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From the Gunflint Chert and Its Implications for Eukaryote Origins

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From the Gunflint Chert and Its Implications for Eukaryote Origins A ‘Giant Microfossil’ from the Gunflint Chert and its Implications for Eukaryote Origins Mark A. S. McMenamin1,*, Aurora Curtis-Hill1, Sophie Rabinow1, Kalyndi Martin1 and Destiny Treloar1 1 Department of Geology and Geography, Mount Holyoke College, South Hadley, Massachusetts, United States *Correspondence: [email protected]; Tel.: 413-538-2280 Copyright©2019 by authors, all rights reserved. Authors agree that this article remains permanently open access under the terms of the Creative Commons Attribution License 4.0 International License Abstract We report here a ‘giant microfossil’ resembling 2. Geological Setting the conidium of an ascomycete fungus (cf. Alternaria alternata). The specimen is preserved in stromatolitic black The Gunflint Iron Formation is known for its chert of the Gunflint Iron Formation (Paleoproterozoic Eon, microfossiliferous black cherts. These cherts have produced Orosirian Period, ca. 1.9-2.0 Ga) of southern Ontario, compelling evidence for very ancient (ca. 2 Ga) microbial Canada, and the rock that provided the thin section may life [3-11]. Tyler and Barghoorn [3] noted that as “far as we have been collected by Elso Barghoorn as part of the are aware, these [microbes] are the oldest structurally original discovery of the Gunflint microbiota. The large size preserved organisms . and, as such, are of great interest of the fossil sets it apart from other, tiny by comparison, in the evolutionary scheme of primitive life.” This paper [3] Gunflint microfossils. The fossil is 200 microns in length is considered one of the great classics of American earth and has cross walls. Individual cells are 30-46 microns in science (“a benchmark, a monumental ‘first’” [5]). Indeed, greatest dimension. The apical ‘spore’ is cap-shaped, and this was one of the first definitive descriptions of very has partly separated from the rest of the structure. ••• gen. ancient Precambrian life, and the fossils of the Gunflint nov. sp. nov. may provide early evidence for eukaryotes continue to serve as the oldest fully non-controversial (fungi) in the fossil record, and may also represent the evidence for life on earth. Research is ongoing to earliest evidence for asexual reproduction in a eukaryote by distinguish authentic ancient microfossils from, say, means of mitospores. todorokite filament pseudofossils, and reliable criteria for recognizing ancient microbial life forms will be essential in the search for extraterrestrial life [12]. Keywords Gunflint Chert, stromatolites, black chert, The Animikie Group, associated with the Cuyuna, origin of eukaryotes, evolution of fungi, Precambrian Gunflint, Mesabi, and Vermilion Ranges in the Lake microfossils Superior region of Minnesota, USA and the Thunder Bay district of Ontario, Canada, was deposited in the Animikie Basin during the Paleoproterozoic between 2.5-1.8 Ga. The stratigraphic sequence in the Mesabi Range has the 1. Introduction Pokegama Quartzite at its base, overlain in turn by the The earliest known eukaryotes are sphaeromorph Biwabik Iron Formation and the Virginia Formation. The acritarchs assigned to Valeria lophostriata from the Gunflint Range’s similar stratigraphic sequence consists of Changzhougou Formation, Changcheng Group, China. a basal conglomerate, overlain in succession by the Gunflint These fossils are dated to 1673±10-1638±14 Ma [1]. The Iron Formation and the Rove Formation. oldest reported fungi are assigned to Ourasphaira giraldae The Gunflint Iron and Biwabik Iron Formations are from the Shaler Supergroup (Grassy Bay Formation) of correlative rock units [4]. The Gunflint “basal clastics Canada, and are dated to 1013±25-892±13 Ma [2]. We consist of a half meter or so of poorly sorted and poorly describe here a ‘giant microfossil’ from the 1900-2000 Ma lithified massive shaly material consisting of angular-to- Gunflint Iron Formation of Ontario, Canada that is subangular greenstone fragments . associated with well- considerably older than either Valeria or Ourasphaira. The rounded-to-angular grains of quartz, Jasper, chert, and great size of this fossil indicates that it should be evaluated greenstone with occasional grains of fresh pink feldspar. as a candidate for the earliest known eukaryote. The clastics are cemented by a minor amount of carbonate and chert” [4]. Animikie strata deposited at approximately 2.0 Ga are remarkable for their banded-iron formations (BIFs), marine preCambrian/from Barghoorn”. The specimen was deposits that reflect the oxidation of dissolved marine iron evidently presented as a gift to Mount Holyoke College by due to large scale oxygenation of the ocean and atmosphere Barghoorn. Recognizing its importance (in spite