New data on the morphology, taphonomy, and phylogeny of Saghacetus osiris (Cetacea, Basilosauridae) and other archaeocetes from the late Eocene (Priabonian) of Fayoum, Egypt

In this study, we present a revision of so far unpublished archaeocete material from the middle to late Eocene deposits of Al-Fayoum (41.2–33.9 Ma), including two previously unpublished incomplete crania, which was collected in the early 20th century and is currently housed in the Natural History Museum of Vienna. We used measurements and morphological analysis of the bones and teeth, complemented by µCT scans to assign one of the crania and much of the other material tentatively to Saghacetus osiris, while referring the other cranium to Basilosaurus cf. isis. The morphological description of the S. osiris material is the first thorough morphological study of this species since Kellog’s description in 1936. It also includes an analysis of its taphonomy according to which the S. osiris specimen probably died due to a B. isis attack that followed a typical cetacean hunting pattern. The tooth and skull morphology and the depositional environment of S. osiris indicate a less piscivorous but more durophagous lifestyle. Based on sedimentological analyses, both crania most likely originate from the lagoonal upper Eocene Qasr-El-Sagha Formation (late Priabonian). Thus, they represent the first evidence of the occurrence of Basilosaurus in this temporal and spatial context and extend the prey spectrum of this species to adult cetaceans.