Geographic control on phenotype expression. The case of Hybonoticeras mundulum (Oppel) from the Mexican Altiplano

Hybonoticeras mundulum (Oppel) (m) s.s. is reported from the Mexican Altipl ano on the basis of material collected bed-by-bed in sections in the States of Durango and Zacatecas. The known range of this species in the Mexican A ltiplano is interpreted to be uppermost Kimmeridgian (upper to uppermost Be ckeri Zone) to lowermost Tithonian (basal to lower Hybonotum Zone). The Mex ican specimens studied are the most complete and valuable collection of H. mundulum (Oppel) s.s. known from a given area, and reveal that Mexican popu lations show phenotypic features different from European ones. Vicariant ev ents accord with data available about the areal and biostratigraphic distri bution of this species, as well as with the combination of allocyclic and a utocyclic factors influencing Mexican seas, the breaking of populations rat her than colonization events, and the impoverished ammonite assemblages sho wing endemic traits within the stratigraphic interval studied.