Biostratigraphy of Guadalupian (Middle Permian) ammonoids from the Olinalá Formation, southern Mexico

An ammonoid biostratigraphic analysis was conducted for the Permian Olinalá Formation, Guerrero state, southern Mexico. Bed-by-bed sampling yielded 113 ammonoids belonging to 14 different taxa. The biostratigraphic analysis allowed us to identify two ammonoid Zones: Paraceltites elegans and Coahuiloceras sp., and one subzone: Pseudagathiceras spinosum. According to the GSSP-defined boundaries, these biostratigraphic zones extend from the Roadian-Wordian to the lower Capitanian interval. To support the biostratigraphic analysis, several key taxa were reviewed from a taxonomic perspective. This taxonomic assessment validated the genus Coahuiloceras. A quantitative analysis based on morphometric ratios of the species of the genera Timorites and Coahuiloceras reveals a clear divergence between both groups. In addition, the biostratigraphic framework of the Olinalá succession is compared with previously published biostratigraphic data from Sonora, Coahuila and Texas, highlighting similarities and differences among these areas. The identified fauna in the Olinalá Formation is mainly associated with the American Realm.