Abstract
AbstractConodonts are an important biostratigraphic tool for many Phanerozoic stages. Along with graptolites, they define all global Ordovician Stage boundaries. Within the Upper Ordovician interval, a known species ofAmorphognathus tvaerensis(Bergström) is present in both Sandbian and Katian stratotype sections. Study of changes in the succession ofA. tvaerensisrevealed that elements in the upper part of its range differ morphologically quite distinctly from those in its lower part. Here, they are described as a new conodont species,A. viiraesp. nov. This new species is recognized in several Estonian and Swedish sections, with apparent occurrence also in Mójcza Quarry, Holy Cross Mountains, Poland and Black Knob Ridge, Oklahoma, USA. Detailed analysis of earlyAmorphognathuselements from Estonian and Swedish sections revealed the absence ofA. inaequalis(Rhodes) in both regions, although a conodont subzone based on this species was identified earlier by some authors. Both the absence ofA. inaequalis(Rhodes) and recognition of the new speciesA. viiraesp. nov. resulted in the revision of the conodont zonation, and a new version of it is proposed for the Sandbian Stage in the Atlantic Realm. The new zonation includes (from below)Pygodus anserinus,Baltoniodus variabilis,A. tvaerensis,B. gerdae,A. viiraeandB. alobatusConodont zones.
Published Version
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