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Journal of Paleontology, 89(5), 2015, p. 695–729 Copyright © 2016, The Paleontological Society 0022-3360/16/0088-0906 doi: 10.1017/jpa.2015.56


A new approach to ancient microorganisms: taxonomy, paleoecology, and biostratigraphy of the Lower Cambrian Berkuta and Chulaktau microbiotas of South Kazakhstan


J. William Schopf,1 Vladimir N. Sergeev,2 and Anatoliy B. Kudryavtsev3


1Department of Earth, Planetary, and Space Sciences, Center for the Study of Evolution and the Origin of Life, and Molecular Biology Institute, University of California, Los Angeles 90095 USA; PennState Astrobiology Research Center, Deike Building, University Park, PA 16802 USA;


and University of Wisconsin Astrobiology Research Consortium, Madison, WI 53706, USA; ⟨schopf@ess.ucla.edu⟩ 2Geological Institute, Russian Academy of Sciences, Pyzhevskii per., 7, Moscow, 119017, Russia, ⟨sergeev-micro@rambler.ru⟩ 3Center for the Study of Evolution and the Origin of Life, University of California, Los Angeles 90095 USA; PennState Astrobiology Research


Center, Deike Building, University Park, PA 16802 USA; and University of Wisconsin Astrobiology Research Consortium, Madison, WI 53706, USA; ⟨kudryavtsev@ess.ucla.edu⟩


Abstract.—Peritidal cherts and silicified phosphorites of the Early Cambrian Kyrshabakta (Berkuta Member) and Chulaktau formations of South Kazakhstan contain diverse assemblages of cellularly permineralized microorganisms that like other microbiotas of similar age and setting are dominated by cyanobacteria, both filamentous (oscillatoriacean trichomes and empty sheaths) and coccoidal (chroococcaceans). Although these near-shore assemblages contain sphaeromorph acritarchs, they differ from Neoproterozoic and Early Cambrian microbiotas of more open-marine environ- ments by lacking morphologically complex planktonic eukaryotes such as the acanthomorphic acritarchs that are abun- dant in the Chulaktau-overlying Shabakta Formation. In general composition, the Berkuta and Chulaktau assemblages are similar both to microfossil assemblages of the pre-trilobite Rovno and Lontova Regional Stages of the East European Platform and to Early Cambrian microbiotas known from chert-phosphorite deposits worldwide. The studies reported here are based on use of optical microscopy combined with techniques recently introduced to


paleobiology: confocal laser scanning microscopy, and Raman and fluorescence spectroscopy and imagery. Taken together, these provide information in situ, in three dimensions and at high spatial resolution, about the organismal morphology; cellular anatomy; chemical composition; and mode, fidelity, and environment of preservation of the permineralized microfossils. Data are presented suggesting that the substitution of samarium for calcium in fossil- preserving apatite may provide evidence of its oxic or anoxic paleoenvironment of formation. We interpret the Berkuta and Chulaktau assemblages to be composed of 27 taxa assigned to 17 genera of microscopic prokaryotes and eukaryotes that include one new genus and species, Berkutaphycus elongatus.


Introduction


Lowest Cambrian rocks are distinguished by the presence of the earliest-known diverse assemblages of shelly fossils. This benchmark, the global appearance of metazoans having robust carapaces and “hard parts,” is perhaps the sharpest and arguably one of the most important biostratigraphic boundaries in Earth history. Phytoplankton also exhibit major changes across the Precambrian-Phanerozoic boundary that are evidenced in Ediacaran-Cambrian successions worldwide (e.g., Australia, China, India, the East-European Platform, India, North America, and Siberia). Of these, one of the most notable is that of the Neoproterozoic to Early Paleozoic succession of the Maly Karatau Range of South Kazakhstan and its stratigraphic equivalents in the Tian-Shan mountains of Kirghizia and China. In these regions, the upper parts of this succession contain abundant shelly fossils that evidence its Cambrian age. The units of the Maly Karatau Range studied here not only preserve


such shelly fossils, but, also, diverse assemblages of chert- and phosphate-permineralized Early Cambrian microorganisms— the focus of our report. In previous studies we described the chert-permineralized


microbiota of the Cryogenian (Neoproterozoic, Upper Riphean) Maly Karatau Range Chichkan Formation (Schopf et al., 2010a; Sergeev and Schopf, 2010). We here extend our investigations of the microfossil assemblages of this region by describing the permineralized microbiotas of the Early Cambrian Kyrshabakta (Berkuta Member) and Chulaktau formations, microbial assemblages that are appreciably less diverse than that permineralized in the underlying Chichkan cherts. The results reported document the use of standard optical microscopy combined with those obtained by confocal laser scanning microscopy (Schopf et al., 2006) and Raman and fluorescence spectroscopy and imagery (Schopf and Kudryavtsev, 2010; Schopf et al., 2002, 2005), techniques that permit analyses of the submicron-scale morphology and carbonaceous composition of


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