Les ammonites du Niger (Afrique occidentale) et la transgression transsaharienne au cours du Cénomanien-Turonien

Abstract

Our study is a paleontological revision of the ammonite faunas connected with the Sahara Cenomanian-Turonian transgression. It is also a refinement of the biostratigraphical framework in the sedimentary Iullemeden basin (Western and Southern Niger: Mt Iguelela and Damergou). About 700 well preserved ammonites have been collected from 8 outcrops (Berere, Tanout Aviation, Birgimari, Dan Dordo, Bakatchiraba in the Damergou and towards West Abalak, Korahane and Iguelela Mountains). It is possible to recognize, locally, nine horizons permitting the description of three new taxa. The Metoicoceras geslinianum (d’ Orbigny) Nigericeras gadeni ( Chudeau) association in particular allows an improved correlation with the N. gadeni ( Chudeau) levels with North-Western Europe and also with the Western Interior and Texas - New Mexico (USA). In addition the presence of such ammonites as Cibolaites (?) africaensis nov. sp. or Coilopoceras inflatum Cobban & Hook also allows acute correlations with USA. However authors admit that the ammonite faunas from Niger essentially have their affinities with the Tethyan realm and even some taxa such as Nigericeras gadeni ( Chudeau) and Thomasites nigeriensis ( Woods) are endemic in the Saharan countries of the Sahel (Algeria, Niger, Northeast Nigeria). Although the connection between the Tethys and the South Atlantic via the Benue trough is at the moment disputed by some authors for the Cenomanian - Turonian, the ammonite distributions lead to the conclusion of their livinghence sea-connection there. This connection began during the upper part of the Gadeni zone (Upper Cenomanian) and ended about the upper limit of the Nigeriensis zone (Uppermost Lower Turonian). This study also deals with ontogenetic and evolutionary analysis. On the one hand Nigericeras gadeni ( Chudeau) - N. jacqueti Schneegans - N. jacqueti involutes nov. subsp. tend to lose their juvenile ornamentation by peramorphosis during their evolution and on the other hand, in Vascoceras (Paravascoceras) cauvini ( Chudeau)- V. (P.) proprium ( Reyment), the strong adult ornamentation seems to disapear by virtue of an opposite tendency: paedomorphosis. The general tendency shown by the Niger ammonite faunas, is to become smoother and to adopt simplified suture lines. It is probably a response to the extrinsic constraints (synergy of climatic, ecological and physiographical constraints) by development of quite different and perhaps more efficied morphologies for the group (channelling of morphological tendency) as a function of their intrinsic constraints (ontogenetic potential).