Abstract

The Prout Dolomite of north-central Ohio lies disconformably above the lowest Givetian (upper Middle Devonian) Plum Brook Shale and below the lowest Famennian (upper Upper Devonian) Ohio Shale. A sample from its base yielded over 4,000 diverse conodont specimens. Included is Polygnathus ansatus Ziegler and Klapper, 1976, the index species for the Middle varcus Subzone, which is not reported from strata of this age in Ontario and Indiana, a fact that long caused their miscorrelation with the Lower varcus Subzone. Also present is P. rhenanus Klapper, Philip, and Jackson, 1970, considered to be also indicative of the Middle varcus Subzone in North America, and P. ovatinodosus Ziegler and Klapper, 1976. Lowest occurrences of the latter are in the middle part of this subzone; its presence indicates correlation with the lower Tully Limestone of New York, the basal unit of the Taghanic Series. The Prout and equivalent strata in the region therefore represent a long unrecognized continuous time-rock unit created by Johnson's (1970) Taghanic onlap. The collection includes a new species of Ancyrolepis, A. huntleyi; a new species of Polygnathus is left in open nomenclature, as are nine specimens assigned to Tortodus but of otherwise uncertain taxonomic status. Givetian conodont correlation between North America and the Global Stratotype Section and Point established by the Subcommission on Devonian Stratigraphy (SDS) in Morocco is extremely problematical because of relatively erratic distribution (probably owing to limited ecologic adaptability) of P. ansatus and P. hemiansatus Bultynck, considered herein to be an early morphotype of P. ansatus. The base of the Givetian Stage has been defined by the SDS as coinciding with the lowest occurrence of P. hemiansatus. The only possible evidence for the SDS's hemiansatus Zone in North America involves reported occurrence of that morphotype in the uppermost Arkona Shale of Ontario, a position above the top of the Plum Brook Shale, which has been considered to be of Givetian age for decades. Also it appears that the interval between the Eifelian (lower Middle Devonian) kockelianus Zone and the hemiansatus Zone at the SDS's global-stratotype section in Morocco is of questionable age and probably too thin to represent continuous sedimentation. Adoption of a widely recognized faunal break at the base of strata deposited during the If T-R cycle of North America and Europe as the base of the Givetian could provide a sound alternative.

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