Taxonomy, Paleoecology and Paleobiogeography of the Middle Eocene Nautiloid Cephalopods from the Midawara Formation, Fayum, Egypt

This paper focuses on the nautiloid fauna of the Middle Eocene (Lutetian) Midawara Formation in the Fayum Depression, Egypt. Four species belonging to four genera were identified: Euciphoceras regale, Eutrephoceras (Simplicioceras) centrale, Aturoidea parkinsoni, and Deltoidonautilus sowerbyi. Two species of them (E. regale and D. sowerbyi) are recorded for the first time in Egypt. Paleoecologic analysis reveals that these nautiloids were deposited in a shallow-marine environment and experienced various diagenetic processes. Evidence of abrasion, bioerosion, and encrustations suggests limited post-mortem transportation. Paleoecological reconstruction indicates a nekton-benthic lifestyle for these nautiloids. Furthermore, the presence of rhyncholites (calcic jaws of cephalopods) indicated that these nautiloids inhabited shallow-water areas, and most likely died during nocturnal migrations. Paleogeography of the studied nautiloid in the Lutetian Stage shows a widespread distribution, adaptability and resilience in diverse marine environments across the Tethys Sea, Africa, Asia and Europe. The co-occurrences of predators (such as whales, sharks, gastropods and cephalopods) and scavengers (e.g., crabs) indicate a complex food web and the prosperity of marine life. The significant turnover of the abundance and diversity of nautiloid assemblage from Lutetian to Bartonian ages may be corresponded to severe environmental conditions that dominate during brief warming event of the Middle Eocene Climatic Optimum (MECO).