Calcareous nannofossil biostratigraphy (Upper Bajocian-Lower Bathonian) of the Ravin du Bès section (Bas Auran, Subalpine Basin, SE France): Evolutionary trends of Watznaueria barnesiae and new findings of “Rucinolithus” morphotypes

A biostratigraphic study based on calcareous nannofossils was performed on the Ravin du Bès section (Bas Auran area, SE France). Semi-quantitative estimates of total nannofossil and single species abundances showed that calcareous nannofloras are rare to common and their preservation is poor to moderate. The following biohorizons were identified and calibrated to ammonite biozonation: the first occurrence (FO) of Watznaueria aff. W. communis, the FO of Pseudoconus enigma; the FO of “Rucinolithus”; the last occurrence (LO) of Carinolithus magharensis; the FO of Stephanolithion speciosum octum and the FO of Watznaueria barnesiae. These results confirm that calcareous nannofossils are good biostratigraphic markers for the Bajocian/Bathonian boundary interval. Moreover, the occurrence of P. enigma allows a direct calibration between Tethyan and Boreal nannofossil events and biozones. Morphometric analyses were performed on the W. communis-W. barnesiae group in order to quantify changes between the two taxa. The intermediate form W. aff. W. communis has been identified; it comprises coccoliths very similar to W. communis, but with central area length ≤ 0.9 μm. This study allowed the detailed reconstruction of the evolutionary trends from W. communis to W. barnesiae: the intermediate taxon W. aff. W. communis appears abruptly in the latest Bajocian and persists after the FO of W. barnesiae in the earliest Bathonian. Our results support the model of punctuated equilibrium rather than a phyletic gradualism. The finding of “Rucinolithus” morphotypes, very similar to the Cretaceous taxa R. terebrodentarius and R. terebrodentarius youngii, is intriguing. They might represent nannoliths strongly affected by a recurrent “Lazarus effect” or be diagenetic artifacts or of bacterial bioprecipitation.