Chronostratigraphical ranges of Lower Jurassic larger benthic foraminifera and lithiotid bivalves calibrated by carbon and strontium isotope stratigraphy (Southern Alps, Italy)

Following the end-Triassic biotic crisis, the first larger benthic foraminifera (LBF) emerged in shallow-water carbonate platforms of the Early Jurassic Tethys, during the Hettangian and Sinemurian. In inner-platform settings, these LBF frequently coexisted with large aberrant bivalves of the Lithiotis Facies, characterized by thick, gregarious accumulations. This study re-assesses the biostratigraphy of LBF and lithiotid bivalve faunas in a c. 200 m-thick Pliensbachian shallow-water sedimentary succession of the Rotzo Formation in the Trento Platform (Southern Alps, Italy). By integrating carbon and strontium isotope stratigraphy, we refine the chronostratigraphical framework for key LBF taxa that are widely used in Lower Jurassic carbonate platform biostratigraphy. Strontium isotope stratigraphy reveals that Orbitopsella primaeva first occurs in the lowermost Pliensbachian Jamesoni Zone, younger than the late Sinemurian age typically cited. Similarly, the last occurrence of Orbitopsella praecursor is in the lowermost upper Pliensbachian (early Margaritatus Zone), later than the traditionally accepted uppermost lower Pliensbachian. This study also constrains the timing of the appearance, development and demise of lithiotid bivalve associations in the Trento Platform. Lithioperna first appears in the lowermost Pliensbachian Jamesoni Zone, followed by Opisoma near the boundary between the Jamesoni and Ibex zones. Higher in the succession, Lithiotis becomes prominent near the Davoei–Margaritatus zone boundary, forming thick accumulations intercalated with brachiopod-rich beds. The final phase of the Lithiotis Facies is marked by Cochlearites – Lithioperna mounds, starting from the Subnodosus Subzone of the Margaritatus Zone. The disappearance of the lithiotid bivalves coincides with the onset of relatively deeper water conditions at the Rotzo Formation–Misone Limestone boundary in the uppermost Pliensbachian Spinatum Zone.