Onustus plioextensus (Sacco, 1896) (Gastropoda: Caenogastropoda): a Mediterranean Pliocene xenophorid with western Atlantic relationships?

Onustus plioextensus (Sacco 1896) is a Mediterranean Pliocene xenophorid characterized by a large fragile shell with wide thin peripheral flange, porcellanous below. It is only known from the Piacenzian (Middle Pliocene) of central northern Italy and is very similar to the west Atlantic Onustus longleyi (Bartsch 1931). If the morphological similarity between these two xenophorids is not a result of convergent evolution and testifies to real affinity, then they constitute a group of closely related species with amphi-Atlantic distribution, a biogeographical pattern suggested for certain gastropod species mainly belonging to the tonnoideans.