PALAEONTOLOGY AND BIOSTRATIGRAPHIC SIGNIFICANCE OF SILURIAN GRAPTOLITE OKTAVITES EXCENTRICUS
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Abstract:
The Ziyang-Langao area,in southern Shaanxi Province,is palaeogeographically located on the northwestern margin of the Yangtze Platform,where Telychian rocks(upper Llandovery,Silurian)with abundant graptolites are beautifully exposed.Oktavites excentricus Bjerreskov,1975 is an important associated graptolite species of the Oktavites spiralis Zone,Telychian stage.Recent investigations in multiple Telychian profiles in the Ziyang-Langao area reveal that Oktavites excentricus,the species that combined rapid evolution with a global distribution,has a great potential for accurate subdivision of Oktavites spiralis Zone.In addition,the thecae of Oktavites excentricus is typcial morphology of Oktavites,however,the morphologic rhabdosome of this species is similar to the latter graptolite index species Cyrtograptus lapworthi Tullberg,1883(the representative of early Cyrtograptids),which having agreat significance for discussing the origin of Cyrtograptus.
