Paleobiology of the Mesoproterozoic-Neoproterozoic transition: the Sukhaya Tunguska Formation, Turukhansk Uplift, Siberia

Premmbriun Reseurth ELSEVIER

Precambrian Research 85 (1997) 201-239

Paleobiology of the Mesoproterozoic-Neoproterozoic transition: the Sukhaya Tunguska Formation, Turukhansk Uplift, Siberia V.N. Sergeev a,,, A.H. Knoll b, P.Yu. Petrov a a Geological Institute, Russian Academy of Sciences, Moscow 109017, Russia b Botanical Museum, Harvard University, Cambridge, MA 02138, USA

Received 14 January 1997; accepted 27 June 1997

Abstract

Silicified carbonates of the latest Mesoproterozoic Sukhaya Tunguska Formation, northwestern Siberia, contain abundant and diverse permineralized microfossils. Peritidal environments are dominated by microbial mats built by filamentous cyanobacteria comparable to modern species of Lyngbya and Phormidium. In subtidal to lower intertidal settings, mat-dwelling microbenthos and possible coastal microplankton are abundant. In contrast, densely woven mat populations with few associated taxa characterize more restricted parts of tidal fiats; the preservation of vertically oriented sheath bundles and primary fenestrae indicates that in these mats carbonate cementation was commonly penecontemporaneous with mat growth. Eoentophysalis mats are limited to restricted environments where microlaminated carbonate precipitates formed on or just beneath the sediment surface. Most microbenthic populations are cyanobacterial, although eukaryotic microfossils may occur among the simple spheroidal cells interpreted as coastal plankton. Protists are more securely represented by large (up to 320 ~ n in diameter) but poorly preserved acritarchs in basinal facies. The Sukhaya Tunguska assemblage contains 27 species in 18 genera. By virtue of their stratigraphic longevity and their close and predictable association with specific paleoenvironmental conditions, including substrates, Proterozoic cyanobacteria support a model of bacterial evolution in which populations adapt rapidly to novel environments and, thereafter, resist competitive replacement. The resulting evolutionary pattern is one of accumulation and stasis rather than the turnover and replacement characteristic of Phanerozoic plants and animals. © 1997 Elsevier Science B.V. Keywords: Cyanobacteria; Micropaleontology; Proterozoic; Siberia

1. Introduction

Microfossils document a m a j o r transition in the biological world near the M e s o p r o t e r o z o i c Neoproterozoic (Middle Riphean-Upper Riphean) boundary (Knoll and Sergeev, 1995). This change is most evident in the diversification * Corresponding author. 0301-9268/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S0301-9268 (97)00035-1

of eukaryotic fossils (Knoll, 1992), but a shift in prokaryotic assemblages is also observed (Sergeev et al., 1995). In part, changes in cyanobacterial assemblages reflect the evolution of green, red and chromophyte algae that competed with them for space and nutrients; however, other changes m a y be a consequence of long-term environmental alteration--specifically, directional change in patterns of carbonate cementation and, hence, both sub-

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strate conditions and preservation potential on Proterozoic carbonate platforms (Knoll and Sergeev, 1995). In a first test of carbonate-control hypothesis, Kah and Knoll (1996) demonstrated that early-cemented carbonates in proximal parts of a late Mesoproterozoic to earliest Neoproterozoic tidal flat from Arctic Canada supported microbenthic prokaryotes of a type common in older Proterozoic successions but rare in the Neoproterozoic, whereas uncemented sediments in more distal parts of the same platform contained microfossils widely encountered in Neoproterozoic successions. Beautifully exposed late Mesoproterozoic to early Neoproterozoic (late Middle Riphean to early Late Riphean) successions in the Turukhansk Uplift, northwestern Siberia (Fig. 1), provide an opportunity to investigate further the influences of evolution and ecology on the stratigraphic and facies distributions of Proterozoic microfossils. Carbonaceous shales throughout the succession contain well-preserved acritarchs, cyanobacteria and possible multicellular algae (Timofeev et al., 1976; Yankauskas et al., 1989; German, 1990; Veis and Petrov, 1994a,b; Petrov and Veis, 1995); among other things, these assemblages document increasing diversity in the eukaryotic phytoplankton. Early diagenetic chert nodules in carbonates of the Turukhansk succession preserve a complementary record of coastal microbenthos and subordinate plankton (Mendelson and Schopf, 1982; Golovenok and Belova, 1992, 1993; Petrov et al., 1995). In this paper, we evaluate the systematic paleontology and environmental distribution of silicified microfossils from the Sukhaya Tunguska Formation. Sukhaya Tunguska carbonates accumulated in environments ranging from supratidal to subtidal below wave base on a ramp-like platform. Preserved microbial remains thus permit a direct test of postulated relationships between microbenthos and carbonate cementation near the Mesoproterozoic-Neoproterozoic boundary.

2. Previous research

Microfossils were first reported from Sukhaya Tunguska cherts by Schopf et al. (1977), who

Fig. 1. Geographic and stratigraphic position of silicified microfossils of the Sukhaya Tunguska Formation. (A) Index map of Eurasia; the box indicates the location of the Turukhansk Uplift. (B) Map of the Turukhansk Uplift (box in A) showing locations of fossiliferous outcrops of the Sukhaya Tunguska Formation. (C) Generalized stratigraphic column of the MesoNeoproterozoic (Riphean) deposits of the Turukhansk Uplift, with stars indicating fossiliferous horizons of the Sukhaya Tunguska Formation. Abbreviations of formation names: bz, Bezymenyy; ln, Linok; sh, Sukhaya Tunguska; dr, Derevnya; br, Burovaya; sr, Shorikha; mr, Miroyedikha; tr, Turukhansk. See Fig. 2 for legend.

illustrated and briefly described specimens from three samples provided by B.B. Nazarov (Geological Institute of the Academy of Sciences, Moscow). The samples, of intraclastic conglomer-

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ate only broadly related to stratigraphic or facies development within the formation, were subsequently described in more detail by Mendelson and Schopf (1982), who reported 11 taxa--most treated informally and confined to a single locality along the Sukhaya Tunguska River. More recently, Golovenok and Belova (1992, 1993) collected systematically from all available sections through the formation; they documented 19 taxa, including the morphologically and environmentally distinctive cyanobacterium Polybessurus bipartitus. Golovenok and Belova also provided a broad indication of stratigraphic and geographic variation in assemblage composition, but in the absence of sedimentological data on chert localities, it is not easy to interpret their observations in terms of environmental variation or evolution. Detailed studies by Petrov (1993) now provide the necessary stratigraphic and sedimentological framework for paleontological interpretation of Sukhaya Tunguska microfossils, enabling Petrov et al. (1995) to demonstrate the strong facies control of fossil distributions within the formation. In the following pages, we integrate new and existing data from systematic paleontology, stratigraphy/sedimentology and petrology in an attempt to understand more completely the distribution of life on the Sukhaya Tunguska carbonate platform. We report the presence of at least 27 distinct taxa, documenting hitherto unrecorded microfossil populations and providing new information on previously known forms. New sedimentological and petrological observations permit us to refine our understanding of paleontology and paleoecology near the MesoproterozoicNeoproterozoic boundary. Our study is based principally on material collected during field excursions by V.N. Sergeev in 1988, P.Yu. Petrov in 1989 and all three authors during a joint U.S.-Russian expedition in 1995.

3. Geological setting, depositional environments and age of the Sukhaya Tunguska Formation

3.1. General geology The Turukhansk Uplift is located near the northwestern margin of the Siberian Platform (Fig. 1).

203

Proterozoic (Riphean and Vendian) deposits in this region comprise a westward-deepening monocline or asymmetrical syncline 130-150 km long and c. 30 km wide disrupted by three major and a series of minor meridionally trending faults. The nearly 4000 m thick Mesoproterozoic to Neoproterozoic (upper Middle to lower Upper Riphean) sedimentary package begins with the Bezymenyy Formation, a thick (up to 1000m) succession of carbonaceous shales, sandstones and subordinate gravelstones that accumulated in midto inner-shelf environments subject to episodic storms (Petrov, 1993). The lower boundary of the formation is a fault contact; thus, its true depositional thickness is unknown. Above the Bezymenyy Formation follow the carbonate-dominated Linok and Sukhaya Tunguska formations. An erosional unconformity separates these lower units from the succeeding carbonate-rich Derevnya, Burovaya, Shorikha, Miroyedikha and Turukhansk formations. The entire Turukhansk succession is overlain unconformably by terminal Proterozoic (Vendian) to Lower Cambrian rocks of the Platonovskaya and Kostino formations (Dragunov, 1963; Petrakov, 1964; Khomentovsky, 1990; Bartley et al., in press).

3.2. Stratigraphy and depositional environments The 530-670 m thick Sukhaya Tunguska Formation consists mainly of limestones and dolomites, with abundant nodular chert in its upper part. The formation terminates a large transgressive-regressive sedimentary cycle that also includes underlying formations (Serebryakov, 1975; Petrov, 1993); it is conventionally divided into lower and upper members [described in detail by Petrov (1993) and Petrov et al. (1995); see also Dragunov (1963); Serebryakov (1975)]. The lower member consists predominantly of limestones, with dolomites increasing in abundance upward and to the north (Fig. 2). The lowermost of the three lithological units that compose this member is a 120-130 m thick interval of mechanically laminated calcisiltites and fiat-pebble conglomerates with rare thin lenses of late diagenetic chert. Deposition is inferred to have taken place below storm wavebase in a relatively deep inner

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Fig. 2. Detailed stratigraphic sections of the Sukhaya Tunguska Formation (localities I-VI) in the Turukhansk Uplift. The six sections correspond to six outcrops labeled in Fig. 1. Key: 1, limestones; 2, dolomites; 3, cherts; 4 10, lithofacies associations: 4, fine-grained carbonates; 5, intraclastic carbonates; 6, mixed micritic and intraclastic limestones from the upper unit of the lower member; 7, grainstone/stromatolitic association; 8, thin-bedded laminites and dark, thinly laminated dolomites; 9, wavy bedded dolomites; 10, sandy dolomites; 11, tepee structures; 12, faults and brecciated rocks. Roman numbers are locations of the outcrops (see Fig. 1): I, Nadporozhnaya River; II, Kamennaya River; III, Nizhnyaya Tunguska River downstream from the mouth of Gremyachii Creek; IV, Nizhnyaya Tunguska River downstream from the Strelnya Mountains; V, the lower stream of Miroedikha River; VI, Sukhaya Tunguska River. Arabic numbers denote horizons of fossiliferous samples.

shelf environment. Mechanically emplaced calcisiltites were cemented by nodular calcite cement prior to significant compaction, imparting an irregular or undulatory appearance to bedding. Early cementation also isolated limestones from later dolomitizing fluids that affected uncemented portions of the unit. The lower unit becomes increasingly sandy up section, with ripples, low-angle cross-lamination, intraformational breccia and phosphatic nodules all increasing concomitantly.

