FOSSIL ORGANISMS FROM PRECAMBRIAN SEDIMENTS 



Elso S. Barghoorn 

 Department of Biology, Harvard University, Cambridge, Mass. 



Stanley A. Tyler 

 Department of Geology, University of Wisconsin, Madison, Wise. 



In widely scattered outcrops of the Gunflint Iron Formation, Lake Superior 

 region, Ontario, Canada, dense, black, nonferruginous, fossiliferous cherts oc- 

 cur as thin units in the sedimentary sequence of black shales, argillites, and 

 dolomites. In its geological setting, the Guntiint Iron Formation is generally 

 regarded as comprising the middle unit of the Animikie Series (Middle Hu- 

 ronian equivolent) of the Lake Superior region. Absolute age of the Gunflint 

 Formation has been determined by P. W. Hurley by measurement of the po- 

 tassium-argon ratios in authigenic minerals which occur in direct association 

 with the cherts and interbedded in the Gunflint sedimentary sequence. Repli- 

 cate determinations have yielded consistent values of 1900 M years (1.9 X 10^ 

 years) . 



The cherts have been studied with the use of thin sections, acid maceration, 

 and a variety of chemical techniques. Thin sections of the chert, when viewed 

 in transmitted light, reveal that its black color, as seen en masse, is caused by 

 the abundance of finely disseminated organic matter that appears light amber 

 to dark brown in color in sections 50 /x or less in thickness. In this respect the 

 chert behaves petrographically much as a typical bituminous coal, which in 

 thin section exhibits a range in color of the petrographical components from 

 light yellow through amber to dark orange red to opaque. In the Gunflint 

 chert a large fraction of the organic constituents reveal a distinct morphological 

 organization consisting of filaments, septate and nonseptate, spheroidal or 

 spherical bodies, and more complex asymmetrical structures. The discrete en- 

 tities are all microscopical in size and present an appearance analogous to 

 masses of anastomosing algal filaments in which are enmeshed other microor- 

 ganisms. The chert matrix in which the organisms are embedded varies from 

 clear and hyaline to granular and crystalline. In polarized light the chert is 

 microcrystalline. Crystals of pyrite, calcite, and apatite vary in abundance, 

 but in no case are more than minor petrographical constituents. 



The biological affinities of the organisms preserved in the Gunflint chert 

 present a curious paleontological problem inasmuch as a number of the distinct 

 entities or "types" possess a morphology that is quite unlike that in existing 

 microscopical crganisms, either plant or animal. In this connection it should 

 be emphasized that the organic structures are 3-dimensionally preserved and 

 not flattened or unilaterally distorted. They are hence amenable to morpho- 

 logical and histological study. 



The most abundant organisms in the assemblage are filaments ranging in 

 diameter from 0.6 to 6.0 n. In the most favorably preserved state these are 

 found to be both septate and nonseptate. The septate types exhibit a form 

 indistinguishable from that of filamentous blue green algal {vis., Oscillator ia, 

 Lyngbya, etc.). The nonseptate types are more difficult to interpret in terms 

 of biological affinities. With exceedingly few exceptions they are unbranched 



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