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Gondwana Research 14

Occurrence of phosphatic microfossils in an Ediacaran–Cambrianmid-oceanic paleo-atoll limestone of southern Siberia

Yuko Uchio a,b,⁎, Yukio Isozaki c, Michael M. Buslov d, Shigenori Maruyama a

a Department of Earth and Planetary Sciences, Tokyo Institute of Technology, O-okayama, Meguro, Tokyo 152-8550, Japanb National Museum of Nature and Science, Ueno Park, Taito, Tokyo 110-8718, Japan

c Department of Earth Science and Astronomy, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japand Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Science, Novosibirsk 6300900, Russia

Received 21 July 2007; received in revised form 14 December 2007; accepted 21 December 2007Available online 15 January 2008

Abstract

New phosphatic microfossils were recently discovered in an Ediacaran–Cambrian mid-oceanic paleo-atoll limestone in the southern GornyAltai Mountains in southern Siberia. Microfossils with calcium phosphate shells are abundant in the limestone that occurs as an exotic blockwithin a Cambrian accretionary complex in the Kurai area. SEM observations confirm that the calcium phosphatic shells are ellipsoidal and equal-sized, about 200–300 μm in diameter. Shell walls are about 1 μm thick. As the absence of external and internal structures hinders a detailedcomparison/identification, these microfossils are tentatively treated here as paleontological problematica. EPMA-analysis confirmed theconcentration of elements P and Ca in microfossil shells and the absence in the matrix, suggesting the primary phosphatic composition of theshells. Because phosphatic microfossils are generally scarce in the Ediacaran but abundant from the Lower Cambrian, in particular within pre-trilobitic SSF assemblages, the phosphatic fossil-bearing limestone in the Kurai area possibly belongs to the Lower Cambrian. The present findproves that mid-oceanic paleo-seamounts as well as continental shelf domains had already been inhabited by diverse metazoans in the Ediacaran–Cambrian transitional interval.© 2007 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Keywords: Cambrian; Ediacaran; Phosphate; Mid-ocean; Atoll; Altai

1. Introduction

The Precambrian–Paleozoic (or Ediacaran–Cambrian)boundary (ECB) event has been a major focus among manyresearchers because this boundary marks a unique time im-mediately after the Neoproterozoic snowball Earth (Kirschvink,1992; Hoffman et al., 1998; Hoffman and Schrag, 2002) andjust before the rapid diversification of both hard-tissued andsoft-bodied metazoans (e.g., Sepkoski, 1992; Li et al., 2007;Shu, 2008). One of the most dramatic events in Earth's life'shistory was the appearance of animals with hard skeletonsaround the ECB, in particular (e.g., Bengtson, 1994, 2004).As to the dramatic evolution of metazoans in the Cambrian

⁎ Corresponding author. National Museum of Nature and Science, Ueno Park,Taito, Tokyo 110-8718, Japan.

E-mail address: yuchio@kahaku.go.jp (Y. Uchio).

1342-937X/$ - see front matter © 2007 International Association for Gondwana Rdoi:10.1016/j.gr.2007.12.009

period, fossil faunas from the Burgess Shale in Canada and theChengjiang (Maotianshan) Lagerstätte in South China are thetwo major sources of information (e.g., Conway Morris, 1993;Shu et al., 1999). Both these world-famous fossil faunas werederived from continental shelf/shelf edge environments adjacentto lands, but in comparison little information is known from thecoeval mid-oceanic domains. There is no oceanic crust olderthan 200 Ma remaining in modern oceans, because all theseafloor existing at that time has been lost by oceanic sub-duction. The lack of fossil information from open oceans is fatalfor clarifying the global diversification pattern of animalsaround the ECB. Nonetheless, it is possible to recover frag-ments of pre-200 Ma mid-oceanic sedimentary rocks, such asdeep-sea chert and shallow marine atoll-type carbonatescapping paleo-seamounts, from allochthonous blocks withinancient accretionary complexes (ACs). Such exotic blocksprovide valuable information on the paleo-environment of lost

esearch. Published by Elsevier B.V. All rights reserved.

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mid-oceans, of both deep- and shallow-water, as demonstratedby comparable studies of the Permo-Triassic boundary event(e.g., Isozaki, 1997; Musashi et al., 2001; Isozaki et al., 2007).

Our current field research project organized in 1997 aimedto study the Cambrian AC in the Gorny Altai Mountains insouthern Siberia (e.g., Ota et al., 2007), deploying exactly thesame approach as the Permo-Triassic boundary studies to re-cover the pre-accretion primary stratigraphy from allochthonousblocks of mid-oceanic sediments (Uchio et al., 2004), and toreveal the environmental change in mid-ocean across the criticalboundary in life's history. Except for the rare occurrence ofCambrian Epiphyton and archaeocyathids, limestone blocksin the AC have long been regarded as near-barren in fossils(Buslov et al., 1993); however, we recently found microfossilsmade of calcium phosphate from the allochthonous limestoneblocks. Calcium phosphate represents one of the majorskeleton-forming material, as well as calcium carbonate andsilica for Paleozoic to modern animals. This paper preliminarilyreports the first occurrence of Ediacaran–Cambrian phosphaticmicrofossils from a paleo-atoll limestone in southern Siberiathat primarily formed in the lost ocean that once existed inEdiacaran–Cambrian time.

