ORTHOPHRAGMINIDS (FORAMINIFERA) ACROSS THE PALEOCENE–EOCENE TRANSITION FROM NORTH AFRICA: TAXONOMY, BIOSTRATIGRAPHY, AND PALEOBIOGEOGRAPHIC IMPLICATIONS

摘要

The platform margin and slope deposits in Galala, Egypt, record a larger benthic foraminiferal turnover (LFT), previously proposed to coincide with the Paleocene-Eocene (P/E) boundary based on the negative carbon isotope excursion (CIE) consistent with the Paleocene-Eocene Thermal Maximum (PETM). The LFT in these deposits was established on the basis of supposed synchronous first appearances of such taxa as Alveolina, Nummulites, and Orbitolites at or around the P/E boundary, although the response of orthophragminids, assigned only to the genus Discocyclina, was not studied. The orthophragminids in the late Paleocene–early Eocene interval in some of these deposits (sections B3, D5, D6, and D8) are represented by species of Discocyclina Gümbel, Orbitoclypeus Silvestri, and Nemkovella Less, reported for the first time from the southern margin of the Tethys. The Thanetian orthophragminids consist of Orbitoclypeus multiplicatus, O. schopeni, Discocyclina seunesi, and D. tenuis, while assemblages in the early Eocene are characterized by Discocyclina archiaci, D. dispansa, Orbitoclypeus schopeni, and Nemkovella stockari, suggesting a significant change in composition through the sections. The assemblages below and above the P/E boundary, only recorded in sections B3 and D8, mark the orthophragminid zones OZ 1B and 3, respectively. The OZ 2, correlated to shallow benthic zones SBZ 5 and 6 in the basal part of the Eocene and recognized by the first appearance of asterocyclinids, nemkovellids, and some ribbed orbitoclypeid species in northern Tethyan platforms, was not found. This may suggest either a hiatus, recorded previously in section D8 based on calcareous plankton, across the P/E transition in section B3 or an artifact of sampling. Consequently, our data raise some doubts about the aforementioned compositional changes related to the LFT and P/E boundary. We discovered an advanced developmental stage of N. stockari (N. stockari bejaensis n. subsp.) from a new section in Tunisia. This subspecies seems to be a key taxon for the early Eocene of the southern Tethyan platforms and extends the geographic and stratigraphic range of the species. We conclude that orthophragminids at Galala bear a close resemblance to those in the northern Tethyan platforms, especially assemblages from Turkey.