摘要
前言百余年来,虽然人们对栗蛤类(Nuculids,双壳纲)代表的构造特征及分类位置多有研究和报道,但正如L. R. Cox(1959)在“原鳃类的地质历史及栉齿类瓣鳃纲的双重起源”一文中所说:“Nuculacea超科和Nuculanacea超科的早期化石代表是相当难以区分的,如某些侏罗纪的Nuculana按分类位置属Nuculanacea超科,但未见均质结构。
The discovery of a partially aragonitic specimen of Palaeoneilo cf. cuneata J. Chen 1976, in an Upper Triassic formation Yunnan (Southwest China), provides an opportunity for precise microstructural description of the external layer of this ruculid, previously known only by morphological characteristics.Microstructural patterns of the fibrous tissue have been studied by SEM and mineralogic composition by microprobe analysis.The external layer appears to ne homogeneous, with no indication of the classical 'composite prismatic structure'. Observations carried out on diversely oriented surface (radial, frontal and symmetric to the growth direction), polished and slightly etched by dilute formic acid, demonstrate that the successive growth layers are built of sub-parallel fibers, perpendicular to the growth surface (cf. Pl.Ⅱ, figs. 1 to 4).This complete lacking of differentiation in the arrangement of the fibers by which the external layer is built up allows us to trace the possible relationship of this species with some other nuculid species such as: Acila cobboldiae, Nucula nucleus, N. hanleyi, Neilo australis and Malletia cumingi. New data on the microstructural features of the external layers of these species are provided. These results suggest some possible evolution of the nuculid family, from a microstructural point of view.Mineralogy of the aragonitic part of the external layer: After characterizing the aragonite by X-ray diffraction, the diagenetic transformation of the described material is studied by punctual analysis with EDS X-ray spectrometer, LINK AN 10000, using the ZPB method (peak to background) to obtain valuable quantitative results on trace elements concentrations.It appears clearly that the remaining aragonite is slightly modified with respect to measurements done on the external layers of recent specimens of the nuculid genus. The strontium content of the aragonitic part of the shell, for instance, is everywhere above the mean values of the corresponding points of the recent specimen. Moreover, the same strontium level can be observed in the calcite (i. e. completely recrystallized) parts of the shell. Obviously, the high strontium content of the aragonitic part is due to diagenetic transformation.On the contrary, the sodium level appears to be normally low: sodium being an easily migrating chemical element in biogenic strutures. Comparison between X-ray mappings for the three usual ele ments involved in diagenetic processes of aragonite confirms the punctual analysis: Sr repartition is uniform (Pl. Ⅱ, fig. 6) when Na (Pl. Ⅱ, fig. 5) and S (Pl. Ⅱ, fig. 7) exhibit a localization in conformity with the growth surfaces.Despite these slight chemical changes, the fibrous aragonite of the external layer authorizes precise microstructural comparison, providing arguments for a hypothesis on the microstructural evolution of the nuculids.
出处
《古生物学报》
CAS
CSCD
北大核心
1990年第1期35-42,共8页
Acta Palaeontologica Sinica