2. Geologic setting and fossil locality

The Gorny Altai Mountains are located in the Paleozoic–Mesozoic Altai–Sayan orogenic belt in southern Siberia thatcorresponds to the western segment of the extensive CentralAsian Orogenic Belt (CAOB) as shown in Fig. 1a (e.g.,Mossakovsky et al., 1993; Khain et al., 2003; Jahn, 2004;Windley et al., 2007). The GornyAltai Mountains formed part ofthe ancient active continental margin around the Siberian craton(e.g., Maruyama et al., 1989; Sengör and Natal'in, 1996), andcontain various orogenic components such as the Cambrian AC,Ediacaran to Cambrian high-P/T metamorphic rocks, ophiolite,Ediacaran to Devonian island arc-type igneous rocks, and

Fig. 1. Index map of the Gorny Altai Mountains in southern Siberia (a, modified frosoutheastern Gorny Altai Mountains (b, simplified from Ota et al., 2007). The GornySiberian craton as part of the Central Asian Orogenic Belt (CAOB). Around Kuraitectonically sandwiched between high-P/T metamorphic rocks associated with an opsouthwest. The microfossils were found in a limestone block in the upper unit of th

Cambrian to Carboniferous fore-arc sedimentary rocks (e.g.,Buslov et al., 1993; Buslov and Watanabe, 1996; Buslov et al.,2002; Fig. 1b). All these units formed in the Late Neoproterozoicto Early Paleozoic post-Rodinian convergent tectonic frame-work (e.g., Hoffman, 1991; Maruyama et al., 1997, 2007) thatinvolved oceanic subduction beneath the Siberian craton and/orsurrounding arcs. For more details of the general aspects ofgeology and tectonics of the Gorny Altai Mountains, see Otaet al. (2007) who synthesized the tectonic evolution of theorogenic belt as a result of their detailed field mapping.

The Cambrian AC occurs in a narrow belt in the central-southern Gorny Altai Mountains; it is composed of basalticgreenstones, limestone and terrigenous clastics. The AC un-derwent regional metamorphism at greenschist to sub-greens-chist facies, but only weak deformation. We studied theCambrian AC in detail in the Kurai area located at ca. 5 kmwest of Kurai village along the Chuya River (Kosh–Agachregion, Altai Republic) (Fig. 1b). This area is underlain mainlyby a weakly metamorphosed Cambrian AC that is in faultcontact with high-P/T metamorphic rocks to the northeast,and with unmetamorphosed Devonian sandstone/mudstone of afore-arc assemblage to the southwest.

The Cambrian AC is divided into two parts, i.e. lower andupper units that are mutually separated by a subhorizontal fault(Ota et al., 2007; Fig. 1b). The lower unit is mostly composed ofa N200 m-thick greenstone derived from basaltic volcaniclas-tics, whereas the upper unit comprises pillowed basalticgreenstone capped by shallow marine carbonates (the Baratallimestone of Uchio et al., 2004). These greenstones and lime-stones were broken into various-sized blocks and sheets, andstructurally repeated with relatively minor terrigenous clastics.As to the paleo-environmental reconstruction, stratigraphicanalyses can be undertaken solely within an individual block orslice of limestone. Fossils are very rare not only in the limestonebut also in the other components of the AC; however, someEarly Cambrian archaeocyathids occur in limestone clasts in

m Maruyama et al., 1989; Sengör and Natal'in, 1996) and of the Kurai area inAltai Mountains belong to the Altai–Sayan orogenic belt developed around thethe Cambrian accretionary complex (AC) extends in a WNW–ESE direction,

hiolite to the northeast and Lower Paleozoic arc-related sedimentary rocks to thee AC (Fig. 2).

Fig. 2. Geologic map of the Kurai area (a, modified from Ota et al., 2007) and outcrop sketch of the microfossil locality at KR4 (b). The Cambrian AC in the Kurai area is mainly composed of basaltic greenstones andassociated Ediacaran–Cambrian limestone (the Baratal limestone) that contains the microfossil-bearing ooid packstone.

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Fig. 3. Field occurrence of the microfossil-bearing ooid packstone at KR4.a: exposures of the microfossil-bearing limestone (Fig. 2b); b: weathered surfaceof the limestone with emphasized relief of microfossils.

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conglomerates, and Early Cambrian sponge spicules are insiliceous mudstone. These fossil occurrences suggest that theAC formed no earlier than the Early Cambrian.

