On the Validity and Evolution of Assilina Species during the Ypresian–Lutetian: Example from Bir Dakhl, Eastern Desert, Egypt

Abstract

AbstractThe genus Assilina is a taxon within the Nummulitacea that appeared early in the Ypresian (Early Eocene) and continued until the end of the Lutetian (Middle Eocene). Thus, this taxon could be useful for the chronostratigraphy of this time interval. Lower Eocene rocks in southern Galala, Egypt are exposed at Bir Dakhl. This section includes marl sediments with debris flow shallow‐marine facies deposits laid down during early Eocene times and includes fossils of large foraminifera: Assilina placentula Deshayes, 1838 and Nummulites burdigalensis de la Harpe, 1926. These are systematically treated, described and illustrated. Nummulites burdigalensis belongs to the N. burdigalensis group, and Assilina placentula belongs to the group of Assilina exponens. This assumption is based on qualitative morphology and quantitative measurements. Both species, together with Operculina libyca Schwager, 1883, enable the assignment of the Bir Dakhl (D5‐40 Section) to the Early Eocene, Ypresian (SBZ10 of Serra‐ Kiel et al., 1998) supporting an earlier opinion that Assilina placentula belongs to that zone in the calibrated larger foraminiferal biostratigraphic zonation.