扬子台地北缘志留纪笔石Oktavites spiralis居群差异研究

通过对陕西紫阳、岚皋地区多个志留系特列奇阶剖面的研究，根据更为精确的化石定义，奥氏笔石属新种Oktavites ziyangensis sp. nov可以从Oktavites spiralis 居群中识别出来。新种O. ziyangensis 在笔石枝盘旋方式和胞管形态上与O. spiralis 和Oktavites contortus 均存在一定的相似性，三者共生层位为特列奇阶Spirograptus turriculatus 笔石带至Cyrtograptus lapworthi 笔石带。尽管新种O. ziyangensis 地层延限较长，但胞管的快速演化仍使其具备较强的地层对比意义。根据“深水起源、浅水扩散”的笔石演化模式推测，O. contortus-O. ziyangensis-O. spiralis 的演化过程是笔石在纵向上的生态位占据过程，而O. spiralis 居群形态差别则是笔石深度分带的结果。同时，关于O. spiralis 居群差异的研究，也为讨论弓笔石的起源和分类提供更多的参考依据。

Population variations of the Silurian graptolite Oktavites spiralis on the northern margin of the Yangtze platform,South China

Recent investigations in multiple Telychian profiles in the Ziyang-Langao area, along the northern margin of the Yangtze platform, South China, revealed a new species Oktavites ziyangensis sp. nov which was further distinguished from the population of O. spiralis by using a more comprehensive and precise set of criteria. Oktavites ziyangensis is analogous to O. spiralis and O. contortus in having a similar coiled rhabdosome and thecal morphology, and these three species coexisted from the base of the Spirograptus turriculatus Biozone to the upper part of the Cyrtograptus lapworthi Biozone. The new species has been considered of great significance for stratigraphic correlation of Telychian strata owing to its rapid thecal evolution, although this species has an even longer stratigraphic range. According the graptolite evolutionary mode of“deep water origin and shallow water dispersal”, the evolutionary trend of O. contortus-O. ziyangensis-O. spiralis has been speculated a process of vertical niche occupation, and the various morphology of population variations are largely due to the graptolite depth zonation. Moreover, the study on population variations of Oktavites spiralis also has a great significance for discussing the origin and classification of Cyrtograptus genus.