Exceptional fossil preservation is defined by the preservation of soft to lightly sclerotized organic tissues. The two most abundant types of soft-tissue preservation are carbonaceous compressions and replicates in authigenic minerals. In the geological record, exceptionally preserved soft fossils are rare and generally limited to only a few stratigraphic intervals. In the Fezouata Shale (Lower Ordovician, southern Morocco), we found that deposits yielding pyritized soft tissues contain iron-rich silicate minerals. These minerals played a crucial role in inhibiting the decay of dead individuals and are comparable to those found in formations yielding carbonaceous soft parts around the world. Furthermore, we found that iron-rich minerals show a cyclic pattern of occurrence (of ~100 k.y. periodicity) implicating a short-period eccentricity control on iron availability through the general oceanic and atmospheric circulations. Our results identify, for the first time, an external climate forcing on exceptional preservation and show that orbital forcing may be a level-selective parameter responsible for the discontinuous occurrence of horizons preserving soft parts around the world.

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