Abstract

It has recently been proposed that the northern Adriatic shelf is a living laboratory in which to test the causes of the evolutionary shift from Paleozoic-like, stationary suspension feeders on the sediment surface to modern, infauna-dominated assemblages. The suggestion is that today's “Paleozoic” ecosystems, composed of a modern fauna, are a regular feature in environments of low nutrient levels and predation intensity. We, however, argue that a high-biomass epifauna is not restricted to oligotrophic settings in the northern Adriatic Sea and that predation intensities are instead at Mediterranean levels, which are neither pre-Cenozoic nor similar to those at high latitudes. Environmental requirements of modern, suspension-feeding epifauna do not support the low-nutrient hypothesis, and we suggest that this striking epifauna depends on the presence of stable hard substrata on the seafloor, is very sensitive to sediment input by flood events and storm-induced sediment resuspension, and is related to seasonally high productivity. Elevation above the sediment-water interface has the advantage of feeding from higher-tier levels and helps these organisms to survive hypoxia, which is a typical seasonal feature of the Adriatic shelf and of many ancient epeiric seas. We hypothesize therefore that the gradual disappearance of large, epicontinental seas, along with their low sedimentation rates and frequent bottom-water hypoxia during the Mesozoic, supported the replacement of the archaic epifauna by modern, bivalve-dominated infaunas.

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