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.2014 Jun 25;9(6):e99438.
doi: 10.1371/journal.pone.0099438. eCollection 2014.

The 2.1 Ga old Francevillian biota: biogenicity, taphonomy and biodiversity

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The 2.1 Ga old Francevillian biota: biogenicity, taphonomy and biodiversity

Abderrazak El Albani et al. PLoS One..

Abstract

The Paleoproterozoic Era witnessed crucial steps in the evolution of Earth's surface environments following the first appreciable rise of free atmospheric oxygen concentrations ∼2.3 to 2.1 Ga ago, and concomitant shallow ocean oxygenation. While most sedimentary successions deposited during this time interval have experienced thermal overprinting from burial diagenesis and metamorphism, the ca. 2.1 Ga black shales of the Francevillian B Formation (FB2) cropping out in southeastern Gabon have not. The Francevillian Formation contains centimeter-sized structures interpreted as organized and spatially discrete populations of colonial organisms living in an oxygenated marine ecosystem. Here, new material from the FB2 black shales is presented and analyzed to further explore its biogenicity and taphonomy. Our extended record comprises variably sized, shaped, and structured pyritized macrofossils of lobate, elongated, and rod-shaped morphologies as well as abundant non-pyritized disk-shaped macrofossils and organic-walled acritarchs. Combined microtomography, geochemistry, and sedimentary analysis suggest a biota fossilized during early diagenesis. The emergence of this biota follows a rise in atmospheric oxygen, which is consistent with the idea that surface oxygenation allowed the evolution and ecological expansion of complex megascopic life.

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Conflict of interest statement

