Dubiofossils from a Mars?analogue subsurface palaeoenvironment: The limits of biogenicity criteria

The search for a fossil record of Earth's deep biosphere, partly motivated by potential analogies with subsurface habitats on Mars, has uncovered numerous assemblages inorganic microfilaments and tubules inside ancient pores fractures. Although these enigmatic objects are morphologically similar to mineralized microorganisms (and some contain organic carbon), they also resemble abiotic structures. Palaeobiologists have responded this ambiguity evaluating problematic filaments against checklists “biogenicity criteria”. Here, we describe material that tests the limits approach. We sampled Jurassic calcite veins formed through subseafloor serpentinization, water–rock reaction can fuel biosphere is known occurred widely Mars. At two localities ~4 km apart, contained curving, branched composed Mg-silicate Fe-oxide minerals. Using wide range analytical techniques including synchrotron X-ray microtomography scanning transmission electron microscopy, show features meet many published criteria biogenicity comparable fossilized cryptoendolithic fungi or bacteria. However, argue processes driven serpentinization could account same set lifelike features, report chemical garden experiment supports view. These are, therefore, most objectively described as dubiofossils, designation here defend from criticism recommend over alternative approaches, but which nevertheless signifies an impasse. Similar impasses be anticipated in future exploration palaeo-habitats Earth To avoid them, further studies required biomimetic geochemical self-organization, microbial taphonomy micro-analytical techniques, focus habitats.