Published online by Cambridge University Press: 14 July 2015
The Pericyclidae are distinctive ancestral (middle Tournaisian-lower Viséan) Goniatitina characterized by prominent transverse sculpture. Ancestry of the group was in Goniocyclus Gordon, 1986 (type species, Goniatites blairi Miller and Gurley, 1896). These diminutive (generally less than 35 mm), coarsely ribbed pericyclids are distinguished by sharply angular ventral sinuses and sutures with primitive, incipiently bifid ventral lobes. Goniocyclus characterizes the late Kinderhookian (late middle Tournaisian) Siphonodella isosticha—Upper Siphonodella crenulata conodont Zone where it is represented by at least six distinctive species in the North American Cordillera and Midcontinent (Gordon, 1986). The earliest record, which extends the generic range back to the base of the traditional middle Tournaisian, is a widely discussed, but previously undescribed, species from the Hannibal Shale of Missouri (Work et al., 1988; Becker, 1993a, 1993b; Popov and Kusina, 1997) described herein as Goniocyclus hannibalensis new species.