Ammonoid faunas of late Kinderhookian (Early Mississippian) age, virtually unknown previously in the American West, are described from several localities in New Mexico, Nevada, Utah, Idaho, and Montana. This study is based upon approximately 130 specimens from 33 collections. These specimens are variously assigned to at least 15 species in the following taxa: Protocanites, Imitoceras, Gattendorfia (Gattendorfia), Kazakhstania, Pericyclus (Goniocyclus), Rotopericyclus (Rotopericyclus), and R. (Hammatocyclus). The genus Muensteroceras, once widely regarded as appearing in the Kinderhookian, is unrepresented in these collections.
The western faunas are characterized by the predominance of the pericyclids and relative scarcity of Imitoceras. Midcontinent faunas of the same age are characterized by the predominance of Imitoceras, scarcity of Pericyclus (Goniocyclus), and presence of Prodromites. Ammonoid faunas of early Osagean age differ in the presence of abundant Muensteroceras and rare Pericyclus (Pericyclus) and in the absence of Prodromites and Pericyclus (Goniocyclus).
Most of the late Kinderhookian ammonoids, both in the West and in the Midcontinent, are assignable to the Siphonodella isosticha–Upper S. crenulata Conodont Zone of Sandberg et al. (1978). A few are referable to the Lower Siphonodella crenulata Zone of the same authors. No ammonoids have been described from equivalent beds in Europe which are included in the Siphonodella crenulata Zone of Voges (1960), despite the fact that Voges regarded this conodont zone as equivalent to the ammonoid Zone CuIIa of Schmidt (1925, p. 493). Based upon conodont correlations, the American Pericyclus (Goniocyclus) fauna is middle Tournaisian (Tn2b, c) in age and, with its Midcontinent equivalents, represents a part of the section between the Gattendorfia and the Pericyclus zones of Europe, from which ammonoids are as yet undescribed.
One new subgenus is described in this paper: Pericyclus (Goniocyclus), its type species Goniatites blairi Miller and Gurley, 1896. New species described include Pericyclus (Goniocyclus) cooperi, P. (G.) costulatus, P. (G.) antelopensis, P. (G.) rockymontanus, P. (G.) filaris, P. (G.)? decipiens, Rotopericyclus (Rotopericyclus) pinyonensis, and R. (Hammatocyclus) brazerensis.