Its top is marked by stratiform stromatolites, calcisiltites and grainstones showing marked bedding truncation, microspar-filled syneresis cracks and early diagenetic chert nodules; these features collectively record shallowing to near sea level. The second unit (4 6 to 20-30 m) consists of columnar stromatolitic bioherms [Baicalia prima and Tungussia nodosa; Semikhatov (1962)], grainstones and micrites that accumulated in peritidal, including lagoonal, environments. Above

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this, the upper unit (100-150m) of the lower member begins abruptly with interbedded stratiform stromatolites and intraformational breccias, along with subordinate cross-bedded grainstones. Dolomite is more common in this unit than in underlying intervals; however, limestone remains a dominant lithology, thanks to extremely early diagenetic cements that occluded pore space. Abundant chert nodules in this unit contain wellpreserved microfossils. Petrov et al. (1995) proposed that the upper unit accumulated in restricted coastal environments within local depressions separated by elevated ridges subject to subaerial exposure. The total thickness of the lower member varies from 240 to 300m, increasing to the northwest. The upper member (300-380 m) of the Sukhaya Tunguska Formation consists predominantly of thin- to medium-bedded dolomites with few columnar stromatolites, but abundant fiat-pebble conglomerates and nodular black chert. Its lower unit (30-70m) comprises thin- to medium-bedded dolomicrites and dolosiltites interbedded with fiatpebble conglomerates and oolites. Fine-grained dolomites are fetid and commonly contain syneresis cracks. The upper unit (270-310 m) comprises thin-bedded dolomites, with stratiform stromatolites, tepee structures and gypsum casts in its uppermost part. The entire upper member records peritidal depositional environments, with its uppermost beds indicating restricted inter- to supratidal conditions. For the most part, Sukhaya Tunguska cherts preserve fossil populations from a limited range of peritidal environments. Fossils representing more open marine environments are limited to a few samples from the lowermost unit of the formation. The taphonomic bias imposed by early diagenetic silicification is similar to that found in many other Proterozoic carbonate platforms (Horodyski and Donaldson, 1980, 1983; Knoll, 1985; Southgate, 1986; Maliva et al., 1989; Knoll et al., 1991; Sergeev, 1992b, Sergeev et al., 1995). 3.3. Age

The age of the Sukhaya Tunguska Formation is constrained by radiometric as well as paleontological and chemostratigraphic data. Reported Pb-Pb

205

age dates for early diagenetic carbonates within the formation are 1017+91 and 1035+60 Ma [11 point and 16 point isochrons, respectively; Ovchinnikova et al. (1994, 1995)]. Glauconites from the overlying Derevnya and Burovaya formations and the underlying Bezymenyy Formation yield K-Ar ages of 800-900 Ma, which is interpreted to reflect late diagenetic resetting of isotope systematics (Mendelson and Schopf, 1982; Gorokhov et al., 1995). Biostratigraphic data are broadly consistent with Pb-Pb dates for the Sukhaya Tunguska Formation. To date, Sukhaya Tunguska samples have not yielded age-diagnostic microfossils, but the subjacent Bezymenyy Formation contains leiospherid and other simple acritarchs consistent with a late Mesoproterozoic (Middle Riphean) age of deposition, whereas shales of the unconformably overlying Derevnya Formation yield abundant acanthomorphic acritarchs, including the widely distributed Neoproterozoic species Trachyhystrichosphaera aimika (Petrov and Veis, 1995). Stromatolites in the Sukhaya Tunguska and overlying formations have been used to support several different stratigraphic interpretations [see review in Semikhatov (1991)]. Early workers drew the Middle Riphean/Late Riphean (Mesoproterozoic/Neoproterozoic) boundary above the last appearances of Conophyton, Jacutophyton and Baicalia and below the first appearance of Minjaria; using these criteria, the boundary in the Turukhansk region was placed at the base of the Shorikha Formation [e.g. Krylov (1963, 1975, 1985); Semikhatov (1962)]. Following the discovery of Conophyton, Jacutophyton and Baicalia in carbonates of the Upper Riphean stratotype in the Southern Urals, emphasis in stromatolite biostratigraphy shifted from last to first appearances. Bioherms in the Derevnya Formation contain Baicalia lacera, lnzeria tjomusi and Gymnosolen sp.--forms that first appear in Upper Riphean (Neoproterozoic) carbonates in the Southern Ural stratotype and elsewhere (Semikhatov, 1991). On this basis, the Middle Riphean/Upper Riphean boundary was repositioned between the Sukhaya Tunguska and Derevnya formations (Semikhatov, 1991). A third point of view has been championed by Komar (1990), Veis (1988), and Khomentovsky

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et al. (1985). Using stromatolite microstructures, acritarch size and broad lithostratigraphic and tectonic arguments, respectively, these authors have argued that the entire Turukhansk succession falls within the Upper Riphean (Neoproterozoic). A proposal to place the Middle Riphean/Upper Riphean (Mesoproterozoic/Neoproterozoic) boundary in the Tnrukhansk region at the unconformity between the Sukhaya Tunguska and Derevnya formations was approved by the AllUnion Conference on the General Problems of the Precambrian Stratigraphy of the U.S.S.R., held in Ufa in 1990 (Semikhatov et al., 1991). Carbon isotopic profiles for multiple sections through the Turukhansk succession (Knoll et al., 1995) show repeated stratigraphic variations between c. - 1 and + 3%o, similar to those of other late Mesoproterozoic to early Neoproterozoic successions (Beeanus and Knauth, 1985; Kah, 1997; Schidlowski et al., 1975), but distinctly different from either older (e.g. Veizer et al., 1992; Knoll et al., 1995; Buick et al., 1995) or younger (Kaufman and Knoll, 1995) Proterozoic intervals. Thus, radiometric, paleontological and chemostratigraphic data for the Sukhaya Tunguska Formation converge on a depositional age near the Mesoproterozoic-Neoproterozoic boundary.

4. The composition of the Sukhaya Tunguska microbiota The Sukhaya Tunguska microbiota contains at least 18 genera and 27 species of morphologically simple filamentous and coccoidal fossils (tabulated in Fig. 3). Most are interpreted as cyanobacterial cells, trichomes and sheaths, but some simple spheroids may be eukaryotic. Several orders of cyanobacteria are recognized with confidence in Sukhaya Tunguska cherts. Five taxa (Calyptothrix sp., Eomicrocoleus sp., Uluksanella sp., Oscillatoriopsis media, Palaeolyngbya sp.) are assigned to the Oscillatoriales; Archaeoellipsoides dolichos is interpreted as the preserved akinete of a nostocalean cyanobacterium; and four taxa (Siphonophycus

robustum, S. typicum, S. solidum, Circumvaginalis sp.) belong to either the Oscillatoriales or the

Nostocales. The Chroococcales are represented by

Eoaphanocapsa oparinii, Gloeodiniopsis lamellosa and Gyalosphaera golovenokii n. sp. and--with less certainty--Eosynechococcus moorei, E. medius and Sphaerophycus parvum. Eoentophysalis arcata and E. cf. belcherensis document the Entophysalidales, and

the

Pleurocapsales

are

represented

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Polybessurus bipartitus. The distribution of these taxa on a phylogeny constructed using comparisons of nucleotide sequence in genes for 16S rRNA (Giovannoni et al., 1988) shows that Sukhaya Tunguska fossils occupy nearly all major branches of the cyanobacterial tree. [With the exception of the placement of some simple coccoids and undifferentiated trichomes, the 16SrRNA tree is broadly congruent with phylogenies based on morphology (Wilmotte and Golubic, 1991).] The inescapable conclusion is that 1000 Ma ago, the principal lines of cyanobacterial diversity were already in place. Indeed, there is reason to believe that they existed substantially before the Mesoproterozoic/Neoproterozoic transition (Schopf, 1992a). Fossils assigned to the uppermost principal branch of the cyanobacteria-elongate ellipsoids of the genus Archaeoellipsoides interpreted as nostocalean akinetes--occur in rocks that are 1500 Ma or older (Sergeev et al., 1995; Golubic et al., 1995). As discussed further below, stratigraphic first appearances of specific cyanobacterial species near the Mesoproterozoic/Neoproterozoic boundary may reflect changing substrate and taphonomic conditions and, therefore, bear no necessary relationship to evolutionary origins [see also Kah and Knoll (1996)]. Peritidal Sukhaya Tunguska assemblages may contain eukaryotic remains, but this remains uncertain. Myxococcoides minor, M. inornata , M. grandis, Myxococcoides sp., and Leiosphaeridia sp. are all simple spheroidal fossils that could be the preserved cells of protists, cyanobacterial cell walls or the extracellular envelopes of coccoidal cyanobacteria. Large (up to 320 lam in diameter), poorly preserved acritarchs in open-shelf facies near the base of the formation are more securely placed among the protists. Of course, the paucity of eukaryotic remains in Sukhaya Tunguska cherts does not mean that nucleated organisms were

V.N. Sergeev et al. / Precambrian Research 85 (1997} 201-239 Phylum CYANOBACTERIA Class HORMOGONEAE Order OSCILLATORIALES Family OSC ILLATORIACEAE Order NOSTOCALE S Family NOSTOCACEAE Class COCCOGONEAE Order CHROOCOCCALE S Family CHROOCOCCACEAE Family ENTOPHYSALIDACEAE-

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C

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Eoaphanocapsa

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Gloeodiniopsis lamellosa

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Gyalosphaera golovenokii

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Fig. 3. Synopsis of microfossil taxa in cherts of the Sukhaya T u n g u s k a Formation. The form marked by an asterisk was described by Mendelson and Schopf (1982), but not identified in our collection. The size ranges of microfossils are displayed on a logarithmic scale.

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inconspicuous in all late Mesoproterozoic to early Neoproterozoic ecosystems--unambiguously eukaryotic acritarchs (Xiao et al., in press), carbonaceous macrofossils (Walter et al., 1990; Han and Runnegar, 1992), and biomarker molecules produced by eukaryotic organisms (Summons and Walter, 1990) occur in both contemporaneous and older successions elsewhere. The predominance of cyanobacteria in Sukhaya Tunguska cherts reflects the restriction of most early diagenetic silica nodules to peritidal environments of a type dominated today as well as in the Proterozoic by prokaryotes.

5. Environmental influences on microfossil distribution

As noted by Mendelson and Schopf (1982) and Golovenok and Belova (1993), different samples of Sukhaya Tunguska chert contain differing subsets of the total biota. Petrov et al. (1995) placed these observations in sedimentological perspective, showing that most Sukhaya Tunguska fossils occur in intertidal to shallow subtidal facies in the upper units of the lower and upper members. Petrological observations of microfossil distributions within samples allow us to track populations with still greater environmental resolution. In cherts collected from the upper unit of the lower member along the Sukhaya Tunguska River (Fig. 1), filamentous sheaths of Siphonophycus spp. are the overwhelmingly dominant fossils. At this locality, stratiform microbial mats are intimately interbedded with thin calcisiltites containing syneresis structures, thicker (to 36 cm) but less frequent grainstones with low-angle cross-lamination, and thin edgewise conglomerate beds (Fig. 4A). Sedimentary features suggest a broad peritdal flat subject to episodic inundation by mechanically produced carbonate sediments. Within this milieu, the filaments are preserved

in silicified stratiform mats, within which they form densely interwoven populations oriented subparallel or, less commonly, perpendicular to bedding (Fig. 4 C - E ) . There is little evidence of sediment trapping, but the preservation of vertically oriented filament bundles and primary fenestrae indicates that carbonate cementation proceeded penecontemporaneously with mat accretion. Siphonophycus robustum and S. typicum were principal mat-builders in these mats, with S. solidum serving as a subsidiary mat builder. Circumvaginalis sp. is confined to a single sample from a proximal peritidal setting (Petrov et al., 1995); consistent with sedimentological evidence, its pattern of pigmentation suggests a higher frequency of subaerial exposure. Mat dwellers and possible plankton are rare in this assemblage, although Gloeodiniopsis and Myxococcoides occur within mats and thin (originally) calcisiltites that drape mat laminae. A completely different mat assemblage occurs locally in this locality. Within samples dominated by Siphonophycus mats, thin horizons contain mats built by E. arcata and, less commonly, E. cf. E. belcherensis. These occur in specific association with thin horizons of microlaminated, acicular carbonate precipitated locally at or just below the sediment/water interface. Eoentophysalis fossils found in the Sukhaya Tunguska Formation and elsewhere appear to be closely related to extant species of Entophysalis that form mats along tidal flats in Abu Dhabi, Persian/Arabian Gulf (Golubic, 1973, 1976; Kinsmann and Park, 1976) and Shark Bay, Australia (Playford and Cockbain, 1976; Golubic, 1976, 1983, 1985). Entophysalis mats also occur on tidal flats (Golubic, 1983) and in supratidal marshes ( M o n t y and Hardie, 1976) of the Bahama Banks. In all cases, Entophysalis populations dominate in warm, shallow, hypersaline bodies of quiet water in the intertidal zone or in coastal ponds

Fig. 4. Outcrop and petrographicviews of fossiliferouscherts in the Sukhaya Tunguska Formation. (A) Early diageneticchert nodules in mixed pertidial microbial laminites and event beds in the upper unit of the Lower Member along the Sukhaya Tunguska River (locality VI of Fig. 1). (B) Early diagenetic chert nodules in microbially laminated carbonates of the Upper Member along the Sukhaya Tunguska River (locality VI in Fig. 1). (C-E) petrographicviews of denselyinterwovenfilamentousmats at locality VI. (F and G). Low and high magnificationviews of cherts dominated by E. arcatamats, from locality III along the NizhnyayaTunguska River. 15 cm ruler and hammer provide scale in A and B. Bars in C, E and F= 1 mm;bars in D and G=20 ~tm.

