Data from: Exceptional preservation requires fast biodegradation: thylacocephalan specimens from La Voulte‐sur‐Rhône (Callovian, Jurassic, France)

Konservat-Lagerstätten are seen as snapshots of past biodiversity for a
given location and time. Yet, processes leading to the exceptional
morphological preservation of fossils in these deposits remain
incompletely understood. This results in the deficient assessment of
taphonomic biases and limits the robustness/relevance of palaeobiological
reconstructions. Here, we report the mineralogical characterization of
crustacean fossils preserved within carbonate-rich concretions from the
Jurassic Konservat-Lagerstätte of La Voulte-sur-Rhône (Ardèche, France).
The combination of SEM-EDS, TEM, synchrotron-based XRF, XRD and XANES
allows identifying the mineralogical phases composing these fossils (i.e.
fluorapatite, Fe-sulfides (pyrite, pyrrhotite) and Mg-calcite) and the
surrounding matrix (i.e. Mg-calcite, clays and detrital silicates).
Fluorapatite and pyrite (and pyrrhotite) precipitated during decay under
anoxic conditions, replacing delicate organic structures and preserving
anatomical details. These mineral structures were subsequently
consolidated by a Mg-calcite cement. Of note, histologically similar
tissues were replaced by the same mineral phases, confirming that
‘fossilization [in La Voulte] occurred rapidly enough to be influenced by
tissue composition’. Altogether, the present study shows that exceptional
preservation requires fast biodegradation, thereby confirming recent
experimental evidence.