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708


Journal of Paleontology 89(5):695–729


this genus occur as tightly packed spheroidal colonies of single- walled cells 10- to 15-µm in diameter that in some instances are enclosed by multilayered envelopes. Given their simple morphology and small cell size, these fossils are probably of cyanobacterial, chroococcacean affinity. Nevertheless, morpholo- gically similar colonies occur also in other cyanobacterial families (e.g., the Pleurocapsaceae and Entophysalidaceae) as well as among microalgal eukaryotes. Described originally from the Neoproterozoic Draken Conglomerate of Spitsbergen (Knoll, 1982), species of Synodophycus have been reported less commonly from permineralized microbial communities than those of the other colonial Chulaktau taxa noted above. Similarly, unlike the benthic habit of the other colonial taxa, that of the Chulaktau Synodophycus specimens has yet to be established.


Planktonic acritarchs and other unicells.—Although the Berkuta and Chulaktau strata, unlike some Early Cambrian microbiotas, lack abundant spiny (acanthomorphic) acritarchs, the Chulaktau cherts contain the planktonic sphaeromorphs Leiosphaeridia minutissima (Fig. 13.10) and L. tenuissima (Fig. 13.9) as well as the problematic acanthomorphs Vandalosphaeridium koksuikum and Cymatiosphaera sp., all of probable of eukaryotic affinity. Leiosphaeridia, of broad strati- graphic range and the principal form genus of unornamented sphaeromorphic acritarch known from Proterozoic and Cambrian sediments, is commonly interpreted to be a unicellular prasinophycean (e.g., Tappan, 1980) or chlorophycean alga (Talyzina and Moczydłlowska, 2000;Moczydłlowska, 2010; Moczydłlowska et al., 2010). Although for most specimens of Leiosphaeridia such affinities are likely correct, some leiosphaerids exhibit wall ultrastructure seemingly unlike that of eukaryotic green algae (e.g., Javaux et al., 2004). In addition to specimens permineralized in the Chulaktau cherts, carbonaceous compression-preserved vesicles assigned to Leiosphaeridia have been reported from interbedded shales of the formation (Korolev and Ogurtsova, 1981, 1982; Ogurtsova, 1985). Of particular interest among the acritarchs of the Chulaktau


assemblage is Vandalosphaeridium koksuicum (Fig. 14.3, 14.6), a form typified by its evidently single-layered more or less spheroidal vesicles, 40- to 45-µm in diameter, in which the vesicle-defining envelope appears to be sculptured by promi- nent semicrescent to transverse ramparts and short appendage- like possible processes. This characteristic sculpture pattern, exhibited also by specimens reported from the underlying Neoproterozoic Chichkan Formation (Schopf et al., 2010a; Sergeev and Schopf, 2010), coupled with the occurrence in Chulaktau and Chichkan specimens of a globular interior cyst- like body, suggests their probable chlorococcalean affinity (cf., Moczydłlowska, 2010). A morphologically rather similar 55- to 65-µm-diameter acritarch co-occurring in the Berkuta and Chulaktau cherts is cf. Cymatiosphaera sp. (Fig. 14.1, 14.2, 14.4, 14.5). Because almost all previously described species of Cymatiosphaera have been defined on the basis of flattened compression-preserved specimens in shales, rather than three- dimensionally chert-permineralized specimens such as those studied here, we do not assign the Tamda Group specimens to a previously defined species. Two species of the spheroidal-celled colonial or unicellular form genus Myxococcoides, M. minor and M. inornata, occur


rather commonly in small colonies scattered among the benthic members of the Chulaktau assemblage, an irregular distribution suggesting that they may represent allocthonous plankton derived from overlying waters. The epithet Myxococcoides has been applied both to envelope-enclosed many-celled colonies of closely packed spherical cells such M. minor (Fig. 14.11), composed of 8.5- to 14-µm diameter cells, and to isolated cells and cell pairs like those of M. inornata (Fig. 14.10), 15–20 µm in diameter. Although all described species of Myxococcoides are plausibly chroococcacean (Сhrооcосcus-or Gloeocapsa- like) cyanobacteria, as such forms were originally interpreted (Schopf, 1968), some bear resemblance also to extant small-celled eukaryotic chlorophycean algae (Knoll et al., 1991; Knoll, 1996; Schopf et al., 2010a; Sergeev and Schopf, 2010).


Filamentous microfossils of uncertain affinities.—This category includes only two of the 27 taxa here reported, Botominella lineata and Berkutaphycus elongatus. Botominella lineata (Fig. 13.1–13.5) exhibits a filamentous trichome-like body composed of cell-like segments 20- to 60-µm wide and 2- to 5-µmlong. Berkutaphycus elongates (Figs. 4.1–4.20, 13.6–13.8, 13.11–13.16) is a previously unreported taxon here described from the Berkuta Member of the Kyrshabakta Formation where it is evidently represented by life cycle and preservational


variants and is interpreted to include 11- to 34-µmbroad filamentous tubes aswell as spheroidal and cask-like structures 25- to 70-µm wide and 2- to 5-µm long. Like Botominella lineata, Berkutaphycus elongatus is of uncertain affinities, resembling large-diameter cyanobacteria and some filamentous eukaryotic algae.


Evolutionary and biostratigraphic significance of the Berkuta and Chulaktau microbiotas


The microbiotas of the Berkuta Member of the Kyrshabakta Formation and the overlying Chulaktau Formation are composed largely of morphologically simple filamentous and coccoidal microorganisms, mainly cyanobacteria, augmented by unornamented spheroidal planktonic acritarchs, presumably prasinophycean or chlorophycean algae. Thus, both resemble so-called ‘typical Proterozoic microbiotas’ (Mendelson and Schopf, 1982)—assemblages dominated by and in some instances composed entirely of filamentous and coccoidal, evolutionarily conservative (hypobradytelic) cyanobacteria (Schopf, 1994). Despite their ‘Proterozoic-like’ appearance, however, their Phanerozoic age is well established by the pre- sence of lowermost Cambrian-defining small shelly fossils in the Kyrshabakta Formation Berkuta Member, specimens of which have also been reported to occur in the stratigraphically underlying tillite-overlying cap dolomite of the formation. Both of the microbiotas studied here lack the richly diverse


assemblage of morphologically relatively complex unicellular eukaryotes, including acanthomorphic acritarchs, of the under- lying and much older Neoproterozoic (~750 Ma-old) Chichkan Formation (Schopf et al., 2010a; Sergeev and Schopf, 2010). Indeed, and although the Chulaktau assemblage includes sphaeromorphic acritarchs (viz., Leiosphaeridia minutissima


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