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Late Precambrian Microfossils: a New Stromatolitic Biota from Boorthanna, South Australia
Abstract
OF the four dozen fossiliferous Precambrian sediments now known1, only a very few contain diverse microfossil assemblages preserved in situ on which inferences of ecologic setting and evolutionary status can be based reliably. The time gaps between these deposits are enormous, commonly approaching or exceeding a duration equivalent to that of the entire Palaeozoic. Present inferences regarding the course of early evolutionary development are therefore highly speculative. There is obvious need for the detection of additional evidence with which to evaluate such inferences and to fill in the skeletal picture that has emerged in recent years. We report here the discovery of a well preserved, Stromatolitic microbiota from the late Precambrian of Boorthanna, South Australia, that should contribute measurably toward this goal.
... Scissilisphaera sp. belonging to family Pleurocapsaceae Schopf and Fairchild (1973) reported unnamed forms with few similar morphological features and arc comparable to extant cyanobacterial form Myxosacrcina. Size of cells is smaller in comparison to specimens reported by Schopf and Fairchild (1973). Typical mode of occurrence in rosette or radially arranged cells is the diagnostic feature of this form and assignable to genus Scissilisphaera. ...
... Scissilisphaera sp. belonging to family Pleurocapsaceae Schopf and Fairchild (1973) reported unnamed forms with few similar morphological features and arc comparable to extant cyanobacterial form Myxosacrcina. Size of cells is smaller in comparison to specimens reported by Schopf and Fairchild (1973). Typical mode of occurrence in rosette or radially arranged cells is the diagnostic feature of this form and assignable to genus Scissilisphaera. ...
... Discussion- Schopf and Fairchild (1973) described from the Skillogalee Dolomite, South Australia an unnamed form, which was compared with the living cyanobacterial form Merismopedia. In dimensions, the Deoban microfossils fall in range of Sphaerophycus parvWll (Schopf. ...
Forty-five additional microfossils are being described from the Meso-Neoproterozoic Deoban Limestone (Formation). Microfossils are preserved in black bedded chert, occurring as thin lenses and bands intercalated with dolostones. The assemblage comprises both filamentous as well as coccoid forms. The assemblage is dominated by cyanobacterial population along with a number of microfossils belonging to other affinities represented by bacterial, algal, fungal and acritarchean forms. Out of the forty-five forms there are forty-four species belonging to 33 genera and one micro-organism is informally described as Form 'A'. Five taxa are described as new genera. Additional Deoban forms are as follows: Cyanobacteria- Archaeoellipsoides minor, A. major, Palaeopleurocapsa sp., Scissilisphaera gradata, Scissilisphaera sp., Coniunctiophycus gaoyuzhuangense, Archaeophycus sp., Gloeodiniopsis lamellosa, Palaeomerismopedia misrai gen. & sp. nov., Siphonophycus septatum, S. robustum, S. solidum, S. typicum, Polytrichoides lineatus, Oscillatoriopsis amadeus, O. obtusa, O. breviconvexa, Rhicnonema antiquum, Nostocomorpha sp., Eomicrocoleus crassus, Cyanonema sp., Palaeolyngbya catenata, Circumvaginalis sp., Obruchevella parva, O. valdaica, O. minor and Glomovertella glomerata. Incertae sedis- Paleosphaeridium zonale, Germinosphaera sp., Myxococcoides chlorelloides, Myxococcoides stragulescence, Palaeococcus indicus gen. & sp. nov., Dumbellina deobanensis gen. & sp. nov., Maithea indica gen. & sp. nov., Eophycomyces herkoides, Bulgenia septata gen. & sp. nov. and Form 'A'. Acritarchs- cf. Cymatiosphaeroides sp., Micrhystridium sp ., Favosphaeridium favosum, Trachysphaeridium sp., Leiosphaeridia crassa, L. jacutica, Caudosphaera sp. and cf. Ovulum saccatum. The Deoban Microfossil Assemblage is characterised by dominance of mat building cyanobacterial population, exhibiting evolutionary conservatism. Extensive size variation from specimens of less than one micron to 265 microns in case of coccoids and from less than one micron to 48 microns among filamentous forms indicate the most favourable preservational conditions for silicification of the biota. There are a number of forms, which show some resemblance with extant chlorophycean, bacterial and rhodophycean forms. Presence of large sized sphaeromorphs ranging in diameter from 105 to 265 µm and rare occurrence of acanthomorph or spinose acritarch (represented by Micrhystridium sp.) and small-sized, moderately developed spirally coiled filaments of Obruchevella support a pre-Vendian age to the Deoban microfossil assemblage.
... The pseudofilamentous arrangement of some spheroids (pI. 1: c) is similar to living pleurocapsalean cyanobacteria (Waterbury and Stanier 1978) and Precambrian microfossils interpreted as representatives of that group of cyanobacteria (Schopf and Fairchild 1973;Knoll et aI. 1975). ...
... They are usually interpreted as remnants of coccoid cyanobacteria. Many have been found in association with stromatolites, often as their integral components (Hofmann 1975;Schopf and Sovietov 1976). The most instructive examples are clusters of unicells described as Myxococcoides minor Schopf from the Upper Proterozoic Bitter Springs Formation of Australia (Schopf 1968) and ...
... Many have been found in association with stromatolites, often as their integral components (Hofmann 1975;Schopf and Sovietov 1976). The most instructive examples are clusters of unicells described as Myxococcoides minor Schopf from the Upper Proterozoic Bitter Springs Formation of Australia (Schopf 1968) and ...
Stromatoporoid stromatolites; new insight into evolution of cyanobacteria. Acta Palaeont. Polonica, 25, 2, 243-251 July, 1980. Common enigmatic fossils called stromatoporoids are recognized as calcareous stromatolitic structures build by coccoid cyanobacteria (= Cyanophyta). The diversified internal structures of stromatoporoids reflect various growth patterns of cyanobacterial cell aggregates or colonies preserved due to a rapid in situ calcification. Stromatoporoid stromatolites are evolutionary advanced descendants of early precamprian stromatolites generated by weakly differentiated Rtratiform mats of coccoid cyanobacteria. The presence of stromatoporoid stromatolites in ancient subtidal environments, often in association with normal marine biota, is a non-actualistic phenomenon which needs to be explained in other than present-day ecological terms.
... The pseudofilamentous arrangement of some spheroids (pI. 1: c) is similar to living pleurocapsalean cyanobacteria (Waterbury and Stanier 1978) and Precambrian microfossils interpreted as representatives of that group of cyanobacteria (Schopf and Fairchild 1973;Knoll et aI. 1975). ...
