Bathyspinula excisa (Philippi, 1844) (bivalvia, protobranchia): A witness of the plio-quaternary history of the deep Mediterranean benthos

Bathyspinula excisa (Philippi) is a nuculoid bivalve known from the Mediterranean Plio-Pleistocene bathyal deposits. It appeared in the Middle Pliocene and reached great success in the Late Pliocene to the Early Pleistocene, dominating the deepest Plio-Pleistocene molluscan communities so far known. Both the appearance and the spreading of this species correspond to a general development and prosperity of the deep Mediterranen benthos. Regarded in past times as an Atlantic species, because of the misidentification of a closely related Atlantic species, Bathyspinula excisa should be considered as a probable deep-water Mediterranean palaeoendemism, belonging to a typical oceanic deep-water genus. The case of Bathyspinula excisa is not isolated, since many Plio-Pleistocene deep-sea species from the Mediterranean show, close taxonomic and morphologic affinities with living deep oceanic species. Their disappearance from the Mediterranean, during the Late Pleistocene, is referred to the change from a cold deep environment, to a "warm" homothermic one: that is from a widely Ocean-connected Mediterranean Sea, to its present "threshold basin" condition.