The Pennsylvanian ammonoids Gastrioceras magoffinense n. sp., Maximites nassichuki n. sp., Dimorphoceratoides adamsi n. sp., Bisatoceras? sp., Phaneroceras chesnuti n. sp., and Christioceratidae gen. indet. occur in the Magoffin Member of the Four Corners Formation in eastern Kentucky. The interval represented by the Magoffin ammonoid fauna should be known as the Gastrioceras magoffinense Assemblage Zone. This overlies the well-known Diaboloceras neumeieri Zone, represented in the Kendrick Shale Member of the Hyden Formation in eastern Kentucky. The Gastrioceras magoffinense Zone correlates to the Diaboloceras varicostatum–Winslowoceras henbesti Zone in the Winslow Member of the Atoka Formation in northwestern Arkansas. Ammonoids and associated conodonts, including Declinognathodus marginodosus and its descendant D. donetzianus, indicate an early Atokan age corresponding to the basal Bolsovian (basal Westphalian C) Substage in western Europe and the Bashkirian–Moscovian Stage boundary interval in eastern Europe and the South Urals.