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that have only limited access to the open ocean [see Knoll and Golubic (1979)]. It is true, however, that in the Persian/Arabian Gulf region, at least, the distribution of Entophysalis mats is determined in part by grazing gastropods confined to subtidal environments by high salinity in the intertidal zone (Kinsmann and Park, 1976). In the absence of macroscopic grazers, Eoentophysalis mats might have colonized shallow subtidal environments on the Sukhaya Tunguska platform. On the other hand, modern Entophysalis species live along the air/water interface (Golubic, 1983); thus, their extension to subtidal waters could be explained by seasonal changes in sea level (Kinsmann and Park, 1976). Independent evidence for a frequently flooded coastal habitat can be sought in the dark brown pigments commonly observed to darken upper surfaces of Eoentophysalis colonies. This pigmentation compares closely in form and distribution to scytonemine pigments produced by the cells of modern Entophysalis in response to high light intensity (Fritsch, 1945; Golubic and Hofmann, 1976; Golubic, 1983) and is present in many Proterozoic populations (Hofmann, 1976; Oehler, 1978; Zhang, 1981; Sergeev, 1993, 1994; Sergeev et al., 1995). It is rare in Sukhaya Tunguska populations, supporting sedimentological evidence that E. arcata extended into subtidal environments. The specific association of Eoentophysalis populations with carbonate precipitates mirrors a correspondence observed in other Mesoproterozoic to lower Neoproterozoic successions. For example, Kah and Knoll (1996) observed this association in peritidal dolomites from the Society Cliffs Formation, Baffin Island and speculated that subtrate preference might control the observed relationship. Modern Entophysalis species are not confined to firm substrates (S. Golubic, personal communication, 1997), however, suggesting that other factors that correlate with penecontemporaneous precipitation must be involved. Influx of fine-grained carbonate sediments is one such factor. Also present day coastlines, Entophysalis species form mats where rates of sediment influx are relatively low, but are excluded from peritidal environments characterized by high rates of sedi-

ment influx. Eoentophysalis may have colonized such habitats throughout the Proterozoic Era; however, these populations were preserved as fossils only when penecontemporaneous precipitation enhanced their otherwise low preservation potential. This explanation is consistent with the observation by Kah and Knoll (1996) that the stratigraphic and environmental distribution of Eoentophysalis-dominated mat assemblages coincides with that of seafloor carbonate precipitates. A different association of microfossils occurs in the upper part of the upper member along the Nizhnyaya Tunguska River (Fig. 1). Here, interlaminated stratiform mats, dolosiltites and dolarenites record deposition in a wave-dominated peritidal setting (Petrov et al., 1995). Rare dissolution voids indicate episodic subaerial exposure. Diversity is greatest within mat horizons built by thin-walled (and, consequently, poorly preserved) Siphonophycus filaments in lower intertidal to shallow subtidal environments. Eo. oparinii and G. lamellosa are locally abundant as mat dwellers, and these species also occur in lenses of finegrained sediment interpreted as ephemeral pond deposits formed during intervals of emergence [Knoll and Golubic (1979); Fig. 4]. The small rodlike fossils of Eosynechococcus are common, as well. Such fossils are often interpreted as Synechococcus- or Gloeobacter(= Gloeothece)-like cyanobacteria (Hofmann, 1976; Golubic and Campbell, 1979; Golovenok and Belova, 1984) that lived within peritidal mats. This may be correct, but the simple morphology of these fossils is shared by many other bacteria, making such interpretations tentative [e.g. Fairchild et al. (1980); Krylov and Sergeev (1986); Knoll et al. (1991); Sergeev, 1992a,b, 1994). Coniunctiophycus conglobatum and Sphaerophycus spp. found in the same mats are also plausibly if not unambiguously interpreted as cyanobacteria. Fine-grained event beds interlaminated with the mats contain abundant particulate organic matter, including fragmental Siphonophycus sheaths, Myxococcoides spp. and rare Gloeodiniopsis. Locally, E. arcata mats also occur (Fig. 4F-G). This locality is biologically similar to Mendelson and Schopf's Locality A (Mendelson and Schopf, 1982).

V.N. Sergeev et al. / Precambrian Research 85 (1997) 201-239

A closely comparable assemblage is found in the c. 700 Ma Draken Formation, Spitsbergen, and its correlatives in East Greenland (Green et al., 1989; Knoll et al., 1991). In these younger assemblages, thin-sheathed Siphonophycus mats in lower intertidal environments host diverse mat dwellers, including species of Coniunctiophycus, Gloeodiniopsis, Myxococcoides and Eosynechococcus; the morphologically distinctive, stalked cyanobacterium Polybessurus bipartitus forms monospecific crusts in the upper intertidal zone, but occurs as isolated individuals in less frequently exposed environments (Green et al., 1987, 1988; Knoll et al., 1991 ). Consistent with sedimentological and other paleontological comparisons between the Sukhaya Tunguska and Draken formations, isolated Polybessurus stalks occur in Sukhaya Tunguska cherts as dwellers in thin-walled Siphonophycus mats. Along the Sukhaya Tunguska River, silicified peritidal carbonates in the upper member provide a third principal locus of microfossil diversity (Fig. 4B). Here, stratiform mats, intraformational breccias and associated grainstones are again the dominant lithologies, but the presence of tepee structures, erosional discontinuities and gypsum casts indicates more restricted tidal fiat conditions. Much of this assemblage is comparable to those described above. Low diversity mats of thicksheathed Siphonophycus are widespread (Petrov et al., 1995); these are comparable to the sheaths of oscillatorian cyanobacteria found in frequently exposed portions of other ancient [e.g. Oehler et al. (1979); Knoll et al. (1991 )] and modem tidal fiats. Thick sheaths appear to confer protection from desiccation and harmful radiation. More diverse Siphonophycus mat assemblages containing Gloeodiniopsis, Myxococcoides and Eosynechococcus occur locally, as do Eoentophysalis mats. Trichomes (Palaeolyngbya sp., Calyptothrix sp.) are rare components of these assemblages; they may (or may not) be preservational variants of the filamentous cyanobacteria more commonly preserved as evacuated sheaths (Siphonophycus). Local cushions of Eomicrocoleus and Uluksanella filament fascicles also record variations in mat-building communities. These taxa compare closely with sheaths of the polytrichomous

211

cyanobacterium Microcoleus, an active builder in many modern mats. Microcoleus may have been abundant along Proterozoic tidal fiats (Horodyski and Donaldson, 1980; Hofmann and Jackson, 1991; Kah, 1997); however, expectations for a widespread Proterozoic record are tempered by the low preservation potential of this taxon (Horodyski et al., 1977; Venetskaya and Gerasimenko, 1988). Allochthonous elements in the restricted facies include Gyalosphaera golovenokii, Archaeoellipsoides dolichos, Leiosphaeridia sp. and Myxococcoides species. Gyalosphaera golovenokii is comparable to some genera of modern planktonic chroococcacean cyanobacteria in freshand brackish-water habitats, in particular Gomphosphaeridium and Coelosphaera (Strother et al., 1983). Archaeoellipsoidesdolichosis interpreted as the akinetes, or resting spores, of Anabaenalike cyanobacteria that lived in ephemeral brackish tidal pools (Sergeev et al., 1995; Golubic et al., 1995). Although some Myxococcoides species probably represent the remains of chlorococcalean green algae (Knoll et al., 1991), most cannot be unambiguously placed even at the level of kingdom or domain. Large spheroidal acritarchs preserved in relatively deep water facies of the lower Sukhaya Tunguska Formation (Petrov et al., 1995) provide a glimpse of eukaryotic phytoplankton in open shelf environments. These fossils are large (averaging 50-100 gm, but reaching maximum diameters of 320 gm; Fig. 6A-D) and have a pigmented outer membrane that surrounds the main vesicle. In general aspect, they resemble populations of the widely distributed Neoproterozoic acanthomorph Trachyhystrichosphaera; however, the Sukhaya Tunguska fossils do not have processes. Whether or not processes were originally present but decayed preferentially [as has been documented elsewhere in Trachyhystrichosphaera populations; Knoll (1984)] cannot be ascertained from the Sukhaya Tunguska material. We have, therefore, chosen to treat this population informally. It is important to note, however, that even if future investigations indicate that processes originally arose from the inner vesicle wall, these fossils would still remain taxonomically distinct from

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Trachyhystrichosphaera aimika and other previously described species of the genus. In a previous publication, two further taxa were listed as occurring in Sukhaya Tunguska cherts: Veteronostocale sp. and Eohyella cf. reetoclada [Petrov et al. (1995), P1. I, Figs. 6-8]. After careful reexamination, we concluded that the 'trichomes' of Veteronostocale sp. are chains of Eosynechococcus moorei ellipsoids. The branched pseudofilaments originally assigned to Eohyella cf. rectoclada are here reinterpreted as poorly preserved organic structures whose relationship to endolithic cyanobacteria cannot be established. In summary, Sukhaya Tunguska microfossils display regular and predictable paleoenvironmental distributions comparable to those inferred for other Mesoproterozoic and Neoproterozoic chert biotas. The following general characteristics can be noted: (1) Silicified microfossils predominantly document peritidal environments, with rarer glimpses of more open shelf settings. (2) Within the peritidal realm, assemblage diversity and composition vary along a paleoenvironmental gradient from restricted upper intertidal flats to shallow subtidal settings. Frequency of exposure, rates of sediment influx and patterns of seafloor carbonate precipitation all appear to have been important in determining population distributions. (3) Different peritidal assemblages occur in intimate stratigraphic relationship to one another, reflecting the close association of biologically distinct communities in space. Commonly, several discrete microfossil associations are represented in different laminae of a single thin section. (4) At the lower end of the peritidal range, mat builders have thin sheaths and the diversity of mat dwellers and allochthonous elements is high. (5) The upper end of the same gradient is dominated by low-diversity mats of tightly interwoven, thick-sheathed filamentous cyanobacteria. (6) Eoentophysalis mats occur principally in peritidal environments where cemented substrates were available.

6. Evolution, environment and silicified cyanobacteria in proterozoic cherts Silicified microfossils in Proterozoic carbonate successions are dominated by morphologically simple and stratigraphically long-lived cyanobacteria. From this, it might be concluded that such assemblages are not as evolutionarily interesting or informative as the eukaryote-rich biotas found in siliciclastic rocks from less restricted environments. In contrast, we argue that great evolutionary and paleoenvironmental interest attaches to the details of chert assemblages in space and time. The close morphological and behavioral comparisons between many Proterozoic and modern cyanobacterial species, when coupled with their comparable environmental distributions, suggests that the cyanobacteria diversified early and have been tracking specific environments for > 2000 Ma (Knoll and Bauld, 1989; Knoll and Golubic, 1992; Schopf, 1994; Kah and Knoll, 1996). Schopf (1994) interpreted this as an indication that cyanob a c t e r i ~ a n d by extension, prokaryotes in gener a l - a r e evolutionarily conservative and slow to evolve. This view contrasts strongly with laboratory experiments which demonstrate that bacterial populations evolve rapidly when introduced to novel environmental conditions (Mortlock, 1984; Clarke, 1985; Lenski and Travisano, 1994). How can we reconcile geological and laboratory observations? One possibility, earlier suggested by Knoll and Bauld (1989), is highlighted by a combination of insights in two recent papers published in a tribute to George Gaylord Simpson's pioneering Tempo and Mode in Evolution. In the first paper, Lenski and Travisano (1994) demonstrated that Escherichia coli populations inoculated onto a novel substrate rapidly evolve increasing fitness; however, within c. 1300 generations, they achieve a level of adaptedness beyond which further fitness increases are unlikely under the imposed environmental conditions. These experimental results nicely complement a mathematical simulation by Niklas (1994) in which he showed that the shape of the adaptive landscape [sensu Wright (1931)] varies as a function of the number of potentially conflicting structural or physiological adaptations.

