Acknowledgement. AB fieldwork has been supported by the project PGC2018-093903-C1, additional support came from the projects DR3AM (CGL2014-54118-C2-2-R) MAGIBERII (CGL2017-90632-REDT) and UKRIA4D (PID2019-104693GB-I00/CTA) all of them from the Spanish Science National Plan. The study of calcareous nannofossils was supported by the Research Group of the Basque University System IT-1602-22. Pablo Calvín gave us a hand with the reversal test in Python. The accurate and constructive comments from Johannes Pignatti, Elisabet Beamud, the Journal Manager Laura Rincón, and the editors Carles Martín Closas, Eulàlia Gili and Miguel Garcés helped to improve the original manuscript.An integrated study including magnetostratigraphy, larger benthic foraminifera and calcareous nannofossil biostratigraphy is presented herein. This work was performed in shallow marine siliciclastics rich in larger foraminifera, around the Ypresian/Lutetian boundary in the Ésera valley (South-Central Pyrenees). Although the calcareous nannofossil content in the studied interval is low, not allowing a precise Y/L boundary to be recognised, the taxa found are enough to support the chronostratigraphic attribution. Data obtained in the Ésera valley section has improved the knowledge of larger benthic foraminifera (Nummulites and Assilina) distribution through chron C21. SBZ 11 to SBZ 12 transition took place at the lowermost C21r, as shown in previous works. SBZ 12 assemblages extend into C21n, where the SBZ 12 to SBZ 13 boundary occurs. These data, obtained in shallow marine siliciclastic facies, with in situ fauna, results in a shift of the SBZ 12/SBZ 13 boundary to the Lower Lutetian, younger than previously believed. Accordingly, the Ypresian/Lutetian boundary occurs in SBZ 12
