EVIDENCE OF RADIATION AFTER A LATEST EIFELIAN (EARLY MIDDLE DEVONIAN) BIO-EVENT, NORTH-CENTRAL OHIO

A latest Eifelian bio-event widely studied in Europe and Morocco involved deteriorating ecology and diversity followed by radiation and new taxa, including the goniatite genus Maenioceras, long a marker for the overlying Givetian Stage. Contrary to claims that neither goniatite nor conodont evidence provides precise placement in North America, the conodont genus Icriodus exhibits remarkable radiation in the basal Plum Brook Shale of north-central Ohio. The underlying Delaware Limestone, deposited during the Ie transgressive-regressive cycle of Johnson et al., contains 3 species of Icriodus that range only to a disconformity at its top. The Plum Brook (T-R cycle If) contains 10 species and 1 subspecies of Icriodus not found in the kockelianus Zone to which the Delaware is probably restricted, and specimens of Polygnathus xylus in the Plum Brook indicate assignment to the upper ensensis Zone, long considered lowest Givetian in age in North America. The lower Givet Limestone in the Givetian type region (Ardennes) exhibits a comparable radiation: 10 species of Icriodus absent from the kockelianus Zone (several in common with the Plum Brook).

The Givetian base established by the IUGS is at the lowest occurrence of P. hemiansatus, which is above Maenioceras at the Global Stratigraphic Section and Point in Morocco but over 40 m below the base of the Givet Limestone in the Ardennes. Only 2 lowest occurrences have been published in North America, one at the top of the Arkona Shale in Ontario (somewhat younger than the top of the otherwise equivalent Plum Brook) and another in northwestern Canada at the base of the Middle varcus Subzone. Use of the event interval to mark the base of the Givetian, as has been suggested in Europe, would provide a safer criterion in North America. The index fossil could be any taxon that does not exist below the upper ensensis Zone.