Aptian (lower Cretaceous) ammonite biostratigraphy of the Francisco Zarco Dam stratigraphic section (Durango State, northeast Mexico)

This work deals with the study of the ammonoid record of a classical stratigraphic section of Aptian age, the Francisco Zarco Dam (Durango State, northeast Mexico). A detailed biostratigraphical analysis considered 1281 specimens systematically sampled on a bed-by-bed basis. Taphonomy, taxonomic composition and paleocology of the studied assemblage, were also stressed. We formally propose an ammonite zonation for this section, which consists of two interval zones, Dufrenoyia justinae and Gargasiceras? adkinsi, one taxon range zone Caseyella aguilerae, and one biohorizon Huastecoceras trispinosoides. Among those biostratigraphic units, three are of new definition. Another topic discussed herein concerns to the problematic definition of the Lower-Upper Aptian boundary using the first stratigraphical appearance of the genus Epicheloniceras. From a paleoecological point of view, we recognized the ammonoid assemblage of the Francisco Zarco Dam section, to be representative of a distal platform environment. Finally, we compared the zonation built for this section with that considered the standard zonation for the Tethyan Domain, concluding that the differences are so substantial that an independent zonation for the Central Atlantic is necessary.