... They are usually interpreted as remnants of coccoid cyanobacteria. Many have been found in association with stromatolites, often as their integral components (Hofmann 1975;Schopf and Sovietov 1976 (Muir 1976;Oehler 1977). ...
A simple technique is described enabling the identification in scanning electron micrographs (SEM) of combined Iractured and etched stromatoporoid sections, the cyanobacteria-hkc coccoid aggregates building the entire stromatoporoid structure. The coccoid aggregates from stromatoporoids are closely related to extant calcified coccoid cyanobacteria (Plcurocapsales) forming stromalolitic mats in Laguna Mormona (Baja California) and Sabkha Gavish (Sinai). Stromatoporoids, stromatolites, cyanobacteria.
... From a paleontological perspective, much of our knowledge of early life on Earth has been gleaned from bacterial fossils in Precambrian cherts (Schopf and Fairchild, 1973;Schopf and Packer, 1987;Westall et al., 1995). Some of these cherts are thought to have originated as thermal deposits analogous to the modern hot spring systems in Yellowstone National Park, Wyoming (Walter, 1976a). ...
... This affinity for silica is important for mineralization, and has obvious significance for their long-term preservation potential as fossils. Exquisitely preserved, bacterial filaments have been recognized in Precambrian cherts (Schopf and Fairchild, 1973;Schopf and Packer, 1987;Westall et al., 1995), and may owe their preservation to this propensity to silicify. ...
Numerous siliceous hot spring systems in the Norris and Lower Geyser Basins of Yellowstone National Park, Wyoming, provide insights into spring geometries, depositional facies, and lithofacies associated with modern hot springs. Analyses of active (Cistern Spring, Octopus Spring, Deerbone Spring, and Spindle Geyser) and inactive (Pork Chop Geyser) siliceous hot springs have facilitated the construction of a facies model for siliceous hot spring deposits at Yellowstone.Yellowstone's siliceous springs tend to group into four broad morphological categories: siliceous spires and cones, domal mounds, terraced mounds, and ponds. Siliceous spires/cones are subconical accumulations up to 5–7 m high and about 2 m in diameter, and are common deposits in Yellowstone Lake. Domal mounds are characterized by siliceous precipitates with a broad lens or shield geometry (2–3 m in vertical relief), discharge channels, and an areal accumulation of approximately 150 m2. In contrast, terraced mounds have a stair-step morphology, a substantial pool (∼8–10 m in diameter), “shrubby” precipitates, and occupy areas of ∼2000 m2. Siliceous ponds are variable in size, have little outflow, and exhibit low amounts of silica precipitation. Of these morphological varieties, domal mounds and terraced mounds are thought to have the best long-term preservation potential.The four spring morphotypes are composed of up to eight cumulative hot spring depositional facies: (1) vent (>95 °C), (2) proximal vent (<95 °C), (3) pool (∼80–90 °C), (4) pool margin (∼80 °C), (5) pool eddy (<80 °C), (6) discharge channel/flowpath (<80 °C to ambient), (7) debris apron (variable temperatures), and (8) geyser (variable temperatures). This facies model based on numerous springs facilitates our ability to interpret ancient hot spring deposits and to infer depositional conditions.Precipitation of siliceous sinter is the result of abiotic and biotic processes. Abiotic precipitational processes are dominant in the vent area, whereas biotic influences on the precipitate fabric become progressively more important downflow.
... We are aware of three Meso-and Neoproterozoic (~1.6-0.65 Ga) microfossils that have been compared to Merismopedia. One of them, from siliceous crusts of the Yenisej Range in eastern Siberia, has been named Distichococcus minutus Golovenoc et Belova (1985) and compared to M. glauca (Schopf 1992(Schopf : p. 1147, while the second is an unnamed form reported by Schopf & Fairchild (1973) from the Boorthanna cherts of South Africa. The third fossil, Tenuocharta cloudii Horodyski et Mankiewicz (1990), comes from the Pahrump Group, Kingston Range, in southeastern California, USA. ...
Rhyniococcus uniformis from the Lower Devonian Rhynie and Windyfield cherts, Scotland, occurs in the form of unistratose, plate-like colonies comprised of (8–)16–>250 cell units (3–5 μm in diameter) situated in rows more or less perpendicular to one another; quadruplets and 16-cell unit clusters are often recognizable as modular units. The fossil resembles extant cyanobacteria in the genus Merismopedia (Synechococcales), but material sufficient for a thorough assessment of R. uniformis has hitherto not been available. Newly discovered specimens provide detailed insights into the morphology and colony organization of R. uniformis, together with specific developmental details, and thus now permit a precise characterization of this fossil that underpins the structural similarity to Merismopedia. Merismopedia-like life forms are exceedingly rare as fossils, likely because the colonies are readily fragmented and destroyed. The new specimens of R. uniformis all occur within small inclusions in silicified substrate, suggesting that the substrate served as a microscopic conservation trap by shielding the enclosed colonies from destructive mechanical forces such as water movement. © 2018 J. Cramer in Gebrüder Borntraeger Verlagsbuchhandlung, Stuttgart, Germany.
... All in transmitted plane light The aggregates of spheroiUs are morphologically similar to some coccoid cyanophytes. The pseudofilamentous arrangements of some spheroids are particularly similar to extant pleurocapsalean blue-green algae (e.g., Geitler 1925Geitler , 1942Beck 1963;Bourelly 1972) and Precambrian microfossils ascribed to that group of cyanophyceans (Schopf and Fairchild 1973;Knoll et a1. 1975). ...
Stromatoporoids are calcareous fossils common in Early Paleozoic (?Cambrian — Lower Carboniferous) shallow-water carbonate deposits. They have been regarded as remnants of enigmatic organisms possibly related to hydrozoans or sponges (e.g., Galloway 1957, for review). The recent attempts of Hartman (1978), Hartman and Goreau (1966, 1970) and others to homologize stromatoporoids with living sclerosponges are basically ill-founded, since instead of proper stromatoporoids (Early Paleozoic) the authors used as comparative material dubious Late Paleozoic and Mesozoic fossils somewhat resembling stromatoporoids in gross morphology and perhaps being sponges.
