Schopf et al.—Berkuta and Chulaktau microbiotas
dant fossils are helical filaments of Obruchevella and empty sheaths, trichomes, and colonial unicells of other cyanobacteria. Assured acanthomorphic acritarchs, abundant in other Early Cambrian units, were not detected and have not been reported from strata either of the Kyrshabakta or Chulaktau formations. The relative abundance of the morphotypes dectected in the Berkuta and Chulaktau microbiotas varies greatly among the samples here investigated. In our taxonomy of the cyano- bacterial morphotypes of these units, we follow the classifica- tions of Butterfield et al. (1994) and Sergeev et al. (2012).
cyanobacteria, (3) planktonic acritarchs and other unicells, and (4) filamentous microfossils of uncertain affinities. Many of these Early Cambrian morphotypes have long time ranges, including virtually all of the cyanobacteria and such sphaer- omorph acritarchs as Leiosphaeridia spp., known also from the Meso- and Neoproterozoic. Throughout the Chulaktau strata studied, the most abun-
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taxa of trichomes and sheaths of Siphonophycus, Oscillatoriopsis and Palaeolyngbya co-occurring in the Chulaktau cherts represent differing parts of single biological entities. Thus, for example, the cell size–defined 13- to 18.5-µmbroad taxon Oscillatoriopsis cuboides may have been a Lyngbya-like oscillatoriacean that was originally ensheathed by 16- to 32-µmdiameter Siphonophycus solidum sheaths thatmight also have originally enclosed the 22- to 30-µm broad cellular trichomes of O. longa. Like their modern counterparts, these taxa were evidently mat-forming, in some instances preserved in the Chulaktau cherts in eroded and redeposited rounded or subrounded mat fragments.
Mat-forming filamentous cyanobacteria.—In the Chulaktau assemblage, well-preserved spiral filaments of Obruchevella are particularly abundant. Four species of Obruchevella have been detected: O. parva (Figs. 3.1–3.5, 7.1–7.7, 8.1, 8.2, 8.5, 8.6, 9.1–9.6), O. parvissima (Fig. 8.3, 8.4, 8.7, 8.8), O. delicata, and O. cf. meishucunensis (Fig. 3.6–3.10). As in genera of similarly helically coiled modern cyanobacteria (e.g., Spirulina and Arthrospira), such fossil trichomes typically exhibit little evidence of cell-defining transverse cell walls. Such septa, however, are preserved in some Chulaktau specimens of O. parva, showing their cells to be 5–10 µm in diameter. This species of Obruchevella is predominant in all Chulaktau samples here studied whereas other taxa of the genus— differentiated from O. parva by their filament diameters (viz., O. parvissima,3–5 µm, O. delicata,10–13 µm, and O. cf. meishucunensis,20–22 µm)—are relatively uncommon. Other filamentous Chulaktau cyanobacteria are here
assigned to Eomicrocoleus (Fig. 11.9), Siphonophycus, Oscillatoriopis and Palaeolyngbya. Siphonophycus, most recently defined taxonomically by Butterfield et al. (1994) and Sergeev et al. (2012) and interpreted to be the extracellular trichome-enclosing tubular sheaths of oscillatoriacean cyano- bacteria, is represented by three species differentiated by their breadths: S. robustum (2–4 µm; Fig. 11.11); S. kestron (8-16 µm); and S. solidum (16–32 µm; Figs. 3.21–3.24, 10.7, 10.8, 10.11, 10.12). These taxa, like those of Obruchevella spp., are of common occurrence in the Chulaktau cherts. The assemblage also includes three species of Oscillatoriopsis, each exhibiting well defined rounded terminal cells and disc- or cube- shaped medial cells and differentiated by their characteristic cell diameters: Oscillatoriopsis sp. (5–7 µm), O. cuboides (13–18.5 µm; Fig. 9.7–9.9), O. longa (22-30 µm; Figs. 10.5, 10.6, 11.7, 11.8). The Chulaktau cherts also contain two species of Palaeolyngbya, characterized by its sheath-enclosed uni- seriate trichomes composed of discoidal to cylindrical cells: P. sp. (6–7 µm in diameter; Fig. 11.12, 11.13) and a much broader form, P. catenata (28–35 µm in diameter; Figs. 9.10, 9.11, 10.1–10.4, 10.9, 10.10, 11.1–11.6). Given the phototaxis of the trichomes of oscillatoriaceans
and their capability by gliding motility to vacate their encompassing tubular sheaths, it is possible that various of the
Colonial and single-celled chroococcacean cyanobacteria.— Of the taxonomic families of coccoidal cyanobacteria identified in the Chulaktau microbiota, the mat-forming Entophysalida- ceae is represented by Eoentophysalis belcherensis (Fig. 12.3, 12.6, 12.7–12.13), a taxon having spheroidal to ellipsoidal 3-to 10-µm diameter cells occuring in palmelloid colonies enclosed by multilayered envelopes. Of limited distribution in the assemblage, distinctive crustose pustular laminae like those formed by E. belcherensis in Proterozoic stromatolitic deposits (Hofmann, 1976; Sergeev et al., 1995, 2012) have not been observed in the Chulaktau cherts. Among all types of fossil coccoidal cyanobacteria
represented in the Chulaktau biota, colonial members of the Chroococcaceae are particularly abundant, their affinity to this family indicated typically by their occurrence in small clusters of 2, 4 or as many as 8 close-packed and commonly individually ensheathed cells produced by cell division in two mutually perpendicular planes and surrounded by a colony-enclosing originally mucilaginous envelope. Of these, Tetraphycus acutus (Figs. 3.11–3.20, 12.4, 12.5), having a cell size-range of 10-20 µm, is especially abundant, occurring in groups of small colonies spread laterally over hundreds of square microns. Although Chulaktau specimens of the morphologically similar colonial chroococcacean
Archaeophycus yunnanensis
(Fig. 12.1, 12.2), characterized by 8- to 15-µm-diameter cells, may represent apatite-replaced preservational variants of Tetraphycus acutus, the individual mineralized cells of such colonies are not sheath-enclosed and are typically somewhat smaller than those of T. acutus. The colonial chroococcacean Eoaphanocapsa molle
(Fig. 14.12) exhibits multilamellated spheroidal and ellipsoidal cells 12- to 17-µm in diameter that occur in loose clusters of a few to many tens of individuals embedded in a diffuse organic matrix and surrounded by a colony-defining envelope. Eoaphanocapsa, regarded as a fossil analogue of the modern chroococcacean Aphanocapsa, provides a useful form genus for colonies of multilamellated spheroidal cells that lack evidence of a cell division pattern like that known for such fossil chroococcaceans as Gloeodiniopsis. Like colonies of modern Aphanocapsa, those of fossil Eoaphanocapsa typically occur as benthic components of mat-building microbial communities dominated by filamentous cyanobacteria, not only in the Chulaktau cherts but also in those of the Proterozoic Min’yar and Sukhaya Tunguska formations (Nyberg and Schopf, 1984; Sergeev, 2006). One additional coccoidal colonial Chulaktau taxon, Synodophycus sp. (Fig. 14.7–14.9), deserves mention. Fossils of
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