basal cambrian microfossils from the yangtze gorges area (south china) and the aksu area (tarim...

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J. Paleont., 83(1), 2009, pp. 30–44Copyright � 2009, The Paleontological Society0022-3360/09/0083-30$03.00

BASAL CAMBRIAN MICROFOSSILS FROM THE YANGTZE GORGES AREA(SOUTH CHINA) AND THE AKSU AREA

(TARIM BLOCK, NORTHWESTERN CHINA)LIN DONG,1 SHUHAI XIAO,1 BING SHEN,1 CHUANMING ZHOU,2 GUOXIANG LI,2 AND JINXIAN YAO3

1Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg 24061, �[email protected]�, �[email protected]�;2State Key Laboratory of Paleobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences,

Nanjing 210008, and 3College of Life Sciences, Peking University, Beijing 100871, China

ABSTRACT—The basal Cambrian marks the beginning of an important chapter in the history of life. However, most paleontological work onthe basal Cambrian has been focused on skeletal animal fossils, and our knowledge about the primary producers—cyanobacteria and eukaryoticphytoplankton (e.g., acritarchs)—is limited. In this research, we have investigated basal Cambrian acritarchs, coccoidal microfossils, and cya-nobacteria preserved in phosphorites and cherts of the Yanjiahe Formation in the Yangtze Gorges area (South China) and the Yurtus Formationin the Aksu area (Tarim Block, northwestern China). Our study confirms the occurrence in these two formations of small acanthomorphicacritarchs characteristic of the basal Cambrian Asteridium–Comasphaeridium–Heliosphaeridium (ACH) assemblage. These acritarchs includeabundant Heliosphaeridium ampliatum (Wang, 1985) Yao et al., 2005, common Yurtusia uniformis n. gen. and n. sp., and rare Comasphaeridiumannulare (Wang, 1985) Yao et al., 2005. In addition, these basal Cambrian successions also contain the clustered coccoidal microfossil Ar-chaeophycus yunnanensis (Song in Luo et al., 1982) n. comb., several filamentous cyanobacteria [Cyanonema majus n. sp., Oscillatoriopsislonga Timofeev and Hermann, 1979, and Siphonophycus robustum (Schopf, 1968) Knoll et al., 1991], and the tabulate tubular microfossilMegathrix longus L. Yin, 1987a, n. emend. Some of these taxa (e.g., H. ampliatum, C. annulare, and M. longus) have a wide geographicdistribution but occur exclusively in basal Cambrian successions, supporting their biostratigraphic importance. Comparison between the strati-graphic occurrences of microfossils reported here and skeletal animal fossils published by others suggests that animals and phytoplanktonradiated in tandem during the Cambrian explosion.

INTRODUCTION

THE HISTORY of life is punctuated by a number of evolutionaryradiations and mass extinctions. Of these evolutionary

events, the Cambrian explosion is perhaps the most significant;essentially all animal phyla evolved in the first 25 million yearsof the Cambrian (Valentine, 2004), and arguably no new animalphyla evolved in the subsequent 500 million years after the Cam-brian Period (Marshall, 2006). Although much has been learnedabout animal radiation patterns during the Cambrian explosion,our knowledge about the diversity of primary producers—cya-nobacteria, macroalgae, and phytoplankton—is limited. This is inpart because Cambrian primary producers were dominated bynon-biomineralizing photosynthesizers that have generally lowpreservability compared to contemporaneous skeletal animals.Nonetheless, Cambrian acritarchs can be useful index fossils forbiostratigraphic correlation (Moczydłowska and Zang, 2006), andthey can also provide insights into the evolution of primary pro-ducers (Vidal and Moczydłowska-Vidal, 1997; Knoll et al., 2006).To improve our knowledge about early Cambrian evolution ofprimary producers and to facilitate biostratigraphic correlationsbetween early Cambrian faunal and floral assemblages, we inves-tigated the basal Cambrian Yanjiahe Formation in the YangtzeGorges area and the Yurtus Formation in the Aksu area.

The Yanjiahe and Yurtus formations both consist of black chert,phosphorite, siltstone, and carbonate. Previous paleontologicalstudies (L. Yin, 1987a; L. Ding et al., 1992; C. Yin et al., 2003;Yao et al., 2005) suggest that these two formations may be broad-ly correlated with each other on the basis of the common occur-rence of several microfossil taxa, including the genera Helios-phaeridium Moczydłowska, 1991, Comasphaeridium Staplin,Jansonius and Pocock, 1965, and Megathrix L. Yin, 1987a. Inaddition, both formations contain similar small shelly fossils(SSFs) characteristic of the traditionally defined MeishucunianStage sensu Y. Qian et al., 2001 (Chen, 1984; J. Qian and B.Xiao, 1984; Gao et al., 1985; B. Xiao and Duan, 1992; Yue andGao, 1992, 1994; Conway Morris and Chapman, 1996, 1997;Conway Morris et al., 1997), which has been correlated with theNemakit–Daldynian and Tommotian stages in Siberia (Y. Qian etal., 2001; Steiner et al., 2007). Thus, the Yanjiahe and Yurtusformations record evolutionary history during the pre-trilobiteCambrian or the unnamed Cambrian series 1 (Babcock et al.,

2005), and have the potential to illuminate the earliest episodesof the Cambrian explosion. The goal of this research is to system-atically describe microfossils recovered from the black cherts ofthe Yanjiahe and Yurtus formations. Our study complements pre-vious paleontological investigation on these units, which largelyfocused on SSFs extracted from the carbonate facies in the twoformations. Together, these paleontological data can be used tofacilitate biostratigraphic correlation of early Cambrian strata andto test evolutionary hypotheses about possible ecological inter-actions between primary producers and animals during the Cam-brian radiation (Butterfield, 1997, 2001; Moczydłowska, 2001,2002).

STRATIGRAPHIC SETTINGS

The Yurtus Formation in the Aksu area.⎯The Aksu area islocated near the northern margin of the Tarim Block (Fig. 1.1,1.2).Thick Neoproterozoic–Cambrian carbonate successions outcrop inthis area. The lower Cambrian Yurtus Formation is about 12–35m in thickness (Fig. 2.1). It unconformably overlies dolostone ofthe Ediacaran Qigeblaq Formation and underlies trilobite-bearinglimestone of the lower Cambrian Xiaoerblaq Formation (Gao etal., 1985). Both the Qigeblaq and Xiaoerblaq formations are char-acterized by abundant fenestrae, indicating intertidal to supratidaldepositional environments. The Yurtus Formation consists of twolithostratigraphic units—the lower chert-phosphorite unit (1–3 min thickness) and the upper siltstone and carbonate unit (Fig. 2.1).Detailed sedimentary facies analysis of the Yurtus Formation isnot available, but the abundance of phosphatic peloids and intra-clasts in the Yurtus Formation indicates a shallow subtidal de-positional environment.

Previous study (Yao et al., 2005) shows that the lower unit ofthe Yurtus Formation contains abundant small acanthomorphicacritarchs, including Heliosphaeridium ampliatum and Comas-phaeridium annulare, as well as the SSF Kaiyangites novilis Y.Qian and G. Yin, 1984, and the enigmatic tubular microfossilMegathrix longus. Yao et al. (2005) also reported Asteridium tor-natum (Volkova, 1968) Moczydłowska, 1991, H. ampliatum, H.cf. lubomlense (Kirjanov, 1974) Moczydłowska, 1991, K. novilis,and M. longus from the basal Cambrian Xishanblaq Formationnear the northeastern margin of the Tarim Block, which they cor-related with the lower Yurtus Formation. Yao et al. (2005) thus

31DONG ET AL.—CAMBRIAN MICROFOSSILS FROM CHINA

FIGURE 1—1, Map showing the location of the Tarim and South China blocks. 2, Simplified geological map of the Aksu–Yurtus area. 3, Simplified geologicalmap of the Yangtze Gorges area. The Yurtus VI, Jiuqunao, Jijiapo, and Yemaomian sections are marked by arrows.

established the basal Cambrian Asteridium–Heliosphaeridium–Comasphaeridium acritarch assemblage zone on the basis of thelower Yurtus and Xishanblaq material, and considered this assem-blage of Meishucunian age (and probably equivalent to SSF as-semblage zones I and II: the Anabarites trisulcatus–Protohertzinaanabarica and Paragloborilus subglobosus–Purella squamulosaassemblage zones). This age assignment is consistent with thepresence of the SSF K. novilis in the Asteridium–Heliosphaeri-dium–Comasphaeridium acritarch assemblage. K. novilis is theeponymous taxon of the SSF Protohertzina anabarica–Kaiyan-gites novilis assemblage zone. This SSF assemblage zone occursin deep shelf Meishucunian successions and has been correlatedwith the SSF assemblage zone I (Anabarites trisulcatus–Proto-hertzina anabarica assemblage zone) in shallow shelf facies ineastern Yunnan and with the lowest Nemakit–Daldynian SSF An-abarites trisulcatus zone in Siberia (Steiner et al., 2007), althoughK. novilis may extend to the next SSF assemblage zone in Gui-zhou (Y. Qian and G. Yin, 1984).

