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.