The Imo Formation of northeastern Arkansas and the Rhoda Creek Formation of south-central Oklahoma contain rich ammonoid assemblages of approximately equivalent age. The Imo ammonoids include Anthracoceras discus Freeh, 1899; Cravenoceras friscoense (Miller and Owen, 1944); Cravenoceras mapesi n. sp.; Cravenoceras bransoni n. sp.; Rhadinites n. gen. (type species: Cravenoceras miseri Gordon, 1965); Eumorphoceras richardsoni McCaleb, Quinn and Furnish, 1964; Eumorphoceras imoense n. sp.; Peytonoceras ornatum Saunders, 1966; Syngastrioceras imprimis n. sp.; Somoholites cadiconiformis (Wagner-Gentis, 1963); Delepinoceras bressoni Ruzhencev, 1958, and Paradimorphoceras sp. Of these, the Rhoda Creek assemblage contains Cravenoceras friscoense; Cravenoceras bransoni n. sp; Rhadinites miseri; Eumorphoceras imoense n. sp. (?); Somoholites cadiconiformis and Delepinoceras bressoni. Ammonoid faunas which include common or similar elements are known from the British Isles, France, Spain, Belgium, Holland, Germany, Poland, Yugoslavia; from the Soviet Union, in the Donetz Basin, the Southern Urals, Novaya Zemlaya and Central Asia, and from the Algeria-Morocco border area.
The interval represented by the Imo-Rhoda Creek ammonoid fauna should be known as the Eumorphoceras richardsoni-Cravenoceras friscoense Assemblage Zone. This overlies the well-known Eumorphoceras bisulcatum-Cravenoceras richardsonianum Zone, represented in the Sand Branch Member of the Caney Formation in Oklahoma, and in the upper portion of the Pitkin Limestone, north-eastern Arkansas. The E. richardsoni-C. friscoense Zone correlates to the Arnsbergian Stage (upper Eumorphoceras, E2) of the lower Namurian, between the E. bisulcatum s.s. (E2a) and Nuculoceras nuculum (E2 c) Zones of the standard European succession.