The Mont-des-Récollets section (N France): a key site for the Ypresian-Lutetian transition at mid-latitudes - reassessment of the boundary criterion for the base- Lutetian GSSP Author Steurbaut, Etienne Royal Belgian Institute of Natural Sciences, OD Earth and History of Life, Vautierstraat 29, B- 1000 Brussel (Belgium) and Department of Earth, and Environmental Sciences, KU Leuven, Celestijnenlaan 200 E, B- 3001 Leuven (Belgium) etienne.steurbaut@naturalsciences.be Author Nolf, Dirk Royal Belgian Institute of Natural Sciences, OD Earth and History of Life, Vautierstraat 29, B- 1000 Brussel (Belgium) text Geodiversitas 2021 2021-05-25 43 11 311 363 journal article 10.5252/geodiversitas2021v43a11 fc56a00d-cf6b-4fda-aa56-d0492e4781cb 1638-9395 4891107 urn:lsid:zoobank.org:pub:07D1F5A8-98E2-40F1-A716-D25CB975EA4C Braarudosphaera stylifera Troelsen & Quadros, 1971 ( Fig. 18 A-D) Braarudosphaera stylifera Troelsen & Quadros, 1971: 212 , pl.1, figs 8-14. DISTRIBUTION. — This taxon is recorded in several middle Eocene outcrop and borehole sections in Belgium (Steurbaut in Herman et al. 2001 and in Smith et al. 2004 ; Steurbaut et al. 2015 ). In the Mont-des-Récollets section it consistently ranges from the uppermost part of the Brussel Sand Formation (base of Unit B4) to the base of the Wemmel Sand Member. In terms of calcareous nannoplankton zones, its occurrence encompasses the upper part of NP14 and the lower part of NP15, as it appears to do in the Paris Basin ( Aubry 1983 , 1986 ) and the Aquitaine Basin ( Lin et al. 2017 ). In the Paris Basin it seems to recur in the Bartonian (Chavençon Marls: NP17 or NP18, Aubry 1983 : pl. 8, figs 10-11), although this has not been observed in the other basins. DISCUSSION B. stylifera differs from all up to now known Braarudosphaera taxa by its very high slightly tapering outline, resembling a pentagonal pyramid with truncated top, the margin sides of which are often slightly bowed inward ( Fig. 18B ). Both pentagonal ends can clearly be observed viewed from above when over-focusing and under-focusing with the microscope.