Some Acanthoceratid Ammonites from Upper Cenomanian (Upper Cretaceous) Rocks of Wyoming

Ammonites of the genera Calycoceras, Dunueganoceras, and Metoicoceras are useful in zoning the Frontier Formation of central and north-central Wyoming and the mostly undifferentiated rocks of northeastern Wyoming that are equivalent in age to the Belle Fourche Shale and Greenhorn Formation. The oldest of the four zones identified contains Dunueganoceras pondi Haas, Calycoceras canitaurinum (Haas), C. newboldi (Kossmat), and Metoicoceras praecox Haas. Calycoceras newboldi has not previously been reported from North America. The second zone is characterized by the new species Dunueganoceras problematicum, Calycoceras rubeyi, C. aff. C. canitaurinum, and Metoicoceras frontierense. A subspecies, D. problematicum natronense, is recognized in this zone in one area of central Wyoming. The third zone has Dunueganoceras albertense (Warren) and Metoicoceras mosbyense Cobban, as well as a new geographic subspecies, D. albertense regale. The fourth and youngest zone may be that of Dunueganoceras conditum Haas. Dunueganoceras is not known in the western interior of the United States above the zone of D. conditum. Metoicoceras, however, ranges higher and terminates in the species M. geslinianum (d'Orbigny).