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710


Journal of Paleontology 89(5):695–729


their deposition, the late Atdabanian marine transgression established a more standard oceanic regime, a possible explanation for the absence of assured acanthomorphs from the Berkuta and Chulaktau assemblages and their abundance in basal horizons of the Chulaktau-overlying Shabakta Formation (Korolev and Ogurtsova, 1981, 1982; Ogurtsova, 1985; Sergeev, 1989, 1992).


Conclusions


The Berkuta and Chulaktau assemblages document the composition of a part of Earth’s microbial biota during a key transition in evolutionary history when, with the rise of mega- scopic metazoans, the biosphere changed markedly. This biotic transition was no doubt gradual, rather than abrupt—occurring over tens of millions of years—with the two assemblages stu- died here providing insight into the adaptation of microbes to this global event. Because of their exceptional preservation, a result of environmental settings that promoted permineralization by both silica and phosphate, the microbiotas of the two units provide a clear view of Early Cambrian shallow-water microbial ecosystems. In comparison with microbiotas permineralized in the


underlying Neoproterozoic Chichkan Formation and the over- lying Early Cambrian Shabakta Formation, those of the Berkuta and Chulaktau cherts and phosphorites are depauperate, most notably lacking assured acanthomorph acritarchs. Although the relatively low diversity of these two cyanobacterium-dominated “Proterozoic-like” communities in part reflects their occurrence in units deposited in the aftermath of the latest Proterozoic phytoplankton extinction event, paleoenvironmental con- siderations suggest that it may also have been a result of their preservation in shallow near-shore settings where the restricted basin served to inhibit an influx of acanthomorphs from distal open-marine environments. In addition to documenting two previously undescribed


microbial assemblages, this study demonstrates the use of new techniques to analyze permineralized microscopic fossils in situ at submicron spatial resolution. Thus, in addition to standard optical microscopy we have used three techniques recently introduced to paleobiology: confocal laser scanning microscopy, to document the three-dimensional organismal and


cellular morphology of the microfossils; and both Raman and fluorescence spectroscopy and imagery, to document their carbonaceous composition, the geochemical maturity of the kerogen of which they are composed, and the composition of the fossil-enclosing matrix and of fossil-permineralizing, -infilling, and -encrusting minerals. For the first time, fluorescence spectroscopic data are provided here that suggest their use to infer the oxic or anoxic paleoenvironment of fossil-preserving apatite-formation. This report of the Berkuta and Chulaktau microorganisms


adds new information about the biological composition and evolutionary status of Early Cambrian (Nemakit-Daldynian and Tommotian, 542- to ~530-Ma-old) microbiotas preserved in restricted shallow-water chert- and phosphate-precipitating environments. The new approach to such studies documented here, the application of diverse newly applied techniques to analyze individual microscopic fossils, can provide useful


insight into their biological affinities, paleoecology, taphonomy, and environment of preservation.


Systematic paleontology


Kingdom Eubacteria Woese and Fox, 1977 Phylum Cyanobacteria Stanier et al., 1978 Class Hormogoneae Thuret, 1875 Order Oscillatoriales Elenkin, 1949


Family Oscillatoriaceae (S.F. Gray) Kirchner, 1900 Genus Eomicrocoleus Horodyski and Donaldson, 1980


Type species.—Eomicrocoleus crassus Horodyski and Donaldson, 1980.


Eomicrocoleus crassus Horodyski and Donaldson, 1980 Figure 11.9


Eomicrocoleus crassus Horodyski and Donaldson, 1980, p. 154, figs 15A, 15B; Sergeev, 2001, p. 442, fig. 9.5; Sergeev, 2002,


p. 559, pl. 2, fig. 6; Sergeev, 2006, p. 208, pl. 18, fig. 5, pl. 25, fig. 6; Sharma, 2006, p. 91, fig. 10d; Sergeev, Sharma and Shukla, 2012, p. 289, pl. 15, figs. 4–6, 9.


Description.—Bundles of tube-like trichomes having very rare cross-walls closely grouped within a common cylindrical sheath or without a surrounding sheath. Parallel or subparallel trichomes are 2–3 µm in diameter, mostly hollow, and have psilate walls ~0.5 µm thick; trichome-encompassing common sheaths, when present, are 25–30 µm in cross-sectional dia- meter, up to 80 µm long, ~1 µm thick, and are typically fine-to medium-grained.


Material examined.—Several well-preserved specimens.


Occurrence.—Widely distributed in Proterozoic and Lower Cambrian chert-permineralized organic-walled assemblages.


Remarks.—Trichomes and sheaths of the Chulaktau Formation are of slightly larger diameter than those of the type population and the bundles of trichomes commonly lack encompassing sheaths, an absence attributable to preservational alteration (Gerasimenko and Krylov, 1983, Sergeev et al., 1997).


Genus Obruchevella Reitlinger, 1948, emend.


Yakschin and Luchinina, 1981, emend. Kolosov, 1984, emend. Yankauskas, 1989, emend. Burzin, 1995, emend. Nagovitsin, 2000


Type species.—Obruchevella delicata Reitlinger, 1948. Obruchevella parva Reitlinger, 1959, emend.


Golovenok and Belova, 1989, emend. Burzin, 1995 Figures 3.1–3.5, 7.1–7.7, 8.1, 8.2, 8.5, 8.6, 9.1–9.6


Obruchevella parva Reitlinger, 1959, p. 21, pl. 6, figs. 1, 2; Kolosov, 1977, p. 73,74, pl. 6, fig. 1; 1982, pl. 16, figs. 1a, 1б; Cloud, Awramik, Morrison and Hadley, 1979, p. 87-89, figs. 5J and 5K; Yakschin and Luchinina, 1981, p. 30, pl. 10, figs. 1–3; Golovenok and Belova, 1983, p. 1464, figs. 1B–1D;


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