The age of the Argiles a Bucaillella of Normandy, the systematic position of the Cretaceous ammonite genera Bucaillella and Arcthoplites, and the delimitation of the Aptian/Albian boundary

The Argiles a Bucaillella of Bec de Caux, Normandy (NW France) are of Early Albian age and represent a local horizon (Subzone of Bucaillella cayeuxi) lying within the Zone of Leymeriella tardefurcata of the standard European sequence. This age determination rests mainly on the recognition of a species of Arcthoplites among the fauna of the Argiles a Bucaillella, described herein as Arcthoplites (A.) marechali sp. nov. Both Bucaillella and Arcthoplites are referred to the family Hoplitidae as primitive members of the Gastroplitinae, a subfamily that reached its peak of development in the Boreal-Arctic regions. The Argiles a Bucaillella are thus the repository of a gastroplitinid fauna that is unique in western Europe and critical for the study of the origin and dispersal of the Albian ammonite superfamily Hoplitaceae. Records of the Aptian genus Acanthohoplites from the Argiles a Bucaillella are incorrect, though its successor, Hypacanthoplites, is well represented. Owing to the limited geographical distribution of early Leymeriella and the extended upwards range of species of the Hypacanthoplites jacobi group, the H. jacobi/L. schrammeni junction-the currently accepted Aptian/Albian boundary-is difficult to recognize outside northern Germany. From a global perspective, the first occurrence of the ammonite genus Hypacanthoplites offers a more satisfactory marker for the base of the Albian Stage. Accordingly, the Zone of H. jacobi is here removed from the Aptian and attached to the Lower Albian. The H. anglicus Subzone of southern England and northern France, previously regarded as the upper part of the jacobi Zone, is redefined and recognized as a subdivision of the tardefurcata Zone, and considered to embrace the Subzone of B. cayeuxi (Argiles a Bucaillella) and probably part of the underlying Sables ferrugineux. (C) 1999 Academic Press.