Abundant SSFs have been described from a number of horizonsin the upper unit of the Yurtus Formation, which consists of phos-phatic dolostone and is amenable to fossil extraction using aciddigestion technique (J. Qian and B. Xiao, 1984; Gao et al., 1985;Duan and B. Xiao, 1992; B. Xiao and Duan, 1992; Yue and Gao,1992, 1994; Conway Morris and Chapman, 1996, 1997; ConwayMorris et al., 1997). Some of the biostratigraphically useful taxainclude Anabarites Missarzhevsky, 1969, Archiasterella Sdzuy,1969, Archicladium J. Qian and B. Xiao, 1984, CambroclavusMembetov in Membetov and Repina, 1979, Chancelloria Walcott,1920, Halkieria Poulsen, 1967, Jiangshanodus Yue and He, 1989,

Lapworthella Cobbold, 1921, Ninella Missarzhevsky and Mem-betov, 1981, Protohertzina Missarzhevsky, 1973, TannuolinaFonin and Smirnova, 1967 and Zhijinites Y. Qian, 1978. Rareoccurrences of trilobite genal spines have also been reported fromthe upper Yurtus Formation (Y. Qian, 1999), although these havenot been illustrated in any formal publications. Together, thesefossils indicate that the upper Yurtus Formation may be correlatedwith the upper Meishucunian to lower Qiongzhusian stages ineastern Yunnan of South China, or upper Tommotian to lowerAtdabanian in eastern Siberia (Yue and Gao, 1992; Conway Mor-ris and Chapman, 1996; Steiner et al., 2007).

The Yurtus microfossils described in this paper were collectedfrom the lower chert-phosphorite unit at the Yurtus VI sectionlocated at 40�49.079�N, 79�25.380�E (Fig. 1.2). This section wasdescribed in detail by Gao et al. (1985), Conway Morris andChapman (1996), and Yao et al. (2005).

The Yanjiahe Formation in the Yangtze Gorges area.⎯TheYangtze Gorges area is located in the northern Yangtze Block(Fig. 1.1 and 1.3), where thick successions of Neoproterozoic–Paleozoic rocks are incised by the Yangtze River. Ediacaran andCambrian rocks are exposed around the Huangling anticline (Figs.1.3 and 2.2). Previous work on the Ediacaran–Cambrian transitionhas been focused mostly on the eastern flank of the Huanglinganticline, where a depositional hiatus exists between the sub-tri-lobite Cambrian Tianzhushan Member and uppermost EdiacaranBaimatuo Member of the Dengying Formation (Xing et al., 1984).In the southern flank of the Huangling anticline, however, the sub-trilobite Cambrian strata are represented by the �40-m-thick Yan-jiahe Formation (Chen, 1984), which is equivalent to but much

32 JOURNAL OF PALEONTOLOGY, V. 83, NO. 1, 2009

FIGURE 2—1, Stratigraphic column of the Yurtus Formation at Yurtus VI section. 2, Stratigraphic column of Yanjiahe Formation at the Jiuqunao section.Sample numbers and fossil occurrences are indicated to the right of stratigraphic columns. The exact stratigraphic location of float samples (JQN-chert-1,-2, -3) is uncertain, but they were likely derived from the sampled chert interval between 12.90 m and 15.13 m because they were collected next to theoutcrop. Thickness of gray bars depicts relative diversity of small shelly fossils, on the basis of Yurtus VI section data from Gao et al. (1985) and Jiuqunaosection data from Chen (1984) and C. Yin et al. (2003).


thicker than the �4-m-thick Tianzhushan Member in the easternflank (Y. Qian et al., 1979; Xing et al., 1984). At the Jijiaposection (for locality see Fig. 1.3), the Yanjiahe Formation overlieslight grey, thick-bedded dolostone of the Dengying Formation,and the contact is interpreted to be conformable (Chen, 1984; L.Ding et al., 1992). It is overlain by trilobite-bearing black shalesof the Shuijingtuo Formation. The Yanjiahe Formation is dividedinto three lithostratigraphic members. The lowest member (bed 3of Chen, 1984) is about 12 m thick, and is composed of chertand dolostone intercalated with carbonaceous shale. The middlemember (bed 4 and 5 of Chen, 1984), about 4.5 m thick, is com-posed of phosphatic dolostone, chert, and minor shale. The upperunit (bed 6-7 of Chen, 1984), about 23 m thick, is characterizedby carbonaceous limestone and carbonaceous shale, with a 0.5—1 m thick phosphatic dolostone bed at the top.

Sedimentary structures indicative of an intertidal and supratidaldepositional environment, including tepees and fenestrae, occurin the Baimatuo Member. The Yanjiahe Formation likely repre-sents an early stage of sea level rise during the early Cambriantransgression. In petrographic thin sections, Yanjiahe cherts pre-serve original packstone fabrics. No evidence of subaerial expo-sure is found in the Yanjiahe Formation. Thus, we infer that itwas deposited in a subtidal environment.

Microfossils were recovered in two previous studies from thelower member of the Yanjiahe Formation. One chert horizon atthe base of the Yanjiahe Formation at Jijiapo in the southern flankof the Huangling anticline (for locality see Fig. 1.3) yields abun-dant acritarchs described as Micrhystridium regulare L. Yin, 1987(L. Ding et al., 1992), a taxon that was later synonymized withHeliosphaeridium ampliatum (Yao et al., 2005). A similar butmore diverse microfossil assemblage has been described from achert horizon at the base of the Yanjiahe Formation (�Tianzhu-shan Member) at the Yemaomian section (for locality see Fig.1.3) in the western flank of the Huangling anticline, about 30 kmto the northwest of Jijiapo (C. Yin et al., 2003). The Yemaomianassemblage includes the acritarchs Micrhystridium ampliatum andseveral species of Paracymatiosphaera Wang, 1985, the tubularmicrofossil Megathrix longus, and the SSF Kaiyangites multis-pinatus Y. Qian and W. Ding in W. Ding and Y. Qian, 1988, whichhas been synonymized with K. novilis (Yao et al., 2005).

The middle and upper members of the Yanjiahe Formation atJijiapo yield abundant SSFs characteristic of the MeishucunianStage, including Anabarites trisulcatus Missarzhevsky, 1969,Protohertzina anabarica Missarzhevsky, 1973, and Archaeospirasp. (Chen, 1984). The SSFs from the middle Yanjiahe Formation(bed 4 and 5 in Chen, 1984) were regarded as characteristic ofthe Meishucunian SSF assemblage zone I (the Anabarites trisul-catus–Protohertzina anabarica assemblage zone) in eastern Yun-nan, and those from the upper Yanjiahe Formation (bed 7 of Chen,1984) characteristic of the Meishucunian SSF assemblage zone II(the Paragloborilus subglobosus–Purella squamulosa assemblagezone) (Steiner et al., 2007). If correct, then the microfossils, in-cluding the small acanthomorphic acritarchs, described from thelower Yanjiahe Formation (L. Ding et al., 1992; C. Yin et al.,2003) can be no younger than Meishucunian SSF assemblage I.This age interpretation is also consistent with the presence in thebasal Yanjiahe Formation of the SSF Kaiyangites novilis (�K.multispinatus), although this species may extend into SSF II (Y.Qian and G. Yin, 1984).

