Chemotrophic Filamentous Microfossils from the Hollard Mound (Devonian, Morocco) as Investigated by Focused Ion Beam

The biologic origin of objects with microbe-like morphologies from the oldest preserved terrestrial sedimentary rocks remains a matter of controversy. Their biogenicity has been questioned, as well as the claim that they are convincing evidence of early life. Though minerals with microbe-like morphologies represent ambiguous evidence of life, they are, in a number of conditions, the only achievable information. In this study, the focused ion beam (FIB) electron microscopy technique was used for nano- and micrometer-scale high-resolution imaging and in situ microsectioning of filamentous microfossils. The structural elements of these filaments, their spatial relationships with the host rock, and artifacts produced by alteration of the original morphology due to laboratory sample processing have been clearly defined. The in situ sectioning provided a means by which to investigate surface and subsurface microstructures and perform different analytical techniques on the same object, which minimizes sample destruction and avoids excessive manual handling and exposure of the specimen during analysis. Improvement in the morphological and compositional evaluation of the filaments has facilitated the development of a hypothesis regarding the metabolic pathway of the filamentous microfossils preserved in the Middle Devonian-aged Hollard Mound deposit, Anti-Atlas, Morocco. The results of this study demonstrate the potential of the FIB/SEM (scanning electron microscopy) system for detecting microbial-scale morphologies.