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 , E 2 ) of the lower Namurian, between the E. bisulcatum s.s. (E 2 a) and Nuculoceras nuculum (E 2 c) Zones of the standard European succession.

At several localities in southwestern Nevada and adjacent California the Mississippian-Pennsylvanian boundary coincides with the contact between the Eleana Formation and the overlying Tippipah Limestone, or their equivalent strata. Near Red Canyon southeast of the Eleana Range, southern Nye County, Nevada, a thin limestone in the Eleana containing the ammonoids Cravenoceras hesperium Miller and Furnish and C. merriami Youngquist of late Chester age is separated by approximately 100 ft of shale and minor quartzite from platy limestone of the Tippipah, containing Diaboloceras aff. D. neumeieri Quinn and Carr of Morrow age. Three inches of conglomeratic limestone marks the base of the Tippipah. In the hills northwest of Frenchman Flat, southern Nye County, 14 ft of shale separates a limestone in the Eleana containing late Chester brachiopods from the basal Tippipah containing the ammonoids Bisatoceras, Diaboloceras, and Stenopronorites. In the hills southwest of Indian Springs, northwestern Clark County, Nevada, we refer a similar sequence (the Indian Springs Member of Longwell and Dunbar, 1936, of the Bird Spring Formation) to rocks equivalent to part of the Chainman Shale and Tippipah Limestone. The Chainman equivalent is about 85 ft thick and contains Mississippian (Chester) brachiopods 25 ft below the top. The basal Tippipah equivalent contains Bisatoceras, Diaboloceras, Stenopronorites, and Syngastrioceras. One foot of conglomeratic limestone marks the base of the Pennsylvanian. The same lithologic sequence is recognized on the northeast flank of the Nopah Range, southeastern Inyo County, California. The Tippipah ammonoids appear closest to late Morrow forms, indicating a hiatus at the base of the Pennsylvanian in this region. A Mississippian-Pennsylvanian disconformity is further suggested by the abrupt lithologic change and the conglomeratic limestones.