Mono– to pauci-specific communities in the Upper Cretaceous of Wadi Qena, Egypt: The combined effect of food and oxygen deficiency

Abstract

Understanding the dynamics of ancient marine systems is vital for assessing biodiversity changes in modern ecosystems. This study examines the paleoenvironments of the Upper Cretaceous deposits in the Wadi Qena region (Eastern Desert, Egypt), integrating lithofacies, biofacies, and taphofacies. Based on quantitative analyses of two sections, Wadi Hawashiya and Gabel Qreiya, four firm-bottom and two soft-bottom macrobenthic associations were identified. In Wadi Hawashiya, the Nicaisolopha tissoti association thrived in a shallow, moderate-energy inner ramp lagoonal environment of the Coniacian–Santonian, transitioning to high-energy shoals (calcareous quartz arenite). Two monospecific associations, Pycnodonte (Ph.) vesicularis and Gyrosoria gracilis, along with one pauci-specific Porospherae globularis association, were found in the Campanian–Maastrichtian. These associations indicate a shift to a deeper, outer low-energy ramp environment with a firm substrate and dysoxic, oligotrophic conditions. In Gabel Qreiya, the lower Campanian reflects low-energy tidal flats, while the upper Campanian–lower Maastrichtian contains the Pycnodonte (Ph.) vesicularis association, which lived in a low-energy, firm-bottom inner ramp lagoon. The upper Maastrichtian is dominated by the Lyropecten (A.) acutiplicatus association, which inhabited a deeper outer ramp environment with soft substrates and dysoxic, eutrophic conditions. Overall, the Coniacian–Santonian is marked by higher diversity, while the Campanian–Maastrichtian is characterized by reduced diversity and a dominance of suspension feeders. This shift is interpreted as the result of a combined effect of oxygen deficiency and food shortage, likely caused by a sudden deepening from inner ramp lagoons to outer ramp settings, alongside with declining trophic resources, and oxygen levels.