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 Sphenolithus spiniger Bukry, 1971 ( Fig. 19P ) Sphenolithus spiniger Bukry, 1971: 321 , pl. 6, figs 10-12; pl. 7, figs 1-2. DISTRIBUTION. — The lowest occurrence of typical Sphenolithus spiniger (as defined below) is in the lower part of the Brussel Sand Formation (at Mont-des-Récollets in Unit B2). It is known from many outcrop and borehole sections of the Brussel Sand Formation and increases in numbers during the Sphenolithus bloom, as recorded in the Diegem and Zaventem outcrop sections (Steurbaut, detailed in forthcoming study). It occurs rarely in the Lede Sand Formation. DISCUSSION This sphenolith differs from the other Sphenolithus taxa described herein by its small triangular outline and very distinctive extinction pattern under crossed nicols. Viewed parallel to the polarisation directions it consists of 4 lobes separated by an extinction cross. The 2 proximal lobes are large and oval; the upper 2, which correspond to the lateral elements and spine, are much smaller and triangular. The extinction cross is much broader at its proximal end, so that the separation between the two lower lobes increases in proximal direction. Forms with a similar outline, but with a clearly different extinction pattern, are attributed to Sphenolithus aff. spiniger , known from the upper part of the Aalter Sand Formation and the Brussel Sand Formation. In these foms the lower 2 lobes are also triangular and the extinction cross is not enlarged proximally.