Barnea (Anchomasa) parva (Pennant, 1777) (Pholadidae, Bivalvia) in the Pleistocene sediments of northern Latium (central Italy)

The study considers the occurrence of Barnea (Anchomasa) parva (Pennant, 1777) in a Pleistocene deposit cropping out in the Tarquinia area (northern Latium, Italy) and ascribed to the Marine Isotope Stage 7 (MIS 7). The finding represents the first quotation of the species in the Pleistocene and also in the fossil record of the Mediterranean region. The systematics, ecology and biogeographical distribution of B. (A.) parva are discussed in detail. The present-day distribution of B. (A.) parva is characterised by its highest frequency in the Boreal and northern Lusitanian regions decreasing progressively towards lower latitudes. The planktonic stage of the larval development, the adaptation to a wide variety of substrata and the absence of evident geographical barriers suggest that the seawater temperature is one of the most important factors limiting the southward dispersal of the species. The palaeoecological analysis of the associated mollusc fauna revealed that the deposit formed in the upper part of the infralittoral zone, which is consistent with the ecological data known for B. (A.) parva. That consistency and the good state of preservation of the specimens strongly suggest that they probably underwent only limited displacement, allowing the extraction of the valves from their borings. All the above suggests that the occurrence of B. (A.) parva in Pleistocene proximal sediments of the Tarquinia area would correspond to an event of dispersal during a time relatively cooler than today. However, the MIS 7 was, in general, a warm stage having maximum temperatures slightly higher than today. The available data are insufficient to clear up that contradiction. As a matter of speculation, one can presume that the species reached the Tarquinia area during the rise of temperatures at the beginning of MIS 7 or during one of its stadial-interstadial transitions.