V.N. Sergeev et al. / Precambrian Research 85 (1997) 2 0 1 ~ 3 9

When numerous features must be optimized, many phenotypic solutions of roughly comparable fitness are possible and adaptive landscapes are low, rolling hills. In contrast, when fitness is maximized in terms of a single parameter, the adaptive landscape looks like Mt Fuji--a single steep peak that rises from the adaptive plain. If any real organisms approach the one-parameter endmember of Niklas' model, it is the bacteria, many of which are remarkably well adapted to exploit a single or limited array of resources. In consequence, the adaptive landscape for cyanobacteria in physically harsh intertidal environments may contain a single steep peak. If so, then the results of Lenski and Travisano (1994) suggest that the first cyanobacterial colonists of tidal fiat environments would have moved rapidly up the fitness peak. Having arrived at its summit--the point at which continuing mutation is unlikely to produce an increase in fitness--they would be difficult to dislodge. The result would be the fossil record actually observed morphologically and behaviorally indistinguishable populations that track specific environments through time. Thus, the fossil record of cyanobacteria may tell us something fundamental about the evolution of bacteria. They adapt quickly to novel environments and then, remain stable as long as those environments exist. The expected evolutionary pattern is then one of accumulation as new environments-either biological or physical--come to exist, rather than the continuing turnover and evolutionary replacement long documented for eukaryotic fossils in both Proterozoic and Phanerozoic rocks (Knoll and Bauld, 1989). On the other hand, the observed composition of cyanobacterial fossil assemblages does change through time. As hypothesized by Knoll and Sergeev (1995) and supported observationally by Kah and Knoll (1996) and in this paper, changing conditions of precipitation and diagenesis in Proterozoic carbonate platforms may be important determinants of microfossil assemblage composition. Given our current understanding of the Proterozoic fossil record, the null hypothesis for explaining secular variation in cyanobacterial microbenthos must be that the cyanobacteria diversified early and subsequently tracked environ-

213

ments through time. It may be possible to document true evolutionary change in cyanobacteria through the Proterozoic Eon, but this can only be done within a highly resolved sedimentological and petrological framework. In silicified microbiotas, then, paleontologists find both a record of tempo and mode in bacterial evolution and finely tuned guides to paleoenvironmental interpretation.

7. Systematic paleontology All specimens illustrated in this paper occur in thin sections of black chert from the Sukhaya Tunguska Formation. Microfossils were photographed in transmitted light with a Zeiss microscope REM-5 and measured with an eyepiece reticle to the nearest micrometer. The coordinates here cited refer to numbered points on paper overlays attached to thin sections. For type specimens, England Finder coordinates are also provided. Illustrated specimens are reposited in the Paleontological Collection of the Geological Institute of Russian Academy of Sciences, Collection No. 4694; additional materials are housed in the Paleobotanical Collection of the Harvard University Herbaria.

7.1. Kingdom: Bacteria; division: Cyanobacteria; class: Coccogoneae; order: Chroococcales; family: Chroococcaceae Ngigeli (1849) 7.1.1. Genus Coniunctiophycus Zhang (1981) 7.1.1.1. Type species. Coniunctiophycus gaoyuzhuangense Zhang ( 1981 ). Coniunctiophycus conglobatum Zhang, 1981 Figs. 13D, F

Coniunctiophycus conglobatum Zhang (1981), p. 499, P1. 4, Fig. 11, P1. 5, Figs. 1 and 2; Sergeev et al. (1995), pp. 26-27, Figs. 13.15 and 13.16. Eomicrocystis parvulus Yakschin (1991), p. 23, PI. VIII, Fig. 9. 7.1.1.2. Description. Single-layered spheroids occurring in spherical to ellipsoidal colonies of a few to > 40 individuals. Vesicle diameter 2-4 pm,

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maximum dimension of colonies 10-25 gin. Colonies, in turn, gathered into aggregates of several colony units.

Gyalosphaera golovenokii sp. nov. Spheroids N=IO0

7.1.1.3. Remarks. Zhang (1981) described the genus Coniunctiophycus as complex aggregated colonies of numerous small, spheroidal cells and compared this Proterozoic genus to extant planktonic cyanobacteria of the genera Microcystis, Coelosphaerium and Aphanothece. 7.1.1.4. D&tribution. Mesoproterozoic: Gaoyuzhuang Formation, China; Kotuikan and Yusmastakh formations, Anabar Uplift, Siberia; Meso-Neoproterozoic: the Sukhaya Tunguska Formation, Turukhansk Uplift; Neoproterozoic: Draken Formation, Spitsbergen.

• inner envelopes [] o u t e r e n v e l o p e s

1

1.5

2

Z5

3

3.5

4

4.5

5

5.5

6 6.5

Diameter of Spheroids (gin)

a

Colonies N=17

7.1.1.5. Material. Approximately 100 colonies. 7.1.2. Genus Gyalosphaera Strother et al. (1983) 7.1.2.1. Type species. Gyalosphaera fluitans Strother et al. (1983). Gyalosphaera golovenokii Sergeev and Knoll (n. sp.) Figs. 5 and 6E, F. Gyalosphaera cf. fluitans Petrov et al., 1995, P1. I, Fig. 13. 7.1.2.2. Diagnosis. A species of Gyalosphaera characterized by relatively large vesicles (1-6.5 gm) arranged along the colony periphery and by colony interiors empty, as opposed to filled by radiating stalks. 7.1.2.3. Description. Spheroidal to ellipsoidal colonies 9-26 gm in diameter, with 10-20 small vesicles arranged evenly along the colony periphery, leaving its interior empty. Vesicles consist of two concentrically arranged envelopes with an outer diameter of 2-6.5 gm; the outer envelope is spherical or ellipsoidal (compressed tangential to colony surface), with chagrinate translucent walls c. 0.5 gm thick; inner envelopes (1 3.5 grn in diameter) are similar in structure, but more coarsely grained and 1-1.5 lam thick. In some speciemns, an opaque, spheroidal inclusion 1.0-1.5gm in diameter is attached to the inner envelope.

10

15

20

25

Diameter of Colonies (gm)

Fig. 5. Histograms showing size frequency distribution of the diameter of spheroids (upper) and colonies (lower) of Gyalosphaera golovenokii from the Sukhaya Tunguska Formation.

7.1.2.4. Etymology. This species honors the memory of the late Victor Golovenok, who made many contributions to our understanding of Proterozoic microfossils in the Sukhaya Tunguska Formation and elsewhere. 7.1.2.5. Holotype. The specimen illustrated in Fig. 5E has been designated as the type for this species; Paleontological Collection of the Geological Institute, Russian Academy of Sciences (Collection No. 4694), sample No. 40, specimen No. 503, slide No. 613, England Finder Coordinates H-26-2. 7.1.2.6. Type locality. Left bank of the Sukhaya Tunguska River c. 20 km upstream from its mouth;

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I~ N. Sergeev et al. / Precambrian Research 85 (1997) 2 0 1 - 2 3 9

ii....

....~

vE ............ !H!•~!i~ '!ilia•¸ !ii~,~

~ ,~ i~:i~ ~ ~

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Fig. 6. Problematic acritarchs and coccoidal microfossils from the Sukhaya Tunguska Formation. In this and all subsequent figures, 'p' followed by an arabic numeral designates microfossil location as indicated by numbered ponts on paper overlays attached to thin sections. (A-D) Unnamed planktonic forms from open shelf carbonates (A) sample 69, thin section 661, p. 1, GINPC 500. (B) Sample 69, thin section 690, p. 1, GINPC 501. (C) Sample 250, thin section 710, p.2, GINPC 513. (D) sample 250, thin section 712, p.1, GINPC 502. (E, F) Gyalosphaeragolovenokii Sergeev and Knoll sp. nov., sample 40, thin section 613, p. 5; (E) holotype, GINPC 503, (F) GINPC 504 (left colony) and GINPC 505 (right colony). Single scale bar = 10 Ixm, double bar = 50 lam.

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V.N. Sergeev et al. / Precambrian Research 85 ( 1997) 201-239

the uppermost part of the Sukhaya Tunguska Formatiom (locality IV, Fig. 1).

7.1.2. 7. Discussion. Gyalosphaera golovenkii differs from G. fluitans by the larger size of its constituent vesicles, its lack of bifurcating stalks within colonies, and the smaller size range of its colonies. Despite these differences, the overall structural organization of the Sukhaya Tunguska population compares closely with that found in the type species. Indeed, we cannot completely rule out the possibility that the Turukhansk and Greenland populations represent degradational variants of a single taxon. On the basis of available data, we prefer to separate the Sukhaya Tungu~ka population as a second species of the genus Gyalosphaera. 7.1.2.8. Material. About 20 colonies. 7.1.3. Genus Gloeodiniopsis Schopf (1968), emend Knoll and Golubic (1979) 7.1.3.1. Type species. Gloeodiniopsis lamellosa Schopf (1968), emend. Knoll and Golubic (1979), emend Sergeev (1992b.) Gloeodiniopsis lamellosa Schopf (1968) emend, Knoll et Golubic emend Sergeev. Figs. 7 and 8A-H. G. lamellosa Schopf (1968), p. 684, P1.84, Fig. 2; Schopf and Blacic ( 1971 ), P1. 110, Figs. 1-5; Knoll and Golubic (1979), p. 147, Figs. 6 and 7; Mendelson and Schopf (1982), p. 66, 68, P1. 1, Figs. 13 and 15; Golovenok and Belova (1993), P1. II, Fig. b; Petrov et al. (1995), P1. I, Fig. 10. Gloeodiniopsis magna Nyberg and Schopf (1984), pp. 763, 765, Fig. 15C-G; Hofmann and Jackson (1991), p. 377, Figs. 13.1-13.7, 13.11-13.14; Golovenok and Belova (1992), pp. 116, 117, Fig. 1, 2, 1993, P1. II, Fig. a; Schopf (1992b), P1. 45, Figs. D, F, G. Gloeodiniopsis grandis Sergeev and Krylov (1986), pp. 90, 91, P1. X, Figs. 8 and 9; Yankauskas et al. (1989), p. 93, P1. XXII1, Fig. 7; Knoll et al. (1991), pp. 550-553, Fig. 19.4; Schopf (1992b), P1. 45, Fig. B. Chroococcus-like morphotype: Mendelson and

Schopf (1982), p. 68-69, P1. 2, Fig. 5; Schopf (1992b), P1. 10, Fig. L. Globophycus-like morphotype: Mendelson and Schopf (1982), p 69, P1. 1, Fig. 12. "Larger Chroococcacean Cyanobacteria" (partim): Mendelson and Schopf (1982), pp. 71-72, PI. 1, Fig. 14. Tetraphycus giganteus (partim) Golovenok and Belova (1992), p. 117, Figs. 1, 3; 1993, P1. II, Fig. c. [For complete synonymy, see Sergeev (1992b, 1994).]

7.1.3.2. Description. Multilamellated spheroidal to ellipsoid vesicles surrounded by a hyaline zone with one or more thin envelopes of differing density, giving a ringed appearance in cross-sectional view. Lamellae in outer portion with uniform curvature, innermost layers more irregular, in some cases containing a centrally or eccentrically located inclusion of dark matter. Spheroids and ellipsoids may be solitary, but commonly occur in colonies of a few to several hundred individuals. Spheroids arranged in monads, dyads, triads and tetrads (cross and planar tetrads), sometimes enclosed in thin common envelopes; larger colonies commonly embedded in a diffuse organic matrix. Diameter of inner envelopes 8-36 tma (x= 18 ~tm, N = 195); outer envelopes 13-42 ~tm (x=28 ~tm, N=200); inclusions 2-5 ~trn (x=3.5 ~tm, N=32). 7.1.3.3. Discussion. Gloeodiniopsis lamellosa was erected by Schopf (1968) for mutlilamellated spheroids preserved in silicified coastal playa lake carbonates of the c. 800Ma Bitter Springs Formation, Australia. Knoll and Golubic (1979) emended this taxon to include species of the genera Bigeminococcus, Eozygion, Eotetrahedrion and Caryosphaeroides (in part), recognizing that previously described differences among these taxa reflect a cell division cycle and variable postmortem decay within a single population. Many species of Gloeodiniopsis have been described, but their reported size ranges overlap. One of the most abundant Proterozoic species, G. lamellosa commonly occurs with the larger species G. magna. In a study of coccoidal microfossils from the Avzyan and Min'yar formations of the Southern Ural Mountains, Sergeev (1992b, 1994) concluded that

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Gloeodiniopsis lamellosa N = 200

25

[--I 20 -

Outer envelope Inner envelope

15N

10

5

0

I

0

5

°ill I

10

I

15

20

25

I

I

30

35

MM !

