Morphological trends in reticulate <i>Nummulites</i> across the Eocene–Oligocene transition

Abstract At the Eocene–Oligocene transition (EOT), c . 34.44–33.65 Ma, the Earth's climate shifted from ‘greenhouse’ to ‘icehouse’ conditions. This change coincides with the first continental ice growth on Antarctica, global cooling, and widespread overturning of faunal and floral assemblages. Studies have focused on the timing and mechanism of larger foraminiferal extinctions across the EOT, but there has been little work on surviving taxa. Reticulate Nummulites are a morphologically distinctive and biostratigraphically important lineage ranging from the lower Eocene to the Oligocene, surviving the climatic change. While general long‐term trends across the entire lineage are well established, no previous study has examined in detail the potential of climatic events on their evolution. The Tanzania Drilling Project recovered three drill core records crossing the EOT. These are exceptionally well‐constrained stratigraphically, and contain specimens of well‐preserved reticulate Nummulites throughout. Using traditional oriented thin sections and micro‐computed tomographic scanning, we examined the morphology of 157 individual specimens from before, during and after the EOT. Significant increases are seen in proloculus size, radii of inner whorls, and total number of whorls. Our results also show a clear expansion in morphospace across the EOT, which we interpret as a speciation event in progress linked to the climatic shift. Modelling suggests that there is a relationship between the δ 18 O and δ 13 C records and morphological change, indicating that factors such as temperature and nutrient cycling may play a role. However, driving mechanisms appear to vary both through time and maybe even within the growth of the foraminifera.