The nepionic stage of Solenomeris Douvillé, 1924 (Acervulinidae, Foraminiferida): new observations from the uppermost Maastrichtian-early Danian of Austria (Kambühel Formation, Northern Calcareous Alps)

In the shallow-water limestone succession of the Kambühel Formation (upper Maastrichtian to upper Thanetian, Northern Calcareous Alps), the benthic-vagile to encrusting-sessile acervulinid foraminifer Solenomeris Douvillé was identified in upper Maastrichtian and, more commonly, lower Danian rocks. We report on the free nepionic (juvenile) stage of this foraminifer, its relation to the adult encrusting stage, and competitive interaction with coralline algae. Whereas morphology and chamber arrangement of the free nepionts clearly assign the Alpine specimens to Solenomeris, the chamber shape in axial sections differs from the Eocene type-species S. ogormani, accounting for classification in open nomenclature. Nepiont chamber shape can be used as a further specific criterion besides size of adult chambers and, perhaps, growth habit. A two-stage concept of ontogenetic development is presented. Solenomeris flourished in early Danian high-stress environments (perhaps related to cooler waters and/or elevated nutrient level) under conditions less favorable for encrusting corallines that competed for substrate. Absence of Solenomeris in upper Danian limestones and in upper Thanetian coralgal limestones that form the top of the Kambühel Formation may be related to recovery of coralline algae in shallow-marine environments of low to moderate nutrient level.