The Yanjiahe microfossils described in this paper were col-lected from the lower Yanjiahe Formation at the Jiuqunao sectionlocated near the roadcut of the Zigui-Chongqing highway. Thestratigraphic thickness of the Yanjiahe Formation at Jiuqunao andJijiapo may be different. However, based on lithostratigraphic fea-tures, the sampled horizons at Jiuqunao (Fig. 2.2) are probablyequivalent to the lower member of the Yanjiahe Formation atJijiapo.

SYSTEMATIC PALEONTOLOGY

Since the phylogenetic and high-level taxonomic positions ofthese microfossils are uncertain, in systematic description webreak down the Yurtus and Yanjiahe microfossils into four mor-phological groups: acanthomorphic acritarchs, clustered coccoidalmicrofossils, filamentous microfossils, and tubular microfossilswith both complete and incomplete cross-walls. All specimensdescribed in this paper were studied in thin sections and are re-posited at the Virginia Polytechnic Institute Geosciences Museum(VPIGM). Thin sections (prefixes YTS- from Yurtus Formation,JQN- from Yanjiahe Formation, and YKG- from Xishanblaq For-mation) and England Finder references (e.g., P35/4) are given foreach specimen. If a specimen is located outside the range of Eng-land Finder, coordinates measured on an Olympus BX-51 micro-scope, with slide labels oriented to the right, are given (e.g., 143.0� 29.8). All specimens were digitally photographed and dimen-sions were measured using TPSDIG software.

ACANTHOMORPHIC ACRITARCHS

Genus COMASPHAERIDIUM Staplin, Jansonius and Pocock, 1965Type species.⎯Comasphaeridium cometes (Valensi, 1948) Sta-

plin, Jansonius and Pocock, 1965, Middle Jurassic, France.

COMASPHAERIDIUM ANNULARE (Wang, 1985) Yao et al., 2005Figure 3.1 (lower specimen), 3.2

Paracymatiosphaera annularis WANG, 1985, p. 42, fig. 3.4.Comasphaeridium annulare (Wang, 1985) YAO et al., 2005, p. 692, pl. 1, figs.

5–7, and synonyms therein.

Description.⎯Spherical vesicle with densely, evenly, and radially arrangedprocesses. Processes solid, simple, slender, somewhat stiff, of uniform length,and terminally truncated. No outer envelope is present. Vesicles 8.6–14.0 min diameter (mean � 11.6 m, S.D. � 1.54 m, n � 16). Processes 1.8–3.0m in length (mean � 2.38 m, S.D. � 0.41 m, n � 16 measurementsmade on 16 specimens) and about 0.25 m in thickness. About 70–90 pro-cesses in a circumference (mean � 81, S.D. � 5.96, n � 9). Vesicle diameternot correlated with process length (r � 0.278, p � 0.3 for H0: r � 0).

Neotype.⎯The specimen illustrated in the lower part of Figure 3.1 is heredesignated as a neotype, reposited at Virginia Polytechnic Institute Geosci-ences Museum (VPIGM-4593; thin section number YTS-54-1; England Find-er Reference Q29/2).

Material.⎯Five specimens from the Yanjiahe Formation at Jiuqunao sec-tion: JQN-chert-1 (P35/4) and JQN-chert-2 (R26/1, M32/2, P33/2, N23/3).Eleven specimens from the Yurtus Formation at Yurtus VI section: YTS-54-1 (Q29/2), YTS-54-3 (L35/4, M9/1, J25, L10/1, L9/1), YTS-54-4(N10/4, P18/2, K23/2), YTS-54-5 (T19/2), and YTS-54-6 (G10/2).

Distribution.⎯Meishucunian successions in South China andTarim (Yao et al., 2005).

Discussion.⎯Yao et al. (2005) transferred this species to thegenus Comasphaeridium on the basis of its solid processes thatare densely and evenly distributed on the vesicle surface. Thisspecies lacks an outer envelope; clotted organic carbon at theterminus of processes (Wang, 1985) was a degradational artifact(Yao et al., 2005). Unfortunately, the type material has been lost(F. Wang, personal communication, 2007). We thus designate aneotype and follow Yao et al.’s (2005) emended diagnosis.

Genus HELIOSPHAERIDIUM Moczydłowska, 1991Type species.⎯Heliosphaeridium dissimilare (Volkova, 1969)

Moczydłowska, 1991, Lower Cambrian, Protolenus zone, Poland.

HELIOSPHAERIDIUM AMPLIATUM (Wang, 1985) Yao et al., 2005Figure 3.1 (upper specimen), 3.3,3.4

Micrhystridium ampliatum WANG, 1985, p. 39, fig. 3.6,3.7.Heliosphaeridium ampliatum (Wang, 1985) YAO et al., 2005, p. 693, pl. 1,

figs. 8,9, and synonyms therein.

Description.⎯Single-walled spherical vesicle with processes that are hol-low, straight, and stiff and taper toward a sharply pointed distal end. Processesare sparsely but evenly distributed on vesicle and communicate with vesicleinterior. Yao et al. (2005) provided measurements of Heliosphaeridium am-pliatum from the Yurtus Formation. They are indistinguishable from mea-surements of the Yanjiahe population (Fig. 4): vesicle diameter 6.5–11 m


(mean � 8.8 m, S.D. � 1.37 m, n � 23 measurements made on 23 spec-imens), process length 9–17 m (mean � 12.6 m, S.D. � 2.17 m, n �23 measurements made on 23 specimens), and process thickness 0.8–1.1 mat base. Vesicle size not correlated with process length (correlation coefficientr � 0.04, n � 23, p � 0.85 for H0: r � 0). About 4–16 processes in acircumference.

Material.⎯About 300 specimens from the Yanjiahe Formation at Jiuqunaoand 100 specimens from the Yurtus Formation at Yurtus VI section.

Distribution.⎯Meishucunian successions in South China andTarim (Yao et al., 2005).

Discussion.⎯The genus Heliosphaeridium was established onthe basis of material extracted from shales and mudstones (Moc-zydłowska, 1991). Permineralized acritarchs preserved in chert,such as those from the Yanjiahe and Yurtus formations (L. Dinget al., 1992; Yao et al., 2005), cannot be extracted and speciesidentification is only based on thin section observations. None-theless, Heliosphaeridium ampliatum can be differentiated fromother species in this genus by its relatively sparse and stiff pro-cesses (Yao et al., 2005). With several exceptions (for example,Heliosphaeridium coniferum), most Heliosphaeridium speciespreserved in shales and mudstones tend to have flexible processes.In contrast, permineralized Heliosphaeridium tends to have stiffprocesses, although the processes of Heliosphaeridium cf. lubom-lense show some degree of flexibility (Yao et al., 2005). Thedifference in process flexibility seems to reflect underlying bio-logical features, but it is also possible that early diagenetic per-mineralization may have assisted in the preservation of rigid pro-cesses in their original shape. Regardless, the morphologicalfeatures of this species—hollow processes that open to the vesiclecavity but are closed at the distal end—definitely fit the diagnosisof the genus Heliosphaeridium.

Genus YURTUSIA new genusType species.⎯Yurtusia uniformis n. gen. and n. sp.Diagnosis.⎯Small acritarchs characterized by a vesicle and an

outer envelope, with stiff and evenly distributed processes in be-tween.

Etymology.⎯The genus name refers to the locality (Yurtus) and strati-graphic unit (lower Yurtus Formation) where the type species occurs.

Discussion.⎯Paracymatiosphaera was described as having anouter envelope (Wang, 1985). However, the type species P. re-gularis Wang, 1985 clearly does not have an outer envelope(Wang, 1985, fig. 3.5, 3.9, 3.10). The presence of an apparentouter envelope in the three other species of Paracymatiosphaeradescribed in Wang (1985)—P. irregularis Wang, 1985, P. annu-laris Wang, 1985, and P. hunanensis Wang, 1985—is question-able and is likely a degradational artifact (Yao et al., 2005). Thus,it is likely all four Paracymatiosphaera species should be trans-ferred to the genus Comasphaeridium; indeed, Yao et al. (2005)synonymized them under the combination Comasphaeridium an-nulare (Wang, 1985) Yao et al., 2005.

The new specimens described here under Yurtusia uniformis n.gen. and n. sp. have a bona fide outer envelope. Because the typematerial of Paracymatiosphaera has been lost, a re-examinationis impossible. Our examination of the published illustration showsthat Paracymatiosphaera lacks an outer envelope. Thus, we erectthe new genus Yurtusia to accommodate the new material.

