Interstitial and peloid microfossils from the 2.0 Ga Gunflint Formation: Implications for the paleoecology of the Gunflint Stromatolites

Precambrian Research, 45 (1989) 291-318

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Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Interstitial and Peloid Microfossils from the 2.0 Ga Gunflint Formation: Implications for the Paleoecology of the Gunflint Stromatolites WILLIAM P. LANIER Department of Earth Science, Emporia State University, Emporia, KS 66801 (U.S.A.) (Received January 23, 1989; revision accepted June 6, 1989 )

Abstract Lanier, W.P., 1989. Interstitial and peloid microfossils from the 2.0 Ga Gunflint Formation: implications for the paleoecology of the Gunflint Stromatolites. Precambrian Res., 45:291-318. Silicified peloidal arenite sediments from the 2.0 Ga Gunflint Formation contain morphologically diverse and exceptionally well-preserved microfossil assemblages. Peloids are current-deposited, sand-sized and larger intraclasts of organic-rich siliceous sediment which were derived from low- to high-energy sedimentary environments within the Gunflint depositional basin. Two distinct occurrences can be differentiated: (1) an interstitial assemblage dominated by well-preserved autochthonous Gunflintia minuta filamentous microfossils, but also containing larger filaments, spheroidal to elongated vesicles and other unusual microfossil-like structures, and (2) intragranular/peloid assemblages containing essentially all previously described Gunflint stromatolite texa including clonal groupings of Huroniospora (Type E cells) showing over 12% dividing cells, and two undescribed filamentous morphotypes which resemble "actinomycetes" eubacteria. The size distribution of clonal Type E populations is almost identical to a major modal population of Huroniospora from exceptionally well-preserved Gunflint stromatolite materials. Both assemblages show a range of textures reflecting post-mortem degradation, but locally the interstitial communities show better preservation. Interstitial filaments differ from stromatolitic G. minuta insofar as they show (1) occasional cross-walls; (2) a size-frequency distribution which seems to reflect post-mortem/pre-permineralization shrinkage; (3) preferred growth orientations; and (4) lengths of up to 2.5 millimeters. The interstitial microbiota is also permineralized by early-diagenetic silica cement. The recognition of this new habitat contributes substantially to our understanding of Gunflint microbial ecosystems by providing new information concerning the ecology, growth and reproductive patterns of Huroniospora and Gunflintia. Detailed comparisons of interstitial Gunflintia with its stromatolitic counterpart collectively suggest that it was a photoautotrophic microorganism with the capacity to form dense (but very thin) microbial mats and of having directly influenced stromatolite morphogenesis. Similar comparisons suggest the Type E Huroniospora did not contribute directly to stromatolite growth.

Introduction Microfossils were first reported from the 2.0 Ga Gunflint Formation by Tyler and Barghoorn {1954). Later systematic descriptions by Barghoorn and Tyler (1965), and Cloud (1965), and the contributions of numerous other workers (see Hofmann, 1971; Awramik and Barghoorn, 1977; Knoll et al., 1978; Knoll and 0301-9268/89/$03.50

Awramik, 1983) have collectively defined the Gunflint microbiota as one of the most consistently cited if not most intensively studied examples of early life on Earth. Two biofacies are conventionally recognized from the Gunflint: (1) laminated gray to black cherts containing a presumably planktonic/allochthonous assemblage dominated by the asterioform microfossils Eoastrion and Kakebekia, and the glob-

© 1989 Elsevier Science Publishers B.V.

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ular "budding" morphotype Galaxiopsis (Frustration Bay assemblage; Awramik and Barghoorn, 1977); and (2) black to gray stromatolitic chert, designated as the Schreiber Beach biofacies, dominated by the filamentous microfossil G. minuta and the coccoid/spheroidal microfossil Huroniospora. The autochthonous, stromatolite-building affinity of the Schreiber Beach biofacies has been conventionally accepted for some time (Knoll and Awramik, 1983) and by analogy with modern microbial mats and stromatolites it has been suggested that Huroniosporaand Gunflintia may represent the fossilized remains of oxygen-generating cyanobacteria. Recently, however, the autochthonous character of the stromatolite biofacies has been questioned by Strother and Tobin (1987). Citing the lack of Gunflintia filaments showing any of the preferred growth orientations which characterize photosynthetic filamentous organisms from modern microbial mats and stromatolites, and the fact that Huroniospora shows no evidence of cell division and is never found in clonal clusters, Strother and Tobin (1987) suggest that the GunflintiaHuroniospora assemblage was a secondary degradative association of microfossils and not a primary producer/stromatolite-building community. The present article describes a new Gunflint biofacies from silicified peloidal arenitic sediments (hereinafter designated as the peloidal arenite biofacies) from samples collected within the Schreiber Channel Provincial Nature Reserve, Ontario (Schreiber Locality, see Awramik and Barghoorn, 1977, Fig. 1, p. 125). The biofacies contains intragranular microfossils and an interstitial assemblage of allochthonous microfossils which were permineralized by early-diagenetic silica cements. The Schreiber peloidal arenite biofacies is thus remarkably similar to a recent report of intra- and intergranular microfossils from the Lower Proterozoic Duck Creek Dolomite, Western Australia (Knoll et al., 1988). Interstitial filamentous microfossils were also reported by Cloud and

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Licari (1972) from the ~ 2.0 Ga Pokegama Formation, Minnesota. These new occurrences of microfossils contribute significantly to our understanding of Gunflint paleobiology in three ways: (1) the interstitial niche is a new microbial habitat in which G. minuta filaments are found to have been living in situ, and observations of the same fossilized microorganism from two different habitats can be a valuable tool for assessing microfossil affinities; (2) the intragranular (peloid) assemblage contains two new morphotypes of branching filamentous microfossils which were secondary but synsedimentary inhabitants of organic-rich substrates and strongly resemble certain extant "actinomycetes" bacteria; (3) the intragranular assemblage also contains Huroniospora coccoids which occur as clonal populations showing cell division, and which show a size distribution which is identical to a major modal population of stromatolitic

Huroniospora. Geological setting and materials The geological setting and stratigraphic relationships of the Lower Proterozoic Gunflint Formation has been described by numerous workers (e.g. Goodwin, 1956; Barghoorn and Tyler, 1965; Cloud, 1965; Hofmann, 1969; Morey, 1973; Knoll et al., 1978; Stille and Clauer, 1986). The Gunflint is the lowest formation of the Animikie Group and rests unconformably upon an Archean granite-greenstone basement ( ~ 2.7-2.6 Ga). It is conformably overlain by the argillaceous Rove Formation (see Hofmann, 1969 ). The Gunflint consists of terrigenous clastic, volcaniclastic and chemical sedimentary rocks and Goodwin (1956) subdivides the formation into upper and lower members thought to represent two transgressive cycles. A persistent stromatolitic chert horizon is found at the base of each member. The Kakabeka Conglomerate locally underlies the lower member. Although the age of the Gun-

