利用背散射电子(BSE)研究奥陶纪部分笔石的始端发育和分枝方式  

INTERPRETATION OF THE PROXIMAL DEVELOPMENT AND BRANCHING DIVISIONS OF SOME EARLY AND MIDDLE ORDOVICIAN GRAPTOLITES BASED ON BSE IMAGES

作　　者：张元动[1] 骆天天[1] 茅永强[1] 

机构地区：[1]中国科学院南京地质古生物研究所

出　　处：《古生物学报》2005年第1期125-137,共13页Acta Palaeontologica Sinica

基　　金：中国科学院资源环境领域知识创新工程重大项目(KZCX2 SW 129 );国家科技部 973项目 (G2000077700 );国家自然科学基金 (No.40372007);德国马普学会资助课题

摘　　要：背散射电子成像 (BackscatteredElectronImaging,简称BSE),是依托扫描电镜的一种电子成像技术,它的成像原理和特点非常适合用来研究那些表皮尚存的各类笔石标本,是二次电子成像 (SEM)无法替代的。文中简要介绍了这一技术的工作原理,并利用它对我国早奥陶世和中奥陶世部分笔石的始端发育和分枝方式等进行了研究。当前BSE图象显示了许多以往其他途径无法观察到的笔石微细结构,特别是笔石复杂的始端发育特征,结果验证了Psigraptusjacksoni的二分岔式和Rhabdinoporaflabelliformisparabola的四分岔式原始分枝的观点,显示它们都具有最原始的等称笔石式(isograptid type)始端发育型式。此外,BSE图象还显示,底栖固着的树形笔石Airograp tusfurciferus的胞管排列和分枝方式与奥陶纪最早期的漂浮笔石非常相似,表明二者可能存在祖 裔演化关系,或者至少是原始性状上的相似性。Backscattered Electron (BSE) imaging, a technique based on Scanning Electron Microscope, is especially suitable and useful for the study of those graptolites with the periderm preserved. The principle of the technique is briefed in the present paper, and by using this technique a study is conducted on the proximal development and branching of some Early and Middle Ordovician graptolites from the Ordovician Yehli Formation in Dayangcha section of Baishan (formerly Hunjiang), NE China, and Ningkuo Formation in the JCY (Jiangshan-Changshan-Yushan) of West Zhejiang, SE China. The BSE images of juvenile specimens of Psigraptus jacksoni Rickards and Stait show some special microstructures involving complex proximal development, which are unavailable by other approaches, and confirmed the “biradiate” primary branching. Accordingly a reconstruction of the proximal development is proposed. Those BSE images of Rhabdinopora flabelliformis parabola (Bulman) support the reconstruction of proximal development by Maletz (1992), in which a “quadriradiate” primary branching produced by successive dicalycal theca th1~1, th1~3 and th1~4 is suggested.In the dendroid Airograptus furciferus Ruedemann from the Yehli Formation, fine microstructures of the rhabdosome are also preserved. As the BSE images show, the thecal arrangement and stipe branching of the species is identical to that of Rhabdinopora flabelliformis as diagramed by Bulman (1970) and Cooper and Fortey (1983). The high similarities might suggest that the nematophorous Rhabdinopora was derived from Airograptus during the Cambrian-Ordovician transition.The specimens of Airograptus furciferus also show that the thecal apertures seem to open in the same direction-opposite the side where the stolons are located. If the species has presumably a cone-shaped rhabdosome, which is quite likely and common in early nematophorous graptolites, this phenomenon means that the thecal apertures open inwards. This special thecal arrangement is helpful to the feeding and protectin

关 键 词：背散射电子 奥陶纪 笔石 始端发育 分枝方式 

分 类 号：Q915[天文地球—古生物学与地层学]