志留纪笔石Oktavitesexcentricus(Bjerreskov,1975)的古生物学特征及地层意义*

陕西紫阳地区在古地理上位于扬子台地西北缘,区域内志留纪地层发育,其中志留系兰多维列统(Llandovery)特列奇阶(Telychian)笔石相地层出露较为完整,笔石带较为连续。Oktavites excentricus(Bjerreskov,1975)是特列奇阶Oktavites spiralis笔石带重要的伴生分子,在世界范围内广泛分布。文中通过对紫阳地区一系列特列奇阶剖面研究后认为,Oktavites excentricus较短的化石延限和广泛的古地理分布使其具备地层对比潜力,这将为Oktavites spiralis笔石带的进一步细分提供依据。同时,Oktavites excentricus胞管为典型的奥氏笔石式,其笔石体的盘旋方式又与稍晚的Cyrtograptus lapworthi(Tullberg,1883)(笔石带化石,弓笔石类的早期代表)十分接近,因此,对该种的演化过程的研究也为讨论弓笔石的起源过程提供了更多参考。

PALAEONTOLOGY AND BIOSTRATIGRAPHIC SIGNIFICANCE OF SILURIAN GRAPTOLITE OKTAVITES EXCENTRICUS

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.