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 floresiRange Zone (≃ upper Humphriesianum-lower Subfurcatum standard zones), and Parastrenoceras zapotecumAssemblage 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. floresiZone, which also includes Subcollina lucretia, Strigoceras (Liroxyites), Stephanoceras, Phaulostephanus, Stephanosphinctes, and Oppelia. Parastrenocerasand Leptosphinctesare dominant in the P. zapotecumZone, whereas Oppeliaand Strigocerasare less abundant. Stephanosphinctesn. gen., type species S. buitronin. sp., is intermediate between the Stephanoceratinae and Leptosphinctinae. The species Phaulostephanus burckhardtiis also new.