologous structures and re-deposited fossils may be difficult to verify (Qian and Bengt-son, 1989; Bengtson et al., 1990). On various Precambrianplatforms different stratigraphic ranges have been detectedfor some index fossils (compare Brasier, 1989; Bengtson etal., 1990). Although these problems can lead to difficultiesfor international correlation, the great utility of SSFs forregional correlation has been repeatedly demonstrated in thepast (Qian et al., 1999; Alexander et al., 2001).
An accepted SSF-based zonation exists today for at leastthe basal Cambrian of a wide range of sections on theYangtzePlatform (Qian, 1989; Qian and Bengtson, 1989; Qian et al.,1999,2001). Intense research was carried out on SSF faunasof the Yangtze Platform during the 1980s in preparation forthe proposal of different sections as candidates for thePrecambrian-Cambrian Global Stratotype Section and Point(GSSP). However, most taxonomic and biostratigraphic in-vestigations focused on SSFs of the Meishucunian and earli-est Qiongzhusian of East Yunnan (e.g. Luo et al.,1982,1984,1994; Qian, 1989; Qian and Bengtson, 1989; Zhuet al., 2001). Younger SSF associations received less atten-tion, a problem exacerbated by the fact that a fossil extraction
from the mainly siliciclastic sequences of the Qiongzhusianand Canglangpuian is by far more difficult than from carbon-ates.
Previous investigations on SSF of northern Sichuan andsouthern Shaanxi mainly concentrated on taxonomic de-scriptions of new taxa (Yang et al., 1983; Li, 1984; Xing andYue, 1984; Yang and He, 1984; He, 1987; Yue, 1986; Xie,1990; Ding et al., 1992). Well-preserved fossil material fromsouthern Shaanxi was later also re-considered in interna-tional revisions of selected SSF-groups (Conway Morris andChen, 1989,1991,1992). Exceptional egg-stages and em-bryos have been published by Bengtson and Yue (1997).However, most of the early investigations lacked detailedbiostratigraphic documentation.
The aim of the present publication is to provide an over-view of the regional and stratigraphic distribution of the mostimportant SSF taxa, extraordinarily preserved embryos, andphosphatized soft-bodied organisms on the northern edge ofthe Yangtze Platform, and to discuss their biostratigraphicrelevance.
2. SSF associations of Southwest Shaanxi region
Under the term Southwest Shaanxi region the Early Cam-brian outcrops of the area between Kuanchuanpu (NingqiangCounty) to Zhangjiagou (Hexi district, western XixiangCounty) are summarized here (Fig. 1). Due to lithologic andbiostratigraphic similarity this region also includes the north-ernmost range of Sichuan Province (Shatan, Xinli, both Nan-jiang County).
The thickest and most complete sections with preservedstrata of the Meishucunian Stage of the Early Cambrian(equivalent to the Nemakit-Daldynian and Tommotian ofSiberia) are located near Kuanchuanpu. The thickness of theLower Cambrian succession decreases continuously towardthe east or southeast. In the Shizhonggou section, approxi-mately 2 km south of Kuanchuanpu, the Meishucunian com-prises the ca. 55-m-thick Kuanchuanpu Formation and partsof the siliciclastic Guojiaba Formation. The Kuanchuanpu
Formation consists mainly of limestones and phosphaticlimestones, with thick interlayers of cherts and some shales(Fig. 2). The uppermost 10 m of the Kuanchuanpu Formationconsist of clastic limestones, which contain breccia beds andconglomerates. The underlying dolostones of the Neoprot-erozoic Dengying Formation are disconformably overlain bythe Kuanchuanpu Formation. The top of the Dengying For-mation contains numerous phosphatized cloudiniids atKuanchuanpu (Bengtson and Yue, 1992).
The main distribution of Lower Cambrian SSFs is re-stricted to an approximately 15-m-thick horizon in themiddle Kuanchuanpu Formation (Fig. 2). The examinedSSFs belong to the Anabarites trisulcatus – Protohertzinaanabarica Assemblage Zone (Fig. 3). Except for the abun-dant protoconodont Protohertzina, and the common ana-baritids and siphogonuchitids, it also contains numerousphosphatized remains of soft-bodied metazoan eggs, em-bryos, larvae and adult stages, such as possible cnidarianpolyps, algae, and bacteria (Fig. 4). In addition, rare repre-sentatives of enigmatic fossils occur, e.g. Carinachites,Hexaconularia, Cambrothyra, and Punctatus, which locallycan be more frequent in Xixiang or Nanjiang counties. It isimportant to mention that Anabarites obliquasulcatus andA. sulcoconvex, both of which were initially described asseparate species, always appear together with Anabaritestrisulcatus. Additionally, transitional forms can be observed
between A. trisulcatus and A. sulcoconvex, and betweenA. obliquasulcatus and A. sulcoconvex (Fig. 3(15, 16)),which led us to regard the latter two species as synonyms ofAnabarites trisulcatus and to treat them here merely asintraspecific varieties. A similar morphological convergencewas pointed out by Qian and Bengtson (1989) between Ana-barites trisulcatus and Conotheca subcurvata. Specimenspreviously described as Anabarites rotunda, which is hereinalso treated as morphological variant (A. trisulcatus formarotunda), indicate a morphologically intermediate positionbetween the latter two species. Conotheca subcurvata andAnabarites trisulcatus also show a similar stratigraphic dis-tribution in the investigated region. The co-occurrence ofProtohertzina unguliformis with the considerably more rareP. anabarica hints that both protoconodont species maybelong to the same apparatus of one biospecies. Morphologi-cal analogies and the co-occurrence of Siphogonuchites tri-angularis, S. mirus, Lopochites latazonalis, andMaikhanella multa in Kuanchuanpu (Fig. 3(2, 3, 5, 7)), andalso in Xinli and Shatan, further support the suspicion thatthese taxa merely represent different sclerites of the samecoeloscleritophoran organism (Bengtson, 1992). This isstrengthened by the fact that merged sclerites of Siphogonu-chites occur and some sclerites show individual features ofboth Lopochites and Siphogonuchites (Fig. 3(6)). It is re-markable that Siphogonuchites triangularis and Lopochiteslatazonalis, which are both typical representatives of theyounger Siphogonuchites triangularis – Paragloborilussubglobosus Zone in Yunnan (Qian and Bengtson, 1989),occur frequently in the Anabarites trisulcatus – Proto-hertzina anabarica Zone of Southwest Shaanxi. However,indefinite siphogonuchitids were also reported from the olderSSF zone in Yunnan (Qian and Bengtson, 1989). Further-more, no unambiguous Paragloborilus specimens were re-ported from the Southwest Shaanxi region.
