An extraordinary single-celled architect: A multi-technique study of the agglutinated shell of the larger foraminifer Mesorbitolina from the Lower Cretaceous of southern Italy

Orbitolinids are larger foraminifera widespread in Lower Cretaceous shallow-water carbonates of the Tethyan realm. They are among the most important fossil groups used for Biostratigraphy. Despite this and although the structural features of the group have been described in detail, very little is known about the composition of their agglutinated test and the process by which they selected foreign grains. In this study, the test of Orbitolina d'Orbigny, 1850 (subgenus Mesorbitolina Schroeder, 1962) from Aptian shallow-water carbonate deposits of southern Italy has been studied in detail. We combine petrographic techniques (optical microscope and SEM) with energy-dispersive x-ray spectrometry (EDS), electron probe microanalyzer (EPMA), X-ray diffraction and Raman spectroscopy analyses.The results show that the test of Mesorbitolina is composed of carbonate and non-carbonate agglutinated grains with the latter distributed across the test with a specific pattern, moving from the marginal to the central zone. In the marginal zone, non-carbonate grains are found only in the epidermis and along the septa which are composed of quartz, with smaller amounts of illite/muscovite and K-feldspar grains. In the central zone of the test, non-carbonate grains are distributed in two ways. Coarse grains of quartz and K-feldspar are abundant and randomly placed in the endoskeleton embedded in a mosaic of minute carbonate grains. Flat grains, mainly of illite/muscovite constitute the external part of the septa. Our observations indicate that Mesorbitolina did select and place agglutinated grains across its test, mainly according to their shape, whereas it did not select particles according to grain size. The distribution of agglutinated particles according to their mineralogical composition shows some contradictory evidence and therefore, at the moment, grain selection in function of mineralogy cannot be completely confirmed or ruled out. Analogies in the test composition of Mesorbitolina specimens from coeval deposits from different areas of southern Italy indicate that the features of their agglutinated test are typical characters of the genus Mesorbitolina. However, it is still unclear what advantage was obtained by the foraminifer by the described test features. © 2012 Elsevier B.V..