INTERSTITIAL AND PELOID MICROFOSSILS

flint Formation has always been somewhat controversial (see Knoll et al., 1978), a new StaNd isochron age of 2.08 + 0.25 Ga has recently been obtained from argillaceous Gunflint material collected near Kakabeka Falls, Ontario (Stille and Clauer, 1986). This age is in good agreement with earlier independent estimations for the age of the Gunflint sedimentary basin (Sims et al., 1981 ) and is also comparable with previous projected radiometric ages for the Gunflint (see Knoll et al., 1978). The microfossiliferous samples which are considered in this report were collected by the author from the Schreiber Beach locality (see descriptions of Barghoorn and Tyler, 1965; Cloud, 1965) located within the Schreiber Channel Provincial Nature Reserve approximately 6.5 km west of Schreiber Beach proper along the north shore of Lake Superior. Perhaps best known for two excellent shoreline exposures of microfossiliferous stromatolitic chert, several outcrops of silicified peloidal arenite containing well-preserved microfossils are also found within the Reserve. Most of the material described in the present article comes from a shoreline outcrop located approximately equidistant from the two stromatolite exposures. These exposures are easily located in the field insofar as they are the only peloidal sedimentary rocks exposed in this area of the Reserve. At the outcrop scale, silicified peloidal arenites are color mottled from black (N 1) to brownish gray (5 YR 4/1) and are characterized by lenticular to somewhat nodular zones containing black peloidal grains cemented by clear to milky translucent early-diagenetic fibrous quartz. Chert color ranges from medium gray (N 5 ) to dark brownish gray outside of the black chert lenses and contacts range from sharp to slightly gradational. The mottled texture of the rock at outcrop scale combined with the fact that the peloids themselves are otherwise uniformly black and well-sorted, strongly obscures primary sedimentary features. Sedimentary layering is continuous across the nod-

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ule boundaries when textures can be recognized. Petrographic observations reveal that the black nodular layers are composed predominantly of well-sorted, light to dark brown, organic-rich grains ranging from ~0.25 mm to over 1.5 mm in maximum dimension and averaging medium sand size. Occasionally found are larger composite grains reaching centimeter dimensions which consist of from several to hundreds of peloid grains which underwent at least partial early cementation prior to erosion, transportation and deposition. Grain shapes vary from well-rounded to angular or shard-like and the predominance of point and long grainto-grain contacts indicate that the peloids were largely uncompacted prior to lithification. More detailed observations show that the peloidal deposits are texturally heterogeneous with organic-rich siliceous lutite grains and rounded to elongated-plate-shaped stromatolite intraclasts dominating as end-member clast lithologies. Only rarely do the Schreiber peloids show textural pseudomorphs after what appear to have been non-siliceous early diagenetic mineral phases. A large percentage of the peloids show tensional cracks and fractures which apparently resulted from in situ grain shrinkage (Fig. 1B). Between crossed polars the microfossiliferous peloids are seen to be composed of very finegrained microcrystalline quartz anhedra and chert grain-size appears to be inversely proportional to organic matter concentrations. Euhedral pyrite grains and patchy to solitary dolomite rhombohedra are the most common chert impurities. Well-preserved peloidal sediments were cemented by multiple generations of length-fast fibrous quartz (chalcedony), although length-slow chalcedony is occasionally found as a second or third generation cement. Thin ( ~ 25-60 ~m thick) isopachous fibrous quartz rim cements locally characterize the initial cement generation while spherulitic chalcedonic fans up to ~ 100 ]~m in radius generally comprise secondary cement generations and infill most remaining porosity (Figs. 1D,E).

Fig. 1. General texture of silicified peloidal arenitic sediments from the Schreiber Locality. A. Photograph of a thin section containing reasonably well-sorted organic-rich peloid grains. B. Low power photomicrograph in plane polarized light of a large composite grain comprised of a heterogeneous mixture of coarse- and fine-grained sedimentary materials; note the typical shrinkage cracks which characterize the organic-rich peloids (arrow lower right). C. Typical peloid showing point contacts with adjacent grains (arrows). D. Low power photomicrograph between crossed polars showing typical first generation isopachous rim cements and a second generation of fibrous quartz cement which fills remaining porosity. E. Isopachous rim cements (arrows) and subsequent fibrous quartz cement generations shown between crossed polars. Bar = 3 mm for A; bar in B = 800 brn for B and D; bar in C = 50 pm for C and E.

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Spherulitic fan cements show a sweeping extinction and radiate inwards meeting along planar to gently curved interfaces indicating competitive and inward directed growth. Drusy megaquartz locally fills remaining porosity. The precise depositionai setting of the Schreiber peloidal cherts is uncertain. The rocks may represent a lateral facies transition from the basal stromatolite horizons which are exposed in the Reserve, but a lack of vertical exposure precludes a detailed sedimentologic analysis. Petrographic and petrologic evidence strongly favor an origin for the peloid grains as intraclasts of chemical sediments which were originally deposited in quiet water to moderately energetic and possibly inter- to supratidal sedimentary environments. The textural heterogeneity of the peloidal grains, their overall excellent sorting and the typically sedimentary grain-to-grain contacts collectively argues for the deposition of these sediments by turbulent, high- to moderate-energy currents. The complete lack of lutitic interstitial sedimentary materials further suggests a reasonably energetic depositional setting. The fact that the peloidal grains are uncompacted also suggests that these materials underwent early diagenetic cementation. Silica cements commonly preserve between 30-50% of primary porosity (Simonson, 1987) which is comparable to the approximately 40% initial intergranular porosity of well-rounded and well-sorted siliciclastic or carbonate sands (Beard and Weyl, 1973). Numerous lines of evidence (see Simonson, 1987) suggest that silica cementation occurred at or near the sediment-water interface and the silica-cemented composite peloid intraclasts containing weU-preserved interstitial microfossils which are found in the Schreiber peloidal cherts further support this hypothesis.

Interstitial assemblage composition The interstitial assemblage of the peloidal arenite biofacies contains autochthonous filamentous microfossils and an interesting assort-

ment of apparently allochthonous coccoid spheroidal, vesicular and bizarre microfossil morphotypes. In terms of relative abundance, the assemblage is dominated by the filamentous taxon Gunflintia minuta Barghoorn ( > 99% ), but also includes a subordinate population of slightly larger filaments which resemble Gunflintia grandis Barghoorn. The allochthonous component of the biofacies is scattered randomly among the filaments and includes: (1) Huroniospora spp. in the ~ 2-12 #m size range; (2) Leptoteichos golubicii Knoll et al.; (3) a single cluster of large chroococcuslike unicells; (4) an unusually large vesicular morphotype; and (5) a single elongated morphotype with a bizarre tail-like process. Gunflintia minuta Barghoorn The highest concentrations of interstitial microfossils thus far identified are from a large composite peloid intraclast from the Schreiber peloidal cherts. It is approximately 2.2 cm × 1.6 cm and is composed of well-rounded peloidal grains averaging medium sand size. The paragenesis of the clast is complex and the specific details are beyond the scope of this report, but involved early-diagenetic silicacementation by isopachous rim cements, a later phase of peloid shrinkage, partial peloid dissolution and silica recrystallization. The latter three diagenetic changes occurred after the intraclast was deposited. It is clear that the clast was at least partially lithifiedprior to transport and deposition because dense masses of interstitialfilaments are truncated at the intraclast boundaries (Figs. 2 and 4A). Interstitial G. minuta show a considerable range of textures which apparently represent a continuum of shrinkage morphologies. A high percentage of the filaments are very long, uniform, straight to gently curved, seemingly hollow tubes which are circular to slightlyelliptical in cross-section (Fig. 3). However, the majority of filaments, and even segments of filaments, are irregularly shriveled to plicate, twisted or