... Bac te rial iron encrustations have been de scribed in many Phanerozoic suc ces sions, es pe cially oc cur ring in con junction with red sed i men tary strata (e.g., Harder, 1919;Schopf and Fairchild, 1973;Karkhanis, 1976;Awramik et al., 1983;Mamet et al., 1997;Préat et al., 1999Préat et al., , 2000Préat et al., , 2006Boulvain et al., 2001;Yongding et al., 2004;Fortin and Langley, 2005;Mameth and Préat, 2006;Ma son, 2008; Barale et al., 2013). ...
The Albian and Cenomanian marine sediments of the Silesian and Tatric basins in the Carpathian realm of the Western Tethys contain ferric and ferromanganese oxyhydroxides, visible macroscopically as brown stainings. They coat calcareous bioclasts and mineral (lasts, fill pore spaces, or locally form continuous, parallel microlayers, tens of micrometers thick. Light-microscope (LM) and scanning-electron-microscope (SEM) observations show that the coatings contain elongated capsules, approximately 3-5 mu m across and enriched in iron and manganese, which may be remnants of the original sheaths of iron-related bacteria (IRB). Moreover, the ferric and ferromanganese staining observed under LM is similar to bacterial structures, resembling the sheaths, filaments and rods formed by present-day bacteria of the Sphaerotilus Leptothrix group. All of the possible bacteria-like structures are well preserved owing to processes of early diagenetic cementation, if the observed structures are fossil IRB, these organisms could have played an important role in iron and manganese accumulation on the sea floor during Albian Cenomanian time. The most plausible source of metals for bacterial concentration in the Silesian Basin might have been submarine low-temperature hydrothermal vents, as previously was hypothesized for Cenomanian-Turonian deposits on the basis of geochemical indices.
... Bac te rial iron encrustations have been de scribed in many Phanerozoic suc ces sions, es pe cially oc cur ring in con junction with red sed i men tary strata (e.g., Harder, 1919;Schopf and Fairchild, 1973;Karkhanis, 1976;Awramik et al., 1983;Mamet et al., 1997;Préat et al., 1999Préat et al., , 2000Préat et al., , 2006Boulvain et al., 2001;Yongding et al., 2004;Fortin and Langley, 2005;Mameth and Préat, 2006;Ma son, 2008; Barale et al., 2013). ...
The Albian and Cenomanian marine sediments of the Silesian and Tatric basins in the Carpathian realm of the Western Tethys contain ferric and ferromanganese oxyhydroxides, visible macro scopically as brown stainings. They coat calcareous bioclasts and mineral clasts, fill pore spaces, or locally form continuous, parallel microlayers, tens of micrometers thick. Light-microscope (LM) and scanning-electron-microscope (SEM) observations show that the coatings contain elongated capsules, approximately 3-5 μm across and enriched in iron and manganese, which may be remnants of the original sheaths of iron-related bacteria (IRB). Moreover, the ferric and ferromanganese staining observed under LM is similar to bacterial structures, resembling the sheaths, filaments and rods formed by present-day bacteria of the Sphaerotilus-Leptothrix group. All of the possible bacteria-like structures are well preserved owtng to processes of early diagenetic cementation. If the observed structures are fossil IRB, these organisms could have played an important role in iron and manganese accumulation on the sea floor durtng Albian-Cenomanian time. The most plausible source of metals for bacterial concentration in the Silesian Basin might have been submarine low-temperature hydrothermal vents, as previously was hypothesized for Cenomanian-Turonian deposits on the basis of geochemical indices.
... Thomas R. Fairchild, professor at the Institute of Geosciences of USP, has been developing an extensive study of the Brazilian paleobiological record. Former PhD student of J. William Schopf at the University of California at Los Angeles studying stromatolites (Schopf & Fairchil 1973; Fairchild & Schopf 1974), Fairchild has been working with the fossil record, mostly from the Precambrian era, in Brazil and elsewhere (Fairchild & Subacius 1986; Fairchild et al. 1996; Yamamoto et al. 2005). The potential of the Brazilian microbial diversity and life in extreme environments have been traditionally explored for biotechnological applications (Cruz et al. 1988; Tosi et al. 1993; Almeida & de Franca 1999). ...
This review reports the Brazilian history in astrobiology, as well as the first delineation of a vision
of the future development of the field in the country, exploring its abundant biodiversity, highly capable
human resources and state-of-the-art facilities, reflecting the last few years of stable governmental
investments in science, technology and education, all conditions providing good perspectives on continued
and steadily growing funding for astrobiology-related research. Brazil is growing steadily and fast in terms of
its worldwide economic power, an effect being reflected in different areas of the Brazilian society, including
industry, technology, education, social care and scientific production. In the field of astrobiology, the
country has had some important landmarks, more intensely after the First Brazilian Workshop on
Astrobiology in 2006. The history of astrobiology in Brazil, however, is not so recent and had its first
occurrence in 1958. Since then, researchers carried out many individual initiatives across the country in
astrobiology-related fields, resulting in an ever growing and expressive scientific production. The number of
publications, including articles and theses, has particularly increased in the last decade, but still counting
with the effort of researchers working individually. That scenario started to change in 2009, when a formal
group of Brazilian researchers working with astrobiology was organized, aiming at congregating the
scientific community interested in the subject and to promote the necessary interactions to achieve a
multidisciplinary work, receiving facilities and funding from the University de Sao Paulo and other funding
agencies.
... The petrifaction of microfloras within peritidal cherts is common in these Meso-to Neoproterozoic cherts, such as the Boorthanna Chert of Western Australia (Fig. 2b) or the Bitter Springs Chert of central Australia (e.g. Barghoorn and Tyler 1965;Schopf 1968;Schopf and Fairchild 1973;Schopf and Klein 1992). Such cherts seemingly acted as 'traps' that show a taphonomic bias towards small organic structures including cellulose cell walls, mucilaginous sheaths, and possibly even subcellular structures (Oehler 1977) or molecular biomarkers (Hod et al. 1999). ...
A unifying model is presented that explains most of the major changes seen in fossil preservation and redox conditions across
the Precambrian–Cambrian transition. It is proposed that the quality of cellular and tissue preservation in Proterozoic and
Cambrian sediments is much higher than it is in more recent marine deposits. Remarkable preservation of cells and soft tissues
occurs in Neoproterozoic to Cambrian cherts, phosphates, black shales, siliciclastic sediments and carbonates across a wide
range of environmental conditions. The conditions for remarkable preservation were progressively restricted to more marginal
environments through time, such as those now found in stagnant lakes or beneath upwelling zones. These paradoxes can no longer
be adequately explained by recourse to a series of ad hoc explanations, such as those involving unusually tough organic matter
in the Ediacaran, or unusual seawater chemistry, or even the role of microbial biofilms alone. That is because the exceptions
to these are now too many. Instead, we suggest that elevated pore water ion concentrations, coupled with the almost complete
lack of infaunal bioturbation, and hence the lack of a sediment Mixed-layer, provided an ideal environment for microbially-mediated
ionic concentrations at or near the sediment–water interface. These strong ionic gradients encouraged early cementation and
lithification of sediments, often prior to complete decomposition of delicate organic structures. Seen in this way, not only
did the biosphere evolve across the Precambrian–Cambrian transition. Fossilization itself has evolved through time, and never
more dramatically so than across this interval.
