Smooth and rugose wall textures in earliest Danian trochospiral planktic foraminifera from Tunisia

摘要

New scanning electron microscopy (SEM) photographs of planktic foraminiferal speci- mens from Tunisian sections (El Kef, Ain Settara, Elles) have revealed the occurrence of two line- ages of primitive trochospiral species in the lowermost Danian. The first lineage to appear, which evolves at the PO-Pa transition, exhibits a smooth and/or granular wall texture (with pore-murals), and its species were attributed to the parvularugoglobigerinids (Parvularugoglobigerina and Palaeoglo- bigerina). The second to appear at the Pa-P1 transition has a rugose wall texture (with rugosities and isolated irregular pore-mounds) and is herein assigned to the new genus Trochoguembelitria. Both lineages co-occur in the upper part of Pa (middle-upper part of Eoglobigerina simplicissima Sub- zone), containing quasi-homeomorph species - pseudocryptic under stereomicroscopy - only differ- entiated under SEM by their wall texture (e.g., Palaeoglobigerina alticonusa vs Trochoguembelitria alabamensis). The data at our disposal suggest Trochoguembelitria derived from triserial Guembe- litria and its species evolved in parallel with the parvularugoglobigerinids in the earliest Danian.