陕西紫阳－岚皋地区志留系特列奇阶Oktavites spiralis笔石带生物地层学研究

Oktavites spiralis, one of the important representatives of graptolites of the Telychian Age (late Llandovery, Silurian), is of great significance in biostratigraphic studies. However, the poor understanding of its bioloigcal origination and evolutionary hisotry present significant challenges for the definition of the first appearance datum of Oktavites spiralis. On the other hand, the relatively thicker Telychian successions worldwide also lead to a wider range zone of Oktavites spiralis. Silurian strata are well deveoloped in the Ziyang-Langao aera in southern Shaanxi Province, especially at the Longtan section of Ziyang. Stratigraphy of these Silurian successions has been well studied and constrained worldwide. A huge number of graptolite specimens (including Oktavites spiralis) have been found at the Longtan section. Oktavites spiralis in the Ziyang-Langao area are not only preserved in abundance but also presereved exquisete morphological details and thus provides good opportunities to study its developmental sequences and evolutionary history. Based on a preliminary study, we have found the transitional genus between Oktavites and Spirograptus and we suggest that Oktavites spiralis may have originated from one species of the Spiropgraptus genera. More importatnly, our study shows that the fossils previously assiged to Oktavites spiralis can be furthre sub-divided into three sub-genera. A detailed systematic investigation of Oktavites spiralis is the key to confirm the first appearence of datum of this species. Studies of the morphological variations of the theca and stipe as well as the symbiotic combination, are keys to divide the graptolite zones. Furthermore, evolutionary study between the genera Oktavites and Cyrtograptus will provide basic biostratigraphic data for the subdivision of the Telychian strata.

Oktavites spiralis是特列奇阶（兰多维列统上部）带化石分子，具有重要的地层意义。然而，世界范围内该段地层普遍厚度较大，关于该化石定义较为宽泛，对其起源、演化过程研究程度不够，也造成了带化石首现位置的不确定以及延限过长。陕西紫阳、岚皋地区志留系特列奇阶剖面连续，笔石带完整，研究程度较高，与国内外地层都可以进行很好的对比，是研究O. spiralis演化以及笔石带进一步细分的理想场所。目前初步研究认为，O. spiralis可能起源于螺旋笔石属，并可将其分为三种类型。笔者认为，通过精确的属种定义、胞管演化历程的研究以及与共生笔石分子组合特征，可以确定带化石的首现位置及其延限，并完成该笔石带进一步划分。此外，关于奥氏笔石属的厘定以及其与弓笔石之间亲缘关系的研究，也将为地层精确划分提供依据。

由于构造抬升引起的海退事件影响，扬子台地志留系特列奇阶中上部的笔石相沉积地层在很多地区存在缺失。陕西紫阳、岚皋地区在古地理上处于扬子台地西北缘斜坡地带，水体相对较深，且受构造抬升事件影响相对较晚，区域内志留纪笔石生物地层较发育，研究工作开展较早。其中特列奇期笔石相生物地层，以紫阳芭蕉口地区任河两岸剖面出露最为完整，化石丰富，笔石带连续。Oktavites spiralis笔石带位于志留系特列奇阶（Telychian）中部，带化石特征明显、演化迅速、世界范围内分布广泛、地层对比意义较强。然而，该种宽泛的定义和演化过程研究的缺乏也造成了对化石延限的争议，不利于地层的精确对比。笔者通过在陕西紫阳、岚皋地区多个志留系剖面的笔石化石采集和研究，认为Oktavites spiralis在该地区具有更长的化石延限，Oktavites spiralis首现于特列奇阶（Telychian）Spirograptus turriculatus笔石带下部，至Cyrtograptus lapworthi笔石带上部灭绝。尽管该种具有较低的首现位置和较长的化石延限，然而胞管的快速演化使其仍具备较强的地层对比意义。同时，通过更为精确的化石定义，可以对该居群的伴生分子做进一步区分，避免造成属种鉴定的误差。此外，通过始部特征的对比研究，可将Oktavites spiralis和与之共生的Oktavites contortus (Perner，1897）明确区分为2个独立的种。此外，对Oktavites spiralis和Oktavites contortus发育过程的研究，也可成为该属进一步精确厘定的主要依据之一。