ECOSPACE PARTITIONING IN MOLLUSK ASSEMBLAGES ACROSS QUATERNARY TRANSGRESSIVE-REGRESSIVE CYCLES OF PO PLAIN, ITALY

Upper Quaternary sequences of the Po Plain (Italy) include two, well developed transgressive-regressive cycles that record the two most recent interglacial cycles. The two cycles are nearly identical in terms of facies architecture and associated mollusk-dominated faunal assemblages. Using Bush and Bambach’s (2007) theoretical ecospace scheme, with focus on life habits of mollusks, we compared the abundance structure and ecological characteristics of faunal associations, laterally and vertically across interglacial sequences of the Po Plain Basin. A dense network of cores yielded fossiliferous samples representing a wide range of depositional sequences: from terrestrial facies, through brackish-to-lagoonal settings, up to fully marine deposits.

The mollusks that occur in the core samples represent extant taxa that reside in the Mediterranean and are well understood in terms of bathymetric distribution, substrate preference, and mode of life allowing for a reliable ecological classification of virtually all identifiable specimens. At this point, our data include 465 samples acquired from 11 cores, distributed throughout the Po Plain. A total of 71,346 individuals have been collected so far, including 235 species representing 127 genera of mollusks. Each specimen was then classified in terms of substrate relationships (tiering), mobility, shell fixation, and feeding type. Abundance counts at the species and genus level were done separately to determine if taxonomic level of analysis affects quantitative ecological patterns.

Maximum flooding surfaces (MFS) and Transgressive systems tracks (TST) have similar ecological communities of suspension and detritus feeders persistent over the Pleistocene and Holocene. Conversely, Highstand systems tracks (HST) across the two interglacial cycles cover a broad range of environments and subsequently yield variable ecological signatures. Pleistocene HST reveals higher proportions of detritus feeders and predators, whereas in the Holocene there is an increase in suspension feeders and only a few detritus feeders present. The extensive flooding of the basin during the Pleistocene followed by reduced flooding in the Holocene could possibly reflect the transition from detritus to suspension feeders during the highstand system tracks.