Unfortunately, the basal and terminal sequence of theKuanchuanpu Formation did not contain determinable SSFsexcept for Conotheca sp. in the investigated Shizhonggousection. However, Xing and Yue (1984) described an ap-proximately one-meter-thick limestone bed with phosphateclasts, containing Paragloborilus subglobosus, Igorella cf.ungulata, Aldanella sp., Watsonella sp. (as ?Heraultipegmasp.) and numerous other taxa of molluscs from the upperKuanchuanpu Formation of the nearby Yuanjiaping section.In a re-study of SSFs from this interval of the Yuanjiapingsection we could only recover few helcionellid sclerites(Fig. 4(11, 12)), such as Bemella sp. and pustulous, cap-shaped steinkerns, previously described as “Tuberoconus”paucipapillae (Xing et al., 1984). The described horizonprobably correlates with the conglomeratic and partiallybrecciated uppermost part of the Kuanchuanpu Formation ofthe Shizhonggou section (Fig. 2). While the single specimenof Paragloborilus subglobosus figured in Xing and Yue(1984) has to be regarded as questionable, the determinationsas Aldanella sp. and Watsonella (?Heraultipegma) sp. appearplausible.
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Beijing
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Fig. 1. Locality map of SSF-bearing sections in South Shaanxi / NorthSichuan (triangles indicate sections discussed in the text).Fig. 1. Carte des localités où sont situées les coupes contenant les SSF dansle sud du Shaanxi / nord du Sichuan (les triangles indiquent les coupesdiscutées dans le texte).
In the nearby villages of Guojiaba and Zhaojiaba, approxi-mately 5 km south of Kuanchuanpu, several thin interbeds ofphosphatic conglomerates exist within the Guojiaba Forma-tion. Their exact stratigraphic position, however, remainsuncertain because of poor exposure. Mitral and sellate scler-ites of Tannuolina zhangwentangi were recovered from oneof these layers (Fig. 3(9)).
In the Shatan section, the Kuanchuanpu Formation isconsiderably reduced in thickness (5.75 m; Fig. 5). A phos-phatic hardground, indicating a disconformity, exists at thebase of the Kuanchuanpu Formation. A trend toward strongersilicification of the limestones occurs from the top of theformation downward. Determinable SSFs were recoveredfrom only the uppermost 2 m of the formation. In somesamples the SSFs are frequently coated by migrated pyrobi-tumen. Most of the Kuanchuanpu Formation at Shatan repre-sents the Anabarites trisulcatus – Protohertzina anabaricaZone (Fig. 5). Due to the lithologic similarity with the Kuan-chuanpu area, and a comparable SSF association, we applythe term Kuanchuanpu Formation to the limestone sequencein Shatan instead of the formerly used Mofangyan Memberof the Dengying Formation (Yang et al., 1983). The higherpart of the Kuanchuanpu Formation, as present at Kuanchua-npu, seems to be missing in this section. However, the forma-tion is terminated by a 16-cm-thick limestone bed with brec-ciated phosphate clasts, which represents a reworkedhorizon. Except for Protohertzina unguliformis and Conoth-eca subcurvata this horizon contains numerous helcionellidmolluscs, such as Igorella sp., and Bemella costa, but alsoZhijinites longistriatus, Chancelloria cf. eros, and Halkieriasthenobasis (Fig. 6). However, these molluscs, zhijinitids andcoeloscleritophorans occur in higher zones in other regionsof the Yangtze Platform, as for example in Yunnan. There-fore, it can be assumed that a mixing of several SSF associa-tions occurred at the top of the Kuanchuanpu Formation atShatan. It is also worth noting that Anabarites trisulcatuscould neither be detected at Shatan nor at Xinli, although it isa common constituent of the Anabarites trisulcatus – Proto-hertzina anabarica Zone in shallow water environments. AtZhangjiagou anabaritids, which accompany Protohertzinaanabarica, are extremely rare and only few fragments wererecovered among thousands of specimens of Conotheca sub-curvata. However, these individuals are morphologicallycloser related to A. tripartitus / A. trymatus, due to theexistence of depressions on the three lobes. The rare occur-rence or lack of anabaritids east of Kuanchuanpu may beexplained by different facies settings. Anabaritids also onlyoccurred very rarely in SSF associations of deeper shelfsettings on the central and eastern Yangtze Platform (Dingand Qian, 1988; He and Yu, 1992; Steiner et al., 2003).
The basal Cambrian phosphatic carbonates are muchthicker at Xinli compared with the Kuanchuanpu Formationat Shatan (Fig. 7), although the section is only situatedapproximately 15 km east of Shatan. Because of the preva-lence of dolostones in this sequence it was treated as the XinliMember of the Dengying Formation (Yang et al., 1983). Due
to the fact that there occur limestones of the KuanchuanpuFormation in the Zhangjiagou section (Hexi area), approxi-mately 40 km further northeast, again, the existence of LowerCambrian dolostones in Xinli may be attributed to localdolomitization. The recovered SSF association is well com-parable to that of the Anabarites trisulcatus – Protohertzinaanabarica Zone from Kuanchuanpu (Fig. 8). In addition tothis, it also contains Mongolodus longispina and M. platy-basalis (Fig. 8(7)).