40

Diameter of spheroids, pm Fig. 7. Histogram showing size frequency distribution of G. lamellosa Schopf emend. Knoll and Golubic emend. Sergeev from the Sukhaya Tunguska Formation.

these 'species' represent the extremes of intraspecific variation within a single population (Sergeev, 1992b, 1994). In similar fashion, we interpret the range of morphologies found in Sukhaya Tunguska cherts to represent variation within a single species. This interpretation is supported by the presence of both large and small individuals in single cell clusters (Fig. 8F,H). Evidently, cells grew to a maximum size of 35-45 pro, following which two or three binary divisions occurred with little intervening growth. The result was quartets and octads of vesicles with envelope diameters of 12-20 ~tm. On the basis of published reports, it appears Siberian and Uralian G. lamellosa differ from the Bitter Springs population in displaying a greater size range (Schopf, 1968; Knoll and Golubic, 1979); however, a reinvestigation of the type material indicates that the largest Bitter

Springs specimens reach c. 35 pm in diameter. We suspect that many other described species of Gloeodiniopsis are synonymous with G. lamellosa [e.g.G. uralicus Krylov and Sergeev (Krylov and Sergeev, 1986; Sergeev, 1992b) from the lower Mesoproterozoic Satka Formation, Southern Urals, and G. dilutus Ogurtsova and Sergeev from the Neoproterozoic Chichkan Formation, Southern Kazakhstan (Ogurtsova and Sergeev, 1987)]. On the other hand, we cannot rule out the possibility that G. lamellosa is a form taxon that encompasses multiple biological species of cyanobacteria and, perhaps, protists. Golovenok and Belova (1992, 1993) described a population of planar tetrads in Sukhaya Tunguska cherts as Tetraphycus giganteus (Zhang) comb. Golovenoc et Belova. The type population of Paratetraphycus giganteus in the 600-500 Ma

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Fig. 8. Gloeodiniopsis lamellosa Schopf emend. Knoll et Golubic emend. Sergeev from the Sukhaya Tunguska Formation. ( h E , G, H) (A) sample 38, thin section 635, p. 47, GINPC 506. (B) Sample 38, thin section 513, p. 16, GINPC 507. (C) Sample 38, thin section 648, p. 1, GINPC 508. (D) sample 85, thin section 615, p. 12, GINPC 509. (E) Sample 20, thin section 660, p. 1, GINPC 510. (G) Sample 38, thin section 635, p. 49, GINPC 511. (F, H) Sample 85, thin section 615, p. 9, GINPC 512. Scale b a r - 10 ~tm.

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Doushantuo Formation, China, consists of tetraspores produced by a red alga similar to extant Porphyra (Zhang et al., in press). It has little in common with the cyanobacteria-like fossils illustrated by Golovenok and Belova. Their Sukhaya Tunguska tetrad is here interpreted as a stage in the life cycle of G. lamellosa, as described by Knoll and Golubic (1979). A further complication in the circumscription of G. lamellosa is that cross-sections through the stalked cyanobacterium Polybessurus bipartitus (present in the Sukhaya Tunguska Formation) can be mistaken for large G. lamellosa individuals [see Knoll et al. (1991) for discussion]. In a previous report on Sukhaya Tunguska paleontology, one such section of P. bipartitus was misinterpreted as G. lamellosa [Petrov et al. (1995), P1. I, Fig. 2].

7.1.3.4. Distribution. A widespread consitutent of microfossil assemblages in Meso-Neoproterozoic (Middle-Late Riphean) cherts, was found in dozens of formations. 7.1.3.5. Material. Several hundred specimens. 7.1.4. Genus Eoaphanocapsa Nyberg and Schopf (1984) 7.1.4.1. Type species. Eoaphanocapsa oparinii Nyberg and Schopf (1984) Eoaphanocapsa oparinii Nyberg and Schopf, 1984 Figs. 9A and 10E, F. Eoaphanocapsa oparinii Nyberg and Schopf (1984), pp. 759, 761, Figs. 13A-C, D?-F?; Yankauskas et al. (1989), p. 90, P1. XXIII, Fig. 8; Krylov et al. (1989), PI. I, Figs. 3 and 4; Sergeev (1992b), p. 78, P1. XII, Fig. 1 a,B, P1. III, Figs. 1, 4 and 5; Schopf (1992b), P1. 47, Figs. El, E2, F?. "Undifferentiated chroococcacean Cyanobacteria" (partim): Schopf et al. (1977), Fig. 1J-K; Schopf et al. (1979), P1. 7, Figs. K, JI; "Larger chroococcacean cyanobacteria" (partim): Mendelson and Schopf (1982), pp. 71, 72, P1. 2, Figs. 2 and 3. Schopf (1992b), PI.10, Fig. J. 7.1.4.2. Description. Single-walled or multilamellated spheroidal and ellipsoidal vesicles. Diameter

Eoaphanocapsa oparinii N = 100 2s" I

N

BBinner envelope

I

20"[ I t 10,

5

•

~

•

louter

pe

§ 0

10

15

20

Diameter of spheroids, I~m a

Eoentophysalis el. E. belcherensis N = 100

N

2S 20 15 10 S 0

4

5

6

7

8

9 Diameter of spheroids, Ira1

10

b Fig. 9. Histograms showing size frequency distribution of some coccoidal microfossils from the Sukhaya Tunguska Formation.

of inner envelopes 6-22 lain, outer envelopes 10-24 lam; inclusions, commonly attached to the interior of the innermost wall layer, 0.5-3 ~tm. Individual vesicle lamellae 0.5-1 lxm thick; vesicles in loose clusters of a few to many tens of individuals commonly embedded in a diffuse organic matrix.

7.1.4.3. Discussion. The type population of Eo. oparinii from the Neoproterozoic (Late Riphean) Min'yar Formation, Southern Ural Mountains, was interpreted by its discoverers as the remnants of chroococcoidal cyanobacteria similar to species of the extant genus Aphanocapsa (Nyberg and Schopf, 1984). It is possible, however, that these colonies are only a stage in the life cycle of other chroococcoidal fossils, in particular G. lamellosa. Within the Sukhaya Tunguska Formation, A. oparinii colonies are found mainly in Siphonophycus mats, where they co-occur with G. lamellosa. Eoaphanocapsa provides a useful form genus for

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V.N. Sergeev et al. ~/Precambrian Research 85 (1997) 2 0 1 ~ 3 9

Fig. 10. Coccoidal microfossils from et Belova emend. Sergeev et Knoll. GINPC 514, (C) GINPC 515. (D) Schopf, sample 38, thin section 518,

the Sukhaya Tunguska Formation. (A-D) E. arcata Mendelson and Schopf emend. Golovenoc (A, B--left square in A, and C ~ f i g h t square in A) Sample 74, thin section 640, p. 23; (B) Sample 74, thin section 637, p. 39, GINPC 516. (E, F ~ s q u a r e in E) Eo. oparinii Nyberg et p. 33, GINPC 517. Single scale b a r = 10 p.m, double b a r = 100 ~tm.

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221

colonies of multilamellate spheroids that lack the division cycle characteristic of Gloeodiniopsis. Accepting this, the Bitter Springs population described as G. gregaria by Knoll and Golubic (1979) should be transferred to Aphanocapsa. The Bitter Springs fossils differ from A. oparinii only in the size of constituent cells.

and Schopf (1983), p. 347, Photo 14-6N; Krylov and Sergeev (1986), p. 107, PI. I, Figs. 8 and 9; Sergeev (1992b), p. 100, P1. IV, Figs. 7-9; Petrov et al. (1995), PI. I, Fig. 12; Sergeev et al. (1995), p. 27, Figs. 9.8, 9.12 and 9.13. Microphycus curtus Yakschin (1991), p. 28, PI. VIII, Fig. 7.

7.1.4.4. Distribution. Meso-Neoproterozoic, Sukhaya Tunguska Formation, Turukhansk Uplift, Siberia; Neoproterozoic, Min'yar Formation, southern Ural Mountains.

7.1.6.1. Description. Single-layered, rod-like, empty ellipsoidal vesicles, solitary or in pairs, occurring in loose clusters or in densely packed colonies from a few to ten or more individuals; constricted or paired vesicles preserve evidence of binary cell division; vesicles 1.5-6.5~tm long, 1-3 ~tm wide, length/width= 1.5-2.0. Dark inclusions <0.5 ~tm in diameter occur in some vesicles.

7.1.4.5. Material. More than 100 colonies. 7.1.5. Genus Eosynechococcus Hofmann (1976) 7.1.5.1. Type species. Eosynechococcus Hofmann (1976).

moorei

7.1.5.2. Discussion. In his original diagnosis, Hofmann (1976) included all morphologically simple ellipsoidal unicells in the form genus Eosynechococcus. Thus, the paleontological genus Eosynechococcus is much broader than its modern counterpart Synechococcus, which includes only non-colonial unicellular cyanobacteria of ellipsoidal morphology. Many species assigned to Eosynechococcus are colonial, similar to living species of the genus Gloeothece (Golubic and Campbell, 1979; Knoll and Golubic, 1979; Golovenok and Belova, 1984, 1993). In light of this circumstance, Zhang (1988) erected the genus Gloeotheceopsis for colony-forming species originally placed in Eosynechococcus. Sukhaya Tunguska populations of E. moorei and E. medius sometimes occur in densely packed, irregular colonies similar to species of the modern genera Gloeothece and Gloeobacter; however, they lack the dispersed envelopes characteristic of Gloeotheceopsis. 7.1.6. Eosynechococcus moorei Hofmann (1976); Fig. 13E Eosynechococcus moorei Hofmann (1976), pp. 1057-1058, P1. 2, Figs. 1-7, 8?; Golubic and Campbell (1979), Figs. 2E-J, 3C, D; Hofmann

7.1.6.2. Discussion. Eosynechococcus moorei is distinguished from other species of Eosynechococcus mainly on the basis of its small size. Golubic and Campbell (1979) demonstrated the close resemblance of Proterozoic E. rnoorei populations to the modern cyanobacterial species Gloeobacter (Gloeothece) coerula (Geitler) Castenholz (Castenholz and Waterbury, 1989). Modern G. coerula populations inhabit wet rock exposures in fresh water environments, so if the resemblance is more than superficial, it would indicate a shift in environmental distribution through time. 7.1.6.3. Distribution. Widely Proterozoic cherts.

distributed

in

7.1.6.4. Material. More than 100 colonies. 7.1.7. Eosynechococcus medius Hofmann (1976), Fig. 13B, G Eosynechococcus medius Hofmann (1976), p. 1058, P1. 2, Figs. 9 and 10; Mendelson and Schopf (1982), pp. 72-74, P1. 3, Figs. 1 and 2; Knoll (1982), p. 780, P1.8, Figs. 11-13; Hofmann and Jackson (1991), pp. 371-372, Figs. 7.14 and 7.15; Schopf (1992b), P1. 10, Fig. O. 7.1.7.1. Description. Single-layered, rod-like, empty ellipsoidal vesicles, occurring in densely packed, irregular colonies in close association with

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colonies of E. arcata, S. robustum and S. typicum; vesicle length 5-10 ~tm, width 2-5 ~tm, length/width = 1-3; walls are translucent, mediumgrained, and c. 0.5 ~tm thick.

7.1.7.2. Discussion. Eosynechococcus medius is differentiated from other species of Eosynechococcus by its size range, which is intermediate among other species. 7.1.7.3. Distribution. Widely Proterozoic chert assemblages.

distributed

species of Synechocystis, as well as other, physiologically dissimilar bacteria. Sukhaya Tunguska assemblages also include larger Sphaerophycus-like fossils (4-101am in diameter). These populations resemble Sphaerophycus medium in size and shape (Fig. 3), but do not exhibit evidence of cell division in the form of dyads or tetrads. Thus, these fossils are referred to Sphaerophycus with some doubt and are not further considered in this paper.

in

7.1.8.4. D&tribution. Widely Proterozoic chert assemblages.

distributed

in

7.1.7.4. Material. More than 100 colonies. 7.1.8.5. Material. More than 100 colonies. 7.1.8. Genus Sphaerophycus Schopf (1968) 7.2. Family Entophysalidaceae Geitler (1925) 7.1.8.1. Type species. Sphaerophycus parvum Schopf (1968) Fig. 13A, C. Sphaerophycus parvum Schopf (1968), p. 672, P1.80, Figs. 4-10; Hofmann (1976), p. 1058, 1061, P1. 2, Fig. 8?, P1. 3, Figs. 1-6; Oehler (1977), p. 343, Fig. 12H, I; Oehler (1978), p. 293, Fig. 10R, S; Horodyski and Donaldson (1980), p. 140, Fig. 5A-E; Horodyski and Donaldson (1983), p. 140, Fig. 5A, B; Knoll (1982), p. 783, P1.6, Figs. 3 and 4; Hofmann and Jackson (1991 ), p. 374, Fig. 8.9, 8.10, 8.11?, 10.1-10.3; Schopf (1992b), P1. 9, Fig. G, P1. 33, Fig. F. "Smaller chroococcacean cyanobacteria": Mendelson and Schopf (1982), p. 71, P1. 1, Fig. 11. 7.1.8.2. Description. Spheroidal vesicles, 2-4 ~tm in diameter, commonly including dyads, in loose clusters comprising hundreds of individuals or scattered among filaments of Siphonophycus; opaque spheroidal inclusions < 0.5 lam in diameter occasionally present in vesicle interior. 7.1.8.3. Discussion. The genus Sphaerophycus includes small spheroidal unicells that occur as isolated individuals or in dyads, tetrads or larger groups of cells that reflect successive binary divisions (Schopf, 1968; Horodyski and Donaldson, 1980; Knoll, 1982; Knoll et al., 1991). Its type species, S. parvum (Schopf, 1968), is similar to extant chroococcacean cyanobacteria such as

