Unchanging biodiversity of bentich mollusk associations during the last two interglacial ingressions of the Po Plain, Italy

Mollusk-dominated fossil assemblages from the Late Quaternary successions of the Po Plain (Italy) were sampled from cores to evaluate changes in mollusk biodiversity between the previous (late Pleistocene) and most recent (Holocene) interglacial ingressions of the Po Plain, Italy. The cores were sampled at 10 cm intervals and all specimens of bivalves, gastropods, and scaphopods were identified to species level. A total of 284 samples from 8 cores yielded 59534 individual specimens representing 231 species from 120 genera. Sample standardized diversity analyses suggest that mollusk diversity was statistically indistinguishable when the samples from the previous interglacial (isotope stage 5e) and the most recent one (isotope stage 1) were compared with one another. The striking similarities were observed for both alpha and beta diversity regardless of the type of depositional environment or the systems tract compared. Concurrently, multivariate ordinations and other analytical approaches suggest that Holocene and Pleistocene samples are also indistinguishable in terms of their quantitative faunal composition. The results suggest a remarkable stability in diversity and composition of faunal associations during the last two interglacials: despite over a 100 k.y. separation from the previous highstand, virtually identical mollusk associations (i.e., comparably diverse sets of mollusk species with the same dominant species) have reestablished themselves on the Po Plain following the Holocene marine ingression. The Po Plain case suggests that regional marine benthic ecosystems can remain static over notable time scales while shifting back and forth in space, in concert with sea level changes, from more distal to more proximal settings. Regardless of its causation, the fact that marine benthic ecosystems may have remained virtually unchanged regionally over long time intervals contrasts (alarmingly) with the magnitude of recent anthropogenic changes observed in mollusk benthic associations, as suggested by recent dead-live fidelity studies.