The greenstone of the upper unit has a unique geochemistryof OIB (oceanic island basalt) affinity that suggests its origin inan ancient hotspot-type seamount or oceanic plateau (Buslovet al., 1993; Utsunomiya et al., 1998). Shallow marine car-bonates (Baratal limestone) are intimately associated with thegreenstones, and their primary thickness is estimated asN200 m. The Baratal limestone contains, in part, stromatoliteand ooid packstone that suggest a shallow-water origin (Uchioet al., 2004). The basal part of the Baratal limestone directlycovers the pillowed OIB-type greenstone, and the wholelimestone lacks coarse-grained terrigenous clastics. Thesegeologic aspects indicate that the Baratal limestone wasprimarily formed on top of an ancient seamount (or oceanicplateau) located in an oceanic domain far remote from landmargins (Uchio et al., 2004). Nohda et al. (2003) reported a bulkPb–Pb isochron age of 598±25 Ma (Early Ediacaran) from thebasal Baratal limestone at locality KR1 (Fig. 2), this radiometricage has a relatively large error range; nonetheless, it providesthe first direct age constraint on the age of the basal Baratallimestone. The Baratal limestone likely ranged from the EarlyEdiacaran to the Cambrian, although its topmost horizon hasonly been constrained, in contrast to its well-defined base. Referto Uchio et al. (2004) for further details on the stratigraphy ofthe Baratal limestone.

The phosphatic microfossils were found in a limestone blockof the upper AC unit at locality KR4 (50 15′22″N, 87 51′30″E) inthe northern part of the Kurai area (Figs. 2, 3). This block isisolated from other rock exposures on a small hill. The KR4section apparently belongs to the same unit as that at KR1;however, without observable stratigraphic/geologic relationshipswith other rocks and with no reliable index fossils, its precise ageand/or horizon is not clear. Nonetheless, the basal interval of theBaratal limestone (N50m-thick) in theKR1 section (Fig. 2a) lacksooid packstone facies, suggesting that the KR4 section probablybelongs to a much higher stratigraphic horizon.

The limestone at locality KR4 (over 2 m thick; Figs. 2b, 3)and other limestones in the vicinity are composed of gray,massive, ooid packstone that contains abundant ooids (ca.0.5 mm in diameter) that make up over 90% in volume within amicritic calcite matrix (Fig. 4). The limestone lacks coarse-grained terrigenous clastics. The occurrence of abundant ooidssuggests that the limestone likely formed at a shallow water-depth (photic zone), probably in more or less the same depo-sitional setting as other shallow marine carbonates of the Baratallimestone that accumulated on top of an ancient seamount(Uchio et al., 2004). The microfossils occur are concentrated inpart of this ooid packstone (Fig. 5a).

3. Phosphatic microfossils

The microfossils in the ooid packstone at KR4 are sphericaland 200–300 μm in diameter (Fig. 5). They can be recognizedon weathered surfaces with the naked eye, but an optic micro-scope, scanning electron microscope (SEM), and electron probe

microanalyzer (EPMA) are needed to observe their detailed 3-Dmorphology and to analyze their chemical composition. Wemade 70 thin sections from 25 rock samples (no. 98KR-112,141, 142, 99KR-1 to -6, -10 to -14, 00AP-1 to -10) collected atKR4 and observed under the microscope. Abundant uniquespherical shell structures were found in Samples 00AP-1, -2, -5,-6, -8, and -10 (Fig. 2b). For the SEM images, the fossil-bearingparts (Sample 00AP-5) were processed/etched by 10%CH3COOH solution for 20 min in order to make the topographicrelief clearer. For the EPMA analysis the same sample waspolished by #5000 grinding powder.

3.1. Microscope image

As demonstrated in Fig. 4A, the ring-like sections of spher-ical shells are abundant in a particular part of the ooid packstoneat KR4. They are mostly equigranular between 200 and 300 μmin diameter. In close-up views (Fig. 5b, c), they are oval insection. Well-preserved individuals clearly retain ca. 1 μm thickwall structures. There is no remarkable structure in the shells,some of which were recrystallized into coarse-grained calciteprobably during diagenesis. The interior of the shells are filledwith structureless micrite.

3.2. SEM image

The 3-Dmorphology of the shells is better recognized by SEMobservations on etched surfaces (Fig. 6). The microfossils are

Fig. 4. Photomicrographs of the ooid packstone at KR4. a: polished surface of the ooid packstone (00AP-9); b–d: thin section views (b: 98KR141, c, d: 00AP-3).

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more ellipsoidal than spherical and have no angular features;some are slightly twisted like beans (Fig. 6b). The aspect ratioin length between the long axis and short axis is mostly about

Fig. 5. Microscopic views of the fossiliferous part of the ooid packstone at KR4. a:views of the microfossils. Some individuals are relatively well-preserved but some a

2:1. The shell surface is smooth on both exterior and interiorsides without any conspicuous ornamentation. Concave breakage(Fig. 6c) may suggest the post-mortem fragile nature of the shells.

abundant occurrence of microfossils that are mostly equal-sized; b, c: enlargedre replaced by secondary large calcites. All specimens are from sample 00AP-5.

