Organism motility in an oxygenated shallow-marine environment 2.1 billion years ago

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).

Keywords: Francevillian; Paleoproterozoic Era; motility; oxygenation.

Fig. 1.

Reflected-light photographs of pyritized string-shaped specimens from the Francevillian Series, Gabon. White and yellow arrows point to string-shaped specimens and microbial mats, respectively. (A) Slab displaying several straight specimens with straight to convoluted strings. (B–I) Sinuous strings. (C) Straight or slightly contorted strings; frame denotes specimen in D. (D) Enlarged part of C, contour of trace marked in white within frame. Note the termination of the trace showing a small pyrite globule (green arrow). (E) Slab displaying several subparallel specimens in the vicinity of bacterial mats. Note that the relief increase at the surface of the sediment from right to left for each specimen; frame denotes specimen in F. (F) Enlarged part of E, white arrow shows the detail of the trajectory of specimen under the fine clay laminae toward the bacterial mats (yellow arrow). (I and J) Part and counterpart of twinned contorted strings, parting from each other at upper white arrow; box denotes specimen in K. (K) Enlarged part of J; note the contorted strings and braided aspect. (Scale bars: 1 cm.)

Fig. 2.

Micro-CT-based reconstructions of string-shaped structures from the Francevillian Series, Gabon. White and yellow arrows point to string-shaped specimens and microbial mats, respectively. (A) Volume rendering showing the external surface of straight structures. Inset (B) shows enlargement of string ending with a pyrite crystal (black arrows). Same specimen as in Fig. 1 D and E. (C) External surface volume rendering showing weakly sinuous string. (D) External surface volume rendering, frame denotes the position of subvertical tubes. (E) External volume transparencies of the same specimen as in D, lateral view showing the string-shaped specimens inside the host rock. Frame denotes the position of subvertical tubes. (F and G) External volume transparencies of the same sample as in D and E at different heights in the sample. (H) Twinned contorted strings; box denotes portion (cross-section) figured in I. (I) Virtual cross-section of contorted strings, black arrow points to the precompactional deformation of silty shale laminae. (Scale bars: 1 cm.)

Fig. 3.

(A and B) Volume rendering showing continuity between sheet and string morphologies in a single specimen. (Scale bars: 1 cm.)

Fig. 4.

Petrography with SEM and EDX of pyritized string-shaped structures from the Francevillian Series, Gabon. (A and C) Sediment laminae, white arrow, intersected by pyritic strings. Laminae bending around the pyrite string confirm precompactional formation, yellow arrow. (B and D) Bending of sediment layers, yellow arrows, around pyritic strings. (E and F) Element mapping of sections in C and D, showing mineralogical composition of the strings and embedding sediment. White arrow indicates the area of elemental point analyses (G). (Scale bars: 1 cm.) (G) Elemental points analysis indicate the presence of K, Mg, and Fe, which confirm the presence of illite and chlorite, respectively. The picture (Left) shows authigenic illite and chlorite grains and free growth of these minerals within the pore spaces.