YURTUSIA UNIFORMIS new speciesFigure 3.5–3.16

Diagnosis.⎯Spheroidal vesicle about 10 m in diameter andcovered with solid and stiff processes. Processes are uniform inlength (about 1 m in length), evenly distributed on vesicle (witha density of 30–50 per circumference), and surrounded by anouter envelope.

Description.⎯Specimens three-dimensionally preserved, although manyare deflated or deformed. Processes extremely uniform in length (�1 m,representing 10–15% of vesicle diameter) and thickness (�0.4 m). Processesoriented perpendicular to and are confined between vesicle wall and outerenvelope. Maximum vesicle diameter 5.2–14.9 m (mean � 8.7 m, S.D. �

2.0 m, n � 45), minimum vesicle diameter 2.9–14.8 m (mean � 6.4 m,S.D. � 2.3 m, n � 45), process length 0.7–1.6 m (mean � 1.1 m, S.D.� 0.17 m, n � 45), processes thickness 0.3–0.4 m (mean � 0.4 m, S.D.� 0.04 m, n � 24), process spacing 0.9–1.3 m (mean � 1.0 m, S.D. �0.08 m, n � 24), and process density 30–50 per circumference. Processlength is positively correlated with outer envelope diameter (correlation co-efficient r � 0.71; n � 45; p K 0.01 for H0: r � 0; Fig. 5).

Etymology.⎯Latin, uniformis, with reference to the uniform length of pro-cesses.

Type.⎯The specimen illustrated in Figure 3.5 is designated as the holo-type, reposited at Virginia Polytechnic Institute Geosciences Museum(VPIGM-4598; thin section number YTS-58-1; coordinates 143.0 � 29.8).

Material.⎯About 80 specimens from the Yurtus Formation at Yurtus VIsection.

Distribution.⎯The lower Yurtus Formation (Meishucunian) inthe Aksu area, Tarim Block.

Discussion.⎯As discussed above, the existence of an outer en-velope is one important character that differentiates Yurtusia fromother taxa. In addition, the evenly distributed, short, uniform, andrigid processes are also diagnostic. The most similar taxon isprobably Comasphaeridium annulare; however, C. annulare andother Comasphaeridium species lack an outer envelope and typ-ically have longer and denser processes than Yurtusia uniformis(Moczydłowska, 1991, 1998; Yao et al., 2005).

Several other Proterozoic and Paleozoic acritarch genera alsohave two or more envelopes. These include CymatiosphaeroidesKnoll, 1984 and Distosphaera Y. Zhang et al., 1998, both ofwhich are much larger than Yurtusia uniformis. The only speciesof Cymatiosphaeroides described in the literature is C. kullingiiKnoll, 1984 from the Proterozoic successions in Nordaustlandetand Spitsbergen (Knoll, 1984; Knoll et al., 1991; Butterfield etal., 1994). C. kullingii is characterized by a multilaminate (up to12) outer envelope and unevenly distributed, solid processes. Fur-thermore, the vesicle diameter of C. kullingii (30–350 m ac-cording to Butterfield et al., 1994) is much greater than that of Y.uniformis. Distosphaera speciosa Y. Zhang et al., 1998 from theEdiacaran Doushantuo Formation in South China was originallyidentified as C. kullingii (Yuan et al., 1993), although it has hol-low conical processes on the outer envelope, as well as solidslender processes between the inner and outer envelopes. Again,D. speciosa (outer envelope 75–125 m and the inner envelope32–55 m in diameter) is much larger than Y. uniformis.

CLUSTERED COCCOIDAL MICROFOSSILS

Genus ARCHAEOPHYCUS Wang, Zhang, and Guo, 1983Paratetraphycus Z. Zhang, 1985, p. 166, emend. Y. ZHANG et al., 1998, p.

46.Tetraphycoides CAO, 1985, p. 189.

Type species.⎯Archaeophycus venustus Wang, Zhang, andGuo, 1983, from the basal Cambrian Zhujiaqing Formation (for-merly Meishucun Formation) in Jinning, eastern Yunnan, SouthChina. The type species is here synonymized with Archaeophycusyunnanensis (Song in Luo et al., 1982) n. comb.

Diagnosis.⎯The original diagnosis by Wang et al. (1983) isessentially accurate. This genus is characterized by monads, dy-ads, tetrads, octads, and sometimes non-2n cell clusters. Cells aresub-spherical or ellipsoidal in shape and typically 6–30 m indiameter. No sheaths are present surrounding individual cells orclusters. Cells tend to be polyhedral when clustered because theyare tightly appressed against each other.

Discussion.⎯Clustered coccoidal microfossils from the BitterSpring Formation and other Proterozoic successions have beendescribed under numerous taxa, but typically they are not as tight-ly clustered as Archaeophycus, their cells tend to be smaller (�15m, and often �5 m, in diameter), and the cells or the clustersare surrounded by sheaths. Sphaerophycus Schopf, 1968, is di-agnosed as sheathed spherical to ellipsoidal cells commonly sol-itary or loosely associated in clusters or uniseriate aggregates(Schopf, 1968). Individual cells of Myxococcoides Schopf, 1968


FIGURE 3.—Thin section photomicrographs of Comasphaeridium annulare (1, lower specimen; 2), Heliosphaeridium ampliatum (1, upper specimen;3–4), and Yurtusia uniformis new gen. and n. sp. (5–16). For each illustrated specimen in this paper, thin section number, museum number (VPIGM-), andcoordinates (either England Finder references or coordinates measured on an Olympus BX-51 microscope with label oriented to the right) are given. Thinsection prefix YTS- denotes specimens from the lower Yurtus Formation at the Aksu VI section, JQN- denotes specimens from the lower Yanjiahe Formationat the Jiuqunao section, and YKG- denotes specimens from the Xishanblaq Formation at the Yukkengol section in the Quruqtagh area (Yao et al., 2005). 1,YTS-54-1, VPIGM-4593, Q29/2, specimen in the lower part is neotype of Comasphaeridium annulare. 2, JQN-chert-2, VPIGM-4595, R26/1. 3, JQN-chert-2, VPIGM-4595, M39. 4, JQN-chert-1, VPIGM-4594, P31/4. 5, YTS-58-1, VPIGM-4598, 143.0 � 29.8, holotype. 6, YTS-57-10-1, VPIGM-4596, J20/4. 7,YTS-57-10-1, VPIGM-4596, H16. 8, YTS-58, VPIGM-4597, 138.8 � 27.4. 9–11, YTS-58-1, VPIGM-4598, 130.5 � 33.4, same specimen at different focallevels to show deformed vesicle wall and even distribution of solid processes. 12–16, YTS-58, VPIGM-4597, 128.5 � 32.8, same specimen at different focallevels to show deformed vesicle wall. Scale bars are 10 m for 1–5 and 5 m for 6–16.

are not distinctly sheathed, but they are embedded in a well-de-veloped amorphous, non-lamellated organic matrix. Cell clustersof Caryosphaeroides Schopf, 1968 and Glenobotrydion Schopf,1968 are embedded in amorphous organic matrix or distinctsheaths, and their cells are characterized by subcellular structures

of dubious biological significance (Knoll and Barghoorn, 1975;Knoll and Golubic, 1979). Gloeodiniopsis Schopf, 1968, and syn-onymous taxa (Bigeminococcus Schopf and Blacic, 1971; Eozy-gion Schopf and Blacic, 1971, and Eotetrahedrion Schopf andBlacic, 1971) are characterized by multilamellate sheaths (Schopf,


FIGURE 4—Cross-plot and frequency distribution of vesicle diameter and process length of Heliosphaeridium ampliatum from the lower Yanjiahe Formation(filled circle) and lower Yurtus Formation (open circle; data from Yao et al., 2005).

FIGURE 5—Cross-plot and frequency distribution of outer envelope diameter and process length of Yurtusia uniformis n. gen. and n. sp. from the lowerYurtus Formation.