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Fig. 2. Photomicrographs in plane-polarized light showing the distribution of interstitial filaments within a large composite peloid intraclast. A. Low magnification of the upper portion of the intraclast with filaments showing a general overall preferred orientation which is roughly parallel to the long axis of the photograph. B. Higher magnificationshowing truncated filaments at the clast boundary (arrow to right). Bar--800 p m for A, 400 pm h)r B.

shrunken minute threads. Tube width for 99% of the population ranges from 0.3 #m to 2.0/~m while averaging 1.0/tm (285 measurements, Fig. 4). Lengths range from several hundred micrometers to 2.5 m m and surface textures are

psilate, smooth to very finely granular. Crosswalls have been identified in less than 5 % of the filaments (Figs. 3F and 3G), but those which show this structuring tend to be slightly larger in diameter than the population mean, averag-

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Fig. 3. High magnification photomicrographs in plane-polarized light of the interstitial G. minuta filaments. A. Empty filament with a central twisted zone. B. Empty uniform filament together with several smaller shrunken filaments (arrow). C. G. minuta filament showing possible branching. D. Irregularly shrunken empty filament. E. Long sinuous filament with possible internal differentiation. F. Filaments showing clearly defined cross-walls. G. Filaments which illustrate a range of preservational textures. H. Filaments showing typical gaps and slight offsets. I. Empty sinuous filament and an adjacent filament showing a morphological transition which resulted from shrinkage (arrow). 10/zm scale bar in G for A-I.

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intricately interwoven and twined around the peloid grains throughout the intraclast (Fig. 2 ). Filament concentrations locally appear to be sufficiently high to have bound the grains. The filaments are strongly aligned in very high densities immediately adjacent to grain surfaces and when these concentrations are traced to larger interspaces the filaments fan out or disperse yet somewhat still retain their strongly parallel alignments (Fig. 5). The overall filament orientation within the clast also appears to be preferential with more than 50% of the filaments showing a relative sense of alignment which falls within an approximately 30 ° arc. Gunflintia grandis Barghoorn

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ing 1.4-1.8 pro. The cross-walls are straight with a spacing of between 2-3.2 #m (2= 2.6). Only five of the several hundred filaments examined in detail appear to be branched (Fig. 3C), suggesting that these may be artifacts of coincidentally overlapping filaments, or alternatively, false branches. Evidence for shrinkage is two-fold. First, a single long filament can show a continuum of textures throughout its lengthfrom uniform with cross-walls to apparently empty unshrunken sheaths, to empty shrunken and twisted sheaths, to completely shrunken minute threads. Second, the overall distribution of interstitial filament size is strongly skewed toward smaller diameters whereas filaments showing cross-walls are larger than the population mean. These lines of evidence suggest a continuum of structures and textures which resulted from filament shrinkage following evacuation of the sheath by the cellular trichome. Exceptionally high densities of G. minuta are

Larger filamentous microfossils which could be placed within the Gunflint taxon G. grandis (Fig. 6) account for less than 1% of the interstitial filaments. They are relatively broad (2.55.5/~m in diameter) with lengths of up to ~ 300 ~m. Surface textures are identical to those of G. minuta, but the filaments are typically highlytwisted, folded or irregularly shrunken {Figs. 5C, 6B and 6C). Long empty tubes of uniform diameter are relatively rare (Fig. 6A). Only one G. grandis filament has been found which contains the degraded remnants of a trichome (Fig. 6D ). It is about 200/~m in length, 4-4.8 #m in diameter and the trichome remnant is well-defined (compare Fig. 6D with 6A) within a hyaline sheath of about 0.3-0.5 pm in thickness. The trichome appears to be constricted at the nodes with individual cells averaging 3.2/~m in width and 5.7-6.3 /~m in length. The apical cell(?) appears capitate, 19 #m in length and 5.3/xm in maximum width while tapering to a rounded tip. Like G. minuta, much of the morphological variability within the G. grandis population also appears to be attributable to sheath shrinkage following cellular trichome evacuation. G. grandis is almost always found within dense well aligned G. minuta filament populations and shows an identical orientation; evidence suggesting that it grew in situ and thus

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Fig. 5. A. Dense interstitial filament masses twined around a peloid grain which is adjacent to the intraclast boundary (curved arrows); note the general preferred filament orientations for the truncated filaments at the boundary. B. Oriented filaments which "fan-out" within a large interspace; note the peloid grain surface (large arrow) and possible isopachous silica cement layer coating the grain. C. Typically dense interstitial G. minuta occurrence which contains a single shrunken Gunflintia grandis filament (arrow). Higher magnification photomicrograph of a portion of (A) also showing a single G. grandis filament. All photomicrographs in plane-polarized light. Scale bar in A = 100/~m; scale in D = 25/Lm for B-D.

represents a subordinate but autochthonous component of the interstitial community. Huroniospora spp. Spherical to ellipsoidal microfossils which are assigned to the Gunflint form taxon Huroniospora comprise the largest percentage of the allochthonous component of the interstitial assemblage. Microfossils which are considered to have been allochthonous are generally solitary unicells and are widely or randomly scattered between the peloids, or are nested among the interstitial filaments. They are not found in

clonal groupings and only very rarely show evidence of reproductive patterns. All Huroniospora-like microfossils are simple spheres to ellipsoids ranging between 2.1/~m and 12.7/~m, with over 70% being less than 5/Ira. They are conspicuous for their almost complete lack of diagnostic texture. For example, identically sized cells within this range may show smooth, psilate walls (Fig. 7D) or somewhat darker granular walls (Fig. 7E). The majority of Huroniospora-like coccoids show a single variably-sized kerogenous inclusion which probably represents shrunken intracellular organic materials.

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Leptoteichos golubicii Knoll et al. Relatively large ( 17-22 #m ) spherical microfossils which resemble Leptoteichos golubicii occur as a minor constituent of the interstitial assemblage (Figs. 7A and B). Interpreted as a planktonic species (Knoll et al., 1978) on the basis of their scattered occurrence in non-stromatolitic distal chert facies of the Gunflint, interstitial Leptoteichos-like microfossils show a well-defined psilate inner wall, a diffuse outer layer of ~ 1-4/~m in thickness and also contain dark to somewhat diffuse kerogenous inclusions. Similar spheroidal microfossils have been described by Hofmann (1976) from the early Proterozoic Kasegalik and McLeary Formations (Belcher Islands, Canada) and were assigned to the taxon Globophycus Schopf (1968).

