Résumé
The Quercy phosphorites (Eocene-Oligocene of France) have yielded rare examples of tetrapods fossilised in alive body-dimensions and with soft-tissues (referred as “natural mummies”). Preserved in calcium phosphate, replicating tissues to their finest resolution, these mummies have proven great interests to document the paleobiology of these lineages, but also their taphonomy. Recently, new Quercy mummies were reported, providing an unprecedented occasion to (1) finely characterize the morphology of both their hard and soft tissues, including to the ultrastructural level, and to (2) document the process by which a range of fossil remains, organisms, and organs have experienced a rapid phopshatisation. In this latter goal, the MEDUSE-P project aims at comparing the textures, compositions and interaction between phosphate crystals and microorganisms in (a) phosphorites crusts, (b) fossilized tissues, (c) some experimentally-phosphatized tissues. The latter part is led both in lab and in karst external conditions, on fruits of Vitacea and Anacardiacea. Fossilized soft-tissues consist in a selection of tetrapod and arthropods tissues, fruits of the above mentioned plant families, but also fossil droppings. Both fossilized and experimentally-obtained phophates are spatially characterized using PPC-SRμCT and SEM for textures and microbial interaction. Compositions and cristallisations are assesed using EDS, Raman and FTIR. Preservation to the cellular level was identified in many fossil tissues using PPC-SRμCT. Raman and FTIR signals do not reflect a varying cystallisation along the centrifugal axis of fossil tissues in section. SEM and EDS signals tend to distinguish between zoned and layering mineralisation type (richer in C) and a more amorphic phosphate which resulting of completely intertwined fine apatite nanoscopic needles (poorer in C). Different types of microorganisms are observed in association to these two cristal growing phases, as well as different densities (evidenced by tomogrpahic data). SEM and spectral analyses will ultimately be applied onto phosphate experimentally produced, in lab and, in situ, where the mummies naturally formed.