3. SSF associations of the Southeast Shaanxi –Northwest Hubei Region
The Southeast Shaanxi – Northwest Hubei region com-prises an area east of Xixiang and is separated from theSouthwest Shaanxi region by a multiple fault zone (Yin et al.,1993). It extends to the Shennongjia district of northwesternHubei.
The carbonate and phosphatic sequences of the LowerCambrian are thin in the exposures east and southeast ofXixiang. Thicknesses vary strongly, even in short distances.In the Sanlangpu section merely a 5 cm thick conglomeraticlimestone bed of the Xihaoping Member of the DengyingFormation is exposed (Fig. 9), whereas the same interval isapproximately 45 cm thick in Xiaowan (Fig. 10). However,in earlier publications a thickness of 8.47 m was mentionedfor the Xihaoping Member in Sanlangpu, and a thickness ofabout 8.4 m was described for the same member in Xiaoy-angba, Zhenba County, southeast of Xixiang (Qian et al.,1999). Lithology and biostratigraphy indicate a large basalCambrian hiatus for this region.
The SSF associations of the two investigated sections aremainly characterized by the frequent occurrence of Cam-brothyra ampulliformis, C. truncata, Ninella tarimensis,Cambroclavus fangxianensis, the chancellorids Archiaster-ella pentactina, Chancelloria cf. eros, Chancelloria? arida,Allonnia tetrathallis, and partly undetermined or unknownmolluscs (Fig. 11). The occurrence of Yochelcionella cf.chinensis is noteworthy, since it has so far not been reportedfrom the Lower Cambrian of the Yangtze Platform. Repre-sentatives of the early Anabarites trisulcatus – Protohertzinaanabarica Zone, the Siphogonuchites triangularis – Para-globorilus subglobosus Zone, and the Watsonella yunnanen-sis Zone have not been demonstrated for Southeast Shaanxiand Northwest Hubei. In earlier reports, Rhombocorniculumwas repeatedly mentioned from the Xihaoping Member ofSoutheast Shaanxi (Xie, 1990; Ding et al., 1991; Qian et al.,1999). However, this occurrence could neither be confirmedfor Sanlangpu and Xiaowan (this study), nor for the Zhenbaarea (Li, 2000). The only figured specimen from the Xihaop-ing Member of Sanlangpu, which has been assigned toRhombocorniculum (Ding et al., 1991: pl. 3, Fig. 27), ap-pears questionable. However, at Xiaoyangba, ZhenbaCounty, Rhombocorniculum occurs abundantly in the lowerShuijingtuo Formation, which overlies the Xihaoping Mem-
ber. In contrast to Southeast Shaanxi, Rhombocorniculumhas been documented from the top Xihaoping Member ofNorthwest Hubei (Qian and Zhang, 1983; Qian et al., 1999).
In Sanlangpu, black shales of the Guojiaba Formation,about 3 m above the conglomeratic limestone bed of theXihaoping Member, contain the eodiscid trilobite Tsunyidis-cus, as well as sclerites of Allonnia, and bradoriids. Thisassociation is typical for the Tsunyidiscus Subzone of theWutingaspis – Eoredlichia Zone of the Qiongzhusian Stagein Yunnan (Steiner et al., 2001a).
4. Exceptional preservation of eggs, embryos,and soft-bodied organisms
The first description of phosphatic fossils with similarityto eggs of animals was given in the original description ofOlivooides from Kuanchuanpu (Qian, 1977). With the selec-tion of the name for the fossil, Qian gave the first hint of afossil preserved as an egg-stage, although Olivooides wasofficially classified together with Archaeooides and otherremains as “problematic fossils”. During later investigations,Xing et al. (1984) and Yue (1986) observed distinct spinosestructures under a thin external layer in individual specimensof Olivooides. This led to an assignment of Olivooides to thePorifera (Xing et al., 1984; Yue, 1986).
Zhang and Pratt (1994) first discovered phosphatized ani-mal embryos on theYangtze Platform from the Middle Cam-brian Gaotai Formation (Guizhou Province). Finally, Bengt-son and Yue (1997) re-investigated material of Olivooidesand interpreted Olivooides as egg-stages and embryos of theconical fossil Punctatus that co-occurs in the same horizon.Conway Morris and Chen (1992) already had drawn atten-tion to the similarity in the spinose surface structure ofPunctatus with Olivooides but interpreted this to be acciden-tal. Later, Yue and Bengtson (1999) formally synonymizedPunctatus with Olivooides and indicated Punctatus was anadult stage within an ontogenetic cycle of different eggs andembryos of Olivooides.
During this study, both Olivooides and Punctatus havebeen documented from a wide stratigraphic range within the
Anabarites trisulcatus – Protohertzina anabarica Zone inthe Southwest Shaanxi region (Figs. 2, 5 and 7). Thousandsof specimens of Olivooides and several dozens of specimensof Punctatus have been recovered from the Shizhonggousection, Kuanchuanpu (Fig. 4(1–7, 9)). However, their de-tailed description is beyond the scope of this study. Theobserved structures confirm the former interpretation ofOlivooides as eggs or embryos. However, Olivooides multi-sulcatus also shows a great morphologic variability, which is,on the one hand, explained biologically by the differentontogenetic stages, but on the other hand also by numeroustaphonomic variants of the same stages. A preliminary studyof the egg-stages and embryos indicates that at least 3 typesof embryos can be distinguished from the egg-stages. It canbe assumed that the true biological diversity, concealed underthe form-taxon name Olivooides multisulcatus, is consider-ably higher. However, final conclusions can only be drawnwith further detailed investigations on the developmentalbiology of the embryos.
