The planktic foraminifer Planorotalites in the Tethyan middle Eocene
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Morphologically uniform, low-trochospiral foraminifera are a common, though not very well-preserved component of foraminiferal assemblages across the interval including the Middle Eocene Climatic Optimum (MECO) at the Alano (northeastern Italy) and Monte Cagnero (central Italy) Tethyan sections. The morphology of these specimens, combined with their state of preservation, made it difficult to assign them to a benthic or planktic group. We measured their stable isotope (oxygen and carbon) composition and compared them with those of trochospiral, epifaunal benthic (Nuttallides truempyi (Nuttall, 1930)) and planktic foraminifera (Acarinina and Subbotina) in the same samples. Despite the preservation, the isotopic analysis showed that the low-trochospiral forms were undoubtedly planktic foraminifera calcifying in the mixed layer. We assign them to the species Planorotalites capdevilensis (Cushman & Bermudez, 1949). Their abundance, distribution and stable isotope values suggest that P. capdevilensis may have been mixed-layer dwellers without photosymbionts, adapted to slightly more mesotrophic conditions than symbiont-bearing, larger acarininids and morozovelloidids. The oxygen isotope data for benthic and planktic species in the two sections are similar, but the carbon isotopic values of all foraminifera are 1.0–1.5 lower at the Alano section than at Monte Cagnero, probably because the Alano sediments were deposited in the more restricted Belluno Basin, the Monte Cagnero sediments in the more open oceanic setting of the Umbria-Marche Basin.