Palaeoecology of Chondrodonta (Bivalvia) from the lower Aptian (Cretaceous) Apulia Carbonate Platform (Gargano Promontory, southern Italy)

Chondrodonta is a Cretaceous oyster-like bivalve with a predominantly calcitic, dorso-ventrally elongated and slightly inequivalve shell. In the Tethyan carbonate platforms this taxon occurs commonly in rudist-bearing limestone. Often preserved with still articulated valves, the opportunistic Chondrodonta had a cemented and gregarious life habit. In the Apulia Carbonate Platform (Gargano Promontory, southern Italy), Chondrodonta beds occur within lower Aptian lagoonal to platform-basin margin deposits. The studied Chondrodonta concentrations, which recorded the early evolutionary phase and spreading of this bivalve, are made up of specimens ascribed to Chondrodonta glabra Stanton. In the inner platform the meter-thick Chondrodonta accumulations were originated by autochthonous bouquet-like valve aggregates, frequently preserved in living position. These aggregates constitute superimposed carpets characterized by only a few generations which were affected by mass mortality and rapid burial processes. These mass mortalities were related to repeated environmental lethal fluctuations that occurred at the onset of the early Aptian OAE 1a. The quick fluctuations of the Chondrodonta populations are typical of unstable and unpredictable environments characterized by significant changes in trophic resources and hyperthermal and acidification peaks. The studied Chondrodonta specimens show different shell preservation according to their palaeobathymetric location. In the shallow-water inner platform specimens the internal aragonite shell layer was rapidly dissolved while in deeper-water margin ones it was preserved during the early diagenetic phases. This different preservation is interpreted as related to different aragonite saturation state of shallow seawater, and tentatively connected to the acidification peak that occurred at the onset of the early Aptian OAE 1a.