Here we distinguish one embryo type with holoblasticcleavage (type A, Fig. 4(2)), one embryo type with discoidalgerm band development (type B, Fig. 4(4)), as well as anembryo type with spinose surface structure and pentameroussymmetry (Punctatus type, Fig. 4(1, 3, 6)). It is undecidedwhether all embryos with holoblastic cleavage developedinto the Punctatus type. It can be assumed that other organ-isms developed embryos with similar morphology in theearly blastula stage since this cleavage pattern is widelydistributed. At least embryo type B belonged to an organismdifferent from Punctatus emeiensis. Both embryos of type Aand Punctatus type developed smooth external egg cases thatare indistinguishable from Olivooides in the case of completepreservation. Therefore, a synonymization of all Olivooideswith Punctatus, as proposed by Yue and Bengtson (1999),appears unjustified. In addition, no indication of an embry-onic stage is available in the type material of Olivooides.Considering this, Olivooides represents a form-taxon, whichis not monospecific. Punctatus emeiensis, however, may be agenuine biospecies.
The preservation of numerous hollow eggs and embryosindicates that the process of phosphatization was directed
Fig. 3. All specimens from the Kuanchuanpu Formation of Shizhonggou section, Kuanchuanpu, Ningqiang County, South Shaanxi Province, except for Fig. 9,which is from the Guojiaba Formation, Zhaojiaba village, Ningqiang County. Thick scale bar equals 500 µm; thin scale bar 100 µm. Samples stored at TU Berlincollection (TUBACK). 1. Lateral view of Cambrothyra truncata, No Kua 125-143. 2. Maikhanella multa, No Kua 125-57. 3. Siphogonuchites mirus, No Kua123b-6. 4. Coccoidal algae or bacterial colonies, No Kua 125-54. 5. Siphogonuchites triangularis, No Kua 121-02. 6. Form transitional between Lopochiteslatazonalis and Siphogonuchites triangularis, No Kua 123b-3. 7. Lopochites latazonalis, No Kua 125-36. 8. Protohertzina anabarica, No Kua 125-118. 9.Tannuolina zhangwentangi, No Q 6-1. 10. Spindle-shaped spicule (?cnidarian spicule), No Kua 125-71. 11. Protohertzina unguliformis, No Kua 124-01. 12.Protohertzina unguliformis, No Kua 121-02. 13. Hyolithellus sp., No Kua 125-136. 14. Anabarites trisulcatus, No Kua 115-11. 15. Anabarites trisulcatus formasulcoconvex, No Kua 115-06. 16. Anabarites trisulcatus forma obliquasulcatus, No Kua 125-43. 17. Conotheca subcurvata, No Kua 125-131.Fig. 3. Tous les spécimens viennent de la Formation Kuanchuanpu, coupe de Shizhonggou, sud du Shaanxi, sauf 9, (Formation Guojiaba, village de Zhaojiaba,Ningqiang County). Barre d’échelle épaisse = 500 µm, barre fine = 100 µm. 1. Vue latérale de Cambrothyra truncata, No Kua 125-143. 2. Maikhanella multa,No Kua 125-57. 3. Siphogonuchites mirus, No Kua 123b-6. 4. Algues coccoïdales ou conolies bactériennes, No Kua 125-54. 5. Siphogonuchites triangularis, No
Kua 121-02. 6. Forme de transition entre Lopochites latazonalis et Siphogonuchites triangularis, No Kua 123b-3. 7. Lopochites latazonalis, No Kua 125-36.8. Protohertzina anabarica, No Kua 125-118. 9. Tannuolina zhangwentangi, No Q 6-1. 10. Spicule en forme de fuseau (spicule de cnidaire ?), No Kua 125-71.11. Protohertzina unguliformis, No Kua 124-01. 12. Protohertzina unguliformis, No Kua 121-02. 13. Hyolithellus sp., No Kua 125-136. 14. Anabaritestrisulcatus, No Kua 115-11. 15. Anabarites trisulcatus forme sulcoconvex, No Kua 115-06. 16. Anabarites trisulcatus forme obliquasulcatus, No Kua 125-43.17. Conotheca subcurvata, No Kua 125-131.
