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.
Wang Xin Wang Jian He Shiping Gu Pingyang Chen Ruiming. Population variations of the Silurian graptolite Oktavites spiralis on the northern margin of the Yangtze platform, South China[J]. Chinese Journal of Geology, 2018, 53(4): 1440-1454.