Other allochthonous(?) interstitial microfossils

Fig. 6. G. grandis from the interstitial habitat. A. Empty filament of reasonably uniform diameter. B. Irregularly shrunken and twisted empty sheath; note G. minuta filament to the right (arrow). C. Shrunken and twisted sheath. D. G. grandis filament with preserved trichome; arrows indicate cross-walls. All photomicrographs in plane-polarized light. Scale bar in B -- 10 ttm for A-C; bar in D is also 10 pm.

Three other allochthonous microfossils which are noteworthy for their interesting size and morphology are represented by single examples from the interstitial materials. A 41 ttm X 30/~m aggregate of large chroococcus-like microfossils showing a well-defined divisional pattern has been found nested among interstitial G. minuta filaments (Fig. 8). Cell walls are thick (1.5-2.3 #m), robust, and have a reticulate to corrugated texture. The aggregate is surrounded by a partially collapsed, hyaline sheath, the outer surface of which is located between ~ 1/~m and 8/lm from the cluster. G. minuta filaments are twined around the sheath and do not appear to penetrate it. The aggregate is composed of a four-celled colony, two of which are 25.8 #m X 14/zm hemispheroids with adjacent faces flattened and separated by a thick (2.3/tm) vertically oriented (relative to the plane of the thin section) medial cross septum (Figs. 8A and 8B ). Two other hemispheroids (also ~ 25/~m X 14 ttm) are separated by a much thinner cross-septum which dips at an oblique angle (Figs. 8C and 8D ). Each of the four cells contains between one and three

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Fig. 7. Interstitial and intragranular microfossils. A and B. Leptoteichos golubicii (?) from the interstitial habitat. C. Bizarre interstitial microstructure with a tail-like process. D and E. Interstitial Huroniospora. F-H. Intergranular Huroniospora showing evidence of shrinkage. I-K. Intragranular Huroniospora cells which are dividing by transverse binary fission. All photomicrographs in plane-polarized light; bar in B = 10/lm for B and D-H,=25/~m for A; bar in C = 10/~m; bar in I = 10 /~m for I-K.

micrometer-sized kerogenous inclusions which are located directly adjacent of the inner cell walls. It seems reasonable that the cluster originally consisted of two large (28-30/tm in diameter) spherical cells, each of which underwent transverse fission to produce the hemispheroids. The two planes of division

which are defined within the colony do not appear to be precisely orthogonal and the relative thicknesses of the medial septa suggest that the two spheroidal cells did not undergo simultaneous fission. Based on size and reproductive pattern, the classification of this colony as a fossilized Chroococcales cyanobacterium is

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compelling because the only extant, comparably sized non-cyanobacterial prokaryote (Thiovulum) shows a pronounced medial constriction prior to cross-wall formation (Buchanan and Gibbons, 1974). Among extant chroococcoids (e.g., A nacystis dimidiata ) , aggregates of two to four cells which become spherical before division are very common when cellular division products fail to separate promptly (Humm and Wicks, 1980). A second allochthonous microfossil-like structure which has also been found among the interstitial filaments is a large (63 ~m X 39/~m ) organically-walled vesicle containing multiple kerogenous inclusions (Fig. 9). The walls of the structure are psilate, very finely granular, and while relatively thin ( ~ 1/~m) are clearly defined. The shape of the microstructure is rather unusual compared with those of other Proterozoic microfossils. It is asymmetrically bulbous with one relatively flat side and a small, pimplelike node at the broad end of the structure. The microstructure lacks any evidence of crosswalls, but instead contains five kerogenaceous inclusions ranging from 2.5/~m to 18 ~m in long dimension, all of which appear to be located at or near the inner wall and each differing somewhat in texture. In terms of overall size and wall structure the structure resembles the Gunflint form taxon Megalytrum diacenium (Knoll et al., 1978), which are interpreted as the large empty organic sheaths of an unknown microorganism. The 52/~m X 36 #m dimensions of one such organic sheath reported by Knoll et al. (1978) are comparable with the interstitial structure. It is also noteworthy in this regard, that if this were an organic sheath it would conform well to the size and shape of the tetrad of chroococcus-like cells which are described above. The possibility that this is an extraordinarily large unicell should not be dismissed outright, however, because there is no completely satisfactory explanation for the peculiarities of the kerogenous inclusions. This speculation must await the identification of additional specimens. A third interesting microstructure {Fig. 7C)

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Fig. 9. A largevesicularmicrostructurefrom the interstitialhabitat shown in plane-polarizedlight.A. Low power photomicrograph showing interstitialoccurrence; note peloid grain surface (arrow) and aligned interstitialfilaments.B. Intermediate power ofA. D and E. High power opticalcross-sectionthrough the structure;note the fiveinclusionswithin the vesicle (arrows in E). Scale bars in A and B shown; bar-- 10/~m in E for C-E.

found among the interstitial filaments is quite unlike any previously described Precambrian microfossil and as such could fall within the Hofmann and Schopf (1983) category of bizarre microfossils. It is 6.3/an X 15.7/~m with a simple tapering, tail-like process 22.1/Lm in

length. Surface texture is psilate,finely granular and organic pigmentation is heterogeneously dispersed throughout the interiorof the structure. The affinitiesof this microstructure are uncertain. It may be a degradational morphotype formed by the differentialdecomposi-

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tion and shrinkage ofa G. grandis filament (see above). The alternative possibility that this is a single cell should also be considered, but until additional specimens can be identified it is best regarded as a dubiofossil.

Intragranular microfossils The intragranular microfossils which are found within the peloidal arenite biofacies are noteworthy for their extraordinary morphological and taxonomic diversity, and also because they show an impressive degree of degradation/preservational variability both between and within individual peloid grains. The Schreiber peloids contain well- to poorlypreserved examples of the majority of previously documented Gunflint microfossils (see Awramik and Barghoorn, 1977; Hofmann and Schopf, 1983). This reflects, in part, the derivation of the peloids from several subenvironments within the Gunflint sedimentary basin. As such, the peloids represent multiple assemblages rather than a single environmentally or taxonomically constrained community. Three peloidal microfossil morphotypes will be considered in detail: (1) Huroniospora spp.; (2) a sparingly-branched filamentous morphotype showing spherical microstructures which are attached to the branches (Type A filaments ); and (3) a richly-branched filamentous morphotype which inhabited peloid shrinkage cracks (Type B filaments). Huroniospora spp. Spherical to ellipsoidal microfossils which fall within the Gunflint form taxon Huroniospora range from about 2/zm to 16 #m in long dimension and are extremely common in certain peloid grains. They are found as simple solitary unicells showing rigid or shrunken crenulated walls (Figs. 7F, 7G, and 7H ), doublets (Figs. 7I, 7J and 7K ), triads, and occasionally as colonial aggregates of over 100 cells. One particular laminated peloid grain con-

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tains an approximately 85/~m × 90/~m aggregate of Huroniospora cells which are confined within a billowy, diffuse, organic-rich external layer and thus strongly resembles a colonial or clonal population (Fig. 10). This contention is further supported by the fact that 12.5% of the cells are preserved at various divisional stages. The cells are spherical to ellipsoidal in shape, with psilate, finely granular walls. They lack well-defined kerogenous inclusions and range from 2.7/~m to 7.7/zm in width (2= 4.61, s = 0.85, N = 1 2 8 ) and between 4.2/~m and 10.2/~m in length (2=6.28, s= 1.19)-defining a mean ellipsoidal cell shape with a size (L + W/2 ) of 5.44 /lm. Axial ratios (maximum width divided by length) within this population range from 0.41 to 0.98. The distribution of cell size is unimodal (4.75 /~m), skewed to larger diameters and ranges from 3.7/~m to 8.7/lm (Fig. 11 ). Similar distributions are subsequently classified as Type E Huroniospora populations. Reproductive patterns are clearly those of a roughly spherical microorganism dividing by transverse binary fission with cell elongation preceding cross-wall formation. Freshly separated cells generally show evidence of their walls having been mutually compressed.