... c: An unnamed cylindrical filament, probably the degraded trichome of an oscillatoriacean cyanobacterium (Min'yar Formation, 740 Ma old, central Russia) Nyberg and Schopf, 1984). d: An unnamed cylindrical filament, probably the degraded trichome of an oscillatoriacean cyanobacterium (Skillogalee Dolomite, 770 Ma old, South Australia) ( Schopf and Fairchild, 1973;Schopf, 1977;Preiss, 1987;Schopf, 1992c). e: Polybessurus bipartitus, the asymmetrically laminated stalk secreted by a Solentia-like pleurocapsacean cyanobacterium (River Wakefield Subgroup, 775 Ma old, South Australia) (Schopf, J.W., 1975(Schopf, J.W., , 1977(Schopf, J.W., , 1992c. ...
Laser-Raman imagery is a non-intrusive, non-destructive analytical technique, recently introduced to Precambrian paleobiology, that can be used to demonstrate a one-to-one spatial correlation between the optically discernible morphology and kerogenous composition of permineralized fossil microorganisms. Made possible by the submicron-scale resolution of the technique and its high sensitivity to the Raman signal of carbonaceous matter, such analyses can be used to determine the chemical-structural characteristics of organic-walled microfossils and associated sapropelic carbonaceous matter in acid-resistant residues and petrographic thin sections. Here we use this technique to analyze kerogenous microscopic fossils and associated carbonaceous sapropel permineralized in 22 unmetamorphosed or little-metamorphosed fine-grained chert units ranging from approximately 400 to approximately 2,100 Ma old. The lineshapes of the Raman spectra acquired vary systematically with five indices of organic geochemical maturation: (1) the mineral-based metamorphic grade of the fossil-bearing units; (2) the fidelity of preservation of the fossils studied; (3) the color of the organic matter analyzed; and both the (4) H/C and (5) N/C ratios measured in particulate kerogens isolated from bulk samples of the fossil-bearing cherts. Deconvolution of relevant spectra shows that those of relatively well-preserved permineralized kerogens analyzed in situ exhibit a distinctive set of Raman bands that are identifiable also in hydrated organic-walled microfossils and particulate carbonaceous matter freed from the cherts by acid maceration. These distinctive Raman bands, however, become indeterminate upon dehydration of such specimens. To compare quantitatively the variations observed among the spectra measured, we introduce the Raman Index of Preservation, an approximate measure of the geochemical maturity of the kerogens studied that is consistent both with the five indices of organic geochemical alteration and with spectra acquired from fossils experimentally heated under controlled laboratory conditions. The results reported provide new insight into the chemical-structural characteristics of ancient carbonaceous matter, the physicochemical changes that accompany organic geochemical maturation, and a new criterion to be added to the suite of evidence by which to evaluate the origin of minute fossil-like objects of possible but uncertain biogenicity.
... Comparison of the mathematical compressibility (cf.Corsetti & Storrie-Lombardie 2003) of digitally imaged assemblages of microfossil-like structures in petrographic thin sections. (Obtained from the following Precambrian sediments: ca 1900 Myr Gunflint Chert of Canada, containing clusters of possible iron bacteria filaments (slide D 3 ; seeBarghoorn & Tyler 1965;Knoll 2003); ca 800 Myr Boorthanna chert of Australia, containing colonies of coccoid cyanobacteria-like cells (cf.Schopf & Fairchild 1973) that can degrade to pseudofilaments (slide WPM 18/6; cf.Kazmierczak & Kremer 2002); ca 560 Myr Gwna Group jaspilitic cherts from Newborough Warren, Wales containing questionable microfossil-like structures (slide CG4; seeMuir et al. 1979) formed from abiogenic spherulites (figure 3j herein); ca 3460 Myr Apex Chert, Australia, containing questionable microfossil-like structures (slide V.63165; seeSchopf 1993) formed from abiogenic spherulites (figure 2H and 3g-l herein); ca 3430 Myr Strelley Pool Arenite, Australia, containing microtubes (slide SP9a; seefigure 5 ...
The rock record provides us with unique evidence for testing models as to when and where cellular life first appeared on Earth. Its study, however, requires caution. The biogenicity of stromatolites and ‘microfossils’ older than 3.0 Gyr should not be accepted without critical analysis of morphospace and context, using multiple modern techniques, plus rejection of alternative non-biological (null) hypotheses. The previous view that the co-occurrence of biology-like morphology and carbonaceous chemistry in ancient, microfossil-like objects is a presumptive indicator of biogenicity is not enough. As with the famous Martian microfossils, we need to ask not ‘what do these structures remind us of?’, but ‘what are these structures?’ Earth's oldest putative ‘microfossil’ assemblages within 3.4–3.5 Gyr carbonaceous cherts, such as the Apex Chert, are likewise self-organizing structures that do not pass tests for biogenicity.
There is a preservational paradox in the fossil record prior to ca 2.7 Gyr: suitable rocks (e.g. isotopically light carbonaceous cherts) are widely present, but signals of life are enigmatic and hard to decipher. One new approach includes detailed mapping of well-preserved sandstone grains in the ca 3.4 Gyr Strelley Pool Chert. These can contain endolithic microtubes showing syngenicity, grain selectivity and several levels of geochemical processing. Preliminary studies invite comparison with a class of ambient inclusion trails of putative microbial origin and with the activities of modern anaerobic proteobacteria and volcanic glass euendoliths.
A well-preserved microbiota consisting of filamentous Cyanobacteria, viz., Oscillatoriopsis, Cyanonema, Siphonophycus, Eomycetopsis, Gunflintia and Animikiea; spheroidal unicells, viz., Glenobotrydion, Globophycus, Sphaerophycus and Myxococcoides; Eubacteria, viz., Archaeotrichion, Biocatenoides; and acritarch (?plankton) Kildinosphaera, is described from petrographic thin sections of cherts from the Deoban Formation, Garhwal Lesser Himalaya. The assemblage has been compared with other authentic Proterozoic records. The palaeomicrobial community is interpreted to have inhabited a protected shallow intertidal environment.
