The lowermost part of the Molare Formation in the Val Lemme area, northwestern Italy, records the early stages of the Oligocene marine transgression within the Tertiary Piedmont Basin (TPB). The studied successions, Lemme and Sottovalle, are composed mainly of coarse siliciclastic lithologies and contain levels rich in Larger Benthic Foraminifera (LBF). Taxonomic analysis indicates that the assemblages are completely dominated by Nummulites fichteli Michelotti 1841 and Nummulites vascus Joly and Leymerie 1848, whereas Nummulites kecskemetii Less 1991 and Operculina complanata (Defrance in Blainville 1822) occur only sporadically. A biometric study was carried out on 101 megalospheric A-forms. Our results show that N. fichteli forms a relatively coherent biometric group, with broad overlap between the Lemme and Sottovalle samples, whereas N. vascus displays a more dispersed biometric distribution and appears to be separated in the two sections. These differences are interpreted as reflecting local population structure, sample-to-sample variability and possible hydrodynamic or facies-related effects, rather than taxonomic separation. Comparison with published biometric datasets places the studied N. fichteli populations within the middle to upper Rupelian, corresponding to SBZ21–22A, clearly separated from the Chattian Nummulites bormidiensis Tellini 1888 of SBZ22B. Although N. kecskemetii (SBZ22B–23) rarely appears in the Sottovalle section only, this occurrence is interpreted as a subordinate element that does substantially not modify the biostratigraphic interpretation. The studied assemblages therefore support attribution of the analyzed interval of the Molare Formation to the upper part of the fichteli-vascus biozone, i.e., SBZ22A and document the development of shallow-marine, LBF rich environments, during the middle Rupelian transgressive phase of the TPB. This study aims to be a contribution to a comprehensive understanding of the initial colonization strategies along a transgressive sequence from riverine - deltaic settings to a fully marine system. Among the early colonizers, LBF appear predominant, their arrival and initial diversity indicate competition for the newly available niches, in a highly stressed condition, and therefore more data are available to fully comprehend how such protists cope and adapt their life strategies in complex ecological settings.
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