牙形刺的鉴定与乐平统的底界(英文)

CONODONT IDENTIFICATION AND THE BASE OF THE LOPINGIAN

Clarkina postbitteri和 C.dukouensis两个种的正确鉴定是确定乐平统底界的关键。 Mei和 Wardlaw(1994b,c)在建立这两个种时就已明确指出 ,C.postbitteri不同于 C.dukouensis主要是有浑圆而不是钝圆的齿台后端 ,并有明显的后边缘 (brim ) ,并不是依据齿脊特征区别的。金玉 (Jin,2 0 0 0 a)与 Henderson(2 0 0 1)强调 ,C.postbitteri的中后部细齿分离是有别于 C.dukouensis的主要特征。比较 C.postbitteri与 C.dukouensis的正模和 Mei et al.(1994c)所鉴定的 C.dukouensis(图 1a— c) ,可以清楚地看到 ,两者的齿脊几乎相同。 Mei etal.(1994c)所鉴定的 C.dukouensis齿脊的细齿甚至比 C.postbitteri的还分离 (图 1a,b)。两个种的区别 ,应保持原来的定义 :主要依齿台后端形状和后边缘的有无。Clarkina dukouensis由 C.postbitteri逐渐过渡而来 ,Henderson(2 0 0 1)认为 ,由于是逐渐过渡 ,C.dukouensis首次出现的点位就是任意选择的。然而 ,泥盆系 -石炭系界线的定义是在 Siphonodella presulcata- S.sulcata连续演化谱系中 S.sulcata的首次出现 ,无人认为 ,由于是连续演化谱系这个界线点位就是任意的。所有泥盆系的阶的金钉子都是以祖先种到定义种的演化谱系中定义种的首次出现为定义的 ,也无人认为这些?

(Clarkina postbitteri and Clarkina dukouensis cannot be effectively differentiated by their denticulation.The original definitions of these two species, with platform posterior shape and brim as important distinguishing characters for these two species, are retained. The ancestor of Clarkina postbitteri is still unclear. Serrated Mesogondollella (- dinogondolella) granti is not the ancestor of Clarkina postbitteri. The position of GSSP from Clarkina postbitteri to Clarkina dukouensis is not an arbitrary point. We have to consider the characters of the earliest species population at its FAD timespan. An unproven punctuated evolutionary event cannot be used to establish a GSSP. A GSSP cannot be placed at a diastem or at a sequence boundary. The base of the Lopingian should be drawn at the base of LPDll5 6k, and the sequence boundary between 6i lower and upper is a very good reference criterion for the GSSP of the base of the Lopingian.