Competing Interests:The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Geological map of the Francevillian basin and lithostratigraphy of the Paleoproterozoic Francevillian Series.
(A) The location of the fossiliferous quarry is indicated by a star. (B) The Francevillian Series consists of four formations (FA to FD). The star indicates the FB2 Subunit. (C) Detailed lithology of the FB2 Subunit in the fossiliferous quarry.
Figure 2
Figure 2. Pyritized macrofossil assemblage from the FB2 black shales of the Francevillian Series, Gabon.
Photographs and micro-CT volume renderings in semi- or full transparency. Where both part and counterpart are shown, the left side of the pictures shows the stratigraphic surface viewed from above, impressions in the overlying black shale are on the right side. (A–H) Lobate forms showing sheet-like structure, radial fabric (A, B, G, H) and wrinkled appearance (A). Scale bars 1 cm.
Figure 3
Figure 3. Pyritized macrofossil assemblage from the FB2 black shales of the Francevillian Series, Gabon.
Photographs and micro-CT volume renderings in semi- or full transparency. (A–D) Lobate forms showing sheet-like structure and radial fabric (A, B). (E–H) Elongate forms showing sinuous shapes and tightly folded structure. In G, both part (positive epirelief, left) and counterpart are shown. Scale bars 1 cm.
Figure 4
Figure 4. Pyritized macrofossil assemblage from the FB2 black shales of the Francevillian Series, Gabon.
Photographs and micro-CT volume renderings in semi- or full transparency (A–F) Forms with partly or wholly elongated morphology. (A, B) Strongly pyritized specimen with traces of degraded sheet. (C, D) Specimen combining lobate and elongate morphology. (E, F) Strongly pyritized specimen with probably little original morphology preserved. Scale bars 1 cm.
Figure 5
Figure 5. Photographs of pyritized macrofossil assemblage from the FB2 black shales of the Francevillian Series, Gabon.
The size and fragility of these specimens did not permit the use of micro-CT observation. (A, B) Forms combining lobate and elongated, partially sinuous, morphology. Scale bars 1 cm.
Figure 6
Figure 6. Pyritized macrofossil assemblage from the FB2 black shales of the Francevillian Series, Gabon.
Photographs and micro-CT volume rendering in semi- or full transparency. (A–F) Forms combining lobage and elongate morphology. The “knobs” in the elongate middle portion are shown in specimen (E, F). Scale bars 1 cm.
Figure 7
Figure 7. Macrofossil assemblage from the FB2 black shales of Gabon.
Photographs and micro-CT volume rendering in semi- or full transparency and different projections show the disparity of forms from the FB2 Subunit and their diverse inner structural organization. Spatial resolution varies from 30 to 115 µm3. (A–F) Non-pyritized to weakly pyritized disks with a radially striated core encircled (arrow) by a flange-like outer part. Virtual section (bottom) in D shows the sharp contact between the specimen and the laminae of the surrounding black shale. Arrows show a few remains of pyrite. Scale bars 1 cm.
Figure 8
Figure 8. Macrofossil assemblage from the FB2 black shales of Gabon.
Photographs and micro-CT volume renderings in semi- or full transparency (A, B, and C, left) Disks with radially striated core and flange-like outer part. (A) Part and counterpart of specimen. (B) Volume rendering in semi-transparency of the same specimen confirms the sharp contact between the specimen and host sediment. (C, D) Disk (left) and large circular aggregate showing slightly domed circular subunits. Scale bars 1 cm.
Figure 9
Figure 9. Results of δ34S isotopic analyses performed on two pyrite concretions (pyrite “sun” and pyrite “flower”, in blue) and on fossil specimens from Gabon (top, in blue) compared to those from their respective host sediments (orange).
Host sediment values for the pyrite “flower” from and .
Figure 10
Figure 10. Petrography of pyrite crystals.
(A) SEM-BSE image of an euhedral crystal after severe HNO3 etching. Note the spongy texture in the centre and the growth bands in the outer part. (B) SEM-BSE image of floriform pyrite after severe HNO3 etching. Note the radiating texture in the centre and the growth bands in the outer part. (C) Photomicrograph of the spongy pyrite after severe HNO3 etching. This pyrite contains both euhedral and floriform pyrites. Note the orange colour in the centre of euhedral and floriform pyrites. (D) Photomicrograph of coarse pyrite after severe HNO3 etching. Note small orange forms in the middle of some crystals. (E) SEM-BSE image of coarse pyrite after severe HNO3 etching. (F) SEM-BSE view of the transition from spongy (lower left corner) to coarse (upper right corner) pyrite. (G–I) Pyrite “sun” sample (SMNH X4450) under reflected, plane-polarized light, highlighting the growth texture of pyrite. (G, H) Surface of the pyrite “sun” showing apparent growth bands (underlined in red) and elongated radiating crystals (arrow) centrifugally developed perpendicular to the growth bands. (H) Close-up view of G showing the relationships between radial crystals (arrow) and apparent growth bands. (I) Section parallel to the plane of the pyrite “sun” showing a centrifugal arrangement of coarse acicular crystals (arrow). Scale bars 10 µm (A), 50 µm (B, C), 100 µm (D, I), 500 µm (E, F), and 5 mm (G, H).
Figure 11
Figure 11. Results of δ34S analysis performed by secondary ion mass-spectrometry (SIMS) on selected specimens.
Three different isotope patterns are distinguished and related to the stage of pyrite formation during diagenesis. Each case is illustrated by one histogram of δ34S data and the color data plot of a representative specimen. Early growth of pyrite (example from Figure S1 in File S1): pyritization was rapidly completed during early diagenesis. Late growth of pyrite (example from Figure S4 in File S1): pyritization continued with burial through early to somewhat later diagenesis. Prolonged growth of pyrite (example from Figure S9 in File S1): pyritization continued episodically over a large range of burial depth during late stages of diagenesis. An atlas showing specimens and their δ34S data is available in Supporting Information.
Figure 12
Figure 12. Spheroidal carbonaceous palynomorphs extracted from the host sediment by acid maceration.
(A–D) Pictures obtained with a transmitted light microscope (A, B) and environmental SEM (C, D). Folding and wrinkling as well as granular and degraded textures of vesicle walls are likely taphonomic features. V-shaped cuts (B) and holes (D) (arrows) illustrate the vesicle wall structure. Scale bars 50 µm. (Extensive details, including Raman, STXM, FIB, TEM, and FTIR are available in Supporting Information.) (E) Ultramicrotomy section through the organic-wall of a single specimen. Scale bar 5 nm. (F) SEM image (top) of a specimen used to extract and FIB foil.Double arrowhead shows the location from where the FIB foil was extracted. Bright-field TEM image (bottom) of the FIB foil. The dark upper layer, which measures ∼800 nm in thickness, is the platinum strap deposited at the top of the gold-coated sample before FIB milling. Gold coating can be observed as a darker, ∼200 nm thick layer at the top of the specimen. The wall consists of a continuous carbonaceous film (arrows). It is mixed with various mineral particles. The sample lies over a glass coverslip. Scale bars 10 µm (top) and 2 µm (bottom).
See this image and copyright information in PMC

References

    1. El Albani A, Bengtson S, Canfield DE, Bekker A, Macchiarelli R, et al. (2010) Large colonial organisms with coordinated growth in oxygenated environments 2.1Gyr ago. Nature 466: 100–104. - PubMed
    1. Wacey D (2009) Early Life on Earth. A Practical Guide. Springer. 274 p.
    1. Bonner JT (2000) First signals: the evolution of multicellular development. Princeton: Princeton University Press. 156 p.
    1. Knoll AH (2011) The Multiple Origins of Complex Multicellularity. Annu Rev Earth Planet Sci 39: 217–239.
    1. Butterfield NJ (2009) Modes of pre-Ediacaran multicellularity. Precambrian Research 173: 201–211.

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