A revised graptolite biostratigraphy for the lower Caradoc (Upper Ordovician) of southern Scotland

Abstract

SynopsisBiostratigraphical evaluation of graptolites from more than 160 localities in the lower Caradoc of southern Scotland does not differentiate a discrete assemblage diagnostic of the Climacograptus bicornis peltifer Biozone. The lowermost Caradoc Nemagraptus gracilis Biozone is redefined by the partial-range of N. gracilis below the appearance of Climacograptus bicornis s.l. The succeeding C. bicornis Biozone is divisible into (1) a lower Subzone of Orthograptus apiculatus and Dicranograptus ziczac, identified also by the appearance of Amplexograptus leptotheca, Dicranograptus tardiusculus and several other taxa, and (2) an upper Subzone of Climacograptus wilsoni, marked by the first appearance of C. wilsoni. Nemagraptus gracilis and Didymograptus (s.l.) superstes range into the lower part of the apiculatus-ziczac Subzone, above which is a poorly characterized interval that is approximately equivalent of the ‘peltifer’ Biozone of former usage. The bicornis Biozone is the approximate equivalent to the C. bicornis Biozone of North America, and, at least in part, of the foliaceus, (or multidens) Biozone of southern Britain. It is succeeded in Scotland by the clingani Biozone.