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Middle Proterozoic Shale-Facies Microbiota from the Lower Belt Supergroup, Little Belt Mountains, Montana

Robert J. Horodyski
Journal of Paleontology
Vol. 54, No. 4 (Jul., 1980), pp. 649-663
Stable URL: http://www.jstor.org/stable/1304301
Page Count: 15
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Middle Proterozoic Shale-Facies Microbiota from the Lower Belt Supergroup, Little Belt Mountains, Montana
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Abstract

Microfossils are abundant and well preserved in shales of the approximately 1,400-million-year-old Chamberlain Shale and Newland Limestone in the Little Belt Mountains, Montana. The microfossils are compressed parallel to lamination and consist of filamentous and spheroidal forms. The filamentous forms, ranging from 0.5-40 μm wide, are more abundant in shales of the Newland Limestone. Filaments less than 2 μm wide could represent either filamentous bacteria or oscillatoriacean cyanophytes; filaments 2-12.5 μm wide appear to represent preserved sheaths of several species of oscillatoriacean cyanophytes; the rare filaments wider than 12.5 μm probably represent nostocalean cyanophytes. Compressed, 12-440 μm diameter, spheroidal and ellipsoidal microfossils are more abundant in the Chamberlain Shale. Their biologic affinities have not been determined, and they could represent either prokaryotic or eukaryotic organisms. Similarity of some of these sphaeromorphs to the modern colonial coccoid cyanophyte Entophysalis illustrates a possible prokaryotic affinity. The sphaeromorphs and larger filaments in this assemblage differ in appearance from microfossils that occur in stromatolitic cherts of comparable age They are, however, similar to microfossils preserved in Middle and Upper Proterozoic shales in the Soviet Union, Sweden, Australia, and Canada; thus, they provide the opportunity to further evaluate the use of Proterozoic shale-facies microfossils for intercontinental biostratigraphic correlation. The following new taxa of fossil cyanophytes are described: Oscillatoriaceae: Siphonophycus beltensis n. sp.; Siphonophycus crassiusculum n. sp.

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