Surviving strategies of nummulitids. Three-dimensional analysis of the Nummulites fabianii group at the Eocene/Oligocene boundary

The calculation from 3D images of chamber volumes and shapes and their changes during ontogeny provides a large amount of information quite impossible to obtain by the traditional two-dimensional methodology of oriented thin sections. The volume measurement itself does not give information about shell geometry but indicates the influence of temporal changes during the foraminiferal growth. Environmental changes are therefore reflected in deviation from a specific Bauplan. Such deviation can be observed by a three dimensional study of the ontogeny of larger foraminiferal shells through geological time. A pilot study has been done on the N. fabianii group which survives the prominent cooling event at the Eocene/Oligocene boundary. The calculation on the chamber volumes shows a drastic growth speed reduction across the boundary. Among the investigated species, the Oligocene specimens (i.e., N. fichteli) reach the same size of the Late Eocene ones (N. fabianii) 40 chambers later. Such reduction in growth speed may testify to an adaptation by nummulitids to survive in colder environments and possibly lower nutrient levels. A complete trend in speed reduction has been observed from the Late Eocene nummulitids through the late Priabonian to the early Oligocene. As a linear correlation has been confirmed between chamber volume measurements and chamber area in equatorial section, the same trend has been observed through thin section analysis with a larger number of specimens. Size reduction is a well-known adaptation to climatic changes but has never been fully demonstrated so far on larger benthic foraminifers.