The biostratigraphic zonation used to divide the Paleocene–Eocene interval is based on the calcareous nannofossil events defined by Martini (1971). In order to define the base of Zone NP10, characterized by the first occurrence (FO) of Rhomboaster bramlettei, we followed the definition of Bybell and Self-Trail (1995), who included Rhomboaster cuspis of many authors with R. bramlettei. In addition, we adopted the taxonomic remarks of Angori and Monechi (1996), who differentiated three morphotypes of R. bramlettei (R. bramlettei "short arms," R. bramlettei "long arms," and R. bramlettei var. T). The studied interval spans from Zones NP9 to NP10. The upper Paleocene nannofossil assemblages are mainly composed of Coccolithus pelagicus, Toweius pertusus, Fasciculithus tympaniformis, Sphenolithus, and Discoaster multiradiatus. At the P/E boundary, the base of the clay interval, the total nannofloral abundance sharply decreases and an enhancement of dissolution occurs from Sample 208-1266C-17H-3, 112–113 cm, to 17H-3, 98–99 cm. Within this interval the FO of R. bramlettei s.a. was detected in Sample 208-1266C-17H-3, 107–108 cm, where the base of Zone NP10 was recognized. Long rays, asymmetrical, and prominent-stem discoaster species such as Discoaster anartios, Discoaster araneus, Discoaster cf. Discoaster lodoensis, Discoaster salisburgensis, and Discoaster diastypus also have FOs in this interval, as previously documented by Monechi et al. (2000), Kahn and Aubry (2004), and Angori et al. (submitted [N1]). The FO of Fasciculithus thomasii has been recorded at the carbon isotope excursion onset. The genus Rhomboaster is not very common, but several morphotypes of the Rhomboaster-Tribrachiatus lineage were recognized (Table T1). Furthermore, significant changes in the calcareous nannofossil assemblages were observed: (1) an increase in the relative abundance of dissolution-resistant forms such as Discoaster multiradiatus, Sphenolithus primus, and Fasciculithus tympaniformis; (2) a decrease in the relative abundance of C. pelagicus and T. pertusus; and (3) the absence of Zygrhablithus bijugatus. From Sample 208-1266C-17H-3, 101–102 cm, upward, an opposite trend between D. multiradiatus and F. tympaniformis was noted. D. multiradiatus decreases from abundant to few, whereas F. tympaniformis increases from common to abundant up to Sample 208-1266C-17H-3, 57–58 cm, where Fasciculithus decreases. Above this level, only F. tympaniformis is always present throughout the studied interval but in very low abundances. At approximately the same level (Sample 208-1266C-17H-3, 63–64 cm) as the decrease of Fasciculithus, the holococcolith species Z. bijugatus exhibits a significant abundance increase, as observed in several tethyan sections and at high latitudes (Monechi et al., 2000; Bralower, 2002; Orue-Etxebarria et al., 2004; Tremolada and Bralower 2004). The Fasciculithus/Zyghrablithus crossover (N3 event of Zachos et al., 2005) has been placed at 306.28 mcd. Z. bijugatus increases considerably, becoming one of the most abundant taxa together with C. pelagicus, S. primus, and T. pertusus. The genus Toweius above the P/E boundary does not reach the same abundances as in the preboundary interval. See Plate P1 for examples of nannofossils found at Site 1266.