Most of our understanding of evolutionary events before the appearance of Metaphyta and Metazoa is based on the detailed study of microfossils found in cherts. These cherts, though rare in pre-Phanerozoic sediments, are yielding a rich and diverse microbiota from which a story of increased diversity and morphologic complexity with time is unfolding. Very little has been added to this understanding using the other great class of pre-Phanerozoic fossils–the stromatolites. Stromatolites are organosedimentary macrostructures produced by sediment trapping, binding, and precipitation activity of microorganisms–predominantly blue-green algae.
The vast majority of stromatolites found in the pre-Phanerozoic is found preserved as carbonates, rarely as cherts. Consequently, due to the low preservation potential of nonskeletal microorganisms in carbonates, most stromatolites are barren of microfossils. What kind of biological information can we obtain from stromatolites that will add to our understanding of the pre-Phanerozoic?
This chapter focuses on the Precambrian paleobiology of Australia. The Precambrian fossils of Australia are remarkably diverse, abundant, and well preserved. So far the Archaean has yielded only carbonaceous microspheroids of uncertain origin. Sediments of this age are well exposed in the Pilbara region of Western Australia. In Australia, the undisputed microfossil record commences with the Early Proterozoic. Cyanophytes are the main constituents of the Early Proterozoic microfossil record, although bacteria are also abundantly represented. The bacteria are diverse and there are fossils the biological affinities of which are obscure. The microfossil assemblages of the 1500–1550 Ma-old McArthur Group and its correlatives are the earliest in Australia to contain forms which have been considered to be eucaryotes, but only bacteria and cyanophytes can be confidently identified. Because of its age, the Roper Group fossil assemblage is critically significant in this regard. No significant microfossils are known from Australia in the interval of 950–1300 Ma. In the 800–900 Ma-old Bitter Springs Formation and Skillogalee Dolomite and their correlatives, there are two of the most important microfossil assemblages: (1) a wide range of cyanophytes, and (2) the presence of eucaryotes and probably also of sexual reproduction. Increasing attention to suitable rocks of this age may well produce evidence of the earliest metazoans. It is not until the latest Precambrian, in rocks 640–570 Ma old, that the first undisputed metazoan body fossils are found. They are large and diverse, and include representatives of at least three phyla—namely, Coelenterata, Annelida and Arthropoda.
This chapter focuses on the geology of Gawler Province and the Stuart Shelf and Adelaide Geosyncline of South Australia. The principal outcrop of the Gawler Province is in the Gawler orogenic domain. Although the area of basement rock is extensive, it is generally of low relief and commonly blanketed by a thin superficial cover. Thus the geology is relatively well known only in the area of higher relief between the Gawler Range Volcanics and the NW shore of Spencer Gulf. Recent work has shown that Archaean gneisses, forming the basement to the Early Proterozoic metasedimentary sequences, are widespread in the less well-exposed areas in the W and NW of the domain. Some of the granitic gneisses of the Lincoln Complex may represent reworked Archaean gneisses. In the southern part of the Musgrave domain, overprinting by later Musgravian events is weak and therefore this part is also included within the Gawler Province. Early Proterozoic rocks of the Gawler Province also reappear in a major inlier—the Willyama orogenic domain—east of the main part of the Delamerian fold belt. This domain is divided by an area of relatively poor outcrop into the Olary subdomain in South Australia and the Broken Hill subdomain in New South Wales.
It is generally agreed that the first organisms were prokaryotic in nature. However, opinions differ as to the mechanism responsible for the origin of the eukaryotic cell – whether it arose by direct filiation from an ancestral blue‐green alga or by a series of symbioses.
The timing of the emergence of photosynthesis in eukaryotes has largely been based on evidence from Precambrian palaeontology. For this reason attention is drawn to recent reassessments of the Precambrian fossil record, which have suggested that a number of structures previously considered to be remnants of eukaryotic nuclei and organelles may instead have been artifacts that formed in certain blue‐green algae during their lithification.
Evidence regarding the mechanism responsible for the origin of chloroplasts may be sought in the molecular fossil record. Studies employing the techniques of DNA‐RNA hybridization and RNA sequencing (oligonucleotide cataloguing) have established the prokaryotic nature of chloroplast ribosomal RNAs. In addition, a close phylogenetic relationship between the blue‐green alga Aphanocapsa 6308 and the chloroplasts of Euglena gracilis has been indicated. By contrast, the sequence resemblance of prokaryotic and chloroplast 16 S rRNAs to eukaryotic 18 S rRNAs is revealed to be no greater than that expected between completely unrelated molecules. However, the size differences which exist between 16 S and 18 S rRNAs raise doubts as to whether these RNA species are in fact directly comparable. On the other hand, all known examples of mature 5 S rRNA are approximately 120 nucleotides in length, and thus this RNA would appear to bridge the evolutionary discontinuity between prokaryotes and eukaryotes in an almost unambiguous manner. The identification of 5 S rRNA in chloroplast ribosomes has therefore prompted a number of investigations designed to establish the prokaryotic or eukaryotic affinities of this RNA species. These have confirmed the prokaryotic nature of chloroplast 5 S rRNAs, and have indicated the sequence dissimilarity of these species to the cytoplasmic 5 S rRNAs of eukaryotes. However, it is pointed out that such results can only provide conclusive support for the endosymbiotic hypothesis, if rates of change in homologous RNAs have remained approximately constant in all lines.
Combined evidence suggests that nucleotide substitutions in cytoplasmic ribosomal RNAs have occurred more frequently during the evolution of flowering plants, Gramineae in particular, than during the evolution of yeast or vertebrates. By contrast, chloroplast ribosomal RNAs are shown to be very strongly conserved. Moreover, considerable disparities are also noted in the relative amounts of change in prokaryotic and eukaryotic 5 S rRNAs. Although undoubtedly associated with their different architectures and functional roles, it is suggested that the primary explanation for this phenomenon lies in the fact that the cytoplasmic 5 S rRNAs of eukaryotes are not derived from that of the protoeukaryote host, but rather, from that of the protomitochondrion. The cytoplasmic j.8 rRNAs are now considered to be the extant descendants of the original protoeukaryote 5 S rRNA these species having increased in size in a similar manner to the other cytoplasmic ribosomal RNAs of eukaryotes.