Fig. 6. SEM images of the microfossils. a: thin-shelled spherical structures arepopping out from the matrix composed of micrite; b: ellipsoidal shells have asmooth surface on both external and internal sides; c: Shell is partly brokenprobably due to its fragility. All specimens are from sample 98KR141.

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3.3. EPMA image

Fig. 7 illustrates EPMA images of element compositionmapping of a polished surface of the microfossil-bearinglimestone. The most striking feature is the apparent concentra-tion of elements P and Ca in the thin ring-shaped shells. Incontrast, Si is relatively depleted in the shell wall per se withrespect to the shell interior immediately adjacent to the shellwall. Quantitative spot analyses of both shells and surroundingparts by EPMA confirmed this element distribution pattern(Table 1). The shells are composed of 22–28 wt.% P2O5,33–39 wt.% CaO, and 29–42 wt.% SiO2, whereas the matrixlimestone is replete with CaO. The interior of the shell isoccupied mostly by CaO with a small amount of MgO, probablycomposed of dolomite; however, the adjacent areas of the shellwall are dominated solely by silica without any accompanyingP or Ca.

4. Discussion

4.1. Primary calcium phosphate shell

On the basis of the well-defined nature of their thin shellwalls, the geochemistry of the shell material (calciumphosphate), and the ubiquity in morphology and size amongmany individuals, the phosphatic spherical structures found atLoc. KR4 are regarded as bona fide microfossils with bio-mineralized skeletons. The thin shells are likely composed ofcalcium phosphate, probably retaining the primary compositionof the shells. In particular, the enrichment of CaO and P2O5 inthe thin shells (Fig. 7), and the absence of P2O5 in the matrixand the remaining grains in the same sample suggest thatphosphatic shell walls have a bio-mineralized origin. Althoughdetailed micro-mineralogical analysis of a 1 μm domain has notyet been performed, possible candidates for the calcium phos-phate include apatite/fluorapatite (e.g., dahllite and francolite,Ca5(PO4,CO3)3(F, OH)) and amorphous calcium phosphate.On the other hand, the concentration of silica on both sides ofthe shell probably represents secondary precipitation duringdiagenesis.

4.2. Mid-oceanic microfossil fauna

Because the microfossils are no more than simple ellipsoidal/spherical shells with smooth surfaces, it is difficult to specifycertain comparable fossil taxa previously described from theEdiacaran–Cambrian interval. Thus at present, we tentativelydescribe these as paleontological problematica in this article.Nonetheless, possible candidates for comparison include var-ious remains of protists (bacteria, fungi, unicellular algae, pro-tozoans) and early metazoans; e.g., bacterial films, algal cases,eggs of early animals etc.

Bacterial films and algal cases in general are too small to becompared with the phosphatic microfossils from the Kurai area(over 200 μm in diameter). Likewise, most of the acritarchsreported from the Ediacaran (post-Marinoan snowball Earth)limestone in South China (e.g., Yin and Yuan, 2007) are gen-erally smaller (less than 15 μm in diameter) than the Kuraimicrofossils. Some large acritarchs usually have a complicatedstructure on their surface that is not observed in the Kuraimicrofossils. Thus, in terms of shell size and surface orna-mentation, none of the previously reported Ediacaran acritarchsare likely candidates. We cannot find any diagnostic featuresanalogous to metazoan eggs in the Kurai specimens. Recentlyreported Middle Cambrian metazoan eggs from South China(Lin et al., 2006) are apparently larger in size (500–950 μm)than the Kurai specimens by one order of magnitude.

Among the previously described Early Cambrian SSF ele-ments, phosphatocopids of crustacea (arthoropods) may appearsimilar to the microfossils from Kurai by sharing the samephosphatic shell, overall ellipsoidal form, and size. Phosphato-copids have a pair of hemispherical valves with a wide dorsalopening filled with plural antennae and pods (Maas et al., 2003).The phosphatic microfossils from the Kurai area are not suf-ficiently well preserved to be checked with such features and/or

Fig. 7. Composition map of the microfossil-bearing part of the ooid packstone by EPMA (Upper: Specimen 1, Lower: Specimen 2). Both specimens are from sample00AP-5. Note the concentration of elements P and Ca, and relative depletion in Si in the shell wall, in sharp contrast to the surrounding part dominated by Si. Thesesuggest that the shell was made primarily of calcium phosphate and secondarily replaced by silica during diagenesis. The spot analyses by EPMA shown in Table 1 alsoconfirm this pattern of element distribution.

