二叠系乐平统底界全球层型剖面和点位问题

Definition and Position of Conodonts for the Base of the Lopingian Series

乐平统被正式接纳为上二叠统的国际标准年代地层单位后,其底界的全球层型剖面和点位(GSSP)待确定。经金玉玕等多年研究,广西来宾县蓬莱滩剖面被公认为乐平统底界层型的最佳候选剖面;该剖面牙形类化石Clarkina postbitteri带或Clarkina dukouensis带之底均可作为乐平统底界的候选层位。C. postbitteri由Jinogondolella granti(Mei and Wardlaw)演化而来,其首次出现的点位(FAD)可在蓬莱滩剖面得到精确确定。1998年,以C. postbitteri带之底作为乐平统底界的方案被国内外牙形类化石专家一致接受,为国际二叠纪地层分会通过投票正式决定蓬莱滩剖面C. postbitteri带之底为全球层型剖面和点位铺平了道路。令人遗憾的是在此关键时期王成源声称,在原定界线之下3米处发现C. postbitteri,并转而提出以C. dukouensis带之底为界线的另一方案,导致一次无谓的争端,延误了在中国建立乐平统底界的全球层型的进程。后经Henderson重新研究有关样品证明,在原定界线之下3m处并未发现C. postbitteri,从而使牙形类化石专家达成新的共识。乐平统牙形类化石的演化以Clarkina属为主导,属内各种的鉴别应以居群(population)为基础。锯齿型式被证明是最能反映该属居群特征的较为稳定的鉴定标志。而齿台轮廓等其它形态性状在一个居群内?

This paper will review on the paper entitled 'Discussion on the definition of the Lopingian Series basal boundary and the point in the section of Penglaitan, Laibin, Guangxi' by Wang Chengyuan (2001). To offer the readers a true picture of recent discussion on the potential levels for the basal boundary of the Lopingian Series, a number of disputed points are clarified in this paper. First, the author agrees with the view that this debate is indeed unnecessary. As pointed out by Glenister (2000), 'Irrespective of where the GSSP is placed within this interval, it will be fully functional, and this will remain one of the most intensely studied sections in the world. ' Second, three potential horizons for the base of the Lopingian Series, the First Appearance Datums (FAD) of Clarkina postbitteri , Clarkina dukouensis and Jinogondolella altudaensis have been repeatedly discussed (Jin et al. , 1994, 1998). Among them, the former two, and not all the three, were proposed by us as Wang repeated in his papers. Third, the FAD of Clarkina postbitteri was reported by us from a level 55 cm below the top of Bed 116 or the top of the Laibin limestone, which is within Bed 6i at the Penglaitan section (Mei et al. , 1994c). Clarkina postbitteri and Clarkina dukouensis can be differentiated consistently only with a taxonomy based on denticulation of the carina within a population and the recognition of ontogenetic changes. Specimens illustrated from Bed 114. 6-6k and Bed 114-7b by Wang (2000, Pl. Ⅱ, 1 - 8) as Clarkina dukouensis are here regarded as C. postbitteri. Identification of species of Clarkina by Wang (2000a) is based on either the posterior brim, or the platform outline, or rare geronic characters such as the overgrown anterior platform. A re-study of the Guadalupian - Lopingian boundary interval once again confirms that there is no overlap between the stratigraphic ranges of Clarkina and Jinogondolella. The overlap of Clarkina postbitteri and Jinogondolella reported by Wang (2000a, 2001) has been found to be due to misidentification and mislabeling or contamination.