Conodont biostratigraphy and depositional history of strata comprising the Niagaran sequence (Silurian) in the northern part of the Cincinnati Arch region, west-central Ohio, and evolution ofKockelella walliseri(Helfrich)

Abstract

The Dayton, Osgood, and Laurel Formations and the Euphemia, Springfield, and basal part of the Cedarville Dolomites near the axis of the Cincinnati Arch in northeast Preble County, Ohio, belong in the uppermost part of theOzarkodina sagitta rhenanato lower part of theOzarkodina?crassaChronozone and are late early to middle Wenlockian in age. The Dayton–Cedarville succession on the eastern flank of the Cincinnati Arch in north-central Greene County, Ohio, belongs in the uppermost part of thePterospathodus cellonito upper part of theAncoradella ploeckensisChronozone and is late Llandoverian to early middle Ludlovian in age.The sea transgressed across the exposed and eroded Brassfield Formation to begin deposition of the Dayton Formation on the eastern flank of the Cincinnati Arch in Greene County, Ohio, during the late Llandoverian and completely flooded all of west-central Ohio by the late early Wenlockian. The region remained covered by a sea of fluctuating depth during deposition of the Dayton Formation–Cedarville Dolomite succession from the Wenlockian through early middle Ludlovian.Kockelella walliseri(Helfrich) evolved fromK. ranuliformis(Walliser) during the middle Wenlockian (upper part ofOzarkodina sagitta rhenanaChronozone) by development of a lateral process adjacent to the cusp on the Pa element and by minor modification of the Pb element and some of the ramiform elements. Specimens from upper Llandoverian and lower Wenlockian strata previously assigned toK. walliseribelong to a different species,Kockelellasp. A Fordham, 1991. The evolutionary trends in theK. walliserilineage, progressive restriction of the basal cavity and increasing development of the length of the lateral processes in the Pa element, parallel the trends in theK. amsdeni–K. stauros–K. variabilislineage and resulted in the divergence ofKockelellacf.K. staurosBischoff, 1986, from the main lineage in the middle Wenlockian.