Fig. 4. All specimens from the Kuanchuanpu Formation of Shizhonggou section, Kuanchuanpu, Ningqiang County, South Shaanxi Province, except for Figs. 11, 12,which are from the upper Kuanchuanpu Formation of Yuanjiaping village, Ningqiang County. Thick scale bar equals 500 µm; thin scale bar 100 µm. Samplesstored at TU Berlin collection (TUBACK). 1. Late embryo of Punctatus type (late blastula stage), indicating a pentaradial symmetry and initial formation of aspinous surface texture, No Kua 125-95. 2. Embryo type A (early blastula stage) with clearly confined blastomeres, indicating a holoblastic cleavage, No Kua125-93. 3. Partly decayed embryo of Punctatus type within the egg case, No Kua 125-30. 4. Embryo type B, indicating the formation of discoidal germ band, No
Kua 120-01; A, vegetal pole with rectangular folding; B, animal pole with discoidal germ band. 5. Smooth egg of Olivooides multisulcatus, No Kua 121-23. 6.Late embryo of Punctatus type with removed egg case; late blastula stage before hatching; note pentaradial symmetry of apertural region, No Kua 125-30. 7.Hatched individual of Punctatus emeiensis, apertural view; note pentaradial symmetry of aperture, No Kua 115-18. 8. Carinachites spinatus, No Kua 125-40.9. Hatched individual of Punctatus emeiensis, lateral view; note spinous surface texture of antapical part, No Kua 115-11. 10. Possible polyp of a cnidarian, No
Kua 125-56. 11. Bemella sp., NoYug 1-1. 12. Undetermined steinkern of a helcionellid shell with pustulous surface structures, NoYug 1-4. 13. Stalked specimenof Acanthocassis orthacanthus, No Kua 115-01; A, view into the five-tentacled crown (two are broken); B, lateral view.Fig. 4. Tous les spécimens viennent de la Formation Kuanchuanpu, coupe de Shizhonggou, sud du Shaanxi, sauf 11 et 12, Formation Guojiaba, village deZhaojiaba, Ningqiang County. Barre d’échelle épaisse = 500 µm, barre fine = 100 µm. Échantillons dans les collections de TU Berlin (TUBACK). 1. Stadeembryonnaire tardif de type punctatus (stade blastula tardif) indiquant une symétrie pentaradiée et la formation initiale d’une texture de surface épineuse, No
Kua 125-95. 2. Embryon de type A (stade blastula précoce) montrant des blastomères clairement resserrés indiquant un clivage holoblastique, No Kua 125-93.3. Embryons partiellement décomposés de type punctatus (enveloppe enlevée), No Kua 125-30. 4. Embryon de type B, indiquant la formation d’une bandegerminative discoïdale, No Kua 120-01. A, pôle végétatif avec plis rectangulaires. B, pôle animal avec bande germinative discoïdale. 5. Œuf lisse de Olivooidesmultisulcatus, No Kua 121-23. 6. Embryon tardif de type punctatus avec enveloppe enlevée ; blastula tardive avant l’éclosion. Noter la symétrie pentaradiée del’ouverture, No Kua 125-30. 7. Individu éclos de Punctatus emeiensis, vue latérale. Noter la surface épineuse de la partie antapicale, No Kua 115-18.8. Carinachites spinatus, No Kua 125-40. 9. Individu éclos de Punctatus emeiensis, vue latérale; noter la texture de surface épineuse de la partie antapicale. 10.Possible polype de cnidaire, No Kua 125-56. 11. Bemella sp., NoYug 1-1. 12. Moule interne de coquille d’hécionellidé avec des structures de surface pustuleuses,No Yug 1-4. 13. Spécimen de Acanthocassis orthacanthus, No Kua 115-01 ; A, vue d’une couronne à 5 tentacules (2 sont cassées). B, vue latérale.
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mul
ta
Zhi
jinite
s lo
ngis
tria
tus
Zhi
jinite
s sp
.
Rha
bdoc
hite
s ex
aspe
ratu
s
Hyo
lithe
llus
sp.
Igor
ella
sp.
Pur
ella
cf.
squa
mul
osa
Bem
ella
cos
ta
Cha
ncel
loria
cf.e
ros
Oliv
ooid
es m
ultis
ulca
tus
Hex
acon
ular
ia s
p.
Hal
kier
ia s
then
obas
is
Pun
ctat
us e
mei
ensi
s
Hel
cion
ella
sp.
?
Fig. 5. Stratigraphic column of the Kuanchuanpu Formation at Shatan, Nanjiang County (Sichuan Province) including occurrences of Small Shelly Fossils(asterisk: reworked horizon with mixing of zones).Fig. 5. Colonne stratigraphique de la Formation Kuanchuanpu à Shatan Nanjiang County (Province de Sichuan) avec indication de la présence de Small ShellyFossils (astérisque = horizons remaniés avec mélanges de zones).
267M. Steiner et al. / Geobios 37 (2004) 259–275
268 M. Steiner et al. / Geobios 37 (2004) 259–275
from the exterior to the interior. Cavities within egg caseswere maintained or later filled by secondary phosphate. Inthis light, we interpret the structures, initially discussed asprimary tissues (Yue and Bengtson, 1999: Fig. 9) from theinterior of some specimens of Olivooides, as secondary sta-lactitic phosphate growth along an organic matrix derivedfrom the enzymatic or bacterial disintegration of the cells.Similar structures were also found in cavities of other SSFs,as well as in phosphate clasts of inorganic origin. Someindividuals of Olivooides also show distinctive phosphaticmicroglobules, which are interpreted here as algae or bacte-ria. Phosphatized bacterial envelopes and coccoidal algae(Fig. 3(4)) are widespread in the Kuanchuanpu Formation.The figured specimen of a “cleavage embryo” with yolkglobules (Yue and Bengtson, 1999: Fig. 6) probably alsorepresents coccoidal algae or bacterial colonies.
Besides the extraordinary preservation of egg stages, em-bryos, and larva, the undoubtedly very early phosphatizationalso led to the preservation of some soft-bodied organisms, asfor example that of a possible cnidarian polyp (Fig. 4(10)), orof the polyp-like Acanthocassis orthacanthus (Fig. 4(13)).Although the latter organism resembles hydrozoan polyps,its detailed biological assignment remains uncertain becauseof the pentaradial symmetry and the lack of a mouth opening.Fragments of this species were also frequently found in theKuanchuanpu Formation at Xinli (Fig. 8(8, 9)).
