The Late Cretaceous ammonite genus Libycoceras in the Iullemmeden Basin (West Africa) and its palaeogeographical significance

The ammonite species Libycoceras crossense Zaborski occurs in the Late Camp anian part of the Nkporo Shale of southern Nigeria and in the (mid to Late? ) Maastrichtian Farin Doutchi and Dukamaje formations of the Iullemmeden Ba sin in western Niger and north-western Nigeria. After an interval during wh ich Libycoceras was able to migrate between southern Nigeria and the wester n Chad Basin, northern and southern populations of L. crossense became isol ated from one another during the Late Campanian by closure of the connectin g route through the Benue Trough. Libycoceras evolved rapidly in southern N igeria but disappeared from the region early in the Maastrichtian. The nort hern population of L. crossense persisted, being introduced into the Iullem meden Basin during a gradual westerly shift of marine environments. During the later part of the Maastrichtian a regression brought about the disappea rance of the species here. A subsequent Maastrichtian transgressive pulse i ntroduced L. ismaelis into the Iullemmeden Basin from the north, through a seaway located to the west of the Adrar des Iforas. (C) 1999 Academic Press .