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Table 1EPMA-analyzed major element composition of microfossil shells and their surrounding parts of a microfossil-bearing limestone (ooid packstone) at KR4

SiO2 CaO MgO TiO2 Al2O3 FeO MnO Na2O K2O P2O5 Total

Specimen 1shell 1-1 28.894 39.427 0.029 n.d. 0.039 0.026 0.019 0.038 0.011 26.997 95.48shell 1-2 38.678 33.732 0.026 n.d. 0.027 0.013 0.014 0.024 0.021 24.369 96.904shell 1-3 30.899 37.482 0.037 n.d. 0.051 0.413 n.d. 0.054 n.d. 27.864 96.8interior (gray) 1-1 96.941 0.092 0.001 n.d. 0.012 n.d. n.d. 0.016 n.d. n.d. 97.062exterior (gray) 1-1 97.286 0.08 0.025 n.d. 0.234 0.019 0.014 0.021 n.d. n.d. 97.679

Specimen 2shell 2-1 75.834 13.405 0.014 n.d. 0.054 0.029 n.d. n.d. 0.018 10.655 100.009shell 2-2 42.353 33.128 0.061 n.d. 0.029 n.d. n.d. 0.008 n.d. 22.341 97.92interior (gray) 2-1 99.298 0.435 0.005 0.006 n.d. 0.008 n.d. 0.02 0.012 0.073 99.857interior (gray) 2-2 99.006 0.283 n.d. 0.016 0.038 0.02 0.019 n.d. 0.002 n.d. 99.384exterior (gray) 2-1 n.d. 55.762 0.245 0.028 0.017 0.036 0.032 0.002 n.d. 0.196 56.318

Both analyzed fossil specimens are from Sample 00AP-5. Small domains were analyzed at 5 spots for Specimen 1 and Specimen 2. Note the remarkable concentrationof P2O5 and relative depletion of Si in shells in both specimens. These results support the composition mapping images by EPMA (Fig. 7).n.d.: not detected.

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internal structures, and this hinders direct comparison withphosphatocopids. However, the coprolitic mode of occur-rence recently reported from the Middle Cambrian in Sweden(Eriksson and Terfelt, 2007) may suggest a possible analogy tothe Kurai specimens. More detailed paleontological analysesare definitely needed with better-preserved material for propercomparison and identification.

4.3. Age of the phosphatic microfossils

The age of the microfossil-bearing part of the Baratal lime-stone is still unclear; however, the lithofacies dominated byooid-packstone suggests that this interval belongs not to thebasal part of the Baratal limestone with a Pb–Pb age of 598+−25 Ma (Nohda et al., 2003), but to much higher horizons of thelate Ediacaran to Early Cambrian.

Calcium phosphate is one of the major skeleton-formingmaterials not only for Phanerozoic metazoans including moderncreatures, but also for the Cambrian SSFs. Following the veryearly pioneer, skeleton-bearing protists/metazoans utilizingsilica and calcium carbonate that appeared already in the lateNeoproterozoic, fossils with calcium phosphate skeletons joinedin the marine biological community in the Early Cambrian (Liet al., 2007), particularly from Stage 2 (ca. 528–521 Maaccording to Zhu et al., 2007) (Fig. 8). In fact, some lateEdiacaran tube-form fossils, such asCloudina andNamacalathus(possible cnidarians) are phosphatic (e.g., Grotzinger et al., 2000);however, these are regarded to have been primarily lessmineralized or mineralized as calcium calcitic if at all, and havebeen coated or replaced by phosphate secondarily duringdiagenesis (Steiner et al., 2007).

The ECB marks a remarkable time of the outbreak of varioushard-tissued animals in history (e.g., Bengtson, 1994, 2004; Liet al., 2007; Zhu et al., 2007; Shu, 2008). In particular, the pre-trilobitic basal Cambrian is characterized by the abundantoccurrence of the so-called SSFs (small shelly fossils). Thisinterval was assigned by Babcock and Peng (2007) as Stage 2 in

Series 1 of the Cambrian that corresponds to the traditionalTommotian to early Atdabanian in Siberia or to the early–middle Meishucunian in South China (Fig. 8). Various geo-chemical proxies also suggest that seawater chemistry wasperturbed remarkably along with the innovation in bio-mineralization (e.g., Brasier and Sukhov, 1998; Maloof et al.,2005; Zhu et al., 2007; Ishikawa et al., 2008). In the youngerpart of Stage 2 immediately before the appearance of trilobites,in turn, SSFs rapidly decreased diversity partly due to tapho-nomic effects related to the global decrease in phosphatic de-posits (Li et al., 2007), highlighting their dominant occurrencesolely in Stage 2.

In sharp contrast to the Ediacaran fauna, the Cambrian SSFassemblages include various bona fide phosphatic elements;e.g., sclerites of tommotiids and lobopods (e.g., Bengtson,1970; Landing, 1984), dermal elements of paleoscolecid (e.g.,Brock and Cooper, 1993; Mueller and Hinz-Schallreuter, 1993),tubes/conchs of hyoliths (Bengtson, 2004), protoconodonts, andarthropods (Li et al., 2007). Phosphatocopids were reportedmainly from Middle-Upper Cambrian rocks (e.g., Maas et al.,2003), but they range down to the upper Lower Cambrian inSouth China (Li et al., 2007).

