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Acarinina sibaiyaensis is a very distinctive and short-lived planktonic foraminiferal taxon that is restricted to the temporally short Carbon Isotope Excursion (CIE) interval characteristic of the Paleocene-Eocene Thermal Maximum (PETM) event. It is confined to the planktonic foraminiferal Zone El which in southern Egypt corresponds to the Dababiya Quarry Member (DQM). This member yields a complete record of the biotic changes induced by the intense warming during the basal Eocene.

Although this distinctive taxon is widely distributed throughout the tropic to the temperate regions, its morphologic changes and phylogenetic relations during the CIE/PETM interval have not yet been documented. The present study discusses and documents the morphologic changes which affected the short-lived Ac. sibaiyaensis probably caused by the extreme warming during the very brief (175-200 ka) CIE/PETM interval in southern Egypt. These morphologic variations are particularly well developed in the middle phosphatic phase (Beds 2 and 3 of the DQM), in which the CIE curve shows its maximum negative shift. The origin, morphology, wall texture, coiling direction and their changes during ontogeny and evolution of Ac. sibaiyaensis during the CIE-PETM interval at Qreiya, Gebel Abu Had, northeast Qena (southern Nile Valley) and Darb Gaga (southeast of Kharga Oasis, southern Western Desert) are described, interpreted and illustrated. During the PETM, numerous intraspecific varieties of Ac. sibaiyaensis are recorded. Several evolutionary lineages are delineated; some of them stay within the morphologic range of Ac. sibaiyaensis, whereas others lead to other excursion taxa i.e. Acarinina africana and Morozovella allisonensis. In addition, forms transitional to taxa which appear in younger horizons than the PETM (younger than Zone El) are recorded, described and illustrated.