Abstract
The ammonite speciesLibycoceras crossenseZaborski occurs in the Late Campanian part of the Nkporo Shale of southern Nigeria and in the (mid to Late?) Maastrichtian Farin Doutchi and Dukamaje formations of the Iullemmeden Basin in western Niger and north-western Nigeria. After an interval during whichLibycoceraswas able to migrate between southern Nigeria and the western Chad Basin, northern and southern populations ofL. crossensebecame isolated from one another during the Late Campanian by closure of the connecting route through the Benue Trough.Libycocerasevolved rapidly in southern Nigeria but disappeared from the region early in the Maastrichtian. The northern population ofL. crossensepersisted, being introduced into the Iullemmeden Basin during a gradual westerly shift of marine environments. During the later part of the Maastrichtian a regression brought about the disappearance of the species here. A subsequent Maastrichtian transgressive pulse introducedL. ismaelisinto the Iullemmeden Basin from the north, through a seaway located to the west of the Adrar des Iforas.
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