Because all bona fide phosphatic microfossils have beenreported from the Cambrian, the enigmatic microfossils fromthe Kurai area possibly belong to an Early Cambrian fauna. Thisis in accordance with the previously reported occurrence of anEarly Cambrian archaeocyathid and Epiphyton from a lime-stone in the Kurai area (Buslov et al., 1993) (Fig. 8).

The phosphatic microfossils from the Kurai area definitelyprovide rare and valuable information on mid-oceanic shallowmarine fauna at the critical interval for early animal evolution.In addition to the previously reported archaeocyathids, theoccurrence of the SSF-like phosphatic microfossils from anaccreted paleo-atoll shallow marine carbonates may suggestthat metazoans had extensively diversified not only incontinental shelf domains, but also in mid-oceanic realmsduring the Early Cambrian. Further research along this line

Fig. 8. Possible stratigraphic position of the Baratal limestone and phosphatic microfossils from the Gorny Altai Mountains with respect to the latest zoning scheme ofthe Upper Ediacaran and Lower Cambrian by Zhu et al. (2007) and Babcock and Peng (2007). Ranges for representative fossils/fossil groups are after Zhuravlev(1996), Grotzinger et al. (2000), Zhu et al. (2007), and Steiner et al. (2007). As to the Baratal limestone, two age-constraining data (archaeocyathids and Pb–Pb age)were reported by Buslov et al. (1993), Nohda et al. (2003), respectively. Note that the Baratal limestone ranges from the Early Ediacaran to at least the Early Cambrian,and that the phosphatic microfossil horizon reported in this article possibly corresponds to the Early Cambrian (Stages 2 to 4).

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will likely document more diverse features of mid-oceanicfauna.

5. Summary

This article reports a new finding of phosphatic microfossilsfrom an Ediacaran–Cambrian limestone in the Gorny AltaiMountains in southern Siberia that form a western segment ofthe Central Asian Orogenic belt developed around the Siberiacraton. New results are summarized below.

1. Microfossils with calcium phosphate shell with diameterabout 100 μm are abundant in ooid packstone of the Baratallimestone in the Kurai area in the southern Gorny AltaiMountains.

2. As the microfossils from the Kurai area are not apparentlycomparable with previously reported fossils from theEdiacaran–Cambrian interval, they are treated as paleono-tological problematica.

3. The occurrence of phosphatic microfossils from a mid-oceanic paleo-atoll limestone suggests that early diversifica-tion of marine fauna had also occurred in the mid-oceanicdomain during the Ediacaran–Cambrian transition interval.

Acknowledgements

We greatly appreciate Tsutomu Ota, Atsushi Utsunomiya,Akira Ishikawa, and Nikolai Simakov for their help during

fieldwork. We thank Degan Shu and Zhifei Zhang for theirvaluable review comments. Language was checked by Brian F.Windley. This study was supported by Grant-in-Aid of JapanSociety of Promoting Sciences (no. 12126202 to S.M.).

References

Babcock, L.E., Peng, S.C., 2007. Cambrian chronostratigraphy: current andfuture plans. Palaeogeography, Palaeoclimatology, Palaeoecology 254,62–66.

Bengtson, S., 1970. The Lower Cambrian fossil Tommotia. Lethaia 3, 363–392.Bengtson, S., 1994. The advent of animal skeletons. In: Bengtson, S. (Ed.),

Early life in Earth. Columbia University Press, New York, pp. 412–425.Bengtson, S., 2004. Early skeletal fossils. In: Lipps, J.H., Waggonor, B.M.

(Eds.), Neoproterozoic-Cambrian biological revolutions. PaleontologicalSociety of America Papers, vol. 10, pp. 67–77.

Brasier, M.D., Sukhov, S.S., 1998. The falling amplitude of carbon isotopeoscillation through the Lower to Middle Cambrian: northern Siberian data.Canadian Journal of Earth Sciences 35, 353–373.

Brock, G.A., Cooper, B.J., 1993. Shelly fossils from the Early Cambrian(Toyonian) Wirrealpa, Aroona Creek, and Ramsay Limestones of SouthAustralia. Journal of Paleontology 67, 758–787.

Buslov, M.M., Watanabe, T., 1996. Intra-subduction collision and role in theevolution of an accretionary wedge: the Kurai zone of Gorny Altai. RussianGeology and Geophysics 37, 74–84.

Buslov, M.M., Bersin, N.A., Dobretsov, N.L., Simonov, V.A., 1993. Geologyand tectonics of Gorny Altai. Guidebook, 4th International SymposiumIGCP Project 283, Geodynamic evolution of the Paleoasian Ocean, RussianAcademy of Science, Siberian Branch. Novosibirsk. 122 pp.

