RESUMEN

Laser-Raman imagery is a sensitive, noninvasive, and nondestructive technique that can be used to correlate directly chemical composition with optically discernable morphology in ancient carbonaceous fossils. By affording means to investigate the molecular makeup of specimens ranging from megascopic to microscopic, it holds promise for providing insight into aspects of organic metamorphism and biochemical evolution, and for clarifying the nature of ancient minute fossil-like objects of putative but uncertain biogenicity.

Asunto(s)

RESUMEN

In 1859, in On the Origin of Species, Darwin broached what he regarded to be the most vexing problem facing his theory of evolution-the lack of a rich fossil record predating the rise of shelly invertebrates that marks the beginning of the Cambrian Period of geologic time ( approximately 550 million years ago), an "inexplicable" absence that could be "truly urged as a valid argument" against his all embracing synthesis. For more than 100 years, the "missing Precambrian history of life" stood out as one of the greatest unsolved mysteries in natural science. But in recent decades, understanding of life's history has changed markedly as the documented fossil record has been extended seven-fold to some 3,500 million years ago, an age more than three-quarters that of the planet itself. This long-sought solution to Darwin's dilemma was set in motion by a small vanguard of workers who blazed the trail in the 1950s and 1960s, just as their course was charted by a few pioneering pathfinders of the previous century, a history of bold pronouncements, dashed dreams, search, and final discovery.

Asunto(s)

RESUMEN

Ion microprobe measurements of carbon isotope ratios were made in 30 specimens representing six fossil genera of microorganisms petrified in stromatolitic chert from the approximately 850 Ma Bitter Springs Formation, Australia, and the approximately 2100 Ma Gunflint Formation, Canada. The delta 13C(PDB) values from individual microfossils of the Bitter Springs Formation ranged from -21.3 +/- 1.7% to -31.9 +/- 1.2% and the delta 13C(PDB) values from microfossils of the Gunflint Formation ranged from -32.4 +/- 0.7% to -45.4 +/- 1.2%. With the exception of two highly 13C-depleted Gunflint microfossils, the results generally yield values consistent with carbon fixation via either the Calvin cycle or the acetyl-CoA pathway. However, the isotopic results are not consistent with the degree of fractionation expected from either the 3-hydroxypropionate cycle or the reductive tricarboxylic acid cycle, suggesting that the microfossils studied did not use either of these pathways for carbon fixation. The morphologies of the microfossils suggest an affinity to the cyanobacteria, and our carbon isotopic data are consistent with this assignment.

Asunto(s)

Asunto(s)

Asunto(s)

RESUMEN

The oldest filament- and colonial coccoid-containing microbial fossil assemblage now known is described here from drill core samples of stromatolitic cherty limestones of the Neoarchean, approximately 2600-Ma-old Campbell Group (Ghaap Plateau Dolomite, Lime Acres Member) obtained at Lime Acres, northern Cape Province, South Africa. The assemblage is biologically diverse, including entophysalidacean (Eoentophysalis sp.), probable chroococcacean (unnamed colonial coccoids), and oscillatoriacean cyanobacteria (Eomycetopsis cf. filiformis, and Siphonophycus transvaalensis), as well as filamentous fossil bacteria (Archaeotrichion sp.); filamentous possible microfossils (unnamed hematitic filaments) also occur. The Campbell Group microorganisms contributed to the formation of stratiform and domical to columnar stromatolitic reefs in shallow subtidal to intertidal environments of the Transvaal intracratonic sea. Although only moderately to poorly preserved, they provide new evidence regarding the paleoenvironmental setting of the Campbell Group sediments, extend the known time-range of entophysalidacean cyanobacteria by more than 400 million years, substantiate the antiquity and role in stromatolite formation of Archean oscillatoriacean cyanobacteria, and document the exceedingly slow (hypobradytelic) evolutionary rate characteristic of this early evolving prokaryotic lineage.