1968; Schopf and Blacic, 1971; Knoll and Golubic, 1979). Fi-nally, Tetraphycus Oehler, 1978, from the MesoproterozoicMcArthur Group in northern Australia, is characterized by rathersmall cells (0.5–5 m) embedded in an amorphous organic matrix(D. Z. Oehler, 1978). Thus these Proterozoic taxa can be distin-guished from Archaeophycus by their cell size, multilamellatesheaths or amorphous organic matrix, and relatively loose aggre-gation.

Parateraphycus Z. Zhang, 1985 and Tetraphycoides Cao, 1985described from cherts and phosphorites of the Ediacaran Doush-antuo Formation in South China (Cao, 1985; Z. Zhang, 1985; Y.Zhang et al., 1998) are indistinguishable from Archaeophycus incell size, tight clustering, and lack of encompassing sheath ororganic matrix. These two genera are regarded as junior syno-nyms of Archaeophycus. We also agree with Z. Zhang (1985) thatTetraphycus conjunctum Lo, 1980, from the Ediacaran YudomaGroup, should be referred to the genus Archaeophycus (� Par-atetraphycus) because it lacks an amorphous organic matrix thatcharacterize the type species of Tetraphycus (D. Z. Oehler, 1978).Similarly, Tetraphycus yunnanensis Song in Luo et al. (1982)from upper Ediacaran to basal Cambrian successions in easternYunnan Province of South China lacks an amorphous organicmatrix and has cells that are much larger than the type species ofTetraphycus (D. Z. Oehler, 1978); it too should be referred to thegenus Archaeophycus.

ARCHAEOPHYCUS YUNNANENSIS (Song in Luo et al., 1982)new combination

Figure 6.1–6.6

Tetraphycus yunnanensis SONG in Luo et al., 1982, p. 216, pl. 31, figs. 3, 4;LUO et al., 1984, pl. 19, figs. 11, 12.

Archaeophycus venustus WANG, ZHANG, AND GUO, 1983, p. 153–154, figs.5.10, 6.1, 8.3; ZHOU et al., 2004, p. 354, pl. 1, figs. 5–9.

? Bigeminococcus grandis WANG, ZHANG AND GUO, 1983, p. 149–150, fig.13.1–13.4.

Paratetraphycus giganteus Z. ZHANG, 1985, p. 166, pl. I, figs. 1, 4, 6, 7; pl.II, fig. 6; Y. ZHANG et al., 1998, p. 46, fig. 20.4–20.8.

Tetraphycoides multa CAO, 1985, p. 189, pl. 1, figs. 1, 2.

Basionym.⎯Tetraphycus yunnanensis Song in Luo et al., 1982.Diagnosis.⎯Cells are 6–20 m in diameter and typically or-

ganized in cruciform clusters. Irregular clusters can be present,but cells in these clusters may have been shifted during diagen-esis.

Description.⎯Individual cells can be solitary or form dyad, triad, tetrad,or octad clusters. Clusters often occur in aggregates. Cell boundary delineatedby dark cell walls (Figs. 6.1, 6.6). Cell diameter 9–15 m (mean � 11.6 m;S.D. � 1.4 m; n � 52), cluster size 13–30 m (mean � 19.5 m; S.D. �4.56 m; n � 27), and cell wall thickness 0.5–1.0 m.

Material.⎯One specimen from the lower Yanjiahe Formation at Jiuqunao(JQN-chert-1, England Finder references L42/4) and about 20 specimens fromthe lower Yurtus Formation at Yurtus VI section.

Distribution.⎯Ediacaran and Meishucunian successions inSouth China and the Tarim Block, including the Ediacaran Doush-antuo Formation in the Yangtze Gorges and Weng’an areas (Cao,1985; Z. Zhang, 1985; Y. Zhang et al., 1998; Zhou et al., 2004)and the early Cambrian Zhujiaqing (formerly Meishucun) For-mation in eastern Yunnan (Luo et al., 1982; Wang et al., 1983),Yanjiahe Formation in the Yangtze Gorges area (this paper), andYurtus Formation in the Aksu area (this paper).

Discussion.⎯Both Tetraphycus yunnanensis Song in Luo et al.,1982 and Archaeophycus venustus Wang et al., 1983 were firstreported from the early Cambrian Zhujiaqing Formation in easternYunnan, and they are morphologically indistinguishable fromeach other. Thus, they are here regarded as synonymous. Theyare distinct from Tetraphycus in their larger cell size and lack ofsurrounding sheaths. Therefore, they should be assigned to thegenus Archaeophycus, which was originally typified by Archaeo-phycus venustus Wang et al., 1983. Because the species epithet

yunnanensis takes priority over venustus, Archaeophycus yunna-nensis (Song in Luo et al., 1982) n. comb. is created here ac-cording to ICBN article 11.4. Although Archaeophycus venustusis no longer a recognized species, it should remain the type spe-cies of Archaeophycus according to ICBN article 10.5.

Bigeminococcus grandis from the basal Cambrian ZhujiaqingFormation (formerly Meishucun Formation) in Jinning of easternYunnan (Wang et al., 1983, fig. 13.1–13.4) is similar to Archaeo-phycus yunnanensis. Although some B. grandis clusters describedby Wang et al. (1983) appear to be surrounded by sheaths, othersin the same colony are devoid of a sheath. It is not clear whetherthe apparent sheath is simply a thicker cell wall. B. grandis isotherwise indistinguishable from and is tentatively regarded as asynonym of A. yunnanensis. As discussed under the genus re-marks, Paratetraphycus giganteus and Tetraphycoides multashould also be regarded as junior synonyms of A. yunnanensis.Tetraphycus conjunctum has relatively small cells between 3.1and 6.2 m in diameter (Lo, 1980), and if transferred to Archaeo-phycus it may represent a species distinct from A. yunnanensis.

FILAMENTOUS MICROFOSSILS

Genus CYANONEMA Schopf, 1968,emend. Butterfield et al., 1994

Type species.⎯Cyanonema attenuatum Schopf, 1968, emend.Butterfield et al., 1994.

Discussion.⎯Butterfield et al. (1994) classified unbranched,unsheathed, uniserate cellular trichomes into two genera accord-ing to their cell length/diameter ratio. Oscillatoriopsis Schopf,1968 has a length/diameter ratio less than one, and Cyanonemagreater than one. In both genera, cells within the same trichometend to have similar length and diameter, although the trichomesmay taper distally (Schopf, 1968).

CYANONEMA MAJUS new speciesFigure 6.7–6.9

Diagnosis.⎯A species of Cyanonema with cells that are8–20 m in diameter and 20–40 m in length. Cell length/di-ameter ratio is 1.4–3.7. Filaments are sometimes aggregated inbundles.

Description.⎯Based on measurements made on 30 filaments, cells are 8.8–16.0 m in diameter (mean � 11.6 m, S.D. � 1.54 m, n � 30) and 20.0–38.7 m in length (mean � 31.9 m, S.D. � 4.6 m, n � 30). Cell length/diameter ratios range from 1.4 to 3.7, but are relatively uniform in the sametrichome, except where degradational shrinkage occurs (arrow in Fig. 6.9).Slight constriction occurs at cell boundaries. No sheath is present.

Etymology.⎯Latin, majus, larger, with reference to the cells that are largerthan other Cyanonema species.

Material.⎯About 40 specimens from the Yurtus Formation at Yurtus VIsection: YTS-54-1 (K11), YTS-54-2 (J18/1, K11), YTS-54-5 (F18/4), YTS-54-8 (H21/3; multiple specimens), YTS-54-14 (S21/3).

Type.⎯The arrowed specimen illustrated in Fig. 6.8 is designated as theholotype, reposited at Virginia Polytechnic Institute Geosciences Museum(VPIGM-4600; thin section number YTS-54-8; coordinates H21/3).

Distribution.⎯The Meishucunian Yurtus Formation in the Ta-rim Block.