Type A peloidal filaments Type A filamentous morphotypes (Fig. 12) are found in large (generally greater than 5 m m dimension) composite peloid grains composed of a very poorly sorted, unstratified,texturally heterogeneous mixture of fine silt-to medium sand-sized chemical sedimentary particles (Fig. 1B ).These intraclastsdo not contain any other microfossils.Sand-sized grains are supported in a "clotted" matrix of finer grained materials. The peloids are color mottled from dark brownblack to light orange-brown and do not show evidence of post-depositional compaction. The overall texture and compositional heterogeneity of these composite grains appears to have resulted in the formation of intraclasts with a surprisingly high primary interstitialporosity.

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305

Fig. 10. Intragranular and stromatolite Huroniospora shown in plane-polarized light. A-G, J, and M. Huroniospora (Type E cells) from a large colony found in a Schreiber peloid; arrows indicate the outer boundary of the cluster. E, H, I, K and L. Type E Huroniospora from the Schreiber stromatolites. Bar= 10/~m in A for A-M.

306

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Type A filaments are intertwined throughout the peloids, occupying virtually all primary porosity but also penetrating the "clotted" matrix of the grains (Fig. 12D). They do not appear to show any preferred orientation. The filaments are light orange-brown to dark brown, apparently non-septate, straight to gently curved and occasionally appear slightly twisted. Surface textures are variable with most ranging from psilate and smooth to slightly granular. They are sparing but unambiguously branched (Figs. 12A-C) with branch concentrations as high as 3-4 per 50/~m of filament length. As a general rule the diameters of the branches are considerably smaller (10-80%) than that of the main filament from which it grew. This provides at least a partial explanation for filament diameters which range between 0.34/tm and 2.6 #m (2= 0.75; s = 0.34; N= 198). The lengths of unbroken, well-preserved filaments range up to ~ 200 ttm and filament breakage and fragmentation appears to have been preferential at the branch junctions. Spherical to ovoid microstructures from 1.5 #In to 6.7 p m in diameter are occasionally found on branches from the main filaments (Figs. 12D-H). These branches range in length from ~ 2-15 ftm and a single microstructure is borne on a single branch. Microstructure textures range from dark, homo-

geneously pigmented to somewhat clotted and only one such structure has been identified with any well-defined internal structuring (Figs. 12IL). It contains four smaller spheres averaging 1.4 #m in diameter and a minute filamentous tubule (0.2-0.3 #m diameter) is observed emanating from each. On the basis of filament diameter alone it would be reasonable to speculate that Type A filaments are the fossils of a prokaryotic microorganism. Our database of branched Precambrian microfossils is very limited however. The listing of"authentic" early Proterozoic microfossils (Hofmann and Schopf, 1983) includes only 2 of 122 taxa which are considered to show unambiguous branching and their systematic positions are judged uncertain. About 15 branching filamentous "taxa" are known from the later Proterozoic (Schopf, personal communication). Among extant prokaryotes, the diameters of Type A filaments are comparable to those of the Phormidium/Lyngbya group of cyanobacteria, the anoxygenic photoautotroph Chloroflexus, the heterotrophic sheathed eubacteria and the actinomycetes. Only the actinomycetes are truly branched microorganisms. The spherical to ellipsoidal microstructures which are associated with Type A filaments may provide the most important evidence of microbial affinity because they strongly resemble reproductive spores. Among extant prokaryotic microorganisms only certain actinomycetes bacteria produce morphologically comparable spore structures. For example, the family Actinoplanaceae (Couch, 1955) is characterized by slightly to considerably branched filamentous hyphae (0.2-2.6/tm diameter) and sporangia (6-14 #m diameter) containing multiple 1-1.5 p m diameter spores (Buchanan and Gibbons, 1974). The sporangia are borne on branches (1-27 Mm in length) offthe main vegetative filaments (Waksman, 1961). The morphological comparisons between Type A filaments and the actinomycetes are compelling and the similarity between the spore-like structure illustrated in Fig. 12H and an actinoplane

INTERSTITIAL AND PELOID MICROFOSSILS

307

Fig. 12. Type A peloidal filamentous microfossil morphotypes shown in plane-polarized light. A-C. Branching patterns of Type A filaments. C. Branching filaments bearing spore-like structures (small arrows); note filaments penetrating the clotted matrix of the clast (large arrow). E-H. Spore-like structures borne on side branches off main filaments. I and J. Optical cross-section of a spore-like structure which contains four smaller spherical microstructures. Scale bars shown in A and C = 10/~m; scale in D = 25/~m; scale in H = 5/~m for E-L.

308

t~J' I~AXIE~

Fig. 13. An example of Type B filamentous morphotypes taken from a single organic-rich grain of the Schreiber peloidal arenite biofacies. A and B. Branching filaments which apparently grew inwards from a shrinkage crack towards the organicrich interior of the grain. C-G. Branching patterns of Type B filaments; arrows indicate true branches. H. Elongated sporelike structures which are attached to a branched filament. All photomicrographs in plane-polarized light. Scale bars shown in A-C = 10/~m; scale in C h)r C-H,

309

INTERSTITIAL AND PELOID MICROFOSSILS

sporangium is striking (compare with Waksman, 1967, figs. 12.2-12.3, p. 148-149). The actinoplanes are common chemoorganotrophic soil microorganisms a n d their vegetative and reproductive mycelia are largely substrate confined (Waksman, 1967). Type A filamentous morphotypes pervasively penetrate the clast/ sediment substrate and thus strongly resemble an actinomycetes mycelium.