5. Biostratigraphic implication of SSF associations
Comparison of the SSF distribution in the SouthwestShaanxi region and the Southeast Shaanxi – NorthwestHubei region by means of the sections examined herein andthe available literature reveals a striking discrepancy in SSFassociations. Biozonation of the Southwest Shaanxi region(Fig. 12) is comparable with that of East Yunnan and Si-chuan. A disconformity occurs at the Precambrian – Cam-brian boundary interval here. The Anabarites trisulcatus –Protohertzina anabarica Zone (first Chinese SSF zone) iswidespread and generally comprises continuous sedimentsequences. Unlike in East Yunnan, Siphogonuchites triangu-laris, Lopochites latazonalis, and Maikhanella multa al-ready appear in the first SSF zone. These species first occurwithin the Siphogonuchites triangularis – Paragloborilus
subglobosus Zone in Yunnan (second Chinese SSF zone;Qian and Bengtson, 1989). But indefinite siphogonuchitidswere also mentioned in the first SSF zone fromYunnan (Qianand Bengtson, 1989). However, up to now, Paragloborilussubglobosus could not be unambiguously determined fromShaanxi. Therefore, the occurrence of the Siphogonuchitestriangularis – Paragloborilus subglobosus Zone in Shaanxiis not discerned. The presence of Aldanella sp. and Wat-sonella sp. [as ?Heraultipegma sp.] (Xing and Yue, 1984)indicate that the Watsonella yunnanensis Zone is represented
Fig. 6. All specimens from the Kuanchuanpu Formation of Shatan section, Nanjiang County, northern Sichuan Province. Thick scale bar equals 500 µm; thinscale bar 100 µm. Samples stored at TU Berlin collection (TUBACK). 1. Bemella costa, No Sht 100-10. 2. Bemella costa, No Sht 100-08. 3. Purella cf.squamulosa, No Sht 100-23. 4. Purella cf. squamulosa, No Sht 100-24. 5. Igorella sp., No Sht 100-04. 6. 8+1 sclerite of Chancelloria cf. eros, No Sht 100-16.7. Zhijinites sp., No Sht 100-09. 8. Hyolithellus sp., No Sht 100-18. 9. Zhijinites longistriatus, No Sht 100-15. 10. Helcionella sp., No Sht 100-20. 11.Protohertzina unguliformis, No Sht 102-01. 12. Rhabdochites exasperatus, No Sht 100-01. 13. Siphogonuchites triangularis, No Kua 103-01. 14. Undeterminedsteinkern of a helcionellid shell with pustulous surface structures, No Sht 100-31.Fig. 6. Tous les spécimens viennent de la Formation Kuanchuanpu, coupe de Shatan, Nanjiang County, nord de la province de Sichuan. Barre d’échelleépaisse = 500 µm, barre fine = 100 µm. Échantillons conservés dans les collections de TU Berlin (TUBACK). 1. Bemella costa, No Sht 100-10. 2. Bemella costa,No Sht 100-08. 3. Purella cf. squamulosa, No Sht 100-23. 4. Purella cf. squamulosa, No Sht 100-24. 5. Igorella sp., No Sht 100-04. 6. 8+1 sclérite deChancelloria cf. eros, No Sht 100-16. 7. Zhijinites sp., No Sht 100-09. 8. Hyolithellus sp., No Sht 100-18. 9. Zhijinites longistriatus, No Sht 100-15. 10.Helcionella sp., No Sht 100-20. 11. Protohertzina unguliformis, No Sht 102-01. 12. Rhabdochites exasperatus, No Sht 100-01. 13. Siphogonuchites triangularis,No Kua 103-01. 14. Moule interne indéterminé d’une coquille d’helcionellidé avec structures de surface pustuleuses, No Sht 100-31.
NXC-1
NXC-3
NXC-4
NXC-8
NXC-6
NXC-9
NXC-10NXC-11NXC-12NXC-13NXC-14NXC-15NXC-16NXC-17
NXC-5
NXC-7
NXC 10a
NXC-18
NXC-19
NXC-20
NXC-21
NXC-22
NXC-23
NXC-24
NXC-2 2m
Conothecasubcurvata
Archaeooidesgranulatus
Siphogonuchitestriangularis
Siphogonuchitesmirus
Lopochiteslatazonalis
Maikhanellamulta
Protoherzinaanabarica
Protoherzinaunguliformis
Hexaconulariasichuanensis
Punctatusemeiensis
Mongoloduslongispina
Mongolodusplatybasalis
Acanthocassisorthacanthus
Olivooidesmultisulcatus
XinliMember
Dengying
Formation
Guojiaba
Guojiaba
Formation
Formation
Xinli SectionNanjiang CountyN.Sichuan Province
Fig. 7. Stratigraphic column of the Xinli Member of Dengying Formation atXinli, Nanjiang County (Sichuan Province) including occurrences of SmallShelly Fossils.Fig. 7. Colonne stratigraphique de la Formation Dengying de Xinli, Nan-jiang County (Province de Sichuan) avec indication de la présence de SmallShelly Fossils.
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270 M. Steiner et al. / Geobios 37 (2004) 259–275
in the higher Kuanchuanpu Formation. Watsonella yunnan-ensis and Aldanella yanjiaheensis (mistakenly mentioned asPelagiella subangulata, Li personal observation) were alsoboth described from the Dahai Member of Xiaotan, Yunnan(Li et al., 2001). Tannuolina zhangwentangi is here reportedfrom the overlying Guojiaba Formation, and this suggests theexistence of the Sinosachites flabelliformis – Tannuolinazhangwentangi Zone. However, Sinosachites and Lapworth-ella have not yet been found in this region. The occurrence ofbreccia beds at the top of the Anabarites trisulcatus – Pro-tohertzina anabarica Zone or at the base of the directlyoverlying strata is remarkable. Such breccia beds occur inwide parts of the shallow water settings of the Yangtze Plat-form (Steiner, 2001). Deep-seated fault zones formed in theprotected basin south of the carbonate platform, which alsoled to the formation of synsedimentary- hydrothermal ore(Steiner et al., 2001b) during the late Meishucunian (equiva-lent to the late Tommotian of Siberia). Both lines of evidenceindicate a considerable tectonic and seismic activity duringthe Early Cambrian.
In the region east of Kuanchuanpu the strata overlying theAnabarites trisulcatus – Protohertzina anabarica Zone are
increasingly missing. A thin breccia bed occurs at Shatan onthe top of the Kuanchuanpu Formation, and this indicates areworking and mixing of skeletal fossils from the second SSFassociation (Zhijinites longistriatus, Igorella sp., Bemellacosta, Purella cf. squamulosa, Chancelloria cf. eros, Halki-eria stenobasis) with those of the first zone (Protohertzinaunguliformis, Punctatus emeiensis, Hexaconularia sp.).However, this needs further confirmation due to the lack ofimportant index fossils in this section.