Discussion.⎯The current species is assigned to the genus Cy-anonema because its cell length (�32 m) is greater than its celldiameter (�12 m). At least five species of Cyanonema havebeen published previously, but their cells are smaller than the newspecies described here (Fig. 7). For comparison, the type speciesCyanonema attenuatum from the Bitter Spring Formation is char-acterized by terminally tapering filaments with medial cell mea-suring 1.3–2.4 m in diameter and 1.9–4.8 m in length, with amean diameter of 1.7 m and a mean length of 3.3 m, averagedover 55 cells measured in 4 trichomes (Schopf, 1968). Medialcells of Cyanonema inflatum Oehler, 1977, from the Mesoproter-ozoic McArthur Group in northern Australia, are 2.1–3.6 m indiameter and 2.1–5.4 m in length, with a mean diameter of 2.9m and a mean length of 3.6 m, averaged over 40 measurements


FIGURE 7—Cross-plot of cell diameter and length of Cyanonema majus n. sp. from the lower Yurtus Formation. Size range of C. attenuatum, C. inflatum,C. minor, C. ligamen, and C. sp. (Butterfield et al., 1994) are also indicated for comparison (data from Schopf, 1968, J. H. Oehler, 1977, Y. Zhang, 1981,and Butterfield et al., 1994).

←

FIGURE 6—Thin section photomicrographs of Archaeophycus yunnanensis n. comb. (1–6), Cyanonema majus n. sp. (7–9), Oscillatoriopsis longa (10–11),and Siphonophycus robustum (12). 1, YTS-57-10-1, VPIGM-4596, P28/3. 2, YTS-58, VPIGM-4597, N23. 3, JQN-chert-1, VPIGM-4594, L42/4. 4, YTS-58-1, VPIGM-4598, 143.7 � 30.7. 5, YTS-58-1, VPIGM-4598, F47/1. 6, YTS-58, VPIGM-4597, M39/4. 7–8 (8 is close-up of 7), YTS-54-8, VPIGM-4600,H21/3, arrowed specimen is holotype. 9, YTS-54-5, VPIGM-4599, F19/3, arrow points to degradational shrinkage. 10, YTS-54-1, VPIGM-4593, F10/2. 11,YTS-54-1, VPIGM-4593, R26/3. 12, YTS-58, VPIGM-4597, M23/4. Scale bars are 10 m for 1–5, 20 m for 6, 8, 9, 12, and 50 m for 7, 10, 11.

in 5 trichomes (J. H. Oehler, 1977). Cyanonema minor Oehler,1977, also from the McArthur Group, has cells that are 1.1–1.5m in diameter (mean � 1.4 m, n � 38 measurements in 4trichomes) and 1.4–2.9 m in length (mean � 2.0 m). Cyano-nema ligamen Y. Zhang, 1981, from the Mesoproterozoic Gao-yuzhuang Formation in North China, is also characterized bysmall cells, with a cell diameter of 1.2–2.2 m (mean � 1.8 m,n �12) and a cell length of 2.5–4.5 m (mean � 3.2 m) (Y.Zhang, 1981). The single specimen of Cyanonema sp. from theNeoproterozoic Svanbergfjellet Formation of Spitsbergen isslightly larger, with cells 7 m wide and 12 m long (Butterfieldet al., 1994). Finally, Cyanonema disjuncta Ogurtsova and Ser-geev, 1987, has been transferred to Oscillatoriopsis because itscell length is less than its diameter (Butterfield et al., 1994).

Genus OSCILLATORIOPSIS Schopf, 1968,emend. Butterfield et al., 1994

Type species.⎯Oscillatoriopsis obtusa Schopf, 1968, emend.Butterfield et al., 1994

Synonyms.⎯See Butterfield et al. (1994).Discussion.⎯This genus is distinguished from Cyanonema by

its smaller cell length to diameter ratio (�1), and from MegathrixL. Yin, 1987a, by its flat and entirely complete cross-walls (seebelow for description of Megathrix).

OSCILLATORIOPSIS LONGA Timofeev and Hermann, 1979,emend. Butterfield et al., 1994

Figure 6.10–6.11Synonyms.⎯See Butterfield et al. (1994) and Y. Zhang et al.

(1998).Description.⎯Unbranched, unsheathed, uniserate cellular trichome. Cells

length 8.6–18.5 m (mean � 13.7 m, S.D. � 2.73 m, n � 30). Celldiameter 19–35 m (mean � 27.4 m, S.D. � 5.43 m, n � 30). Cellsuniform in length and diameter within the same trichome. No constrictions atcell boundaries.

Material.⎯Three specimens from the Yurtus Formation at Yurtus VI sec-tion: YTS-54-1 (F10/2, R26/3, M23/4).

Distribution.⎯This morphospecies is widely distributed in Pro-terozoic and Paleozoic successions. See Butterfield et al. (1994)and Y. Zhang et al. (1998) for a list of described synonyms.

Discussion.⎯Butterfield et al. (1994) recognized four speciesof Oscillatoriopsis according to their diameter, i.e., O. vermifor-mis (Schopf, 1968) Butterfield in Butterfield et al., 1994, 1–3 m;O. obtusa Schopf, 1968, 3–8 m; O. amadeus (Schopf and Bla-cic, 1971) Butterfield in Butterfield et al., 1994, 8–14 m; andO. longa Timofeev and Hermann, 1979, 14–25 m. Our speci-mens are regarded as O. longa, although one of the three speci-mens is wider than 25 m—the size limit recognized by Butter-field et al. (1994).

Genus SIPHONOPHYCUS Schopf, 1968,emend. Knoll, Swett, and Mark, 1991

Type species.⎯Siphonophycus kestron Schopf, 1968.Synonyms.⎯See Knoll et al. (1991) and Butterfield et al.

(1994).Discussion.⎯The morphogenus Siphonophycus includes all un-

branched, smooth-walled, nonseptate filamentous microfossilswith little or no tapering toward filament termini (Knoll et al.,1991). Butterfield et al. (1994) suggested that species-level clas-sification of Siphonophycus should be based on filament diameter,i.e., S. thulenema Butterfield in Butterfield et al., 1994, �1 m;S. septatum (Schopf, 1968) Knoll et al., 1991, 1–2 m; S. robus-tum (Schopf, 1968) Knoll et al., 1991, 2–4 m; S. typicum (Her-mann, 1974) Butterfield in Butterfield et al., 1994, 4–8 m; S.kestron Schopf, 1968, 8–16 m; S. solidum (Golub, 1979) But-terfield in Butterfield et al., 1994, 16–32 m, and S. punctatumMaithy, 1975, 32–64 m. Our material best fits the definition ofS. robustum.

SIPHONOPHYCUS ROBUSTUM (Schopf, 1968) Knoll et al., 1991Figure 6.12

Synonyms.⎯See Butterfield et al. (1994).Description.⎯Tubular filaments with a diameter of 3.1–4.5 m (mean �

3.7 m, S.D. � 0.44 m, n � 11 filaments). Filaments are open at both ends.Material.⎯Two specimens from the Yurtus Formation at Yurtus VI sec-

tion: YTS58-1 (Olympus coordinates 139.2 � 28.9, England Finder referencesC31/4).

Distribution.⎯This morphospecies is widely distributed in Pro-terozoic and Paleozoic successions.


TUBULAR MICROFOSSILS WITH BOTH COMPLETE AND

INCOMPLETE CROSS-WALLS

Genus MEGATHRIX L. Yin, 1987a, n. emend.

Type species.⎯Megathrix longus L. Yin, 1987a, n. emend.,from the basal Cambrian Yanjiahe Formation (incorrectly de-scribed as uppermost Dengying Formation in L. Yin, 1987a) inthe Yangtze Gorges area.

New diagnosis.⎯Tubular microfossils that are typically lessthan 100 m in diameter. Tubes are subdivided by regularly in-tercalated complete or incomplete (centrally perforated) trans-verse cross-walls. Cross-walls are corrugated or flat, although in-complete cross-walls are typically less strongly corrugated thancomplete ones. Central perforations in incomplete cross-walls areof similar size in the same specimen.

Discussion.⎯The genus was emended by Yao et al. (2005),and the present study provides a new emendation based on ournew observations. It emphasizes the various degrees of corruga-tion and regular intercalation between complete and incompletecross-walls, as well as the uniform size of central perforations,although there are exceptions. The regular intercalation is bestseen by adjusting focal levels, because incomplete cross-wallsmay appear to be complete if cut eccentrically. In addition, mostspecimens do not branch; the unequal branches described in Yaoet al. (2005) are not common in the examined population.