Asunto(s)

RESUMEN

NASA: Recent studies of Precambrian fossils indicate that life on Earth originated earlier than assumed, microscopic life was prevalent in the Precambrian Eon, the tempo and mode of evolution during the Precambrian period were different from other periods, and that only the Precambrian fossil record can be used as evidence of early life. Implications for future research include directing the search for the origin of life away from the geological record, modification of hypotheses about molecular change, use of Precambrian microfossils in dating younger geological units, and progress in defining the nature of major events in early evolution.^ieng

Asunto(s)

RESUMEN

Over the past quarter century, detailed genus- and species-level similarities in cellular morphology between described taxa of Precambrian microfossils and extant cyanobacteria have been noted and regarded as biologically and taxonomically significant by numerous workers world-wide. Such similarities are particularly well documented for members of the Oscillatoriaceae and Chroococcaceae, the two most abundant and widespread Precambrian cyanobacterial families. For species of two additional families, the Entophysalidaceae and Pleurocapsaceae, species-level morphologic similarities are supported by in-depth fossil-modern comparisons of environment, taphonomy, development, and behavior. Morphologically and probably physiologically as well, such cyanobacterial "living fossils" have exhibited an extraordinarily slow (hypobradytelic) rate of evolutionary change, evidently a result of the broad ecologic tolerance characteristic of many members of the group and a striking example of G. G. Simpson's [Simpson, G.G. (1944) Tempo and Mode in Evolution (Columbia Univ. Press, New York)] "rule of the survival of the relatively unspecialized." In both tempo and mode of evolution, much of the Precambrian history of life--that dominated by microscopic cyanobacteria and related prokaryotes--appears to have differed markedly from the more recent Phanerozoic evolution megascopic, horotelic, adaptationally specialized eukaryotes.

Asunto(s)

RESUMEN

Eleven taxa (including eight heretofore undescribed species) of cellularly preserved filamentous microbes, among the oldest fossils known, have been discovered in a bedded chert unit of the Early Archean Apex Basalt of northwestern Western Australia. This prokaryotic assemblage establishes that trichomic cyanobacterium-like microorganisms were extant and morphologically diverse at least as early as approximately 3465 million years ago and suggests that oxygen-producing photoautotrophy may have already evolved by this early stage in biotic history.

Asunto(s)

RESUMEN

Evidence accumulated over the past two decades is now sufficient to permit an initial quantitative assessment of the patterns of biotic diversity and extinction that occurred during Proterozoic time. Because of limitations in both the quality and quantity of data currently available, however, generalizations thus derived must be regarded as tentative. Nevertheless, read literally, available palaeontological data appear to indicate that the global ecosystem experienced a gradual but massive collapse between 1 000 Ma and the beginning of the Phanerozoic, a supposition consistent with other lines of geological and geochemical evidence. A possible forcing agent for such a collapse appears to have been a decrease in ambient levels of carbon dioxide and a resultant decrease in average global temperature, photosynthetic efficiency, and primary productivity.

Asunto(s)

RESUMEN

Cellularly preserved filamentous and colonial fossil microorganisms have been discovered in bedded carbonaceous cherts from the Early Archean Apex Basalt and Towers Formation of northwestern Western Australia. The cell types detected suggest that cyanobacteria, and therefore oxygen-producing photosynthesis, may have been extant as early as 3.3 billion to 3.5 billion years ago. These fossils are among the oldest now known from the geologic record; their discovery substantiates previous reports of Early Archean microfossils in Warrawoona Group strata.

Asunto(s)

