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.
Highlights
Reports of Paleoproterozoic macrofossils tend to be controversial, and considerable uncertainty persists about the nature of such remains
Macrofossils The specimens described here have been excavated from 45 horizons within the 5 m-thick section of the FB2b black shales
Fossiliferous levels may display more than one morphotype (Figure 8C), in particular, non-pyritized or weakly pyritized disks are associated with pyritized specimens
Summary
Reports of Paleoproterozoic macrofossils tend to be controversial, and considerable uncertainty persists about the nature of such remains. 2.1 Ga Francevillian black shales of Gabon [1] portended a new window on the history of macroscopic multicellular life. Their putative biological origin was investigated with the help of noninvasive structural investigations in combination with D34S analysis elucidating the process of pyritization. Multicellularity has arisen a multitude of times in prokaryotes and eukaryotes [3,4]. It has a long geological history [5] and most likely occurred in numerous lineages not represented in today’s biota. The evolutionary pathways from simple coloniality to complex multicellularity are probably diverse, and involve various issues of cell–cell recognition, competition, co-
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