Bathyal large lituolids from the Thanetian of northeastern Italy and their extinction at the Paleocene/Eocene boundary

Several large agglutinated foraminifera resembling the genus Navarella Ciry & Rat (1951), were recently recovered in middle-lower bathyal Thanetian hemipelagites from the Belluno Basin, northeastern Italy. These navarellids first appear in the basal Thanetian and become quite common in the >500 µm washed residue from the uppermost Thanetian. They abruptly disappear at the Paleocene/Eocene boundary, in coincidence with the extinction of Paleocene cosmopolitan taxa (Benthic Foraminiferal Extinction Event). Navarella is a large lituolid belonging to the family Ammobaculinidae Saidova 1981 and attaining a maximum length of 5 mm, with a first streptospirally enrolled test, later uncoiled, and with an aperture varying during ontogeny from slit-like to cribrate. The genus was originally described from the Maastrichtian flysch of the Spanish Pyrenees, and then reported elsewhere in Europe from Campanian to Maastrichtian rocks. The validity of this taxon and its range distribution is, however, controversial and strongly debated. In order to document the internal chamber arrangement and the agglutinated wall microstructure of the Thanetian navarellids and to compare them with similar individuals recovered from the Upper Cretaceous and Danian strata of the same section, the collected specimens were sectioned and analysed using a Scanning Electron Microscope (SEM), equipped with an energy-dispersive X-ray spectrometer (EDX). Our results show a typical bi-layered wall microstructure in the Thanetian specimens, whereas the older Maastrichtian and Danian specimens display a single, thicker agglutinated wall. The taxonomy of the Italian navarellids is discussed and compared with other Navarella-like taxa known in the literature. Based on available data, we infer that bi-layered navarellids, which are morphologically identical to Maastrichtian-Danian specimens but that differ in their internal microstructure, appeared and developed in the Thanetian but they were eventually driven to extinction during the environmental perturbations associated to the Paleocene-Eocene Thermal Maximum. We also argue how the finding of new and well-preserved material from the Paleocene Scaglia Rossa beds of Italy may help shed light on both taxonomy and ecological preferences of the still poorly known deep-water lituolids.