MEGATHRIX LONGUS L. Yin, 1987a, n. emend.Figure 8

Unnamed filamentous fossils, L. YIN, 1986, p. pl. 2, figs. 2, 8, 10.Megathrix longus L. YIN, 1987a, p. 476, pl. 16, figs. 1–7; pl. 17, figs. 2–5,

7, 8, 10; C. YIN et al., 1992, pl. 2, fig. 9; L. YIN, 1997, pl. 1, figs. 7, 8,10; C. YIN et al., 2003, p. 83, pl. 3, figs. 12,13; YAO et al., 2005, p. 697–700, pl. 2, figs. 3–8.

Megathrix sp. A, L. YIN, 1987a, p. 477, pl. 17, figs. 1, 6; pl. 19, fig. 4.Megathrix sp. B, L. YIN, 1987a, p. 477, pl. 18, figs. 1,2, 4–6; pl. 20, figs. 2,

3, 7.Megathrix sp. C, L. YIN, 1987a, p. 478, pl. 20, figs. 1, 4–6, 8.Megathrix sp. D, L. YIN, 1987a, p. 478, pl. 19, figs. 1–3, 5–8.

New diagnosis.⎯Tubular microfossils typically less than 100m wide and several hundred m long. Tubes characterized byevenly spaced transverse cross-walls that are either complete orincomplete. Complete cross-walls are corrugated or flat, and mostof them are regularly intercalated with incomplete ones. Incom-plete cross-walls are flatter or less strongly corrugated than com-plete ones, and they have central perforations that are typicallyof similar size within the same specimen, although the perforationsize can be different between specimens. Tubes rarely branch.

Description.⎯As most specimens are incompletely preserved and curved,it is difficult to reconstruct their three-dimensional morphology on the basisof thin section observation. Some specimens show apparent terminal taperingin thin sections (Fig. 8.1, 8.2, 8.4); either the tubes do indeed taper terminallyor the apparent tapering in thin section is an artifact resulting from obliquecutting of curved cylindrical tubes (Yao et al., 2005). On the other hand,several specimens show a square termination in thin sections (Yao et al., 2005,pl. 2, fig. 6). If the square termination represents true biological terminationrather than physical or taphonomic breakage, then the tube has at least onetruncated (and non-tapering) termination.

In most specimens, complete and incomplete cross-walls are regularly in-tercalated. This is best seen in longitudinal sections (Fig. 8.3, 8.5–8.7, 8.9,8.10) where short spine-like incomplete cross-walls are interspersed betweenmore strongly corrugated complete cross-walls. The intercalation can also beobserved in transverse sections (e.g., Yao et al., 2005, pl. 2, fig. 4) by ad-justing the focal level so that a succession of complete and incomplete cross-walls are brought into focus. In a transverse section (Fig. 8.8), a corrugatedcomplete cross-wall and a flat incomplete cross-wall can both be seen at thesame focal level, indicating the complete cross-wall is strongly corrugated.Some specimens illustrated in Yao et al. (2005, pl. 2, fig. 6) do not appear tohave incomplete cross-walls. Reexamination of those specimens by adjustingthe focal level shows that there is indeed intercalation between complete andincomplete cross-walls. The effect of different focal levels is also shown inFigure 8.5–8.7 and Figure 8.9, 8.10, where the incomplete cross-walls areonly visible in axial (but not in eccentric) focal planes.

A specimen with unequal dichotomous branches from the lower Yurtus

Formation was described in Yao et al. (2005, pl. 2, fig. 6). Unfortunately, thethin section was damaged after photographing, and verification of the branch-ing pattern is difficult. None of our Yanjiahe specimens show evidence ofbranching.

The measurements of the Yanjiahe specimens are presented here for com-parison with the Yurtus specimens (Yao et al., 2005; Fig. 9). Tube length ofthe Yanjiahe population is 19.5–1100 m (mean � 251.2 m, S.D. � 210.0m, n � 20), tube diameter 34–80 m (mean � 64.5 m, S.D. � 12.49 m,n � 20), spacing of complete cross-walls 4–18 m (mean � 12.2 m, S.D.� 3.71 m, n � 20), and spacing of all cross-walls 3–9 m (mean � 6.2m, S.D. � 1.72 m, n � 15). Incomplete cross-walls extend 1/8–2/3 towardthe center of tubes. There is no significant correlation between tube diameterand cross-wall spacing (r � 0.33, p � 0.15 for H0: r � 0).

Material.⎯About 20 specimens from the Yanjiahe Formation at Jiuqunao,and about 40 specimens from the Yurtus Formation at Yurtus VI section andthe Xishanblaq Formation at Yukkengol (Yao et al., 2005).

Distribution.⎯Meishucunian successions in the Yangtze Gorg-es area and the Yurtus and Yukkengol areas in the Tarim Block(L. Yin, 1987a; C. Yin et al., 1992; C. Yin et al., 2003; Yao etal., 2005).

Discussion.⎯Megathrix longus differs from filamentous cya-nobacteria such as Oscillatoriopsis in its relatively large diameter,corrugated cross-walls, the intercalation of complete and incom-plete cross-walls, and rare branches. Liu et al. (2008) describedfive species of tubular microfossils from the Ediacaran Doush-antuo Formation at Weng’an, Guizhou Province, South China.They too have complete and incomplete cross-walls (S. Xiao etal., 2000). However, the diameter of the Doushantuo species(mostly 100–250 m in diameter) is much greater than M. longus,and they all have flat rather than corrugated cross-walls. Of thefive Doushantuo species, Ramitubus increscens Liu et al., 2008and Ramitubus decrescens Liu et al., 2008 are both characterizedby regularly dichotomous branching and rare incomplete cross-walls, Quadratitubus orbigoniatus Xue et al., 1992 by tetragonaltubes, and Crassitubus costatus Liu et al., 2008 by curved cylin-drical tube with a longitudinal ridge. Sinocyclocyclicus guizh-ouensis Xue et al., 1992 is most similar to M. longus except theformer has greater diameter and flat cross-walls (Xue et al., 1992;S. Xiao et al., 2000). Additionally, S. guizhouensis has at leastone tapering termination as shown in extracted specimens, al-though the comparison between these two species is difficult with-out extracted specimens of M. longus.

The phylogenetic affinity of Megathrix longus is uncertain, butit is unlikely to be cyanobacteria (Yao et al., 2005) because ofthe presence of corrugated cross-walls and intercalation of com-plete and incomplete cross-walls. Nor is it certain how the incom-plete cross-walls were formed. Liu et al. (2008) propose that theincomplete cross-walls in the Doushantuo tubular microfossilscould have been formed in two ways. In the intercalary insertionmodel, different incomplete cross-walls in the same tube are in-serted between pre-existing complete cross-walls, either synchro-nously or sequentially. In the terminal addition model, the incom-plete cross-walls are formed sequentially after each subtendingcomplete cross-wall. Regardless, the presence of a central perfo-ration in the incomplete cross-walls indicates that they grew cen-tripetally.

DISCUSSION

The Meishucunian Stage in South China shortly postdates thedisappearance of the Ediacara biota (Narbonne, 2005) and pre-cedes the arrival of trilobite-dominated Cambrian fauna (Sepko-ski, 1992). It is conventionally correlated with the Nemakit–Dal-dynian and Tommotian stages in Siberia (Y. Qian et al., 2001).Therefore, a comprehensive characterization of the Meishucunianbiota is critical to a full understanding of the Tommotian biota(Sepkoski, 1992) and the biostratigraphic significance of bothSSFs and acritarchs. Most previous studies of Meishucunian pa-leobiology have been focused on small shelly fossils. Yao et al.


FIGURE 8.—Thin section photomicrographs of Megathrix longus from the lower Yurtus and lower Yanjiahe formations. 1, JQN-chert-1, VPIGM-4594,P33/4, note tapering ends probably due to oblique cuts of curved tube. 2, YTS-57, VPIGM-4602, M40/2, note very short incomplete cross-walls. 3, JQN-chert-1, VPIGM-4594, T50/3, note short incomplete cross-wall. 4, JQN-chert-3, VPIGM-4601, W34/1, note variation in tube width (arrow) probably due tooblique cuts of curved specimen. 5–7, YKG-67-5, VPIGM-4603, U34/2, same specimen as published in Yao et al. (2005, pl. 2, fig. 3), showing magnifiedviews of longitudinal sections at different focal levels. 5 and 7 are tangential sections and 6 is an axial section. Central perforations of incomplete cross-walls(arrows) are only visible in 6. 8, JQN-chert-1, VPIGM-4594, G41, transverse section showing both complete (corrugate, obliquely cut, black arrow) andincomplete (flat, annular, white arrow) cross-walls. 9–10, YKG-67-5, VPIGM-4603, W13/3, same specimen as published in Yao et al. (2005, pl. 2, fig. 5),showing longitudinal sections at different focal levels. Incomplete cross-walls (arrows) are best seen in axial section (9) but not in tangential section (10).Scale bars represent 100 m for 1 and 4, 50 m for 2, 3, 8–10, and 20 m for 5–7.