7.2.1. Genus Eoentophysalis, Hofmann emend. Mendelson and Schopf (1982) 7.2.1.1. Type species. Eoentophysalis belcherensis Hofmann (1976). Eoentophysalis arcata Mendelson and Schopf (1982), emend. Golovenoc and Belova, emend. Sergeev and Knoll Figs. 10AD, 11 and 12 Eoentophysalis arcata Mendelson and Schopf (1982), p. 76, 77, P1. 2, Figs. la-b, text--Fig. 5; Yankauskas et al. (1989), p. 90, P1. XIX, Figs. 11-12; Schopf (1992b), P1. 10, Fig. E; Petrov et al. (1995), P1. I, Figs. 11, 14, 16 and 17]. Eogloeocapsa arcata Golovenok and Belova (1992), pp. 115-116, Figs. la,6 and 2, 1993, P1. I, Figs. a-d, text.--Fig. 4. Unnamed sedentary forms: Golovenok and Belova (1993), PI. II, Fig. 3. 7.2.1.2. Description. Multilamellated spheroidal, ellipsoidal and polyhedral vesicles in dyads, tetrads and octads, forming colonies of a few to many thousand individuals; colony morphology varies from loose clusters of gloeocapsoid vesicles to large aggregations of regular, cuboidal to palmelloid colonies that form crustose stratiform laminae. Vesicle elongation common and sometimes pronounced laterally or vertically, reflecting burial and attempted escape. Individual spheroids are

IA N. Sergeev et al. / Precambrian Research 85 (1997) 201 239

223

!

e~

m

m

Fig. 11. Coccoidal microfossils from the Sukhaya Tunguska Formation. ( A F ) E. arcata Mendelson et Schopf. (A) Sample 38, thin section 648, p. 3, GINPC 518. (B) Sample 74, thin section 640, p. 44, GINPC 519. (C, D--square in C) sample 74, thin section 640, p. 1; (C) GINPC 520 (central colony), (D) GINPC 521. (E) Sample 4399, thin section 664, p. 18, GINPC 522. (F) Sample 73, thin section 664, p. 12, GINPC 523. (G, H square in G) Eoentophysalis cf E. belcherensis, sample 85, thin section 617, p. 18, GINPC 524. Single scale bar = 10 I~m, double b a r = 50 ~tm.

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V.N. Sergeev et aL / Precambrian Research 85 (1997) 201-239

Eoentophysalis arcata Spheroids N = I 0 0 20.

N

,5.ALL.._.

10.

8

10

12

14

16

18

20

Diameter of spheroids, I~m a Colonies N=40

N

101520253035404550

Diameter of colonies, ~m

b Fig. 12. Histograms showing size frequency distribution of spheroids (upper) and colonies (lower) of E. arcata from the Sukhaya Tunguska Formation.

multilayered and even numbers of envelopes are nested within larger envelopes. The envelopes are translucent and psilate to finely granular; dark spot-like inclusions are sometimes attached to the interior of the central envelope layer. Vesicle diameter 7-21 ~un, diameter of gloeocapsoid colonies 15-45~m; walls 0.5-1 ~m, spot-like inclusions 0.5-3 ~tm.

7.2.1.3. Discussion. Mendelson and Schopf(1982) described these coccoidal microfossils as a species of Eoentophysalis despite the lack of such diagnostic characters as polarized growth and attached palmelloid colonies in the material they illustrated. Mendelson and Schopf stressed that their assignment of E. arcata to the Entophysalidaceae was subjective and that additional data were needed to clarify the taxonomic relationships of this population. Subsequently, Golovenok and Belova (1992, 1993) transferred this species to a new genus, Eogloeocapsa, because of the perceived absence of polarized growth and attached colonies. In our material, we observed E. arcata specimens closely comparable to those described previously, but we also were able to document a range of population variation much broader than reported earlier. Locally, attached, palmelloid colonies showing unidirectional, polarized growth are abundant, and a complete morphological gradation from these to canonical E. arcata can be demonstrated (Fig. 5A, C and D). Thus, the entophysalidacean affinities of this population can be demonstrated, eliminating any need to transfer these fossils to Eogloeocapsa. Attached colonies of E. arcata may, in fact, have been illustrated by Golovenok and Belova (1993), P1. II, Fig. 3), who referred to them as 'unnamed sedentary forms'. Eoentophysalis arcata can be differentiated from E. belcherensis by the larger diameter of its constituent vesicles and envelopes. More difficult is the differentiation of E. arcata from E. dismallakesensis (Horodyski and Donaldson, 1980, 1983, Sergeev et al., 1994), E. yudomatiea (Lo, 1980), E. croxfordii (Muir, 1976), and E. magna (McMenamin et al., 1983)--all can be separated as a group of 'larger' entophysalids, with vesicle diameters of 20 pm or more. Continuing study may show that these species are synonymous. 7.2.1.4. Distribution. Meso-Neoproterozoic (Middle-Upper Riphean), Sukhaya Tunguska Formation, Turukhansk Uplift, Siberia. 7.2.1.5. Material. More than 1000 colonies. 7.2.2. Eoentophysalis cf. E. belcherensis Hofmann (1976), Figs. 9b and l l G - H

V.N. Sergeev et al. / Precambrian Research 85 (1997) 2 0 1 ~ 3 9

7.2.2.1. Description. Spheroidal and ellipsoidal vesicles occurring in loose clusters to densely packed, irregular, broadly globular colonies. Pairs of vesicles enclosed by a common external envelope; pairs of pairs, in turn, enclosed by a more inclusive envelope. Vesicle diameters 4-9.5 ~ma ( x = 6 t.tm, N = 100). 7.2.2.2. Discussion. This form is a rare element of the Sukhaya Tunguska microbiota. Colonies are specifically associated with thin precipitated (originally) carbonate hardgrounds. Eoentophysalis belcherensis is a characteristic fossil in silicified peritidal carbonates of Paleo- and Mesoproterozoic age, but occurs only as a minor component on Neoproterozoic tidal fiats (Knoll and Sergeev, 1995). A second species, E. cumulus, was described from silicified playa lake carbonates of the Neoproterozoic Bitter Springs Formation (Knoll and Golubic, 1979) and subsequently identified in the Neoproterozoic (Late Riphean) Min'yar Formation of the Southern Ural Mountains (Sergeev and Krylov, 1986; Sergeev, 1992b); however, it now appears that E. cumulus is a morphological and preservational variant of E. belcherensis (Butterfield et al., 1994). 7.2.2.3. Material. Ten observed loose clusters and globular colonies. 7.3. Order: Pleurocapsales Geitler (1925);family: Dermocarpaceae Geitler (1925); genus: Polybessurus Fairchild ex Green et al. (1987) 7.3.1. Genus Polybessurus Fairchild ex Green et al. (1987) 7.3.1.1. Type species. Polybessurus bipartitus Fairchild ex Green et al. (1987) Polybessurus bipartitus Fairchild ex Green et al. (1987) Figs. 18H, I. 'Mini-stromatolite-like' structure: Schopf (1975), Fig. 2J. 'Polybessurus' Schopf (1977), Fig. 13H-J, K. Polybessurus bipartitus Green et al. (1987), pp. 938-939, Figs. 5-12, 15-20; Green et al. (1989); Figs. 4G, H and 5A; Knoll et al. (1989),

225

Figs. 6b, c; Knoll et al. (1991), p. 553, Fig. 12; Hofmann and Jackson (1991), p. 378, Fig. 7.8; Sergeev (1992a), pp. 109-110, P1. X, Figs. 1-4; Sergeev (1992b), pp. 85-86, PI. VII, Figs. 3, 4, 7 and 8, P1. IX, Fig. 10; Schopf (1992b), Pls. 36-38; Golovenok and Belova (1992), pp. 117-118, Figs. 1, ~, e; 1993, P1. II, Fig. g; Butterfield et al. (1994), p. 52, Figs. 21C, F-G; Sergeev (1994), pp. 248-249, Figs. 9A-G; Petrov et al. (1995), pl. I, Fig. 5. Gloeodiniopsis lamellosa (partim): Petrov et al. (1995), P1. I, Fig. 2. 7.3.1.2. Description. Multilamellated cylindrical stalks 30-75 ~ n across and up to 150 ~rn long, composed of regularly spaced, upwardly concave, funnel-shaped laminae whose side walls constitute the outer wall of the stalk. Stalks are open at the top and do not terminate with preserved cells, capshaped envelopes or sporangium-like structures. 7.3.1.3. Discussion. Polybessurus bipartitus is widely distributed in silicified peritidal carbonates of Neoproterozoic age (Schopf, 1977; Green et al., 1987; Knoll et al., 1991; Butterfield et al., 1994; Sergeev, 1994). Less commonly, P. bipartitus has been found in cherts of known or inferred late Mesoproterozoic age: for example, in the Avzyan Formation in the type Middle Riphean section of the Southern Ural Mountains, estimated to be 1200Ma old (Sergeev, 1994); in the Uluksan Group, Baffin Island, Arctic Canada, estimated by Hofmann and Jackson ( 1991) to be 1270-1240 Ma old; and in the Hunting Formation, Somerset Island, Arctic Canada, which is broadly correlative with the Uluksun Group (Butterfield et al., 1990). Sukhaya Tunguska specimens occur principally as isolated individuals within S. robustum mats; unlike Polybessurus populations in other assemblages, these have not been found as monospecific crusts [compare to Green et al. (1987)]. Like many other occurrences of this taxon, Sukhaya Tunguska P. bipartitus stalks lack preserved cells at their apices. The rarity of preserved cell walls or baeocytes appears to be a taphonomic bias affecting pleurocapsalean cyanobacteria in general.

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Fig. 13. Coccoidal microfossils from the Sukhaya Tunguska Formation. (A,C) Sphaerophycus parvum Schopf. (A) sample 38, thin section 518, p. 25, GINPC 525 (C) sample 38, thin section 518, p. 25, GINPC 556 (B, G) Eosynechococcus medius Hofmann,(B) sample 38, thin section 635, p. 24, GINPC 528. (G) Sample 40, thin section 612, p. 13, GINPC 529. (D,F) Coniunctiophycus conglobaturn Zhang, (D) sample 38, thin section 626, p. 14, GINPC 531. (F) Sample 38, thin section 648, p. 6, GINPC 532. (E) Eosynechococcus moorei Hofmann, sample 40, thin section 613, p. 9, GINPC 533. Scale bar= 10 ~tm.

Fig. 14. Filamentous microfossils from the Sukhaya Tunguska Formation. (A) Siphonophycus typicum (Hermann)-upper colony, GINPC 534- and S. robustum (Schopf)-lower colony, GINPC 535; sample 94, thin section 541, p. 12. (B) Siphonophycus typicum (Hermann), sample 94, thin section 541, p. 7, GINPC 536. (C) Circurnvaginalis sp., sample 85, thin section 617, p. 7, GINPC 537. (D) Palaeolyngbya sp., sample 38, thin section 518, p. 23, GINPC 538. (E-H) Archaeoellipsoides dolichos (Zhang), sample 38, thin section 639, p. 24, (E) GINPC 539, (F) GINPC 540, (G) GINPC 541, (H) GINPC 542. (I, K) Siphonophycus solidum (Golub), (I) sample 88, thin section 622, p. 1, GINPC 543. (K) Sample 93, thin section 624, p. 5, GINPC 544. (J) Eomicrocoleus sp., sample 38, thin section 513, p. 47, GINPC 545. (L) Uluksanella sp., sample 38, thin section 518, p. 45, GINPC 546. Single scale bar= 10 pro, double bar = 100 ~tm.