Type B peloidal filaments Type B filamentous morphotypes (Fig. 13) and their degradational variants have been identified in about 1 To of the Schreiber peloids because they are generally found within the small shrinkage cracks (see Fig. 1) which are often associated with the grains. They also appear to have grown from the cracks and extend into the organic-rich substrate of the peloids. The filaments may be profusely- or sparinglybranched, or appear unbranched. Diameters are, however, consistently between 0.2/zm and 0.6 /~m and they commonly appear very darkly pigmented in large part because they are so thin. The finer features of surface texture cannot be resolved optically but the majority of the filaments appear to be unsegmented. In examples of the microfossil which are judged to be profusely-branched (Fig. 13), all of the filaments branch, or originated as branches, and branch diameters are about the same as the filament from which it grew. It is a particular characteristic of Type B morphotypes that several branches can issue from a very short length of the filament and the filaments often rejoin or fuse together when coming into contact with one another. Small ellipsoidal microstructures (0.75 /~m by 1.75/~m) with orthogonal long axis orientations are occasionally attached directly to the filaments (Fig. 13H). These structures are quite rare. Among extant prokaryotic microorganisms only the actinomycetes eubacteria show comparable filament diameters and branching patterns. For example, the genera Micromonospora

and Thermomonospora produce a well-defined substrate mycelium consisting of 0.2-0.6/tin diameter filaments (Waksman, 1961) and when two filaments from the same mycelium make contact, this junction is eventually closed forming a complete cell and producing a network in which the filaments branch and rejoin (Waksman, 1961, figs. 60-62, p. 306). Both actinomycetes genera produce single spores (less than about 1.5 #m) which are borne directly on the vegetative filaments or on very short reproductive branches. The organic-rich peloid grains would also have been ideal habitats for decomposer microorganisms such as the actinomycetes. Discussion

Microfossil assemblages from the Schreiber peloidal arenite biofacies contribute to our understanding of Gunflint microbial ecosystems because they contain many of the same taxa which characterize the Gunflint stromatolite biofacies. Insofar as these microfossils occasionally show unambiguous evidence of growth and reproductive patterns and inhabited slightly different benthic paleoenvironmental niches, they should be a valuable tool for assessing microbial affinities. Certainly one of the central controversies presently surrounding the Gunflint stromatolites concerns the stromatolitebuilding, primary producer affinities of Gunflintia-Huroniospora associations (Strother and Tobin, 1987; Knoll et al., 1988). Strother and Tobin (1987) and Knoll et al. (1988) have recently questioned the conventional interpretation of G. minuta as a stromatolite-forming photoautotroph. Citing the complete lack of parallel aligned or strongly oriented filaments, their breakage into short, relatively straight segments with non-tapering termini and their overall hash-like appearance within organic-rich laminae, Strother and Tobin (1987) suggest that G. minuta compares more favorably with a sheathed, heterotrophic (decomposer) iron-bacterium (Sphaerotilus/Leptothrix group) than with any known microbial

310

mat building cyanobacteria. A similar proposal was initially made by Cloud (1965). This skepticism is indeed well justified by the fact that convincing photomicrographic evidence of parallel-aligned a n d / o r alternating prostrate to erect filament orientation has never been published. The recognition of G. minuta from the interstitial habitat does little to constrain the affinities of this microfossil. Similar Lower Proterozoic occurrences of interstitial G. minuta filaments have been reported by Cloud and Licari {1972) from the Pokegama Formation (Minnesota), and by Knoll et al. (1988) from the Duck Creek Dolomite (Western Australia). Neither of these occurrences appear to be as well-preserved as the new Gunflint materials. Evidence from Schreiber interstitial G. minuta suggests that it was a sheathed, gliding filamentous microorganism with a trichome composed of barrel-shaped cells ( ~ 1.5-3.0/~m), which possessed the potential to grow to well over 2 mm in length. Morphologically comparable extant eubacteria include the Phormidium/Lyngbya group of cyanobacteria, the anoxygenic photoautotroph Chloroflexus, and the Sphaerotilus/Leptothrix group of aerobic, heterotrophic, gliding, sheathed filamentous eubacteria. Photosynthetic filamentous microorganisms (largely cyanobacteria) showing prostrate or erect growth orientations play an important role in binding arenaceous sediments on modern shorelines and in subtidal environments which remain undisturbed for long periods of time (Whitton and Potts, 1982, p. 519 ). Interstitial G. minuta compare favorably with occurrences of sediment binding filamentous cyanobacteria from North Sea tidal flats (see Gerdes et al., 1985, fig. 3, p. 267) in terms of overall filament density per unit area of intergranular porosity and intertwined pattern of growth. It is by no means certain to what extent interstitial G. minuta were associated with particle surfaces (epipsammic) or were truly interstitial, but the crude preferred orientation of filament populations over large areas of the

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sediment would suggest a tactic response by the microorganism to an environmental stimulus. Exceptionally high concentrations of these filaments per unit area of the interstitial habitat (Figs. 2 and 5 ) would tend to support an autotrophic affinity for this microfossil; although the possibility that the microorganism was a facultative heterotroph cannot be dismissed. An obligate heterotrophic metabolism is not favored because these filaments do not penetrate the peloid grains - the only apparent source of preformed organic matter within this particular paleoecological niche. Autochthonous interstitial filament communities from the peloidal arenite biofacies are comparable to stromatolitic filament populations insofar as they consist of over 99% G. minuta and less than 1% of the filaments resemble G. grandis. Direct comparisons between the interstitial filaments and G. minuta from exceptionally well-preserved stromatolite materials (Fig. 14) collected at the Schreiber Beach locality indicate that the former differ only in terms of average filament length and possession of occasional but clearly-defined crosswalls. Interstitial G. minuta show a mean population diameter which is almost identical to that of the stromatolite filaments (Fig. 4), but a size - frequency distribution which is skewed towards smaller diameters suggesting considerable filament shrinkage following evacuation of the sheaths by cellular trichomes. In addition, several examples of G. minuta showing aligned vertical growth and alternating prostrate to erect orientations have been identified from the Schreiber stromatolites (Fig. 14A); although the strongly preferred and more typically preserved orientation of stromatolitic filaments is prostrate (horizontal). Strikingly similar orientations within modern photosynthetic microbial mats have been illustrated for the filamentous cyanobacterium Phormidium from Andros Island stromatolites (Monty, 1976, fig. 2, p. 197); but also characterize mats constructed by the anoxygenic eubacterium Chloroflexus which are found in small columnar

INTERSTITIAL A N D PEI.X)IDMICROFOSSILS

311

Fig. 14. Exceptionally well-preserved G. minuta microbial mats from stromatolites collected at the Schreiber Locality. A. Layers of G. rninuta filaments from the surface o f a stromatolite column which show alternating prostrate (P) to erect (E) to prostrate orientations; note the typical inside-out tapering crack towards the center of the photomicrograph. B. Dense G. rainuta filamentous mat viewed in plan. C. Edge of a mat showing an atypically high concentration of Huroniospora spheroids which are larger than about 10/Ira. All photomicrographs in plane-polarized light; bar scale in A = 50/~m, for A-C.

