Mississippian (late Osagean) ammonoids from the New Providence Shale Member of the Borden Formation, north-central Kentucky

The Mississippian ammonoids Polaricyclus conkini new species, P. ballardensis (Gordon, 1965), Winchelloceras knappi new species, and Cantabricanites? greenei (S. A. Miller, 1892) occur with Gnathodus texanus Zone conodonts near the base of the New Providence Shale Member of the Borden Formation in north-central Kentucky. Both ammonoids and conodonts indicate an early late Osagean (lower Keokuk equivalent) age corresponding to the middle or, more probably, the late Chadian (latest Tournaisian or early Viséan) portion of the Fascipericyclus–Ammonellipsites Zone.

Mississippian (middle Osagean) ammonoids from the Nada Member of the Borden Formation, Kentucky

The borden formation in northeastern Kentucky contains a significant, largely undescribed, Osagean (upper Tournaisian to lower Viséan) ammonoid succession. Lower Osagean strata in the Nancy Member contain a diverse Pericyclus Zone assemblage characterized by Muensteroceras oweni (Hall), M. parallelum (Hall), Kazakhstania colubrella (Morton), Imitoceras ixion (Hall), and Masonoceras kentuckiense Work and Manger. This interval, which ranges into the basal Cowbell Member, was referred to the Muensteroceras oweni Assemblage Zone by Gordon and Mason (1985) and Gordon (1986) and indicates correlation to the lower Ivorian Stage of the Belgian Tournaisian succession. The succeeding middle Osagean interval in the middle and upper part of the Cowbell Member contains Merocanites drostei Collinson and Dzhaprakoceras which presumably represent the lower, or Tournaisian (upper Courceyan or lower Chadian), portion of the Fascipericyclus–Ammonellipsites Zone. An even higher middle Osagean assemblage with Polaricyclus bordenensis new species and Winchelloceras allei (Winchell, 1862) occurs in the upper part of the Nada Member, near the top of the Borden Formation, and is the basis for the current report.

Masonoceras, a new karagandoceratid ammonoid from the lower Mississippian (lower Osagean) of Kentucky

Karagandoceratids are a rare offshoot of the Prionoceratinae, resembling that subfamily in general conch form and sutural ontogeny, but differing by possession of an acute ventral margin and an increasingly trifid ventral lobe. The systematic position of the Karagandoceratidae has been controversial [see Bartzsch and Weyer (1988) for an exhaustive review]. The nominate genus, Karagandoceras Librovitch, 1940 (type species, K. galeatum), possesses a weakly divided ventral lobe which has led authors to refer it to both the Praeglyphioceratina (Ruzhencev, 1960, 1962; Bogoslovsky, 1971; Ruzhencev and Bogoslovskaya, 1978; Bogoslovskaya et al., 1999; Kusina, 2000) and the Goniatitina (Weyer, 1965, 1972; Kullmann, 1981). Discovery of an ancestral karagandoceratid genus, gen. nov. I aff. Karagandoceras Bartzsch and Weyer, 1988, in the early Tournaisian Siphonodella sandbergi conodont Zone in Germany provided clarification on the proximate origin of Karagandoceras and provided a plausible link to the early Tournaisian prionoceratin genus Nicimitoceras Korn, 1993 (type species, Imitoceras subacre Vöhringer, 1960). Bartzsch and Weyer (1988) proposed a karagandoceratid phylogeny beginning with gen. nov. I aff. Karagandoceras in the early Tournaisian, progressing through Karagandoceras in the middle Tournaisian, and culminating with a third, descendent genus, gen. nov. II aff. Karagandoceras (typical species, Karagandoceras bradfordi Manger, 1971), early in the late Tournaisian. Bartzsch and Weyer (1988) elected to leave both the initial and final members of this lineage, gen. nov. I and gen. nov. II aff. Karagandoceras, in open nomenclature pending discovery of more completely preserved material. Discovery of superbly preserved representatives of a new species of gen. nov. II aff. Karagandoceras from the Borden Formation in northeastern Kentucky provides additional sutural and morphological details that support Bartzsch and Weyer's phylogenetic interpretation and makes formal description of this terminal karagandoceratid taxon (herein designated Masonoceras new genus) possible.