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

Abstract

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.