Sous-espèces géographiques et/ou contrôle environnemental de la variabilité morphologique chez “Thomasites” gongilensis (Woods, 1911), (Ammonitina, Acanthocerataceae, Vascoceratinae) du Turonien inférieur de la Haute Bénoué (Nigéria)

Abstract

The ammonite “Thomasites” gongilensis ( Woods, 1911) is a classical species from the Lower Turonian limestones of the Upper Benue Basin (Nigeria). In all the known sections, the species characterises a bed considered as isochronous in all this area; within the ammonite fauna, “ Thomasites” always represents the main part with a ratio of 80 to 98%. The huge range of morphological variation of the shell of this ammonite is well known in the type population of the Ashaka quarry and described as a species represented by several “morphs”, which extreme morphologies are gongilense and compressum, and with instermediates like tectiforme, crassicostatum, etc… But, this variability exists not only within the population but in several samples, coming from other localities, situated in a marginal position according to the population of the type section. So, this interpopulation variation seems to be related to several factors: on the one hand, as it is commonly admited, to internal ones (genetical) showing a certain phenotypic potentiality linked to ontogenetical heterochrony; on the other hand, to external constraints, in connection with environmental conditions (energy and deepness, concurence with other organisms). Owing to all these facts, three taxonomical solutions are proposed. The first one, not retained here, considers the possibility of two sympatric species. The second one, refering to a polytypism and to ecological variations is proposed to explain the large variability; “Thomasistes” gongilensis can be split into two geographical groups which can be considered as subspecies. On the one hand, “Thomasites” gongilensis gongilensis which is present in the center, the Norhtern and the North-Western part of the basin; this subspecies can be secondary split into three subgroups which morphology is aconditioned by external constraints and considered as ecological variants (ecophenotypes). On the other hand, “Thomasistes” gongilensis inflatum which is restricted to the South-Eastern quadrant of the basin. The third solution, proposed as an alternative to the previous one, admits a unical species highly variable below internal (ontogenetical heterochrony) and external (environments) constraints. These investigations open the way to new researches on the taxonomical status, and especialy the generic status of the Wood's species.