https://www.pnas.org/doi/full/10.1073/pnas.1815721116

Significance
The 2.1 billion-year-old sedimentary strata contain exquisitely
preserved fossils that provide an ecologic snapshot of the biota
inhabiting an oxygenated shallow-marine environment. Most striking are
the pyritized string-shaped structures, which suggest that the producer
have been a multicellular or syncytial organism able to migrate
laterally and vertically to reach for food resources. A modern analogue
is the aggregation of amoeboid cells into a migratory slug phase in
modern cellular slime molds during time of food starvation. While it
remains uncertain whether the amoeboidlike organisms represent a failed
experiment or a prelude to subsequent evolutionary innovations, they add
to the growing record of comparatively complex life forms that existed
more than a billion years before animals emerged in the late Neoproterozoic.
Abstract
Evidence for macroscopic life in the Paleoproterozoic Era comes from 1.8
billion-year-old (Ga) compression fossils [Han TM, Runnegar B (1992)
Science 257:232–235; Knoll et al. (2006) Philos Trans R Soc Lond B
361:1023–1038], Stirling biota [Bengtson S et al. (2007) Paleobiology
33:351–381], and large colonial organisms exhibiting signs of
coordinated growth from the 2.1-Ga Francevillian series, Gabon. Here we
report on pyritized string-shaped structures from the Francevillian
Basin. Combined microscopic, microtomographic, geochemical, and
sedimentologic analyses provide evidence for biogenicity, and
syngenicity and suggest that the structures underwent fossilization
during early diagenesis close to the sediment–water interface. The
string-shaped structures are up to 6 mm across and extend up to 170 mm
through the strata. Morphological and 3D tomographic reconstructions
suggest that the producer may have been a multicellular or syncytial
organism able to migrate laterally and vertically to reach food
resources. A possible modern analog is the aggregation of amoeboid cells
into a migratory slug phase in cellular slime molds at times of
starvation. This unique ecologic window established in an oxygenated,
shallow-marine environment represents an exceptional record of the
biosphere following the crucial changes that occurred in the atmosphere
and ocean in the aftermath of the great oxidation event (GOE).