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7.3.1.4. Distribution. A widespread constituent of Late Mesoproterozoic and Neoproterozoic microfossil assemblages. 7.3.1.5. Material. Fifteen observed specimens. 7. 4. Class: Hormogoneae," order: Oscillatoriales; family: Oscillatoriaceae (S.F Gray) Dumortier ex Kirchner (1898); genus: Palaeolyngbya Schopf (1968) 7.4.1. Genus Palaeolyngbya Schopf (1968) 7.4.1.1. Type speeies. Palaeolyngbya barghoorniana Schopf (1968). Palaeolyngbya sp. Fig. 14D. Tubular sheath with internal 'trichome': Mendelson and Schopf (1982), p. 63, P1. 4, Fig. 1; Schopf (1992b), P1. 10, Fig. D. Filamentous microorganisms: Petrov et al. (1995); P1. I, Fig. 9. 7.4.1.2. Description. Uniseriate, unbranched trichomes with poorly preserved septa, 1-2 ~ n in cross-sectional diameter, trichomes enclosed within well-defined extracellular sheaths; sheaths 2.5-6 ~tm in cross-sectional diameter and up to 150 ~tm long, hyaline, unlaminated, < 0.5 ~tm thick.

7.4.2.3. Discussion. Rare trichomes of Calyptothrix sp. occur within Siphonophycus robustum mats and may represent cellular remants of the same cyanobacterial species. The Sukhaya Tunguska population is broadly similar to specimens of C. annulata from the Neoproterozoic Bitter Springs Formation, Australia (Schopf, 1968), but poor preservation of the Siberian material precludes its confident assignment to this species. 7.4.2.4. Material. Twelve specimens.

poorly

preserved

7. 4.3. Genus Eomicrocoleus Horodyski and Donaldson (1980) 7. 4.3.1. Type species. Eomicrocoleus crassus Horodyski and Donaldson (1980). Eomicrocoleus sp. (Fig. 14J). 7.4.3.2. Description. Bundles of empty cylindrical tubes closely grouped within a common cylindrical sheath; tube width 1-2 ~tm, tube walls psilate to chagrinate, sometimes broken into sharp fragments that superficially resemble trichomes, < 0.5 ~tm thick; encompassing sheath 10-40 ~tm in crosssectional diameter, fine- to medium-grained, and c. 1 ~tm thick.

7. 4.1.3. Material. Twelve observed specimens. 7.4.2. Genus Calyptothrix Schopf (1968) 7.4.2.1. Type species. Calyptothrix annulata Schopf (1968). Calyptothrix sp. (Fig. 18J). Calyptothrix sp: Petrov et al. (1995), P1. I, Fig. 8. 7.4.2.2. Description. Solitary or gregarious, flexible, unbranched uniseriate trichomes showing little or no constriction between adjacent cells and no encompassing sheath; terminal cells may be slightly enlarged, but are otherwise undifferentiated from medial cells; cells isometric, cylindrical or barrelshaped, 3-3.5 ~tm wide and 2.5-3 ktm long; crosswalls indistinct and sometimes hard to distinguish.

7.4.3.3. Discussion. The form here described as Eomierocoleus sp. differs only slightly from Eomicrocoleus crassus described by Horodyski and Donaldson (1980) from the Mesoproterozoic Dismal Lakes Group, Arctic Canada. In contrast to the type population, Sukhaya Tunguska specimens consist of empty tubes, not trichomes; however, these empty tubes are possibly trichomes that lost their septa during post-mortem degradation (Gerasimenko and Krylov, 1983). These microfossils are interpreted as the remnants of polytrichomatous cyanobacterial filaments, comparable species of the modern genera Microcoleus, Hydrocoleum and Schizothrix. 7.4.3.4. Material. Twenty filaments.

poorly

preserved

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Archaeoellipsoides dolichos

7.4.4. Genus Uluksanella Hofmann and Jackson (1991) 7.4.4.1. Type species. Uluksanella Hofmann and Jackson (1991). Uluksanella sp. (Fig. 14L).

baffinensis

7.4.4.2. Description. Knots of empty unbranched cylindrical tubes 2-4 ~rn in diameter; tube walls fine-grained, <0.5 txm thick; tubes tightly packed in round to oblong, generally isolated clumps 20-30 ~tm in maximum dimension. 7.4.4.3. Discussion. Hofmann and Jackson (1991 ) described U. baffinensis from the late Mesoproterozoic to early Neoproterozoic Bylot Supergroup of Baffin Island, Arctic Canada, pointing out that its filaments are wound nto a ball, unlike those of Siphonophycus, Brachypleganon or Gunflintia. Comparable fascicles of filamentous cyanobacteria occur in the modem polytrichomatous taxa Microcoleus and Hydrocoleum; however, other hormogonian cyanobacteria form similar clumps when grown under unfavorable conditions. Uluksanella may therefore be an ecological variant of more widely distributed Siphonophycus species. 7.4.4.4. Material. Fifteen observed specimens. 7.5. Order Nostocales Family Nostocaceae 7.5.1. Genus Archaeoellipsoides Horodyski and Donaldson emend Sergeev et al. (1995) 7.5.1.1. Type species. Archaeoellipsoides grandis Horodyski and Donaldson (1980). Archaeoellipsoides dolichos (Zhang) Sergeev et al. (1995) Figs. 14E-H and 15. 7.5.1.2. Basionym. Bactrophycus dolichum Zhang (1985), pp. 298-299, Fig. 7Q-U. Filamentous microfossil: Horodyski and Donaldson (1983), Fig. 5Z. Bactrophycus dolichum Zhang (1985),

N=6 •

3

•

E

=.2

0 0

I 5

I 10

I 15

I 20

I 25

I 30

t 35

I 40

Length, p.m Fig. 15. Scatter diagram showing the dimensions of ellipsoids of Archaeoellipsoides dolichos for a single population from the Sukhaya Tunguska Formation, sample 38, thin section 635, p. 24.

pp. 298-299, Figs. 7Q-U; Cao (1992), P1. II, Figs. 11-13. Eomycetopsis robusta (partim) Yakschin ( 1991 ), pp. 35-36, P1. XII, Fig. 3. Archaeoellipsoides dolichos Sergeev et al. (1995), p. 32, Fig. 12.7.

7.5.1.3. Description. Solitary, straight or gently curved, highly elongate rod-like vesicles with rounded ends, often broken in two; length 15-39 ~tm, width 2-3 ~tm, length/width= 5-13. 7.5.1.4. Discussion. The rod-like vesicles of Archaeoellipsoides superficially resemble those of Eosynechococcus and were, in fact, interpreted as the remnants of giant chroococcacean unicells by Golovenok and Belova (1984, 1994); however, Archaeoellipsoides populations lack one feature that characterizes both living and fossil chroococcoids--evidence of cell division. Abundant Archaeoellipsoides fossils in the Mesoproterozoic Billyakh Group, northern Siberia, show convincingly that these distinctive fossils are the preserved akinetes of Anabaena-like nostocalean cyanobacteria (Sergeev et al., 1995; Golubic et al., 1995). Archaeoellipsoides dolichos differs from other species of the genus by its small cross-sectional diameter and high length/width. It is a rare element of the Sukhaya Tunguska microbiota. 7.5.1.5. Distribution. Mesoproterozoic (Early-Middle Riphean): Wumishan Formation,

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China; Kotuikan and Yusmastakh formations, Billyakh Group, Anabar Uplift, Northern Siberia; Dismal Lakes Group, northern Canada; Deoban Formation, India. 7. 5.1.6. Material. Twenty observed specimens. 7.6. Order? Nostocales or Oscillatoriales 7.6.1. Genus Siphonophycus Schopf emend. Knoll et al. (1991) 7.6.1.1. Type

species. Siphonophycus

kestron

Schopf (1968). Siphonophycus robustum (Schopf) Knoll et al. (1991) Fig. 14AFig. 16. Eomycetopsis robusta Schopf (1968), p. 685, P1. 82, Figs. 2-3, P1.83, Figs. 1-4; Knoll and Golubic (1979), p. 149, Fig. 4A, B; Mendelson and Schopf

(1982), pp. 59, 60 and 62, P1. 1, Figs. 9 and 10; Golovenok and Belova (1993), P1. II, Fig. e. Eomycetopsis filiformis Schopf (1968), pp. 685, 686, P1. 82, Fig. 1, 4, P1. 83, Figs. 5 8. Siphonophycus robustum Knoll et al. (1991), p. 565, Figs. 10.3 and 10.5 [for complete synonymies see Sergeev (1992b) and Butterfield et al. (1994)]. Eomycctopsis spp. (partita) Mendelson and Schopf (1982), p. 62, PI. 4, Fig. 2. 7. 6.1.2. Description. Unbranched non-septate cylindrical tubes, occasionally solitary, but mostly gregarious in tangled masses. Cross-sectional diameter 1.5-4.5~tm (x=3~tm, N=100); tube walls psilate to finely granular, <0.5 ~m thick; no septa or cross-walls observed. Dense masses of tubes sometimes aligned parallel or perpendicular to bedding lamination. 7.6.1.3. Discussion. S. robusturn is a predominant mat-building organism in many Proterozoic microbenthic assemblages; it is regarded as the empty sheaths of LPP-type oscillatorian or nostocalean cyanobacteria (Knoll and Golubic, 1979; Mendelson and Schopf, 1982; Knoll et al., 1991).

Siphonophycus robustum a n d Siphonophycus typicum N total = 200

7.6.1.4. Distribution. Widely distributed in proterozoic cherts. 7.6.1.5. Material. Thousands dense mat populations.

N

1

2

3

4

5

6

7

8

9

10

Diameter of Sheaths (~m) Fig. 16. Histogram showing size frequency distribution of sheath diameter for undifferentiatedpopulations of S. typicum and S. robustum from the Sukhaya Tunguska Formation.

of individuals in

7.6.2. Siphonophycus typicum (Hermann) Butterfield (1994) in Butterfield et al. (1994) Figs. 14A, B and 16 Leiothrichoides typicus Hermann (1974), p. 7, P1.6, Figs. 1 2. Siphonophycus inornatum Zhang ( 1981 ), pp. 491-493, Pl. 1, Figs. 1, 3 5; Petrov et al. (1995), P1. I, Fig. 3. Eomycetopsis lata Golovenok and Belova (1985), pp. 94-96, P1. VII, Fig. 4; Yankauskas et al. (1989), pp. 106-107, P1. XX, Fig. 4, Golovenok and Belova (1993); P1. II, Fig. f. Siphonophycus typicum Butterfield (1994) in Butterfield et al. (1994), pp. 66 67, Figs. 23B-D

V.N. Sergeevet al. / PrecambrianResearch85 (1997) 201-239 and 26B, H and I [for complete synonymy, see Butterfield et al. (1994)]. 7.6.2.1. Description. Unbranched non-septate tubes mostly gregarious in tangled masses; crosssectional diameter 4.5-9.5 gm (x=6.25 gm, N = 100); tube walls psilate to finely granular, c. 0.5 gm thick. No septa or wall partitions observed, but sheaths sometimes broken into rectangular fragments with sharp ends that superficially resemble trichomes. 7.6.2.2. Discussion. Like other Siphonophycus species, S. typicum is interpreted as the evacuated sheaths of mat-building filamentous cyanobacteria (Knoll et al., 1991). In the Sukhaya Tunguska Formation as well as in many other Proterozoic microbiotas, S. typicum sheaths are commonly found in close association with those of S. robusturn (Fig. 14A), a recurring spatial relationship that resembles the association of Lyngbya and Phormidium filaments in some modern mats. In a previous publication, we referred to these fossils as S. inornatum (Petrov et al., 1995); however, Butterfield [in Butterfield et al. (1994)] synonymized this species with S. typicum (Hermann) Butterfield [in Butterfield et al. (1994)], and we adopt this usage here. 7.6.2.3. Distribution. Widely Proterozoic cherts.

distributed

in

7.6.2.4. Material. More than 1000 specimens. 7.6.3. Siphonophycus solidum (Golub) Butterfield (1994) in Butterfield et al. (1994) Fig. 141, K 7.6.3.1. Basionym. Omalophyma solida Golub (1979), p. 151, P1. 31, Figs. 1-4 and 7. Siphonophycus solidum Butterfield et al. (1994), p. 67, Figs. 25H-I and 27D. Large-diameter 'oscillatoriacean' sheaths: Mendelson and Schopf (1982), pp. 62-63, PI. 3, Figs. 4 and 5 [for complete synonyny, see Butterfield et al. (1994)].

