A New Silurian Graptolite Species of Oktavites from Northwestern Margin of Yangtze Platform, and Its Stratigraphic Significance
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摘要: 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.
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关键词:
- 志留系 /
- 特列奇阶 /
- Oktavites spiralis /
- Oktavites longtanensis sp.nov. /
- 弓笔石 /
- 古生物学 /
- 地层学
Abstract: 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 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.-
Key words:
- Silurian /
- Telychian stage /
- Oktavites spiralis /
- Oktavites longtanensis sp.nov. /
- Cyrtograptus /
- paleontology /
- stratigraphy
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图 5 陕西紫阳-岚皋地区Oktavites spiralis,Oktavites longtanensis sp. nov.和Oktavites contortus之间的比较
a.Oktavites contortus(Perner,1897),龙潭剖面,38.10 m,O. spiralis笔石带,Gr21454;b.Oktavites spiralis(Geinitz,1842),龙潭剖面,64.15 m,O. spiralis笔石带,Gr21424; c.Oktavites longtanensis sp. nov.,龙潭剖面,44.60 m,O. spiralis笔石带,Gr20300; d.Oktavites contortus(Perner,1897),桥西剖面,2.10 m,Cyr. lapworthi笔石带,Gr21700;e.Oktavites spiralis(Geinitz,1842),龙潭剖面,65.00 m,O. spiralis笔石带,Gr21420; f.Oktavites longtanensis sp. nov.,龙潭剖面,50.50 m,O. spiralis笔石带,Gr20303
Fig. 5. Comparison among Oktavites spiralis, Oktavites longtanensis sp. nov. and Oktavites contortus in the Ziyang-Langao area
图 6 Cyrtograptus lapworthi弓笔石的起源过程推测
a.Oktavites contortus(Perner,1897),龙潭剖面,38.10 m,O. spiralis笔石带,Gr21259; b.Oktavites longtanensis sp. nov.,龙潭剖面,50.5 m,O. spiralis笔石带,Gr20304; c.Oktavites excentritus(Bjerreskov,1975),龙潭剖面,63.5 m,O. spiralis笔石带,Gr21303;d.Cyrtograptus lapworthi(Tullberg,1883),芭蕉口剖面,546.0 m,Cyr. lapworthi笔石带,Gr20166
Fig. 6. Potential origin process of Cyrtograptus lapworthi
图 7 Oktavites spiralis和Oktavites contortus的不同发育阶段
a~d,i.Oktavites spiralis(Geinitz,1842);a.龙潭剖面,70.00 m,O. spiralis笔石带,Gr21414; b.龙潭剖面,42.60 m,O. spiralis笔石带,Gr23583; c.芭蕉口剖面,545.10 m,O. spiralis笔石带,Gr20751; d.龙潭剖面,44.00 m,O. spiralis笔石带,Gr21278; i.龙潭剖面,64.15 m,O. spiralis笔石带,Gr21424;e~h.Oktavites contortus(Perner,1897);e.芭蕉口剖面,546.00 m,Cyr. lapworthi笔石带,Gr23404; f.桥西剖面,2.10 m,Cyr. lapworthi笔石带,Gr20405;g.龙潭剖面,53.50 m,O. spiralis笔石带,Gr23630; h.龙潭剖面,38.10 m,O. spiralis笔石带,Gr21454
Fig. 7. Oktavites spiralis and Oktavites contortus in different developmental stages
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