The marine Bajocian Taberna Formation of west-central Oaxaca consists mainly of shale, intercalated siltstone, and frequent iron concretions with ammonites. This formation overlies the continental Zorrillo Formation and is overlain by the continental to subcontinental Simon and Otatera formations. The best outcrops are in the area of Mixtepec and Diquiyú-Yutatio.
Two new ammonite biozones are distinguished: Duashnoceras floresi Range Zone (≃ upper Humphriesianum-lower Subfurcatum standard zones), and Parastrenoceras zapotecum Assemblage Zone (≃ Subfurcatum Standard Zone). The biogeographic affinities are principally with the Mediterranean Province, and also with both the East-Pacific Andean and North Cordilleran provinces. A high percentage of species is endemic. The Stephanoceratinae, mainly Duashnoceras, are dominant in the D. floresi Zone, which also includes Subcollina lucretia, Strigoceras (Liroxyites), Stephanoceras, Phaulostephanus, Stephanosphinctes, and Oppelia. Parastrenoceras and Leptosphinctes are dominant in the P. zapotecum Zone, whereas Oppelia and Strigoceras are less abundant.
Stephanosphinctes n. gen., type species S. buitroni n. sp., is intermediate between the Stephanoceratinae and Leptosphinctinae. The species Phaulostephanus burckhardti is also new.