Middle and Upper Cambrian Protoconodonts and Paraconodonts from Hunan, South China

More than 4200 kg of limestone, representing 980 productive samples, from the upper Middle Cambrian, Upper Cambrian, and lowermost Ordovician in western and north-western Hunan were processed for paraconodonts, protoconodonts, and euconodonts. The focus of the present paper is on the upper Middle and lower Upper Cambrian interval as it is developed at two, apparently stratigraphically continuous, key sections. The collections studied, which include more than 20,000 specimens and are quite diverse taxonomically, prove that some protoconodonts and paraconodonts are useful biostratigraphically. Previously proposed protoconodont-paraconodont biozones in Hunan are revised and correlated with recently revised trilobite biozones, as well as with protoconodont-paraconodont and trilobite biozones in North China. Twenty-six species and seven conditionally identified species belonging to 13 genera are described. Among these, two genera ( Huayuanodontus and Yongshunella ) and two species ( Westergaardodina elegans and Yongshunella polymorpha ) are new. This study has special interest for Cambrian biostratigraphy because the two studied sections are global stratotype candidates for the Middle-Upper Cambrian series boundary. The level of the Upper-Middle Cambrian Series boundary, as currently recognized in China (at the base of the Linguagnostus reconditus Trilobite Biozone) is well marked in the paraconodont succession and can be traced into the Swedish standard succession using these fossils. By contrast, another level recently proposed as a potential global Middle-Upper Cambrian Series boundary level, the base of the Glyptagnostus reticulatus Trilobite Biozone, does not coincide with any marked change in the Hunan conodont species succession useful for local and regional correlation.     

一些原牙形石、副牙形石和最早的真牙形石的组织学和比较组织学研究

本文主要介绍对采自湘西、辽东和鲁西的寒武纪牙形石进行组织学和比较组织学研究成果中尚未发表的内容。以前认为是原牙形石的属Huyuanodontus的组织学构造与原牙形石、副牙形石以及真牙形石均不相同,而且与任何已知化石类群的组织学构造亦不相同。副牙形石Prooneotodus rotendatus和Prooneotodus gallatini中的一些分子具有厚薄不同的齿冠,应属于真牙形石。在已采集到的副牙形石属Laiwugnathus的所有标本中,既未发现副牙形石生长层构造,也未发现牙本质。在具有典型的副牙形石生长层构造的标本中发现了令人信服的牙本质。在最早的真牙形石Proconodontus muelleri中发现了较复杂的管状牙本质。组织学研究证实Coelocerodontus bicostatus为真牙形石。上述成果以及我国寒武纪牙形石组织学和比较组织学已经发表的成果(如:Dong,2004;Dongetal.,2005等)说明研究寒武纪牙形石必需研究牙形石组织学。目前,正在进行的寒武纪牙形石研究的两项主要工作《厘定寒武纪牙形石的分类》以及《副牙形石和早期真牙形石的谱系分析》,均是以组织学的研究为基础的。