The Southeast Shaanxi – Northwest Hubei region is sepa-rated from the Southwest Shaanxi region by the Sishang –Zhenba – Yudu Fault Zone (Yin et al., 1993) and showsdistinct differences in facies and biostratigraphy. The earliestthree Lower Cambrian SSF zones are completely lacking inthe Southeast Shaanxi – Northwest Hubei region (Fig. 12).Tannuolina, Lapworthella, and Sinosachites also could notbe detected in this region. Instead of this, SSFs of completelydifferent associations were detected in the Xihaoping Mem-ber. They are subdivided here into the following two zones(in ascending order):
ensis Zone is defined here by the joint occurrence of Ninellatarimensis, Cambroclavus fangxianensis, and Cambrothyratruncata. Additional taxa detected in this zone include Cam-brothyra ampulliformis, Yochelcionella cf. chinensis, andnumerous chancelloriids.
Fig. 8. All specimens from the Kuanchuanpu Formation of Xinli section, Nanjiang County, northern Sichuan Province. Thick scale bar equals 500 µm, thin scalebar 100 µm. Samples stored at the collection of Nanjing Institute of Geology and Palaeontology (NIGPAS). 1. Hexaconularia sichuanensis, No NXC9-01. 2.Hexaconularia sichuanensis, No Q5-13. 3. Punctatus emeiensis, lateral view, No NXC12-10. 4. Lopochites latazonalis, No NXC8-23. 5. Siphogonuchitestriangularis, No NXC8-24. 6. Maikhanella multa, No NXC8-22. 7. Mongolodus longispina, No NXC12-12. 8. Acanthocassis orthacanthus, No NXC12-26. 9.Acanthocassis orthacanthus, No NXC12-27. 10. Protohertzina cf. unguliformis, No NXC8-6. 11. Protohertzina unguliformis, No NXC8-4. 12. Protohertzinaanabarica, No NXC8-3. 13. Conotheca subcurvata, No NXC8-6.Fig. 8. Tous les spécimens viennent de la Formation Kuanchuanpu de la coupe de Xinli, Nanjiang County, nord du Sichuan. Barre d’échelle épaisse = 500 µm,barre fine = 100 µm. Exemplaires déposés au Nanjing Institut of Geology and Palaeontology (NIGPAS). 1. Hexaconularia sichuanensis, No NXC9-01. 2.Hexaconularia sichuanensis, No Q5-13. 3. Punctatus emeiensis, vue latérale, No NXC12-10. 4. Lopochites latazonalis, No NXC8-23. 5. Siphogonuchitestriangularis, No NXC8-24. 6. Maikhanella multa, No NXC8-22. 7. Mongolodus longispina, No NXC12-12. 8. Acanthocassis orthacanthus, No NXC12-26. 9.Acanthocassis orthacanthus, No NXC12-27. 10. Protohertzina cf. unguliformis, No NXC8-6. 11. Protohertzina unguliformis, No NXC8-4. 12. Protohertzinaanabarica, No NXC8-3. 13. Conotheca subcurvata, No NXC8-6.
Sanlangpu section,Xixiang County,
South Shaanxi Province
Slp 1
1 m
Con
othe
ca b
revi
ca
Cam
brot
hyra
am
pulli
form
is
Cam
brot
hyra
trun
cata
Cam
broc
lavu
s fa
nxia
ngen
sis
Cha
ncel
loria
? a
rida
Igor
ella
sp.
Hal
kier
ia s
then
obas
is
Cup
ithec
a m
ira
Allo
nnia
err
omen
osa
Allo
nnia
tetr
atha
llis
Arc
hias
tere
lla p
enta
ctin
a
hexa
ctin
ellid
spo
nge
spic
ules
Yoc
helc
ione
lla c
f. ch
inen
sis
Tsun
yidi
scus
Sub
zone
Xihaoping Mb.
DengyingFormation
Gu
ojia
ba
Fo
rmat
ion
poor
ly fo
ssili
fero
us
?
Fig. 9. Stratigraphic column of the Xihaoping Member at Sanlangpu,Xixiang County (Shaanxi Province) including occurrences of Small ShellyFossils.Fig. 9. Colonne stratigraphique de la Formation Xihaoping de Sanlangpu,Xixiang County (Province de Shaanxi) avec indication de la présence deSmall Shelly Fossils.
Xiaowan section,Xixiang County,
South Shaanxi Province
Xiw 103
1 m
XihaopingMember
GuojiabaFormation
Xiw 104
Xiw 105
Xiw 102
Xiw 100
Nin
ella
tarim
ensi
s
Cam
broc
lavu
s fa
ngxi
anen
sis
Con
othe
ca b
revi
ca
Cam
brot
hyra
am
pulli
form
is
Cam
brot
hyra
trun
cata
Turc
uthe
ca lu
bric
a
Igor
ella
sp.
Hal
kier
ia s
then
obas
is
Cup
ithec
a m
ira
Cha
ncel
loria
cf.
eros
Allo
nnia
tetr
atha
llis
Allo
nnia
err
omen
osa
Arc
hias
tere
lla p
enta
ctin
a
hexa
tinel
lid s
pong
e sp
icul
es
cup-
shap
ed m
ollu
scs
inde
t.
DengyingFormation
Fig. 10. Stratigraphic column of the Xihaoping Member at Xiaowan,Xixiang County (Shaanxi Province) including occurrences of Small ShellyFossils.Fig. 10. Colonne stratigraphique de la Formation Xihaoping de Xiaowan,Xixiang County (Province de Shaanxi) avec indication de la présence deSmall Shelly Fossils.
