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 Altiplano 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 Altiplano is interpreted to be uppermost Kimmeridgian (upper to uppermost Beckeri Zone) to lowermost Tithonian (basal to lower Hybonotum Zone). The Mexican specimens studied are the most complete and valuable collection of H. mundulum (Oppel) s.s. known from a given area, and reveal that Mexican populations show phenotypic features different from European ones. Vicariant events accord with data available about the areal and biostratigraphic distribution of this species, as well as with the combination of allocyclic and autocyclic factors influencing Mexican seas, the breaking of populations rather than colonization events, and the impoverished ammonite assemblages showing endemic traits within the stratigraphic interval studied. [ABSTRACT FROM AUTHOR]