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

2005; Paleontological Society of China; Linguagem: Inglês

0001-6616

Yuan Zhang,

Geological and Geophysical Studies

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 protecting, and may be of critical importance for the benthic dendroids. However, in the presumably planktic Psigraptus jacksoni, all the thecae open outwards-probably the result of changes in food resources and feeding behavior.It is also noteworthy that the relatively smaller bithecae of Airograptus are always located nearby the apertures of autothecae and in a regular distance (Text-fig.6). If the postulation that autothecae accommodate female zoids and bithecae male zoids is right, then this phenomenon may imply the presence of ‘simultaneous hermaphrodites’. This sexual relationship may be of particularly critical significance for the benthic dendroids, as the male zoids do not need to extend far to reach their mates, thus guaranteeing a high reproduction rate. While in the subsequent planktic anisograptids and graptoloids, this relatively fixed sexuality becomes no more essential and competitive, and is finally replaced by ‘sequential hermaphrodites’. This ecologically induced sexual change probably accounts for the gradual degeneration and final loss of bithecae in the late Tremadocian.