Because the rates of nucleotide substitution in homologous prokaryotic and eukaryotic ribosomal RNAs are non‐identical, consideration is given to derived character states. The late methylated sequence ‐Gm2 ⁶ Am2 ⁶ AC– has been identified in the 16 S rRNAs of many prokaryotic microorganisms and may possibly be universal in the 18 S rRNAs of eukaryotes. Although its presence has been indicated in the 18 S rRNA of Euglena gracilis , it is absent from the chloroplast 16 S rRNA of this protist, as well as from the 16 S rRNAs of Aphanocapsa 6308 and many other blue‐green algae. Direct filiation would seem unable to provide a satisfactory explanation for this phenomenon. As a result it ii included that chloroplasts, and most probably mitochondria also, are of endosymbiotic origin.
Resumo
A late or middle Proterozoic microflora of narrow, tubular to septate, probably algal filaments and small-celled algal or bacterial colonies has been found in a silicified stromatolite in the lower Bambui Group on Cedral Ranch, near Sâo Domingos, Goiás, west-central Brazil. The very small size and simple morphology of the microfossils suggest an age possibly older than 1400 m.y. for the Bambui Group, an age inconsistent with other available geologic and paleobiologic data. An understanding of the biological affinities, paleoecological setting, and timing of silicification of this microflora may clarify this apparent paradox.
Micro-organisms from the drill core samples recorded from the Bushimay Supergroup (Late Pre-Cambrian) of Kanshi. Zaire are recorded. The micro-organisms consist of remains of algae fungi, acritarcha and indeterminate remains. Among the algae both filamentous and colonial forms are present. The colonial forms are either preserved in the form of globular colony or elongate colony. Many of these colonies are surrounded by a mucilage sheath. The algal remains have been assigned to 9 genera belonging to 14 species. Of these 2 genera Palaeomicrocysts and Chlorogloeaopsis are new. Only 1 genus and 3 species have been referred to fungi. The acritarch remains belong to the groups Sphaeromorphitae, Megasphaeromorphitae. Acanthomorphitae Netromorphitae and the family Tasmanaceae. The spinate forms are rare in the assemblage. The elements belonging to Sphaeromorphitae and Megasphaeromorphitae are common. The assemblage resembles, on the basis of algal and fungal remains. to the Late Pre-Cambrian flora of Bitter Spring Formation of Australia and on the basis of acritarcha remains. the Late Pre-Cambrian (Upper Riphean) acritarcha assemblage of USSR.
We describe evidence of biogenicity in the morphology and carbon content of well-preserved, Neoarchean samples of banded iron formation (BIF) from Carajás, Brazil. Silica-rich BIF layers contain translucent ellipsoidal or trapezoidal structures (∼5–10 μm diameter) composed of silica, hematite, and kerogen, which are arranged in larger ring-like forms (rosettes). Stable carbon isotope analysis yields a δ13C value of −24.5‰ indicating that the contained carbon is likely biogenic. Raman and SEM analyses, as well as wavelength-dispersive X-ray elemental maps, show kerogen inside the rosette forms. Within the iron-rich BIF layers, tubular structures (0.5–5 μm) were observed between hematite granules and blades. Kerogen and kaolinite are present in these structures. Both the rosettes and the tubular structures resemble morphologies that are characteristic of some bacterial species.
Since the mid-1960s, following a century of unrewarded search, impressive progress has been made toward deciphering the Precambrian
fossil record, evidence of life extant during the earliest seven-eighths of geologic time. Hundreds of fossiliferous units
have been discovered containing thousands of microbial fossils—dominantly but not exclusively cyanobacterial — and the documented
antiquity of life has been extended to an age roughly three-quarters that of the Earth. Mutually reinforcing lines of evidence,
paleontological, geological, and isotopic geochemical, indicate that stromatoliticmicrobial ecosystems,evidently including
cyanobacteria and other members of the bacterial domain, were extant ~3500 Ma ago; methanogenic archaeans by ~2800 Ma ago;
and Gram-negative sulfate-reducing bacteria at least as early as ~2700 Ma ago.The discrepancy between these dates and those
suggested for emergence of these groups by a recently proposed amino acid-based “molecular clock” is too great and too consistent
to be ignored. The challengeis to unify the molecular data with the increasingly well-established paleobiologicrecord.
Small, partly silicified, laterally linked stromatolites exhibiting a high degree of inheritance in the shape of successively stacked laminae from the lower Bambuí Group (late Proterozoic), near Unaí, Minas Gerais, south-central Brazil, are here described and classified as Stratifera undata Komar 1966. Rare, moderately preserved microfossils and stratiform concentrations of poorly preserved probable microfossils consist essentially of coccoidal forms, there being no convincing evidence of filamentous forms. If, in fact, coccoidal micro-organisms were the overwhelmingly dominant mat-formers, this represents an unusual situation among silicified microflorules from morphologically distinct Precambrian stromatolites yet is quite similar to that observed in another form of Stratifera (S. biwabikensis from the Gunflint Iron-Formation), which, like the Unaí stromatolites, also formed under permanently submerged conditions. S. undata is now known in Brazil from similar upper Proterozoic settings in two very widely separated localities, which suggests its potential use in regional stratigraphic correlation. As a result of this study, we recommend that greater attention be paid to the ‘simpler’ stromatolite morphologies, especially when micro fossiliferous, since such forms commonly comprise the basal portions of biostratigraphically significant stromatolites and may share a common microstructure with these same, more complex forms. Inasmuch as microstructure is generally acknowledged as the stromatolite property most closely controlled by biological factors, study of silicified microflorules within simpler forms may permit inferences regarding not only the microbial communities responsible for more complex stromatolites having the same microstructure but also possible biological reasons for the succession of distinct stromatolite assemblages observed in upper Precambrian rocks.
Microfossils have been discovered in thin sections cut from a sample of black, laminated, nodular chert collected from the Neohelikian Dismal Lakes Group of the Dismal Lakes area, District of Mackenzie. Two distinct morphological forms are differentiated: filaments and spheroids. The filaments are further subdivided, on the basis of morphology, into four basic varieties: (1) septate, unbranched, uniseriate filaments composed of chains of spheroidal to ellipsoidal cells showing marked constriction between cells, (2) septate, unbranched, uniseriate filaments characteristically lacking constrictions between cells, (3) nonseptate, unbranched filaments of large diameter (> 1.8 μm), (4) nonseptate, unbranched filaments of small diameter (< 1.3 μm). The spheroidal structures occur singly, in pairs and in clusters, and some of them contain blebs of carbonaceous matter which may represent remnants of organelles, reflecting eucaryotic affinities.
