Variabilité et hétérochronies chez l’exogyre Rhynchostreon suborbiculatum (Lamarck, 1801) (Bivalvia : Ostreoidea : Gryphaeidae) du Cénomanien et du Turonien inférieur des Charentes (SW France)

Polymorphism of R. suborbiculatum could be summed up as the double occurrence of individuals partially costulated and individuals completely smooth within the same population and a great inter-population variability of general size. After having put aside all altered assemblages, causes of these polymorphisms are analyzed from a palaeoenvironmental point of view and in terms of heterochronies of development. Environment seems to play a minor role in the control of these parameters. Among the several combinations between these two kinds of polymorphism, it is possible to observe a general trend to hypermorphosis during the Cenomanian-Turonian transgression.