312

stromatolites from hot springs in Yellowstone National Park (Walter et al., 1976, fig. 5, p. 281). Populations of this Gunflint microorganism were capable of forming dense mats which are noteworthy for being very thin (generally < 60/zm ) by comparison with modern filamentous cyanobacterial mats (Lanier, unpublished data ). Viewed in plan (Figs. 14B and 14C ) these Gunflintia mats showing largely horizontal filament orientations consist of 70%-75% filament volume and 25%-30% fossilized coccoid cell volume (Fig. 15B), as determined by measuring the size of every fossil within a given unit area of the mats (see methods of Lanier, 1986). Filament concentration drops significantly above and below Gunflintia mats and accounts for less than 25% of microfossil volume (Fig. 5A). Filament orientations are closely comparable to those which characterize photosynthetic filamentous microorganisms from modern microbial mats when viewed in plan (compare Fig. 15B herein with Gerdes and Krumbein, 1987, fig. 22A, p. 69). Gunflintia lengths often exceed several hundred micrometers but individual filaments may be broken and fragmented numerous times throughout their extent. Similar patterns of fragmentation may also characterize interstitial Gunflintia filaments (Fig. 3H). It is reasonable to assume that the formation and growth of these Gunflintia mats required an extended period of time in the absence of sedimentation. Evidence from interstitial and stromatolitic occurrences of G. minuta collectively suggest that it was a photoautotrophic, primary producer microorganism which under favorable environmental conditions, and given sufficient time, was capable of forming dense microbial mats and therefore having influenced to varying degrees the morphogenesis of the Schreiber stromatolites. These occurrences do not, however, provide any compelling evidence as to whether G. minuta was an oxygen-producing or anoxygenic microorganism or whether it lined in the presence (aerobic) or absence of oxygen. Huroniospora (Barghoorn and Tyler, 1965)

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has long been a problematic Gunflint microfossil. Taxonomically classified as a spherical to ellipsoidal form genus, Huroniospora ranges in size from ~ 1 to 16/lm and is conventionally considered to represent several fossilized biological species. Polymodal size-frequency distributions for the taxa have been obtained by several workers. Schopf (1976) found modes at 1-3 #m, 4-9 ,um and 10-16 /lm. Knoll and Simonson (1981) obtained modes at 3, 5 and 10/xm for Huroniospora from the early Proterozoic Sokoman Iron Formation, Canada. Knoll et al. (1988) distinguished two spatially separated populations from the Lower Proterozoic Duck Creek Dolomite (Western Australia); small spheroids ranging from 1-5/zm in diameter with a mode at about 2 ttm and large Huroniospora from 3-15 #m in size with a mode between 4-7/tm. Strother and Tobin (1987) found a bimodal size-frequency distribution with modes at about 4 and 7 ttm for Gunflint stromatolitic Huroniospora, although these workers did not consider spheroids smaller than 3 /2m in their analyses. Strother and Tobin (1987) have recently suggested that Huroniospora represents spores or encysed cells rather than vegetative cells. Evidence supporting their conclusion is based on vesicle morphology (circular apertures and breaks in cell walls), the fact that they are almost always solitary individuals rarely showing evidence of cell division, and are never found in clonal clusters or colonies. Occurrences of Huroniospora from grains of the Schreiber peloidal arenite biofacies provides good evidence for a single biological species of the microfossil in the 4-9/~m size range with a mode at about 4.75/~m (Type E cells). Given favorable environmental conditions, this microorganism reproduced by transverse binary fission forming roughly spherical to irregularly-shaped mucilage-embedded colonies at or near the sediment-water interface. Type E Huroniospora is comparable with certain chroococcales cyanobacteria (e.g. Microcystis; Desikachary, 1959) in terms of size, reproduc-

313

I N T E R S T I T I A L AND PELOID MICROFOSSILS

tive patterns, occurrence and mode of colony formation. Detailed measurements of coccoid cell size from well-preserved Gunflint stromatolitic materials (Fig. 14) confirm that Type E Huroniospora comprise a well-defined component of the overall size-frequency distribution of cells found within, above and below silicified Gunflintia mats (Fig. 16). These distributions are consistently polymodal, with modes at about 1.5 /zm, 2.25/Ira and between 4.75/tin and 5.75 #m (Type E). Vesicular structures larger than about 9-10/~m are somewhat rare, but may locally occur in large numbers above or below dense Gunflintia mats (Fig. 14C ). Type E cells are generally dispersed within the mats where they comprise about 5% of the total microfossil volume (Fig. 15B). They are also occasionally found as doublets or triads (Figs. 10E, H, K and L), and two to several cells in close proximity suggesting recent division are not uncommon (Fig. 10I). Three Type E "clusters" consisting of over 40 cells often showing flattened adjacent walls have also been found; although the groupings are not bounded by a "mucilagenous" outer wall. A composite size-frequency distribution for these clusters shows a mean width of 4.75/tm (mode at 3.75) and a mean Ao

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0 BBBBI i D. 31.75<4 IJmW+L/2 I.M,.20-L J L ~ ~ E. >4<10 pm W+L/2 O Filaments o~ F. Total filament ovolume A B C D E F'A B C D E F MICROFOSSIL POPULATION Fig. 15. Histograms showing the distribution of fossilized cell volume (FCV) above, (A) and within, (B) a dense G. minuta filamentous microbial mat. Size groupings for coccold cells are based on a preliminary survey of almost 5000 Huroniospora cells and each distribution is normalized on the basis of total microfossil volume per unit area. Note that the apparently sympathetic relationship between A and B is considered purely coincidental at this time.

301

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of Huroniospora for a single sample population from a stromatolite microbial mat; data collected as a step-wise optical cross-section through the mat. Note the scale change at 2 /~m along the ordinate and that cell size is represented as L+W/2.

length of 6.20 pm (mode at 5.75) (Fig. 17). It is interesting to compare these length and width modes with the composite distribution of Strother and Tobin (1987, fig. 6, p. 328) showing modes at approximately 3.9/tm and 5.85/nn. Observations of Type E Huroniospora from the peloidal arenite biofacies and well-preserved stromatolite materials collectively suggest that while these cells represent an important component of the overall microbial mat fossilized biomass they were insignificant as a stromatolite-forming taxon. The cells were probably int}oduced as an allochthonous component (an inoculum), although because they have been found within the Schreiber peloids to have formed mucilage-embedded (presumably benthic) colonies, they were probably not planktonic microorganisms. In several cases they appear to have undergone limited vegetative growth within Gunflintia mats but ecological conditions were apparently inadequate to support gregarious vegetative growth and colony formation. Further evidence supporting this hypothesis lies in the fact that Type E cells are very common components of intercolumnar lutitic sediments (Lanier, unpublished data ). The detailed observations of Huroniospora vesicle

314

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sustained and prolific vegetative growth then the conclusion of Strother and Tobin (1987) regarding spore-like and/or encysed Huroniospora cells should come as no great surprise.

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morphology which were reported by Strother and Tobin (1987) are noteworthy in this regard. If Type E cells were introduced as an allochthonous component into a microbial environment which was largely unfavorable to

Reports of Proterozoic interstitial microfossils have been few (Cloud and Licari, 1972; Knoll et al., 1988; Green et al., 1988) suggesting that this particular benthic paleoenvironmental niche has been largely overlooked. The significance of the Gunflint interstitial assemblage is twofold. First, although the total number of microfossils thus far identified is small the assemblage appears to be taxonomically diverse. In large part this reflects the input of what are considered to have been aUochthonous microorganisms into a diageneticpermineralization setting which favored the detailed preservation of microfossil morphology. Future investigations of this habitat thus have great potential for expanding our existing database with regard to microbiological diversity 2.0 billion years ago. Secondly, the Gunflint materials are permineralized by silica cements suggesting rapid and pervasive silica cementation may have occurred at or near the sediment-water interface. Recent reports of Lanier (1986), Simonson and Lanier (1987), Awramik et al. (1983), and Knoll et al. (1988) of other chalcedony-preserved microfossils underscore its potential as an excellent "permineralizing" silica phase. Other intergranular microfossils. Types A and B intragranular filamentous morphotypes resemble certain "actinomycetes" eubacteria on the basis of morphology (filament diameters, branching patterns, and spore-like structures) and paleoecological habitat. Reports of actinomycetes-like microfossils have been few. Sokolov and Fedonkin (1984) report fossil actinomycetes from the latest Proterozoic (0.680.57 Ga) of Europe. Hofmann and Schopf (1983) suggest that non-stromatolitic Ramacia ( Oehler, 1977 ) filaments from the 1.6 Ga H.Y.C.

