Preservation bias in the Fezouata Shale.

International audienceExceptionally preserved biotas have been providing crucial paleontological data on the earliest major pulses of animal evolution – the Cambrian Explosion and the Ordovician radiation – for over 100 years. The Fezouata Shale is the only Ordovician formationto yield a diverse exceptionally preserved biota, yet the mechanism behind the preservation of soft parts in this formation and the potential of the Fezouata Shale depositsto reveal accurate snapshots of early animal life remain poorly understood. Here, based on two recently drilled cores, we constrain facies controls over the exceptional preservation of theFezouata Biota. We find that the mechanisms leading to soft part preservation in some distal facies of this formation are strikingly different from those at play in Cambrian deposits. In addition to that, based on an original database, we compare the preservation potentialof this new mechanism to the processes operating for Cambrian soft tissue preservation in the Burgess Shale and the Chengjiang Biota. Using a statistical approach,we show that the spatial distribution and type of soft tissues preserved are significantly differentbetweenCambrian biotas and theFezouata Shale, which is distorting to a variable extent our knowledge on the original diversity in ancient paleoenvironments. Constraining this bias is essential to correctly reconstruct paleoecological and evolutionary scenarios