First report of the lituolid Navarella Ciry & Rat (1951) in the Thanetian of northeastern Italy: an upper Paleocene Lazarus occurrence?

Several large lituolids resembling the genus Navarella Ciry & Rat (1951) were recently recovered in Thanetian hemipelagites of the Belluno Basin outcropping in the Forada section, northeastern Italy (Giusberti et al., 2016; Plate 4, figs. 14, 20). These agglutinated foraminifera are quite common in the >500 µm fraction of the washed residues from the uppermost Thanetian and abruptly disappear at the Paleocene-Eocene boundary, in coincidence with the extinction of Paleocene cosmopolitan taxa (Benthic Foraminiferal Extinction Event; Giusberti et al., 2016). Navarella is a large lituolid, attaining a maximum length of 5 mm, with a first streptospirally enrolled test, later uncoiled and with an aperture varying during the ontogenesis from slit-like to cribrate. The genus was originally described from the Maastrichtian flysch of the Spanish Pyrenees (Ciry & Rat, 1951), and then reported elsewhere in Europe from Campanian-Maastrichtian rocks (e.g., Maync, 1954; Sampò, 1972; Radoičić et al., 2010). The validity of this taxon is, however, controversial and strongly debated (e.g., Stacher, 1980; Riegraf, 1998). In order to document the internal chamber arrangement and the agglutinated wall microstructure of the Thanetian navarellids and to compare them with individuals recovered from Upper Cretaceous strata, the specimens were sectioned and analyzed through a scanning electron microscope (SEM), equipped with an energy-dispersive X-ray spectrometer (EDX). Our results confirm the attribution of the Thanetian specimens to Navarella, thus permitting to expand the stratigraphic distribution of the genus. Despite intensive researches, we did not find any record of Navarella in Danian-Selandian strata of the investigated section, implying at least a 7 Myr gap in the stratigraphic range of this peculiar agglutinated foraminifer. Based on available data, we infer that Navarella reappeared as “Lazarus” genus in the Thanetian having survived the Cretaceous-Paleocene mass extinction, but it was eventually driven to extinction during the environmental perturbations associated to the Paleocene-Eocene Thermal Maximum (Giusberti et al., 2016). We also argue how the finding of new and well-preserved material from lower Paleogene Scaglia Rossa beds of Italy may help in shed light on both taxonomy and ecological preferences of this still poorly known deep-water lituolid.