The Pennsylvanian ammonoidsGastrioceras magoffinensen. sp.,Maximites nassichukin. sp.,Dimorphoceratoides adamsin. sp.,Bisatoceras? sp.,Phaneroceras chesnutin. 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 theGastrioceras magoffinenseAssemblage Zone. This overlies the well-knownDiaboloceras neumeieriZone, represented in the Kendrick Shale Member of the Hyden Formation in eastern Kentucky. TheGastrioceras magoffinenseZone correlates to theDiaboloceras varicostatum–Winslowoceras henbestiZone in the Winslow Member of the Atoka Formation in northwestern Arkansas. Ammonoids and associated conodonts, includingDeclinognathodus marginodosusand its descendantD. 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.