The overlying Rhombocorniculum cancellatum Zone,which also extends into the lower Shuijingtuo Formation, isdefined here by the occurrence of Rhombocorniculum can-cellatum. It is reported from Xihaoping, Fangxian County,Hubei (Qian et al., 1999) and is possibly missing in the wholesoutheastern Shaanxi. The few reports of Rhombocornicu-lum cancellatum from southeastern Shaanxi (Xie, 1990;Ding et al., 1991) remain questionable. A detailed correlationof the two zones of the Xihaoping Member with the Sinosa-chites flabelliformis – Tannuolina zhangwentangi Zone ofsouthwestern Shaanxi is not yet possible because Tannuolinahas not been reported from Southeast Shaanxi and Northwest
Hubei (Fig. 12). It is worth noting that both Rhombocornicu-lum and Cambroclavus have been reported from the PararaLimestone of South Australia, which has been interpreted tobe of Botoman age (Alexander et al., 2001). As in SouthChina, the two genera do not co-occur in the same strati-graphical level. Cambroclavus only occurs in the lowerParara Limestone, while Rhombocorniculum was recoveredfrom the top of this formation (Alexander et al., 2001). Asimilar SSF association as represented in the XihaopingMember and Shuijingtuo Formation has been reported fromKazakhstan (Missarzhevsky and Mambetov, 1981), whereCambroclavus and Ninella occur in the lower subzone of theRhombocorniculum cancellatum Zone. This interval hasbeen correlated with the upper Atdabanian of the SiberianPlatform (Missarzhevsky and Mambetov, 1981).
The large stratigraphic hiatus, which comprises almost thewhole Meishucunian, is represented in the entire region ofSoutheast Shaanxi and Nordwest Hubei (Fig. 13). This pri-mary hiatus, present in an area of at least 400 × 80 km, alsoindicates strong tectonic activities on the Yangtze Platformduring the Early Cambrian. This and the wide distribution ofbreccias in the other areas of the carbonate platform may berelated to the assembly of Gondwana following the breakupof the supercontinent Rodinia (Powell and Pisarevsky, 2002).
Acknowledgements
We are grateful to Bernd Weber (FU Berlin), Tanja Gold-berg, Harald Strauss (both Münster), Guo Qingjun (Guiy-ang), Zhang Junming, Yang Aihua, He Hongwei (all Nan-jing), and Yang Xianhe (Chengdu) for discussions andsupport in the field. The manuscript has profited considerablyfrom the detailed review by Loren E. Babcock (Columbus)and an anonymous referee. Technical assistance with fossilpicking and acid treatment by Sandra Schochardt and DavidSchmälzle (both TU Berlin) and with SEM by Jörg Nissen(ZELMI, TU Berlin) is warmly acknowledged. This studywas financially supported by DFG (grant ER 96/32-1), NSFC(grants 49972042 and 40172002), MOST of China (grantG2000077700), and CAS (grant KZCX2-116). This is acontribution to the Sino - German bundle project “FromSnowball Earth to the Cambrian Bioradiation- A Multidisci-plinary Analysis of the Yangtze Platform, China”.
Fig. 11. Fossil specimens from the Xihaoping Formation of Sanlangpu section (abbreviation Slp), or Xiaowan section (abbreviation Xiw), Xixiang County,southern Shaanxi Province. Thin scale bar equals 100 µm. Samples stored at TU Berlin collection (TUBACK). 1. Igorella sp., No Slp1-03. 2. Ninella tarimensis,No Xiw104-01. 3. Yochelcionella cf. chinensis, No Slp1-16. 4. Cambrothyra ampulliformis, No Xiw103-05. 5. Cambrothyra truncata, No Xiw103-06. 6. Sixaggregated individuals of Cambrothyra truncata, No Q5-19. 7. Allonnia tetrathallis, No Slp1-06. 8. Cambroclavus fanxiangensis, No Xiw104-07. 9.Cambroclavus fanxiangensis, No Xiw104-08. 10. Chancelloria? arida, No Slp1-04. 11. Conotheca brevica, No Slp1-05. 12. Conotheca brevica, No Slp1-07.Fig. 11. Spécimens de la Formation Xihaoping de la coupe de Sanlangpu (Slp), ou de la section Xiaowan (Xiw), Xixiang County, sud de la Province de Shaanxi.Barre d’échelle épaisse = 100 µm. Exemplaires déposés au TU Berlin (TUBACK). 1. Igorella sp., No Slp1-03. 2. Ninella tarimensis, No Xiw104-01. 3.Yochelcionella cf. chinensis, No Slp1-16. 4. Cambrothyra ampulliformis, No Xiw103-05. 5. Cambrothyra truncata, No Xiw103-06. 6. Six individus agglutinésde Cambrothyra truncata, No Q5-19. 7. Allonnia tetrathallis, No Slp1-06. 8. Cambroclavus fanxiangensis, No Xiw104-07. 9. Cambroclavus fanxiangensis, No
Xiw104-08. 10. Chancelloria? arida, No Slp1-04. 11. Conotheca brevica, No Slp1-05. 12. Conotheca brevica, No Slp1-07.
Fig. 12. Stratigraphic chart of the Lower Cambrian sections of theSouthwest Shaanxi and Southeast Shaanxi – Northwest Hubei regions(* = SSF association possibly comparable with the Siphogonuchites trian-gularis – Paragloborilus subglobosus Zone of Yunnan).Fig. 12. Tableau stratigraphique des coupes du Cambrien inférieur du SW deShaanxi et des régions SE Shaanxi – NW Hubei (* = association à SSF peutêtre comparable à la zone à Siphogonuchites triangularis – Paragloborilussubglobosus du Yunnan).
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