INTERSTITIAL AND PELOID MICROFOSSILS

Pyritic Shale resemble actinomycetes eubacteria. "Actinomycetes" were reported by Jackson (1967) from the 2.2 Ga Gowganda Formation (Canada) but later reinterpreted as modern contaminants (Schopf, 1975; see Hofmann and Schopf, 1983, table 14.2, p. 327). The new Gunflint microfossils appear to have been secondary but synsedimentary occupants of organicrich substrates; habitats which would have been well-suited to heterotrophic, decomposer microorganisms such as the actinomycetes. The possibility that these are modern contaminants is considered highly unlikely given their occurrence within arenitic grains which underwent early diagenetic silica cementation. The actinomycetes designation of the filaments should be weighted with appropriate caution, however, because a recent reevaluation of eubacterial phylogeny using ribosomal RNA sequence comparisons has suggested that genera and species which were previously assigned to the order Actinomycetales (see Buchanan and Gibbons, 1974) actually appear to fall within four different subgroupings of the high G + C (guanine plus cytosine) subdivision of the Gram-positive eubacteria (Woese, 1987). The acquisition of actinomycetes-like morphological characteristics would thus appear to have evolved at several different times within different pleomorphic groups of Gram-positive microorganisms. Morphology and habitat would not, therefore, seem to be adequate or reliable criteria on which to confidently evaluate the possible affinities of these new Gunflint filamentous microfossils. They are at best regarded as "actinomycetoids" and a formal systematic description must await additional research. However, a non-cyanobacterial prokaryotic and possibly heterotrophic affinity for the filaments is compelling.

Significance and conclusions Microfossil assemblages from the Schreiber peloidal arenite biofacies are significant because they (1) expand our existing record of

315

microbiological diversity during Gunflint time; (2) extend our perception of the variety of taxa which have the potential of being well-preserved in Precambrian sedimentary rocks (i.e. actinomycetes-like eubacteria); and (3) augment our understanding of the range of mineral phases (i.e. early-diagenetic silica cements) which were capable of preserving these microbial fossils and the general conditions under which this may have occurred. These assemblages also contain filamentous and coccoid microfossils which are morphologically comparable with the predominant Gunflint stromatolite biofacies taxa (Gunflintia and Huroniospora) and, as such, they also have important potential implications for the paleoecology of the Gunflint stromatolites. Based on the fact that the former show unambiguous evidence of growth and reproductive patterns while there were few convincing reports of stromatolite microfossils which preserve a record of such activities, an initial working hypothesis was to consider Huroniospora and Gunflintia as allochthonous microorganisms which were introduced into a sedimentary setting in which abiogenic stromatolite-like structures were forming. Moreover, because Gunflintia filament fragments and Huroniospora coccoids were found to be an important component of well-preserved intercolumnar siliceous lutitic sediments this hypothesis seemed compelling; although it was later discredited with the finding of fossilized Gunflintia filamentous mats in the Schreiber stromatolites. Detailed comparisons of interstitial G. minuta and intragranular Huroniospora from the Schreiber peloids arenite biofacies with wellpreserved examples of their stromatolite counterparts has led to the following tentative conclusions: (1) G. minuta filament fragments and Type E Huroniospora unicells were important biological constituents of the finer-grained fraction of Gunflint sedimentary materials. (2) G. minuta was a sheathed, gliding filamentous microorganism which was capable of

316

constructing dense microbial mats and thus of having influenced to varying degrees the growth and morphogenesis of the Gunflint stromatolites; it was a phototrophic microorganism based on evidence of aligned growth and occasional alternating prostrate to erect growth, probably autotrophic based on its very high density of fossilized biomass per unit area of both the interstitial habitat and within the microbial mats, presently available information is insufficient to determine whether G. minuta was oxygenic or anoxygenic. (3) Type E coccoid microorganisms appear to comprise a single and well-defined fossilized biological species within the Gunflint form taxa Huroniospora; given appropriate ecological conditions it reproduced by transverse binary fission forming roughly spherical to irregularly-shaped, mucilage-embedded colonies at or just below the sediment-water interface; it is comparable to certain extant colony-forming coccoid cyanobacteria but does not appear to have been an important stromatolite-forming taxon. (4) These new findings concerning Huroniospora further suggest that Gunflint coccoid microfossils which conform to the morphological and size criteria of Type E clonal populations should be assigned to a separate taxon, regardless of the presence of exastment pores and possible endospores; concomitant with the description of additional size-distinctive and spatially separate populations from the Gunflint stromatolites (Lanier, in prep. ), the form genus Huroniospora should be dropped. Additional research and critical evaluations (e.g. Strother and Tobin, 1987) which focus on conventional notions are clearly needed because after having identified several exceptionally well-preserved Gunflintia mats we must question as to precisely why the record of their former presence is so surprisingly scant within the Gunflint stromatolites. Preliminary analyses suggest that Gunflintia was limited in its ability to respond tactically to repeated, shortterm sedimentation events and also limited in

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its capacity for upwards-growth through sediment layers greater than ~ 1 mm in thickness (Lanier, unpublished data). Time may therefore have been a critical factor which determined the relative degree to which this microorganism was capable of constructing stable microbial mats and also the degree to which structured-stratified decomposer communities were able to establish themselves beneath the primary producer level. Finally, this study provides strong support for a biological interpretation of the Gunflint stromatolites which are found at the Schreiber locality (Awramik, 1976). Hopefully, it will provide the emphasis for further investigations to determine the relative degree to which microbiological communities contributed to stromatolite morphogenesis.

Acknowledgements I would like to thank Stan Awramik for first suggesting that the author investigate the Gunflint interstitial microbiota and for having supplied information concerning outcrop localities. The Ontario Ministry of Natural Resources (Terrace Bay District) is gratefully acknowledged for permission to collect samples at the Schreiber Channel Provincial Nature Reserve (Schreiber Locality). Seth Moran assisted in field work and data collection and Robert Oliver, working under a grant from the FordMellon Research Scholars Program for Minorities, wrote a computer program which greatly assisted the author in microfossil-size data reduction and analyses. Field and petrographic work were supported by Oberlin College. The critical reviews and suggestions of M. Droser, J.W. Schopf, A.H. Knoll and H.J. Hofmann contributed significantly to the present version of the manuscript. I also thank Bruce Simonson for many helpful discussions. Veronica Kusznir typed the manuscript while Hubert Bates and Jim Keith made the petrographic thin sections. I thank them all.

INTERSTITIALANDPELOIDMICROFOSSILS

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