(2005) gave a thorough discussion on the biostratigraphic signif-icance of Meishucunian small acanthomorphic acritarchs and es-tablished the Asteridium–Comasphaeridium–Heliosphaeridium(ACH) acritarch assemblage on the basis of acritarchs from thelower Yurtus Formation. They suggested that the Asteridium–Comasphaeridium–Heliosphaeridium (ACH) acritarch assem-blage is at its minimum age-range correlated with the Meishu-cunian small shelly fossil assemblages I and II (Y. Qian et al.,2001; Steiner et al., 2007) and at its maximum age-range with theentire Meishucunian Stage. They further correlated the Asteri-dium–Comasphaeridium–Heliosphaeridium (ACH) acritarch as-semblage in the lower Yurtus Formation with the Asteridium tor-natum–Comasphaeridium velvetum acritarch zone in the East

Europe Platform (Moczydłowska, 1991, 1998; Yao et al., 2005).The current study provides a test of this correlation through acomparative study of basal Cambrian acritarchs from the YurtusFormation in the Tarim Block and the Yanjiahe Formation in theYangtze Gorges area. The taxonomic similarity between the lowerYanjiahe Formation and lower Yurtus Formation (Fig. 2) clearlysupport their biostratigraphic correlation. Further taxonomic doc-umentation of other basal Cambrian acritarchs, particularly in theTal Formation of the Lesser Himalaya (Tiwari, 1999) and theChulaktau Formation of Kazakhstan (Sergeev, 1989; Sergeev andOgurtsova, 1989), will provide additional tests of the wider bio-stratigraphic significance of the Asteridium–Comasphaeridium–Heliosphaeridium (ACH) acritarch assemblage.


FIGURE 9—Cross-plot and frequency distribution of tube diameter and cross-wall spacing of Megathrix longus from the Yanjiahe Formation (filled circle)and Yurtus Formation (open circle; data from Yao et al., 2005).

In addition to acanthomorphic acritarchs, the tubular microfos-sil Megathrix longus may also have biostratigraphic significance.It has so far been reported from the lower Yurtus and lower Yan-jiahe formations, and it appears to be restricted to the Meishu-cunian Stage. Furthermore, the clustered coccoidal microfossil Ar-chaeophycus yunnanensis and the spiral filamentous fossilObruchevella are common in basal Cambrian cherts and phos-phorites in South China and the Lesser Himalaya (Song, 1984;Tiwari, 1999), although they also occur in the Ediacaran Doush-antuo Formation (Y. Zhang et al., 1998).

It has been proposed that the diversification of Cambrian acan-thomorphic acritarchs is an ecological response to increasing pre-dation pressure through top-down ecological interactions (Butter-field, 1997, 2001) or an ecological fuse to the radiation of animalsthrough bottom-up ecological interactions (Moczydłowska, 2001,2002). These two hypotheses make different predictions about theautecological function of acritarch processes and the relative tim-ing of acanthomorphic acritarch diversification and animal diver-sification. Although the currently available biostratigraphic reso-lution of basal Cambrian is insufficient for a biostratigraphic testof the two ecological hypotheses, there has been some data in-dicating that acritarch diversification appears to precede animaldiversification in the Cambrian (Moczydłowska, 2001, 2002).

The two hypotheses also make specific predictions about theecological functions of acritarch processes. Traditionally, acritarchprocesses are interpreted as cyst formation structures, but Butter-field and colleagues interpreted the processes as defensive struc-tures against animal predators and envisioned two phases of an-imal-acritarch escalation during the Ediacaran–Cambriantransition (Butterfield, 1997, 2001; Peterson and Butterfield,2005). They hypothesize that the first escalation occurred in theEdiacaran Period when benthic macrophagous eumetazoans drove

the diversification of Doushantuo-Pertatataka acanthomorphic ac-ritarchs (Zhou et al., 2007), which according to their hypothesiswere benthic organisms. In the second phase, the escalation stagewas moved to the water column, where zooplankton drove themorphological diversification of phytoplankton represented byearly Cambrian small acanthomorphic acritarchs. A possible testof the top-down ecological coupling hypothesis during the Cam-brian explosion is to comprehensively examine the morphologicalcomplexity of acritarch processes. Acritarchs in the Asteridium–Comasphaeridium–Heliosphaeridium (ACH) assemblage arecharacterized by solid or cylindrical, unbranching processes,much more complex than the Leiosphaeridia-dominated late Edi-acaran acritarch assemblage (Huntley et al., 2006). Planktonic ac-ritarchs with branching processes (e.g., Skiagia), however, do notappear until the trilobite-bearing Atdabanian Stage (Moc-zydłowska and Zang, 2006); some Ediacaran acritarchs (for ex-ample Dicrospinasphaera zhangii Yuan and Hofmann, 1998 fromthe Doushantuo Formation) have branching processes, but theywere interpreted as products of the Ediacaran escalation in thebenthic realm. Still more complex acanthomorphic acritarchs withwell-defined excystment structures occur in Ordovician and Si-lurian rocks (Martin, 1993; L. Yin, 1995). It would be useful tocarry out a comprehensive study of the morphological complexityand taxonomic diversity of Paleozoic acritarchs (cf. Huntley etal., 2006) and compare the patterns with animal morphologicaland taxonomic history. From a qualitative examination of pub-lished data, it appears that acritarchs experienced morphologicaland taxonomic diversification in the Meishucunian, Atdabanian,and Early Ordovician, matching the diversification of the Tom-motian, Cambrian, and Paleozoic evolutionary faunas (Sepkoski,1981, 1992). If confirmed, animals and phytoplankton may haveradiated in tandem during the Paleozoic, at least at the broadest


time scale, although the ecological mechanisms (Butterfield,2001; Moczydłowska, 2001) remain unresolved.

CONCLUSIONS

Eight species, including two new species and a new combina-tion, are described from the Meishucunian lower Yurtus Forma-tion in the Aksu area and lower Yanjiahe Formation in the Yang-tze Gorges area. These two assemblages share a number of taxacharacteristic of the Asteridium–Comasphaeridium–Heliosphaer-idium acritarch assemblage, confirming their stratigraphic corre-lation between the two formations. The acritarch assemblage isdominated by Heliosphaeridium ampliatum, Yurtusia uniformis n.gen. and n. sp., and rare Comasphaeridium annulare. The genusAsteridium does not occur in the two formations, and a previousstudy shows that its occurrence may be slightly lower than Com-asphaeridium and Heliosphaeridium (Yao et al., 2005). The enig-matic tubular microfossil Megathrix longus L. Yin, 1987a, n.emend. may also be biostratigraphically useful. Other taxa, in-cluding Archaeophycus yunnanensis n. comb., Siphonophycus ro-bustum, Cyanonema majus n. sp., and Oscillatoriopsis longa areless useful in biostratigraphic correlation. Preliminary comparisonof acritarch and animal fossil record suggests that metazoans andphytoplankton radiated in tandem during the Cambrian explosion,although the ecological mechanism behind the parallelism remainunresolved.

ACKNOWLEDGMENTS

This research was supported by grants from NASA Exobiology program(NNG05GP21G), National Natural Science Foundation of China (40602001,40572006, and 40628002), National Science Foundation (EAR 0745827),Chinese Ministry of Science and Technology (2006CB806400), and ChineseAcademy of Sciences (KZCX3-SW-141). B. Xiao and Y. Qian provided fieldassistance. We thank M. Head, M. Moczydłowska, and L. Yin for constructivereviews. We would also like to thank K. Grey, T. Servais, P. Strother, F. Wang,Reed Wicander, and X. Yuan for useful discussions.

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ACCEPTED 10 JULY 2008

basal cambrian microfossils from the yangtze gorges area (south china) and the aksu area (tarim...

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