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726


Journal of Paleontology 89(5):695–729


Occurrence.—Widely distributed in Proterozoic chert- permineralized microfossil assemblages.


Remarks.—Although Myxococcoides was established in 1968 to include colonies of simple spheroidal microfossils interpreted to be chroococcacean cyanobacteria (Schopf, 1968), this genus name has since been used to encompass diverse microfossils of heterogeneous origin (Green et al., 1989; Knoll et al., 1991; Butterfield et al., 1994; Sergeev et al., 1995, 2012). Some such species are no doubt chroococcaceans (though this may be uncertain for the type population of M. minor; Knoll, 1981) whereas others also resemble small-celled chlorococcalean green algae (Green et al., 1989; Knoll et al., 1991; Sergeev and Schopf, 2010). Still others have been suggested to have different affinities; for example, specimens of the relatively large- diameter taxon M. grandis have been suggested to represent the empty originally colony-enclosing envelopes of colonial prokaryotes (Fairchild, 1985; Sergeev, 1992, 2006) or the akinetes of nostocalean cyanobacteria (Sergeev et al., 1995; 2012). Because of such uncertainty regarding the affinities of Myxococcoides taxa, M. inornata and M. minor are classified here as Incertae Sedis. To date, more than 30 species of this genus have been described.


Myxococcoides minor Schopf, 1968 Figure 14.11


Myxococcoides minor Schopf, 1968, p. 676, pl. 81, fig. 1, pl. 83, fig. 10; Schopf, 1992b, pl. 32, figs. H, I; Sergeev, Knoll, and Petrov, 1997, p. 234, figs. 18C, 18D; Sergeev, 2001,


p. 443, fig. 8.11; Sergeev, 2006, p. 225 and 226, pl. 15, figs. 2, 5; pl. 20, fig. 11; pl. 48, fig. 5; Sergeev, Sharma, and Shukla, 2008, pl. 5, fig. 8, pl. 6, fig. 9; Sergeev and Schopf, 2010, p. 393, figs 12.3, 12.4; Schopf, Kydryavtsev and Sergeev,


Description.—Spheroidal vesicles, subcircular in cross-section, having a surface folded into numerous polygonal fields from the walls of which protrude unbranched broadly conical processes that are more or less evenly distributed across the surface. Vesicle diameters range from 40 to 45 µm(n = 3, x = 43); processes are commonly indistinct, evidently 5 to 7 µmlong and taper from being 2–5 µm broad at their base to 1–2 µmwidein their distal parts. A globular opaque polygonal (pyrite-like) inclusion, 4–5 µm broad, is present within some vesicles.


Material examined.—Three well-preserved specimens.


Occurrence.—Neoproterozoic: Chichkan Formation, South Kazakhstan; Lower Cambrian, Chulaktau Formation, South Kazakhstan.


2010a, figs 5.5, 5.6; Sergeev, Sharma, and Shukla, 2012, pl. 5, fig. 8, pl. 6, fig. 9.


Description.—Spheroidal closely packed cells 8.5–14.0 μmin diameter, occurring in organic matrix-embedded colonies of a few to tens of individuals, defined by single-layered fine- grained walls ~ 0.5 μm thick. Some cells contain spheroidal to irregularly shaped opaque bodies, evidently condensed cytoplasmic remnants, that partially or nearly completely infill cell lumina.


Material examined.—Tens of well-preserved colonies and hundreds of individuals.


Occurrence.—Widely distributed in Proterozoic chert- permineralized microfossil assemblages.


Genus Vandalosphaeridium Vidal, 1981 Type species.—Vandalosphaeridium reticulatum (Vidal), 1981.


Vandalosphaeridium koksuicum Sergeev and Schopf, 2010 Figure 14.3, 14.6


Vandalosphaeridium koksuicum Sergeev and Schopf, 2010, p. 397, figs. 13.1, 13.2a, 13.2b, 13.3, 13.4a, 13.4b.


Remarks.—The taxonomic status of this genus is uncertain. Described initially from the Late Neoproterozoic Visingsö Group of Sweden as a morphologically distinctive acantho- morphic acritarch (Vidal, 1981), Vandalosphaeridium has been reported from numerous other Neoproterozoic units including the Chulaktau-underlying Chichkan Formation (Sergeev and Schopf, 2010) and Ediacaran-age strata of Australia (Grey, 2005). The Chulaktau taxon, Vandalosphaeridium koksuicum, exhibits distinctive broadly conical processes and differs in size and morphology from such other described species as V. reticulatum and V. varangeri. Although at least one species of the genus (V. walcottii, described from the Neoproterozoic Chuar Group of Arizona; Vidal and Ford, 1985) was defined on the basis of what appear to be rather poorly preserved specimens of Trachyhystrichosphaera aimika, the distinctive morphology of V. koksuicum and the occurrence in many Chichkan speci- mens of a vesicle-enclosed globular organic body (Sergeev and Schopf, 2010) that is similar to that of the (pyrite-replaced?) inclusions of the Chulaktau specimens, suggests that V. koksuicum is a legitimate taxon, most probably allied to planktonic chlorophycean green algae.


Acknowledgments


We thank two anonymous reviewers for constructive reviews of the manuscript and M. A. Semikhatov, M. A. Fedonkin, T. A. Litvinova, P. Yu. Petrov, N. G. Vorob’eva, N. M. Chumakov, and the late V. V. Missarzhevskii for helpful discussions. The four-week visit in 2012 of V.N.S. to the University of California, Los Angeles, where parts of this study were carried out, was supported by the UCLA Center for the Study of Evo- lution and the Origin of Life (CSEOL). Fieldwork involved in the collection of the fossiliferous samples studied and the research carried out by V.N.S. at GIN, the Geological Institute RAS in Moscow, were supported by RFBR Grants #13-05-00127, #14-05-00323, and the Program of the Pre- sidium of Russian Academy of Sciences #28. The participation of A.B.K. is this study was supported by CSEOL and the PennState Astrobiology Research Center (PSARC).


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