RESUMEN

A diverse assemblage of exceptionally well-preserved microorganisms, including several previously unknown taxa, has been discovered in stromatolitic black chert from the ca. 680-790 Ma-old Min'yar Formation (Suite) of the southern Ural Mountains, USSR. Like most ancient and modern stromatolitic communities, the Min'yar microflora is dominated by filamentous and unicellular cyanobacteria. Geologic evidence indicates that the microbial community inhabited a shallow water, presumably marine environment. The microfossils occur in two interlaminated and thinly interbedded sedimentary fabrics: 1, fact to wavy-laminated Stratifera-like stromatolitic laminae that presumably were deposited during periods of little wave action; and 2, intraclast grainstone that formed as a result of desiccation and (or) wave agitation. Microfossils are both better preserved and more abundant in the intraclasts than in the Stratifera-like laminae. The occurrence of probable pseudomorphs after replacement of sulfate minerals provides additional evidence for a shallow water, periodically emergent depositional environment for the Min'yar microbial mats. Kerogenous microfossils are three-dimensionally preserved, permineralized in fine-grained silica of primary or early diagenetic origin. In many aspects the Min'yar assemblage is comparable to that of the well-known ca. 850 Ma-old Bitter Springs Formation of central Australia. The following taxa are herein described: Division? Schizomycophyta or ? Cyanophyta, Biocatenoides sp.; Family Oscillatoriaceae, Eomycetopsis robusta Schopf emend. Knoll and Golubic, Rhicnonema antiquum Hofmann, Entosphaeroides? sp., Palaeolyngbya? sp., Siphonophycus capitaneum n. gen., n. sp.; Family? Oscillatoriaceae or? Rivulariaceae, Caudiculophycus? sp.; Family? Scytonemataceae or? Stigonemataceae, Ramivaginalis uralensis n. gen., n. sp.; Family Chroococcaceae, Sphaerophycus medium Horodyski and Donaldson, Eosynechococcus amadeus Knoll and Golubic, Gloeodiniopsis lamellosa Schopf emend. Knoll and Golubic, Gloeodiniopsis magna n. sp., Eoaphanocapsa oparinii n. gen., n. sp.; Division? Chlorophycophyta or? Rhodophycophyta, Glenobotrydion majorinum Schopf and Blacic. Also discussed in the taxonomic section, but not formally described, are intermediate-diameter tubular sheaths (Oscillatoriaceae), small tubular sheaths enclosed by larger tubular sheaths (Oscillatoriaceae?), and undifferentiated spheroidal unicells.

Asunto(s)

RESUMEN

Two communities of diverse, well-preserved, fossil prokaryotic microorganisms have been discovered in petrographic thin sections of carbonaceous black chert from the ca. 1400-1500 Ma-old Gaoyuzhuang Formation at the stratotype section of the "Sinian Suberathem" near Jixian, northern China. One of these communities is preserved in bedded, essentially flat-laminated, stromatolitic chert; the other occurs in silicified conical stromatolites of the forms Conophyton cylindricum and C. garganicum. The Gaoyuzhuang cherts comprise one of the new microfossiliferous deposits now known from the Precambrian of China; the permineralized microbiotas they contain are among the first such stromatolitic assemblages to be discovered in the Sinian stratotype section.

Asunto(s)

Asunto(s)

RESUMEN

Carbonaceous shales of the late Precambrian Chuar Group of the Grand Canyon, Arizona, contain abundant and well-preserved chitinozoans. The occurrence of these distinctive, tear- and flask-shaped microfossils, the oldest chitinozoans now known and the first to be reported from the Precambrian, seems to suggest that heterotrophic protists (or primitive metaozoans) were extant at least as early as about 750 +/- 100 million years ago.

RESUMEN

Silicified specimens of the Vendian (late Precambrian) "index fossil" Conophyton gaubitza from South Kazakstan contain a diverse assemblage of well-preserved cyanophytic and apparently eukaryotic algae, the first stromatolitic microbiota to be reported from the Soviet Union. Unlike the stromatolites in which they occur, the microorganisms that apparently built this form of Conophyton did not become extinct at the end of the Precambrian.

RESUMEN

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.

RESUMEN

A comparative statistical study has been carried out on populations of modern algae, of Precambrian algal microfossils, of the 'organized elements' of the Orgueil carbonaceous meteorite, and of the oldest microfossil-like objects now known (spheroidal bodies from the Fig Tree and Onverwacht Groups of the Swaziland Supergroup, South Africa). The distribution patterns exhibited by the greater than 3000 m.y.-old Swaziland microstructures bear considerable resemblance to those of the abiotic 'organized elements' but rather markedly differ from those exhibited by younger, assuredly biogenic, populations. Based on these comparisons it is concluded that the Swaziland spheroids could be, at least in part, of non-biologic origin; these oldest known fossil-like micro-structures should not be regarded as constituting firm evidence of Archean life.

Asunto(s)

Asunto(s)

RESUMEN

An assemblage of cellularly well-preserved, filamentous and spheroidal plant microfossils has been detected in a cherty pisolite bed of the late Precambrian Chuar Group from the eastern Grand Canyon of the Colorado River. This newly discovered microflora, probably among the youngest Precambrian biological communities now known, appears to be of both evolutionary and biostratigraphic significance.