Fr Utexites from Stromatolites of the Gunflint Iron Formation of Canada, and Its Biological Affinities*

Walter, M.R. and Awramik, S.M., 1979. Frutexites from stromatolites of the Gunflint Iron Formation of Canada, and its biological affinities. Precambrian Res., 9: 23--33. The microfossil Frutexites is known from many Palaeozoic and some Proterozoic carbonate sedimentary rocks. We recently found unusually well-preserved examples in the early Proterozoic Gunflint Iron Formation of Ontario, Canada. These are preserved in chert, along with other microfossils of the Gunflint microbiota. The Gunflint examples have previously been described as laminated, columnar-branching microstromatolites. We have found narrow tubes (interpreted as trichome moulds) axially placed in many of the microcolumns. By comparison with extant organisms, we interpret Frutexites as a thick-sheathed scytonematacean cyanophyte. These are the oldest known fossil scytonemataceans. They apparently grew within mats of other microorganisms. These probably were photosynthetic organisms, which may have significant implications in the interpretation of some occurrences, such as in the fore-reef facies of Devonian reefs in Western Australia. I N T R O D U C ~ O N The Gunflint microbiota from southern Ontario is famous as the first well-preserved assemblage of Precambrian microfossils to be described (see Awramik and Barghoorn, 1977, for references to earlier work}. Its isotopic age is no t well established, but is older than 1685 + 24 Ma, and may be approximately 2000 Ma old (for a discussion see Knoll et al., 1978}. The microfossils are organically preserved in dark grey to black cherts, and infrequently as iron-oxide replacements or casts in red cherts. The dominant taxa are found in both modes of preservation in the Gunflint Iron Formation * Contribution No. 82, Biogeology Clean Laboratory, University of California, Santa Barbara, Calif., 93106, U.S.A.