扬子台地西北缘志留纪奥氏笔石属新种及地层对比意义
A New Silurian Graptolite Species of Oktavites from Northwestern Margin of Yangtze Platform, and Its Stratigraphic Significance

Oktavites spiralis(Geinitz，1842)是志留系兰多维列统特列奇阶的笔石带化石分子，其独特的盘旋生长模式易于识别，在世界范围内广泛分布，地层意义重要.然而，关于Oktavites spiralis的首现位置始终存在争议，不利于地层的精确对比.在陕西紫阳、岚皋地区，Oktavites spiralis笔石带可以通过带化石丰度变化进行识别，而不是依靠带化石的首现.近来，对陕西紫阳、岚皋地区多个志留系特列奇阶剖面研究后认为，通过幼体特征和盘旋形态对比，一个奥氏笔石新种Oktavites longtanensis sp. nov.可以从Oktavites spiralis和Oktavites contortus中进一步区分出来.新种Oktavites longtanensis sp. nov.化石延限仅限于Oktavites spiralis笔石带，延限较短，并具有重要的演化意义和较强的地层划分对比潜力.
Oktavites spiralis(Geinitz，1842) is an index graptolite species of the Telychian age (late Llandovery，Silurian) that combined marked spiral morphology as well as a global distribution，making it stratigraphically significant. However，the first appearance datum of Oktavites spiralis is controversial, and consequently it is much difficult for regional stratigraphic correlations. In the Ziyang-Langao area of Shaanxi Province，Oktavites spiralis graptolite zone belongs to an abundance zone，rather than the FAD (first appearance datum) of this index fossil. Recent investigations in multiple Telychian profiles in the Ziyang-Langao area，along the northern margin of South China，reveal that a new species Oktavites longtanensis sp. nov. can be further distinguished from typical form of Oktavites spiralis and Oktavites contortus according to the larvas and coiled model. The new species Oktavites contortus according to the larvas and coiled model. The new species Oktavites longtanensis sp. nov. is of a shorter stratigraphic range with great evolutionary significance，therefore has a great potential for accurate correlation of Oktavites spiralis Zone