Schopf et al.—Berkuta and Chulaktau microbiotas
the end of the immediately pre-Ediacaran (pre-Vendian) Cryogenian. This interpretation was based primarily on the presence in Kyrshabakta Formation carbonates of a large negative shift in δ13C values (up to −9‰) that may correspond to the Shuram/Wonoka carbon isotope anomaly of approxi- mately the same age. In contrast, Chumakov (2009, 2010, 2011) correlates the Aktas tillites and similarly aged glacial sequences of Kazakhstan and Kirghizia to Late Ediacaran (Vendian)- Nemakit-Daldyn Baykonurian glacial deposits and, thus, with tillites of the Luoquan Formation (North China), Hankalchough Formation (Tarim, northwestern China), Zabit Formation (East Sayan, northwestern Mongolia-southern Siberia) and the Pourpree de l’Ahnet Group (West African Craton, Algeria). Similarly, rather than correlating the negative carbon isotope excursion in the Kyrshabakta carbonates with the Shuram/ Wonoka δ13C anomaly, Chumakov (personal communication to V.N.S.) correlates this shift to the negative δ13C Dounce anomaly of South China (Zhou et al., 2004) and assigns an age to this glaciation of 550–540 Ma. Although Chumakov’s interpretation is consistent with the
reported occurrence of lowermost Cambrian-defining small shelly fossils of the Protohertzina anabarica biozone in the cap dolomite immediately overlying the Aktas tillites (Mambetov, 1993), subsequent searches of these strata by Missarzhevskii
(personal communication to V.N.S.) were unable to confirm this finding. Eganov and Sovietov (1979) also recorded the presence of small shelly fossils in this cap dolomite—for which, however, they did not provide taxonomic descriptions. At present, therefore, the oldest confirmed and appropriately documented occurrence of Protohertzina anabarica biozone fossils in the succession here studied is that in the Berkuta Member of the Kyrshabakta Formation (Missarzhevskii and Mambetov, 1981). Fossils typical of Early Cambrian Tommotian Stage
faunas occur in phosphorites of the Kyrshabakta-overlying Chulaktau Formation (Missarzhevskii and Mambetov, 1981). The two lower members of the formation, the Aksai and Karatau, correspond, respectively, to the Tiksitheca licis and Pseudorthotheca costata Tommotian faunal zones, and the uppermost Ushbass Member to the Berkutia cristata zone (which has been suggested, however, to occur in the Atdabanian Stage; Cook and Shergold, 2005, p. 318). Lower units of the Chulaktau-overlying Shabakta Forma-
tion contain Late Atdabanian faunas of the Rhombocorniculum cancellatum zone (Missarzhevskii and Mambetov, 1981; Missarzhevskii, 1989; Mambetov, 1993; Popov et al., 2009) whereas the upper strata of the formation contain Microcornus parvulus zone small shelly fossils and, higher in the succession, trilobites of the Hebediscus orientalis, Ushbaspis limbata and Redlichia chinensis-Kootenia gimmelfarbi faunal zones of the Lower Cambrian Botomian and Toyonian Stages (Ergaliev and Pokrovskaya, 1977; Mambetov, 1993; Popov et al., 2009). Such fauna-based biostratigraphic data for the Early
Cambrian age of the Tamda Group strata are supported by organic-walled microfossils of the Chulaktau and overlying Shabakta Formation. The Chulaktau shales contain microfossil assemblages that include such time-diagnostic acritarch taxa as Granomarginata prima, G. squamacea, and Leiomarginata simplex, indicating their temporal correlation to the Early Cambrian Lontova Regional Stage (Horizon) of the East
699
European Platform (Korolev and Ogurtsova, 1981, 1982; Ogurtsova, 1985; Sergeev, 1989, 1992). Similarly, and although none of the chert- and phosphate-permineralized Chulaktau microfossils reported here are time-diagnostic, this microbiota is dominated by the distinctive helically coiled cyanobacterium Obruchevella typical of Early Cambrian phosphorite- bearing deposits worldwide (Sergeev, 1989, 1992; Sergeev and Ogurtsova, 1989). Furthermore, acanthomorphic acritarch taxa present in the Chulaktau-overlying Shabakta Formation correspond well to those of the late Early Cambrian Vergale Regional Stage of the East European Platform. In sum, the Early Cambrian age of Berkuta and Chulaktau
microfossil assemblages described here seem firmly established by radiometric analyses of underlying deposits and by biostratigraphic data, both faunal- and microfossil-based, and from both underlying and overlying strata.
New analytical techniques and preservation of the microbiotas
Permineralized (“petrified”) fossils, studied typically in petrographic thin sections, are among the best preserved and, thus, the biologically and taphonomically most informative. Nevertheless, until recently it had not been possible to document accurately, in situ and at high spatial resolution, the organismal form and cellular anatomy of such three-dimensional fossils, a deficiency particularly detrimental to studies of the morphology and cellular structure of microscopic fossilized microorganisms. Similarly, there had been no means by which to analyze in situ the molecular-structural composition and geochemical maturity of the coal-like carbonaceous organic matter (kerogen) that comprises permineralized fossils, factors crucial to assessment of their fidelity of preservation, nor had there been means— suggested here for the first time—to assess the oxic or anoxic nature of the fossil-permineralizing environment. The studies reported here of the Berkuta and Chulaktau microbiotas document use of three analytical techniques recently introduced to paleobiology that together meet these needs: confocal laser scanning microscopy (CLSM; Schopf et al., 2006), and Raman (Schopf et al., 2002, 2005) and fluorescence (Schopf and Kudryavtsev, 2010) spectroscopy and imagery. Applicable to permineralized organic-walled fossils of all
major biologic groups (animals, plants, fungi, protists, and microbes), whether preserved in quartz, apatite, calcite or gypsum, the four principal matrices in which permineralization occurs, CLSM, and Raman and fluorescence spectroscopy and imagery—used in tandem to study individual specimens—can provide data by which to characterize, in three dimensions and at submicron spatial resolution, a one-to-one match of cellular form and carbonaceous (kerogenous) composition as well as the spatial distribution of permineralizing minerals (Schopf and Kudryavtsev, 2010; Schopf et al., 2010b, 2012). Moreover, their use can elucidate the preservational history (e.g., apatite- permineralization of the soft tissues of a metazoan embryo followed by calcite-infilling of interstices and fluid-filled cavities; Chen et al., 2007) and the fidelity of their geochem- ical preservation (measured by the Raman index of preservation [RIP], a metric that documents the geochemical maturity of their
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