231

7.6.3.2. Description. Cylindrical to slightly compressed, unbranched, nonseptate tubes, 20-33 ktm in cross-sectional diameter, up to 200 ~un long (incomplete specimen), tube wall psilate and c. 1 tam thick. 7.6.3.3. Discussion. Siphonophycus solidum occurs as a subordinate constituent of S. robustum and S. typicum mats. It is probably the empty sheath of large monotrichomatous Lyngbya-like or polytrichomatous Microcoleus-like cyanobacteria. 7.6.3.4. Distribution. Widespread in peritidal mat assemblages of Proterozoic age. 7.6.3.5. Material. More than 100 specimens. 7.6.4. Genus Circumvaginalis Sergeev (1993) emend Sergeev et al. (1995) 7.6.4.1. Type species. Circumvaginalis elongatus Sergeev (1993) emend. Sergeev et al. (1995). Circumvaginalis sp. Fig. 14C. Circumvaginalis sp. Petrov et al. (1995), P1. I, Fig. 4. 7.6.4.2. Description. Solitary, unbranched, nonseptate, flexible cylindrical tubes bearing prominent darkly pigmented rings; cross-sectional diameter 6.5-12.5 lam; diameter of dark-brown rings 9.5-12.5 lam; tube wall medium-grained, transluscent, c. 1 ~tm thick; dark rings are opaque and coarse-grained; length of tubes up to 225 lam. 7.6.4.3. D&cusshgn. The genus Circumvaginalis was diagnosed by Sergeev (1993) and subsequently emended by Sergeev et al. (1995) as tubular microfossils formed by funnel-like segments inserted one in another and terminating in prominent dark rings with a coarse-grained surface texture. The type population is interpreted as a filamentous cyanobacterium comparable to species of the modern nostocalean genus Scytonema. The Sukhaya Tunguska specimen shows prominent dark bands of pigment, but differs from the type material by its lack of well-defined funnel-like segments. It may be more similar to sheaths of

232

V.N. Sergeev et al. / Precambrian Research 85 (1997) 201 239

some Lyngbya species that are also known to secrete dark pigment bands within their sheaths [Kondratyeva, 1975, Fig. 30]; in size and other morphological features these microfossils closely resemble S. kestron and may, therefore, be a pigmented variation of this taxon. T.R. Fairchild [see Schopf (1992b), Plate40, Figs. A1 and A2] described tube-like structures with prominent dark rings from the Neoproterozoic Skillogalee Formation, Australia, and drew comparisons with green algae. In general, Fairchild's population is similar to Circumvaginalis; however, without investigating the type material, formal synonymy is premature.

7.6.4.4. Material. One specimen. 7. 7. Microfossils Incertae Sedis 7. 7.1. Genus Myxococcoides Schopf (1968) 7. 7.1.1. Type species. Myxococcoides Schopf (1968).

minor

7. 7.1.2. Discussion. The genus Myxococcoides was established by Schopf (1968) for colonies of simple spheroidal microfossils without organic inclusions; Schopf (1968) interpreted his Bitter Springs populations as chroococcacean cyanobacteria. With the subsequent discovery of abundant populations in many Proterozoic cherts, Myxococcoides has come to be considered a form genus encompassing microfossils of heterogeneous origin (Green et al., 1989; Knoll et al., 1991; Butterfield et al., 1994; Sergeev et al., 1995). Some species of Myxococcoides may belong to the cyanobacterial family Chroococcaceae, although this is by no means clear for the type population of M. minor (Knoll, 1981). Others closely resemble chlorococcalean green algae (Green et al., 1989; Knoll et al., 1991), while still others, including Myxococcoides grandis, may be akinetes produced by nostocalean cyanobacteria (Sergeev et al., 1995) or the empty envelopes of colonial microorganisms (Fairchild, 1985; Sergeev, 1992a,b, 1994). At least 28 species of Myxococcoides have been described, many of them only superficially.

7. 7.2. Myxococcoides grandis Horodyski and Donaldson (1980) Figs. 17 and 18A Myxococcoides grandis Horodyski and Donaldson (1980), p. 142, Figs. 7A-N and 8; Horodyski and Donaldson (1983); Figs. 5J-P; Zhang et al. (1989), p. 325, P1. 1, Figs. 10-13; Cao (1992); PI. II, Figs. 1 6; Sergeev et al. (1995), p. 33, Figs. 7.1-7.9, 7.13, 9.1-9.5 [for complete synonymy, see Sergeev et al. (1995)]. 7.7.2.1. Description. Single- or (rarely) doublelayered spheroidal vesicles occurring as solitary unicells; diameter 11-52 jam; outer layer spherical or compressed into an irregular ellipsoidal shape; walls fine- to medium-grained, 1-2 jam thick; when present, inner layer similar in composition or collapsed; opaque inclusion 2-3 jam in diameter commonly attached to the inner vesicle wall. 7.7.2.2. Discussion. Myxococcoides grandis vesicles occur principally as scattered individuals within peritidal mats or (originally) micritic event beds. The Sukhaya Tunguska population includes individuals larger than observed in the type population, but is otherwise comparable. 7.7.2.3. Distribution. Mesoproterozoic Dismal Lakes Group, Canada; Kotuikan and Yusmastakh formations, Anabar Uplift, Siberia; and Wumishan Formation, China. Late Mesoproterozoic or early Neoproterozoic Sukhaya Tunguska Formation, Turukhansk Uplift, Siberia. Myxococcoides

N

spp.

N = 120

14

I~

12 10

M. minor and M. inornata M. grandis

8 6 4 2 0 10

m

I

I

20

30

~fl

I

t

40

50

H

Diameter of spheroids, pm

Fig. 17. Histogram showing size frequency distribution of Myxococcoides spp. from the Sukhaya Tunguska Formation.

V.N. Sergeev et al. ' Precambrian Research 85 (1997) 201-239

233

Fig. 18. Coccoidal microfossils from the Sukhaya Tunguska Formation. (A) Myxococcoides grandis, sample 38, thin section 518, p. 41, GINPC 547. (B, G) Myxococcoides inornata Schopf (B) sample 38, thin section 626, p. 15, GINPC 548, (G) sample 38, thin section 626, p. 15, GINPC 552. (C,D--square in C) Myxococcoides minor Schopf, sample 38, thin section 635, p. 47, GINPC 549. (E) Myxococcoides sp., sample 38, thin section 650, p. 9, GINPC 550. (F) Leiosphaeridia sp., sample 51, thin section 506, p. 19, GINPC 551. (H,I) Polybessurus bipartitus Fairchild ex Green et al. (H) sample, thin section 631, p. 24, GINPC 553. (I) Sample 38, thin section 531, p.1, GINPC 554. (J) Calyptothrix sp., sample 38, thin section 518, p. 46, GINPC 555. Single scale b a r = 10 Ixm, double bar = 50 Ixm.

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V..N. Sergeev et al. / Precambrian Research 85 (1997) 201-239

7. 7.2.4. Material. Approximately specimens.

50

observed

7. 7.3. Myxococcoides minor Schopf ( 1968); Figs. 17 and 18C and D Myxococcoides minor Schopf (1968), p. 676, P1. 81, Fig. 1, PI. 83, Fig. 10. 7. 7.3.1. Description. Single-walled spheroidal vesicles 5-17 ~tm in diameter, occurring as solitary unicells or in clusters of a few to many individuals; vesicle wall fine-grained, c. 1 ~tm thick. 7.7.3.2. Discussion. Myxococcoides minor differs from other species of the genus by its size. It is a rare component of the Sukhaya Tunguska assemblage. 7. 7.3.3. Distribution. Widely Proterozoic cherts.

distributed

in

7. 7.3.4. Material. A few dozen specimens. 7. 7.4. Myxococcoides inornata Schopf ( 1968); Figs. 17 and 18B and G Myxococcoides inornata Schopf (1968), pp. 676-677, P1. 84, Fig. 7. 7.7.4.1. Description. Single- or double-layered spheroidal vesicles with envelopes, occurring as solitary unicells, dyads, triads, tetrads (cross and planar) and octads surrounded by a common spherical vesicle. The outer layer of the spheroids is usually transparent, spherical or commonly, flattened or irregularly elliptical in shape; walls are fine- or medium-grained, c. 1 ~rn thick. The inner layer of the spheroids when present is translucent and spherical or irregular in shape; walls are medium- or course-grained c. 2 ~m thick. An opaque, spheroidal inclusion 1-2 ~xn in diameter sometimes occurs attached to the inner or outer layer of the envelope. The outer diameter of spheroids ranges from 5 to 28 ~tm. The vesicles that surround the grouped spheroids are spherical or irregular in shape, single-layered, up to 100 pm in diameter, transparent; walls are medium-grained, c. 1 pm thick.

7.7.4.2. Discussion. In the Sukhaya Tunguska Formation, populations assigned to M. inornatum are similar to those referred to M. grandis, differing principally in size. 7. 7.4.3. Distribution. Widely Proterozoic cherts.

distributed

7. 7. 4. 4. Material. Approximately specimens.

50

in

observed

7. 7.5. Myxoeoccoides sp. : Fig. 18E 7. 7.5.1. Description. Single-lamellated relatively large spheroids (55-70 ~tm) occurring as solitary unicells. The envelope is translusent; wall is medium-grained and up to 2 ~tm thick. 7.7.5.2. Material. A few dozen specimens. 7. 7.5.3. Leiosphaeridia sp.. Fig. 18F Leiosphaeridia sp. Petrov et al. (1995), P1. I, Fig. 15. 7. 7.5.4. Description. Spheroidal single walled vesicles 20-40 jam in diameter (three spheroids measured). Walls are translucent, fine- or mediumgrained, c. 1.5 lim thick; specimens occur as solitary individuals. 7.7.5.5. D&cussion. This is a common, but not ubiquitous component of the Sukhaya Tunguska assemblage. The biological affinities of these microfossils are uncertain; they may be empty eukaryotic cells or empty envelopes of colonial cyanobacteria. The distribution of these fossils within the formation suggests that they were planktonic. 7. 7. 5.6. Material. Approximately ten specimens. 7. 7.6. Unnamed planktonie form Figs. 6A-D and 19 cf. Trachyhystrichosphaera Petrov et al. (1995), P1. I, Fig. 1. 7. 7.6.1. Description. Spheroidal vesicles consisting of two envelopes separated by empty space; inner

V.N. Sergeev et al. / Precambrian Research 85 (1997) 201-239

Unnamed planktonic form N=18

350 • •

300

outer envelope inner envelope

] ]

/

/

•

/

7. 7.6.3. Material. Twenty observed specimens.

/ ~- 200

),/

150

Acknowledgments

/

/

50 0 / 0

be noted that the Sukhaya Tunguska cells are distinctly smaller than those of the typical Neoproterozoic (Late Riphean) species T. aimika and T. polaris.

/*

250

100 -

235

I 50

I 100

t 150

I 200

t 250

t 300

i 350

Length,/m Fig. 19. Scatter diagram showing maximum vesicle dimensions of unnamed planktonic forms from the Sukhaya Tunguska Formation.

envelope translucent, robust, more or less regularly spheroidal or elliptical in shape, c. 2 ~tm thick; outer envelope translucent, irregular in outline, < 1 ~tm thick. Outer envelope diameter 45-320 ~tm; inner envelope diameter 35-265 lam; envelopes separated by 5-25 ~m. No processes were observed to support the outer envelope.

7.7.6.2. Discussion. In their general morphology, these fossils resemble populations of the characteristically Neoproterozoic acanthomorphic acritarch Trachyhystrichosphaera--for example, the vesicles have regular inner and wavy outer envelopes. The obvious difference between Trachyhystrichosphaera and the Sukhaya Tunguska populations is the presence of cylindrical processes that arise from the inner vesicle and support the outer envelope in the former. Within Trachyhystrichosphaera populations, however, as many as 50% of all specimens do not have preserved processes (Knoll et al., 1991; Sergeev, 1992b; Butterfield et al., 1994). Therefore, it is possilbe that the Sukhaya Tunguska populations originally had processes but lost them during post-mortem decay. In this regard, it should

We thank M.A. Semikhatov for his critical reading of successive versions of this paper. We also thank S. Golubic, A.F. Veis and M.S. Burzin for helpful discussions; O.V. Artemova, L.M. Mudrenko and N.G. Vorobyeva for technical assistance; and M.R. Walter and C.V. Mendelson for useful manuscript reviews. Preparation of this article was supported by R F F R Grant No. 95-05-14575; International Scientific Foundation, Grant No. 300; and a PalSIRP Grant from the Paleontological Society to V.N. Sergeev; NASA Grant NAGW-837 to A.H. Knoll; and R F F R Grant No. 96-05-64329 to P.Yu. Petrov.

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