of the fact during the Oxygen Crisis. An eJecta layer associated with that it had been poorly curated prior to my arrival at Mount the Sudbury impact event occurs between the Gunflint and Holyoke), I had this slab thin sectioned to create slide 1 of Rove formations and the Biwabik and Virginia Formations, 1/30/92. respectively. Global catastrophe due to the 1.85 Ga Sudbury Slide 1 of 1/30/92 is already known to the impact is implicated in the cessation of global-scale BIF paleontological community due to its service providing the deposition [13]. first specimen used to demonstrate stereophotomicrograph Gunflint Iron Formation microfossils are preserved in images of microfossils, namely, specimens of the black chert and are associated with stromatolites. Some of filamentous microfossil Gunflintia minuta [14]. There is these stromatolites attain 3 m diameter. A typical Gunflint absolutely no question about, and has been no controversy stratigraphic sequence, for example in the vicinity of Flint about, the fact that fossils in slide 1 of 1/30/92 are derived Island and Winston Point on the Lake Superior shoreline, from the Gunflint Iron Formation. develops a tripartite division beginning with Archean Comparison with Barghoorn’s own descriptions of the greenstone (2.6 Ga Ely Greenstone, intruded by 2.5 Ga Gunflint Formation [3, 4] strongly suggest that this Algoman Granite), Gunflint breccia, and Gunflint specimen belongs to the lower algal member of the black stromatolites and black chert. Many of the stromatolites cherts of Schrieber Ontario. This chert crops out at several initiated growth on a cobble of Archean greenstone, and sites at the same stratigraphic horizon “over a distance of retain the greenstone clast in the core of the stromatolite. approximately 190 kilometers along the strike of the More distal facies, as for example in the vicinity of Trans- formation” [4]. Of these sites, the Schrieber locality has the Canada Highway 11-17 one mile north of Nelson Road, best fossil preservation making it the focus of Barghoorn’s develop interbedded light and dark chert that in selected research and the likeliest source of the slab. horizons shows evidence for lateral sedimentation, The thin section (Fig. 1) shows three long columnar developed as intraformational breccias in the form of stromatolites and, in its stratigraphically lower part, several edgewise conglomerates. low domal stromatolites. The columnar stromatolite on the Specimens from the Gunflint sequences of Biwabik, far right (Fig. 1) appears to have toppled over to the left, as Minnesota, USA show development of centimeter scale its growth axis is no longer parallel to the undisturbed stromatolites in banded red Jasper with either low domal or columns to the left. Allochem grains, including ooids, occur columnar forms. The fossil described here is from a in the spaces between the columns. The ooids are very dark stromatolitic black chert (Gunflint Range, Gunflint Iron in color, and some show spalling of their outer laminations. Formation; Fig. 1), with stromatolite morphology strongly Larger, rounded allochems appear to be faintly laminated resembling that of Biwabik Jasper stromatolites. Barghoorn fragments of microbial mat material. The large microfossil and Tyler [4] reported transitions from black to red cherts, of interest occurs near the base of the columnar stromatolite and inferred that these reflected “a change from reducing to (doubled to two conjoined columns in its middle part) at the oxidizing conditions at the time of deposition.” left edge of the slide. 3. Provenance of Sample 4. Giant Gunflint Microfossil Figs. 3-4 show ••• gen. nov. sp. nov. The fossil somewhat The provenance of the sample is certain. It is derived resembles the conidium of modern Alternaria alternata. from the Gunflint Iron Formation of southern Ontario, The fossil is clearly not a modern contaminant as it is Canada. In fact, the rock sample was probably collected by completely embedded in the chert matrix. As noted above, Elso Barghoorn himself as part of the original Gunflint its color matches that of the other organic material in the microbiota research programme, was later donated to chert, and under cross-polarization cryptocrystalline quartz Mount Holyoke College, and thus bears historical as well as crystals of the chert are visible inside the fossil. scientific interest. The fossil described here is Figure 5 shows a modern conidium of Alternaria for unquestionably embedded in chert as it is the same color as comparison. Alternaria is an extremely widespread modern other organic matter in thin section. It is thus not a modern fungus [15], occurring almost ubiquitously in terrestrial [16] contaminant. and marine [17] habitats. Barnett and Hunter [15] describe After arriving at Mount Holyoke College in 1984, one of Alternaria as developing elongate
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