Morphology and C-isotopic composition affirm biogenicity of Mesoarchean sphaeromorph organic-walled microfossils from the ∼3.0 Ga Farrel Quartzite, Western Australia

Carbonaceous cherts of the ca. 3.0 Ga Farrel Quartzite in the Pilbara Craton, Western Australia host abundant morphologically diverse microfossils including filaments, spheroids, and lenses of unknown biological affinities, as well as films. The Farrel Quartzite microfossils are organically walled, include forms similar to younger (Proterozoic) leiosphaerids, and some can be extracted by palynological acid maceration. Thus, they represent the oldest records of organic-walled microfossils (OWMs) and non-pollen palynomorphs. The Farrel Quartzite OWMs are composed largely of lenses, with subordinate of flexible-walled large spheroids (FWLSs) and films, among which the FWLSs and films have been poorly described and examined. In this study, we report morphology, texture, and chemical features including Raman spectra, Fourier transform infrared spectra, and carbon isotopic composition of the FWLSs, all of which point to their biogenicity. Their dispersed occurrence in petrographic thin sections and the lack of morphologies corresponding to reproduction stages such as budding and division imply that they represent resting stage, which remains to be explored further. The major axis of the extracted FWLSs (n = 185) ranges from 67 µm to 247 µm with the mean value of 180 µm, which are correlative to the Proterozoic and the 3.2 Ga South African OWMs that can be extractable by acid maceration. This study provides additional evidence of early evolution of microorganisms that could have produced flexible and acid-resistant sac-like large vesicles.