Echinoderms of the cretaceous deposits from Le Mans (France) : outstanding preservation conditions for an outstanding fauna
Résumé
The Cenomanian stage of the geological time scale was named for Cenomanum, the Latin name of
Le Mans (Sarthe, France). D’Orbigny (1847) has chosen the locality of Le Mans as a type area for
its outstanding richness in marine fossils, especially molluscs, bryozoans, brachiopods and
echinoderms. The Gueranger collection, at the Musée Vert (Le Mans) houses a unique echinoderm
fauna, including many taxa unknown elsewhere. Preservation is exceptional with the occurrence of
echinoids with still-attached spines (Tetragramma roissyi), starfish body fossils with plates, spines,
and granules, still looking as in living conditions (Cenomanaster cenomanensis, Tethyaster
guerangeri, Alkaidia sp.), two forms of ophiuroids (Ophiotitanos sp., an unpublished taxon) and
Crinoids (“Isocrinus” cenomanensis) known from articulated bodies. All specimens were found in
the so-called “lentilles à Echinodermes” of the “Sables du Mans” Formation. The sedimentary
matrix usually combines clay and sand. Juignet (1974) recognized shallow marine conditions for the
formation, with evidence of tidal currents. Individuals were buried rapidly and coated with clays
brought by River floods, which favoured the preservation. The contrasted ecologies of the
associated taxa suggest some mixing of individuals from distinct, not necessarily distant,
environments, and the transport of bodies by coastal and/or storms currents. Similar preservation
conditions are reported in the literature for other uncommon fossil sites that sample uncommon
echinoderm faunas.
The Cenomanian is a critical time for understanding of Asteroida and Ophiuroida. Their fossil
record remains sparsely and unevenly distributed in time and space during the Early Cretaceous. A
high diversity emerged in the Cenomanian of Europe at the dawn of Chalk age. The varied fauna
described in the Chalk of Anglo-Paris basin, the North Aquitaine margin, German-Polish basin, and
the Elbe valley, all show a high endemicity. The European fauna homogenize later with extension
of the chalk and migration of lineages.