Buslov, M.M., Watanabe, T., Saphonova, I.Yu., Iwata, K., Travin, A., Akiyama,M., 2002. AVendian–Cambrian island arc system of the Siberian continentsin Gorny Altai (Russia, Central Asia). Gondwana Research 5, 781–800.

192 Y. Uchio et al. / Gondwana Research 14 (2008) 183–192

Conway Morris, S., 1993. The fossil record and the early evolution of theMetazoa. Nature 361, 219–225.

Eriksson, M.E., Terfelt, F., 2007. Anomalous facies and ancient faces in thelatest middle Cambrian of Sweden. Lethaia 40, 69–84.

Grotzinger, J.P., Watters, W.A., Knoll, A.H., 2000. Calcified metazoans inthrombolite–stromatolite reefs of the terminal Proterozoic Nama Group,Namibia. Paleobiology 26, 334–359.

Hoffman, P.F., 1991. Did the breakout of Laurentia turn Gondwanaland inside-out? Science 252, 1409–1412.

Hoffman, P.F., Schrag, D.P., 2002. The snowball Earth hypothesis: testing thelimits of global change. Terra Nova 14, 1–27.

Hoffman, P.F., Kaufman, A.J., Halverson, G.P., Schrag, D.P., 1998. ANeoproterozoic snowball Earth. Science 281, 1342–1346.

Ishikawa, T., Ueno, Y., Komiya, T., Sawaki, Y., Han, J., Shu, D.G., Li, Y.,Maruyama, S., Yoshida, N., 2008. Carbon isotope chemostratigraphy of aPrecambrian/Cambrian boundary section in the Three Gorges area, Southchina: prominent global-scale isotope excursions just before the Cambrianexplosion. Gondwana Research 14, 193–208. doi:10.1016/j.gr2007.10.008.

Isozaki, Y., 1997. Permo-Triassic boundary superanoxia and stratified super-ocean: records from lost deep-sea. Science 276, 235–238.

Isozaki, Y., Kawahata, H., Ota, A., 2007. A unique carbon isotope record acrossthe Guadalupian–Lopingian (Middle–Upper Permian) boundary in mid-oceanic paleoatoll carbonates: the high-productivity Kamura event and itscollapse in Panthalassa. Global Planetary Change 55, 21–38.

Jahn, B.M., 2004. The Central Asian Orogenic Belt and growth of thecontinental crust in the Phanerozoic. In: Malpas, J., Fletcher, C.J.N., Ali,J.R., Aitchison, J.C. (Eds.), Aspects of the Tectonic Evolution of China.Geological Society of London, Special Publication, vol. 226, pp. 73–100.

Khain, E.V., Bibikova, E.V., Salnikova, E.E., et al., 2003. The Paleo-Asianocean in the Proterozoic and early Paleozoic: new geochronologic data andpaleotectonic reconstructions. Precambrian Research 122, 329–358.

Kirschvink, J.L., 1992. Late Proterozoic low-latitude global. glaciation: thesnowball Earth. In: Schopf, J.W., Klein, C. (Eds.), The ProterozoicBiosphere. Cambridge University Press, Cambridge, pp. 51–52.

Landing, E., 1984. Skeleton of lapworthellids and suprageneric classification oftommotiids (Early and Middle Cambrian phosphatic problematica). Journalof Paleontology 58, 1380–1398.

Li, G.X., Steiner, M., Zhu, X.J., Yang, A.H., Wang, H.F., Erdtman, B.D., 2007.Early Cambrian metazoan fossil record of South China: generic diversityand radiation pattern. Palaeogeography, Palaeoclimatology, Palaeoecology254, 229–249.

Lin, J.P., Scott, A.C., Li, C.W., Wu, H.J., Ausich, W.I., Zhao, Y.L., Hwu, Y.K.,2006. Silicified egg clusters from a Middle Cambrian Burgess shale-typedeposits, Guizhou, south China. Geology 34, 1037–1040.

Maas, A., Waloszek, D., Mueller, K.J., 2003. Morphology, ontogeny andphylogeny of the Phosphocopina (Crustacea) from the Upper Cambrian“Orsten” of Sweden. Fossil and Strata 49, 1–238.

Maloof, A.C., Schrag, D.P., Crowley, J.L., Bowring, S.A., 2005. An expandedrecord of Early Cambrian carbon cycling from the Anti-Atlas margin,Morocco. Canadian Journal of Earth Sciences 42, 2195–2216.

Maruyama, S., Liou, J.G., Seno, T., 1989. Mesozoic and Cenozoic evolution ofAsia. In: Ben-Avraham, Z. (Ed.), The Evolution of the Pacific OceanMargins. Oxford Monograph Geology and Geophysics, vol. 8. ClarendonPress, Oxford, pp. 75–99.

