Previous to the establishment of the Paleocene-Eocene (P/E) boundary stratotype it was common practice to indicate a boundary interval in anticipation on a formal decision on the position of the P/E boundary, as was also done in the Serra-Kiel et al (1998) zonation. This boundary interval covered SBZ 6-lower7. LBF faunas around this time are characterised by the larger foraminifera turnover, the onset of marked size differences between A and B-forms and dominance of Nummulites, Assilina, Alveolina and Orthophragminids in LBF assemblages. Now that the P/E is defined, the synchronicity of the P/E boundary and LF turnover can be calibrated. Early papers using Spanish sections (Orue-Etxwbarria et al. 2001; Pujalte et al. 2003; Pujalte et al 2009) proposed that both events occurred synchronous, and a more recent paper by Scheibner and Speijer (2009) confirmed this notion in Egyptian sections where an unambigious marker of the P/E-boundary was present. This implies that there is no need to extend the Ilerdian into the Thanetian.
Paleocene-Eocene correlation of deep- and shallow-water biostratigrahic schemes and platoform stages in the Tethyan realm. The main difference with the Serra-Kiel et al (1998) scheme is the position of the early Ilerdian (SBZ5). From Scheibner and Speijer (2009).
What the implications are for biozonation, and the definitions of the boundaries between SBZ4-7 requires an update of all records from this interval.
Scheibner and Speijer (2009) provide a headstart for this discussion. In the Egyptian sections SBZ3 is rather low diverse, characterised by Assilina, Ranikothalia, Miscellanea and Glomalveolina primaeva. SBZ4 has a similar LBF assemblage composition, but is also recognised by, for example, Hottingerina lukasi, Dictyocatina simplex. They combine SBZ5 and 6, and the boundary is characterised by the FO of a number of genera, such as Nummulites, Alveolina and Orbitolites which are important in Eocene biostratigraphy. Notewothy is that Nummulites occurs in SBZ4 deposits in elsewhere in, for example, Egypt, Spain and Turkey. Other taxa, such as Miscellanea and Ranikothalia are not observed in Ypresian sediments in the Egyptian sections, but range into younger sediments elsewhere (e.g. Serra-Kiel et al 1998). Also noteworthy is that samples immediately above the P/E boundary occur Alveolina species characteristic for both SBZ5 and 6.
Stratigraphic ranges of representative shallow-marine benthic foraminifera (from Scheibner and Speijer, 2009)