New insights into the extinction and recovery of marine vertebrates across the Permian-Triassic mass extinction event in the Dolomites, Southern Alps, Italy

Abstract

The impact of the Permian-Triassic mass extinction on marine vertebrates is poorly known due to their scarce fossil record. Here, we report Acrodus sp. [Chondrichthyes], Bobasatrania aff. ladina [Osteichthyes] and an indeterminate actinopterygian from the Changhsingian Bellerophon Formation, and a saurosphargid reptile from the lower Spathian Val Badia Member of the Werfen Formation in the Dolomites, Italy. The saurosphargid is represented by a single vertebra, whose relatively tall neural spine indicates that it might belong to a new species. This is one of the oldest Sauropterygomorpha records worldwide and the first unambiguous occurrence of Saurosphargidae in the western Paleo-Tethys during the Early Triassic. These new findings suggest fishes were more diverse in this paleo-tropical shallow shelf community of the western Paleo-Tethys in the latest Permian than during the Early Triassic. The evolution of the new apex predator niche (Saurosphargidae) relatively shortly after the mass extinction event coincides with the reappearance of predatory cephalopods and precedes the full ecological and taxonomic recovery of benthic groups in the Dolomites community. This contrasts with previous hypotheses about the delayed recovery of higher trophic groups. Furthermore, we dated a surface sample with actinopterygian scales found at a Permian-Triassic boundary outcrop. The U-Pb indicates 248+/-1 Ma as a maximum depositional age, and radiolarian biostratigraphy indicates an Anisian age. This suggests age assignments to surface samples require caution. U-Pb dating further reveals an age discrepancy in the scales and the host rock, likely due to 206Pb enrichment in scales during diagenesis.