Maruyama, S., Isozaki, Y., Kimura, G., Terabayashi, M., 1997. Paleogeographicmaps of the Japanese Islands: plate tectonic synthesis from 750 Ma topresent. The Island Arc 6, 121–142.

Maruyama, S., Santosh, M., Zhao, D.P., 2007. Superplume, supercontinent, andpost-perovskite: mantle dynamics and anti-plate tectonics on the core–mantle boundary. Gondwana Research 11, 7–37.

Mossakovsky, A.A., Ruzhentsev, S.V., Samygin, S.G., Kheraskova, T.N., 1993.The Central-Asian fold belt: a geodynamic evolution and history of origin.Geotektonika 6, 3–33 (in Russian).

Mueller, K.J., Hinz-Schallreuter, I., 1993. Paleoscolecid worms from the MiddleCambrian of Australia. Palaeontology 36, 549–592.

Musashi, M., Isozaki, Y., Koike, T., Kreulen, R., 2001. Stable carbon isotopesignature in mid-Panthalassa shallow-water carbonates across the Permo-Triassic boundary: evidence for 13C-depleted ocean. Earth Planetary ScienceLetters 196, 9–20.

Nohda, S., Uchio, Y., Kani, Y., Isozaki, Y., Maruyama, S., 2003. Pb–Pbgeochronologic study on the carbonaceous rocks in the Kurai area, Altai,Russia: V–C boundary or snowball Earth event? American GeophysicalUnion, Fall Meeting, Abstract, V32C-1037.

Ota, T., Utsunomiya, A., Uchio, Y., Isozaki, Y., Buslov, M.M., Ishikawa, A.,Maruyama, S., Kitajima, K., Kaneko, Y., Yamamoto, H., Katayama, I., 2007.Geology of the Gorny Altai subduction-accretion complex, south of Siberia:tectonic evolution of an intra-oceanic arc in the Ediacaran–CambrianPacific. Journal of Asian Earth Sciences 30, 666–695.

Sengör, A.M.C., Natal'in, B., 1996. Paleotectonics of Asia: fragments of asynthesis. In: Yin, A., Harrison, M. (Eds.), The Tectonic Evolution of Asia.Cambridge University Press, Cambridge, pp. 486–640.

Sepkoski Jr., J.J., 1992. Proterozoic–Early Cambrian diversification of metazoansand metaphytes. In: Schopf, J.W., Klein, C. (Eds.), The Proterozoic Biosphere.Cambridge University Press, Cambridge, pp. 553–561.

Shu, D.G, 2008. Cambrian explosion: Birth of tree of animals. GondwanaResearch 14, 219–240. doi:10.1016/j.gr2007.08.004.

Shu, D.G., Luo, H.L., ConwayMorris, S., Zhang, X.L., Hu, S.X., Chen, L., Han,J., Zhu, M., Li, Y., Chen, L.Z., 1999. Lower Cambrian vertebrates fromSouth China. Nature 402, 42–46.

Steiner, M., Li, G.X., Zhu, M.Y., Erdtmann, B.D., 2007. Neoproterozoic toEarly Cambrian small shelly fossil assemblages and a revised biostrati-graphic correlation of the Yangtze Platform (China). Palaeogeography,Palaeoclimatology, Palaeoecology 254, 67–99.

Uchio, Y., Isozaki, Y., Ota, T., Utsunomiya, A., Buslov, M.M., Maruyama, S.,2004. The oldest mid-oceanic carbonate buildup complex: setting andlithofacies of the Vendian (Neoproterozoic) Baratal limestone in the GornyAltai Mountains, Siberia. Proceedings of Japan Academy 80B, 422–428.

Utsunomiya, A., Ota, T., Ishikawa, A., Maruyama, S., Buslov, M.M., 1998.Petrology of greenstone from the Cambrian accretionary complex, southernmargin of Siberian Craton. Abstract 105th Annual Meeting GeologicalSociety of Japan, vol. 223.

Windley, B.F., Akexeiev, D., Xiao, W.J., Kröner, A., Badarch, G., 2007.Tectonic models for accretion of the Central Asian Orogenic Belt. Journal ofGeological Society London 164, 31–47.

Yin, L.M., Yuan, X.L., 2007. Radiation of Meso-Neoproterozoic and earlyCambrian protests inferred from microfossil record of China. Palaeogeo-graphy, Palaeoclimatology, Palaeoecology 254, 350–361.

Zhu, M.Y., Strauss, H., Shields, G.A., 2007. From snowball earth to theCambrian bioradiation: Calibration of Ediacaran–Cambrian earth historyin South China. Palaeogeography, Palaeoclimatology, Palaeoecology 254,1–6.

Zhuravlev, A.Y., 1996. Reef ecosystem recovery from the Early Cambrianextinction. In: Hart, M.B. (Ed.), Biotic recovery frommass extinction events.Geological Society of London Special Publication, vol. 102, pp. 79–86.