The question of Cyanobacteria in the Apex chert of Australia is
addressed and the results of indepth microscopy, isotope and elemental
analysis as well as detailed geological mapping are presented. The
microfossils seen in this formation are shown to be pseudofossils.
Nineteen species and eight genera of microfossils in the Amelia Dolomite microflora (ca 1.5 × 109 years old) are described and named. Of these, two genera (Ameliaphycus, Myxomorpha) and eleven species (Gunflintia oehlerae, Huroniospora ornata, Sphaerophycus reticulatum, S. tetragonalis, Myxococcoides reniformis, M. minutus, M. kingii, M. konzalovae, Palaeoanacystis plumbii, Ameliaphycus croxfordii, Myxomorpha janecekii) are new. Most of the micro-fossils are interpreted as being the remains of blue-green algae and the assemblage is compared with older and younger microfloras, as well as the Paradise Creek and Bungle Bungle Dolomite assemblages which are of similar age. In addition, mucilaginous moulds of both coccoid and filamentous micro-organisms are described, and some indirect evidence for the activity of sulphate reducing bacteria is presented.
Diverse, cellularly preserved microbial communities are now known from stromatolitic sediments of at least twenty-eight Precambrian formations. These fossiliferous deposits, principally cherts and cherty portions of carbonate units, range in age from Early Proterozoic (Transvaal Dolomite, ca. 2250 Ma old) to Vendian (Chichkan Formation, ca. 650 Ma old) and include units from Australia, India, Canada, South Africa, Greenland, the United States and the Soviet Union. More than three-quarters of these microbiotas have been discovered since 1970. Although few, therefore, have as yet been studied in detail, virtually all of the assemblages are known to be dominated by prokaryotic (bacterial and blue-green algal) microorganisms and to contain three major categories of microfossils: spheroidal unicells, cylindrical tube-like sheaths, and cellular trichomic filaments. Analyses of data now available (including measurements of more than 7800 fossil unicells) indicate that each of these three types of microfossils exhibited a gradual, but marked, increase in mean diameter and size range during the Proterozoic and that taxonomic diversity apparently also increased, especially beginning about 1400 Ma ago. Thus, it now seems evident that (i) the microbial components of Proterozoic stomatolitic assemblages have varied systematically as a function of geologic age and that (ii) such communities are both more abundant and more widespread than had previously been recognized. These observations augur well for the future use of such assemblages in Precambrian biostratigraphy. At present, however, data are sufficient to warrant the provisional establishment of only a few microfossil-based subdivisions of the Proterozoic. Such zones, necessarily relatively long-ranging, are here tentatively defined; it is of interest to note that boundaries between certain of these microfossil-based subdivisions appear to coincide, at least approximately, with previously suggested stromatolite-based boundaries. To some extent, therefore, results of this study seem consistent with, and may be supportive of, the concept of stromatolite-based biostratigraphy. At the same time, however, the study seems to indicate that stromatolites of markedly differing age, whether of similar or of dissimilar morphology, were probably formed by distinctly differing microbiotas. Data are as yet insufficient to indicate whether differing types of coetaneous, stratigraphically useful, stromatolites were formed by differing microbial communities and two what extent the “evolution” of stromatolite morphology was a result of the biologic evolution of stromatolite-building microorganisms. There is thus continued need for investigation of the potential biostratigraphic usefulness of stromatolitic microbiotas and, especially, for more effective integration of results of such studies with those available from studies of stromatolites without preserved microbiotas and from studies of the acritarchs preserved in Proterozoic shales.
During the past decade, important strides have been made toward deciphering the paleobiology of the Precambrian Eon, the earliest seven-eighths of Earth history. This progress has accured chiefly from micropaleontological and organic geochemical studies of fine-grained, ancient cherts. Although understanding of the early biota—of its composition, diversity, paleocology and evolution—still remains far from adequate, three particularly significant generalizations have emerged: (i) Living systems were extant earlier than about 3000 m.y. ago; (ii) between about 3000 and 1000 m.y. ago, the Earth''s biota was dominated by prokaryotic blue-green algae; and (iii) the development of the nucleated, eukaryotic cell type somewhat earlier than 1000 m.y. ago led to a stage of rapid diversification that culminated with the appearance of megascopic life near the close of the Precambrian. Consideration of these generalizations, and of the evidence bearing on them provides a state-of-the-art assessment of the current status of Precambrian paleobiology.
The performance of an autonomous mobile robot in acquiring a meaningful spatial model of its operating environment depends greatly on the accuracy of its perceptual capabilities. A key challenge in the field arises from handling sensor measurements and in particular the handling of measurement noise. The current prevailing approach used when attempting to deal with the problem of noisy readings is to tune the sensory model to explicitly accommodate this sensory noise. An alternative approach, however, is to consider the issue of noisy readings as a separate problem and address accordingly. In this paper such an approach is outlined. Experimental evaluation illustrate the associated improvement in map quality across a number of established paradigms.
The diversity of the Australian Precambrian record is used to empirically assess the application of all available techniques of correlation of late Precambrian rocks — lithofacies, isotopic ages, palaeontology, and palaeomagnetism — within the framework of latest lithostratigraphic/geochronologic correlation charts.Lithostratigraphy is the main tool for intrabasinal correlation and mapping, but isotopic ages will continue for some time yet to provide the main criteria for interregional correlation.Lithostratigraphic correlation is demonstrated over hundreds of kilometres, but is generally constrained by tectonic controls to individual major crustal blocks. Climatic markers, such as tillites, may be correlated interregionally over thousands of kilometres.The most exciting recent development in platform geochronology is the application of U—Pb zircon dating to fine-grained tuffs. Illite may provide reliable minimum estimates, but can be shown to be up to 150 Ma too young. Glauconite and Rb—Sr total-rock shale ages have provided useful broad age estimates of sequences for which better methods are unavailable, but only when based on cautious assessment and multiple sampling; their ages may be either minimum or maximum.Stromatolites continue to demonstrate their potential for broad correlation when based on rigorous taxonomic study, but the framework of time ranges of known taxa must be expanded for stromatolites to achieve their full potential, especially for rocks older than 1000 Ma.Acritarchs in black shales provide diverse and complex assemblages, with excellent correlation potential, but once again the Australian taxonomic framework is still inadequate. Black cherts contain an evolutionarily conservative biota, with only limited correlation potential.Recent palaeomagnetic studies, based on both APWP and magnetostratigraphy, demonstrate excellent potential for detailed correlations, down to at least 1700 Ma old. However, the present chronologically-calibrated data base is sparse and must be supplemented by many more detailed studies through thick continuous rock sequences, spanning substantial intervals of time.The value of multidisciplinary correlation is elegantly demonstrated by very detailed and unambiguous correlation of combined palaeomagnetic, isotopic age, and stromatolite data across the 1700 Ma McArthur Basin. Previous regional lithostratigraphic correlations have been conclusively and significantly revised, with major implications to regional palaeogeography.
