Middle Miocene balanid barnacles (Concavinae) from the Geniefa Formation (Gebel Gharra, Egypt): Systematic and palaeoenvironmental insights

Middle Miocene balanid barnacles of the subfamily Concavinae are described from the carbonate-dominated Geniefa Formation exposed at Gebel Gharra, Cairo–Suez District, Egypt. The study is based on detailed morphological and petrographic investigations of shell walls using thin-section analysis. When opercula are not available, internal shellstructures, wall microarchitecture and growth patterns provide important taxonomic criteria and permit a more reliable assessment of the systematic affinities than the external features of compartments alone. Petrographical observations reveal well-preserved calcareous compartments - carina,first carinolatus,second carinolatus and rostrum with limited diagenetic alteration, allowing recognition of primary skeletal fabrics and ontogenetic features The concavine barnacles commonly occur associated with oysters, pectinids, and abundant infaunal organisms. Their occurrence, together with sedimentological and faunal evidence, indicates deposition in a warm, shallow-marine inner-ramp setting subjected to normal marine salinity and low to moderate hydrodynamic conditions. The predominance of suspension-feeding communities and the occurrence of hard substrates favoured the establishment of dense barnacle populations. Taphonomic and thinsection data further suggest scarcity of postmortem alteration and support anautochthonous preservation of the assemblage. Most specimens retain their original shell microstructure and exhibit minor fragmentation, abrasion, and bio-erosional overprinting. The low abundance and moderate diversity of borings, together with the preservation of delicate compartment features, suggest limited residence time at the sediment-water interface and early burial under relatively stable depositional conditions. Distribution of concavine balanids within the Geniefa Formation reflects favourable ecological conditions that prevailed during the Middle Miocene transgressive phase along the Gulf of Suez region, characterised by shallow, well-oxygenated, and nutrient-rich marine environments.