In this paper, ammonite biostratigraphy and biogeography are used as the basis for an investigation of the origin of the Gulf of Mexico. Three key observations indicate a Pacific rather than an Atlantic origin for the Gulf of Mexico:
1) The Bajocian ammonite Stephanoceras, which occurs throughout the western American margin (Alaska, Canada, United States, Venezuela, Peru, Argentina, and Chile), has also been recorded in the base of the Tecocoyunca Series in Oaxaca (southern Mexico).
2) The Bathonian and Callovian transgressive cycles, which have been recorded in eastern and southeastern Mexico, have been dated on the basis of the ammonites Wagnericeras and Reineckeia, which are of East Pacific affinity and have been recorded in the subsurface of the Gulf of Mexico coastal plain. The transgressions started in Oaxaca and ended with the Gulf of Mexico opening around the Tampico and Campeche areas, as well as in locations around the margins of the Gulf (e.g., southeastern United States and Cuba) during the early Oxfordian, justifying this age for the origin of this paleogeographic province.
3) In Mexico, several groups of known cephalopods, from the Permian to the Jurassic, are related only to fauna in the Pacific province. They occur in isolated sequences located to the west of the present coastline of the Gulf of Mexico.