Abstract
Abstract Oxygen bubbles produced during photosynthesis internally deform filamentous cyanobacterial mats, producing distinctive fenestral patterns. Similar textures preserved in ancient microbialites are useful biosignatures when filaments are no longer preserved, but have typically been observed within stromatolites. This study describes bubble-associated fenestrae within oncoids from the early Cambrian Bayan Gol Formation of Mongolia. Fenestrae appear in mm-scale micritic laminae which contain dense accumulations of large (10 × 300 μm) filamentous Girvanella microfossils. Many laminae are not spherical, often occurring with one flat side opposite a conical peak. Up to six generations of conical geometry are present, with each cone rotated with respect to the previous peak. We hypothesize that the oncoids experienced intermittent disturbances followed by periods of stasis and vertical growth. During resting periods, we hypothesize that flat areas formed the oncoid resting base and peaked areas the top. T...
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