This chapter discusses the evolutionary origin of the mitochondria. Most prokaryotes and essentially all eukaryotes are adapted to life in the presence of free oxygen. The two fundamental biochemical adaptations for such life are the possession of two enzymes, superoxide dismutase and catalase, which protect cellular components from autooxidation, and an electron transport system that allows cells to use atmospheric oxygen as a terminal electron acceptor for energy yielding biochemical oxidations. Evolutionary rates of amino acid or nucleotide substitution have been computed for several macromolecules. Such rates are generally quite constant for any particular molecular phylogeny. This has been illustrated by Dickerson, who correlated sequence changes in cytochromes from diverse organisms with absolute times of phylogenetic divergence obtained from classic paleontological data. According to the symbiotic model, eukaryotic cytoplasm should show evidence of a fundamentally anaerobic nature, since the anaerobic protoeukaryote acquired its oxygen-mediating systems from the aerobic symbiont. Mitochondria contain their own genetic system. This is composed of a specific mitochondrial DNA (mtDNA), as well as the means for its replication and, at least potentially, expression. Thus the organelles possess a DNA-dependent RNA polymerase and a system for protein synthesis, consisting of ribosomes, mRNA(s), tRNAs, aminoacyl-tRNA ligases, and the three classes of protein factors (initiation, elongation, and termination factors) required for its function.
Structures resembling remarkably preserved bacterial and cyanobacterial microfossils from about 3,465-million-year-old Apex cherts of the Warrawoona Group in Western Australia currently provide the oldest morphological evidence for life on Earth and have been taken to support an early beginning for oxygen-producing photosynthesis. Eleven species of filamentous prokaryote, distinguished by shape and geometry, have been put forward as meeting the criteria required of authentic Archaean microfossils, and contrast with other microfossils dismissed as either unreliable or unreproducible. These structures are nearly a billion years older than putative cyanobacterial biomarkers, genomic arguments for cyanobacteria, an oxygenic atmosphere and any comparably diverse suite of microfossils. Here we report new research on the type and re-collected material, involving mapping, optical and electron microscopy, digital image analysis, micro-Raman spectroscopy and other geochemical techniques. We reinterpret the purported microfossil-like structure as secondary artefacts formed from amorphous graphite within multiple generations of metalliferous hydrothermal vein chert and volcanic glass. Although there is no support for primary biological morphology, a Fischer--Tropsch-type synthesis of carbon compounds and carbon isotopic fractionation is inferred for one of the oldest known hydrothermal systems on Earth.
Unlike the familiar Phanerozoic history of life, evolution during the earlier and much longer Precambrian segment of geological time centred on prokaryotic microbes. Because such microorganisms are minute, are preserved incompletely in geological materials, and have simple morphologies that can be mimicked by nonbiological mineral microstructures, discriminating between true microbial fossils and microscopic pseudofossil 'lookalikes' can be difficult. Thus, valid identification of fossil microbes, which is essential to understanding the prokaryote-dominated, Precambrian 85% of life's history, can require more than traditional palaeontology that is focused on morphology. By combining optically discernible morphology with analyses of chemical composition, laser--Raman spectroscopic imagery of individual microscopic fossils provides a means by which to address this need. Here we apply this technique to exceptionally ancient fossil microbe-like objects, including the oldest such specimens reported from the geological record, and show that the results obtained substantiate the biological origin of the earliest cellular fossils known.
Evidence from Precambrian sediments appears to indicate that nucleated (eukaryotic) organisms had become well established and relatively diverse about 850 +/- 100 million years ago and that eukaryotes were probably extant, and may have first appeared, as early as 1400 +/- 100 million years ago.
Lyssoxylon grigsbyi Daugherty, a petrified stem with petiole bases, was originally described from the Upper Triassic Chinle Formation of Arizona and considered to be a member of the Williamsoniaceae. Investigation of additional material from a similar horizon in New Mexico, together with re-examination of preparations of the holotype, suggest that the plant, with its monoxylic stele, girdling leaf traces, and bicelled epidermal hairs is a true cycad. Cells of the New Mexico specimens contain structures interpreted as preserved nuclei.
The zonation of the Proterozoic of the USSR is reviewed and extension to other places is attempted. Preliminary results are: some of the columnar stromatolites of northern Eurasia occur in other continents and in the same stratigraphic ranges; some of the forms ('species') of the distinctive columnar types have intercontinental distribution, such as Gruneria biwabikia from Australia and North America. Pre-upper Proterozoic (pre-Riphean) strata also contain distinctive stromatolites of intercontinental distribution not known in younger strata, such as the new groups ('genera') Gruneria and Katerina. The above, together with progress in microbiota studies offers hope that at least a gross worldwide biological subdivision of Proterozoic will be practicable. The Belt Series seem to represent only one of the Soviet stromatolite zones.
The stratigraphical problem of defining the lower boundary of the Adelaide System is discussed in relation to the geology of several critical areas in the Adelaide Geosyncline and adjacent shelf‐platform.The Precambrian stratigraphical succession and geological history is outlined with the aid of Rb/Sr age‐determinations made by Dr W. Compston of the Australian National University.It is concluded that the lower boundary of the Adelaide System is related to the collapse of older basement positive areas on which a regional erosional surface had developed. This surface is defined by the Callanna Beds, the oldest deposits of Willouran age. Willouran sedimentation began some time between 1,340 m.y. and 1,490 m.y. ago. Erosion of the basement rocks probably occupied a major early part of this time interval.
Both procaryotic and eucaryotic nannofossils occur in chert from the upper Beck Spring Dolomite (Pahrump Group) in northeastern San Bernardino County, California. These fossils include the oldest reasonably certain records to date of chlorophycean and chrysophycean algae, and hence of the eucaryotic or mitosing cell. Associated with these are cyanophycean procaryotes of still more ancient affinities. Indirect radiometric evidence implies an age of 1.2 to 1.4 aeons (1.2 to 1.4 x 10(9) years) for the enclosing rocks. Associated stromatolites are consistent with such an age assignment.