云南曲靖刘家冲剖面关底组中的鱼类微体化石及其地层学意义

Microvertebrate remains from the Kuanti Formation of the Liujiachong Section in Qujing,Yunnan and their stratigraphic significance

云南曲靖地区的关底组产出世界闻名的潇湘脊椎动物群(Xiaoxiang Vertebate Fauna),但其地层的划分对比和时代归属一直存有争议,特别是其下部地层的划分及时代归属争议最大。通过在曲靖城南以关底组下段地层为主的刘家冲剖面上开展详细的野外及室内地层古生物学研究工作,文章厘清了该剖面上的志留纪地层,并依据采自数个层位样品中丰富的鱼类微体化石材料探讨了关底组下部的地质时代。研究结果表明:刘家冲剖面上出露的志留纪地层根据岩性变化特征可划分为关底组Ⅰ段(岳家山段)、Ⅱ段(崇家湾段)和Ⅲ段(彩莲段);Ⅰ段的下部以浅黄色、浅黄绿色含砾粉砂质泥岩、钙质粉砂岩为主,未见化石,而上部以深灰色、灰黑色页岩为主,含少量腕足类Lingula sp.及双壳类;Ⅱ段则以灰绿色、暗紫红色页岩互层、并夹薄层状灰岩为主,从下往上灰岩夹层逐渐增多,含丰富的腕足类、海百合茎等无脊椎动物化石及鱼类微体化石;在关底组Ⅲ段地层中首次发现的鱼类微体化石多样性高,门类比较齐全,包括无颌类、盾皮鱼类、棘鱼类和硬骨鱼类四大早期脊椎动物类群,从鱼群组成面貌上看隶属于潇湘脊椎动物群扬子鱼类组合(Yangtze Fish Assemblage)。笔者主要依据对其中棘鱼类的全球对比分析研究,并在结合牙形类等无脊椎动物化石研究进展及新认识的基础上,深入探讨了关底组下部Ⅰ段及Ⅱ段的地质时代。关底组Ⅱ段的时代应为志留纪罗德洛世高斯特晚期(late Gorstian,Ludlow),关底组Ⅰ段含砾岩层之上、含盾皮鱼类“王氏鱼”(Wangolepis)的地层,则应归入罗德洛世高斯特早期(early Gorstian,Ludlow),而在该剖面上尚未发现鱼类化石的关底组Ⅰ段下部,目前还不能确定全部归入高斯特阶(Gorstian),有可能会下延到温洛克统(Wenlock)之中。文章中相关工作的开展,不仅为中国志留纪生物地层的精深研究提供了古鱼类学方面的可靠证据,而且为探讨早期脊椎动物演化提供了新的地质年代意见。

The Kuanti Formation,with the discovery of the Xiaoxiang Vertebate Fauna in 2007,has become the focus of global research on early vertebrates.However,its stratigraphic subdivision,correlation and age,especially the subdivision and age of its lower part,remain contentious,despite many biostratigraphic attempts made since 1914.In recent years,we conducted a series of extensive geological investigations in the Kuanti Formation near the Xiaoxiang Reservoir in Qujing,Yunnan Province.Based on the recently obtained data from the Liujiachong Section(dominated by the lower part of the Kuanti Formation)and the preliminary study of the newly discovered microvertebrate remains,we documented the sequence of the Kuanti Formation of the Liujiachong Section,and further explored the geological age of the related strata.The Kuanti Formation of the Liujiachong Section can be subdivided into three members,in ascending order according to the main lithological changes:MemberⅠ(Yuejiashan Member),MemberⅡ(Chongjiawan Member),and MemberⅢ(Cailian Member).The lower part of MemberⅠis characterized by light yellow and yellow-green gravelly silty mudstones and calcareous siltstones lacking any fossils,while the upper part is composed of dark grey and grey-black carbonaceous shales with rare Lingula sp.and bivalves.MemberⅡis dominated by grey-green and purple-red shales,intercalated with light grey thin-bedded or lenticular limestones or bioclastic limestones containing many invertebrate fossils(brachiopods and crinoid stems)and abundant microvertebrate remains.MemberⅢ,partially exposed in the section,is composed of purple-red and grey-green silty and calcareous mudstones or marls intercalating with minor purple-red or yellow-green shales or siltstones containing brachiopods,fish,and stout tubular trace fossils.A thin layer of mediumthick-bedded fine sandstone is developed on the bottom of the member,which marks the boundary between MembersⅢandⅡ.The microvertebrate remains from several beds in MemberⅡshow high diversity,including four early vertebrate groups:agnathans,placoderms,acanthodians,and osteichthyans.They are characterized by the Yangtze Fish Assemblage of the Xiaoxiang Vertebrate Fauna.The discovery of the acanthodians Nostolepis spp.in this section reveals the stratigraphic correlation and the precise geological age of the lower part of the Kuanti Formation.Based on the acanthodians from MemberⅡ,coupled with previous palaeontological data,we suggest the age of MemberⅡto be late Gorstian Stage of the Ludlow,in the Silurian.The upper part of Member I,containing the placoderm‘Wangolepis’,might be referred to as early Gorstian.However,the age of the lower part of Member I without fossils is uncertain,and the possibility of its base extending down to the Wenlock is not excluded.This study offers new palaeoichthyological evidence for the Silurian biostratigraphy of China and provides a new chronostratigraphic scheme in the study of early vertebrate evolution.