中-晚奥陶世牙形石属Pygodus Lamont et Lindström和Periodon Hadding的形态演化及其生物地层学意义

ON THE EVOLUTION AND BIOSTRATIGRAPHIC SIGNIFICANCE OF THE MIDDLELATE ORDOVICIAN CONODONT PYGODUS LAMONT ET LINDSTRÖM AND PERIODON HADDING

本文系统梳理了中-晚奥陶世全球广泛分布的两个牙形石属Pygodus Lamontet Lindström,1957和Periodon Hadding, 1913已命名种的形态特征,清理出这两属目前的有效种,探讨了属内演化关系。认为Pygodus属的特征分子Pa分子表现出齿瘤小型化、密集化且齿瘤列数量先减少后增加的演化趋势, Pb分子表现出细齿密集化、增量化和倾斜化的趋势, S分子的细齿仅表现为密集化和增量化的趋势;Periodon属的特征分子Pa和Pb表现出前齿突上细齿逐步增量化的进化特征, M分子基部前缘细齿数量和S分子主齿与最大细齿间的小齿数量虽然也有增加的特点,但该特征并不稳定。实践上, Pygodus属Pa分子齿瘤列的数量和Periodon属Pa及Pb分子前齿突细齿的数量是这两属最重要的定种依据。基于此演化认识,对产自鄂尔多斯盆地西缘台地边缘相区棋探9井达瑞威尔阶顶部的牙形石开展了系统的分类学和生物地层学工作。在连续采集的15件样品中共获得527枚牙形石,鉴定后归入13属20种,以Pygodus和Periodon占绝对优势,可识别出2个生物带:Pygodusserra带和Pygodus anserinus带。此序列可用于检验归纳出的两属属内演化认识和前人据此建立的化石带、化石亚带及化石超带的全球可对比性。经对比,认为基于Pygodus建立的生物带和生物亚带具有区域和全球可对比性,但基于Periodon建立的生物超带穿时性明显。

In this paper,we reviewed the morphological characteristics of all the named species of two widelydistributed,Middle-Late Ordovician conodont genera Pygodus Lamont et Lindström,1957 and Periodon Hadding,1913.The validity of species assigned into these two genera and the evolutional relationship among them are reassessed.We herein conclude that the stelliscaphate Pa element is the most diagnostic element of Pygodus species,whose denticles become gradually more diminutive and denser in the evolutionary history.The number of denticle rows on the platform of the Pa el.of Pygodus species decreases during the early evolutionary stage but increases later.The denticles of the Pb el.of Pygodus species get densified,reclined,and increased in numbers,while those of the S els.show only densified and increased in numbers in the evolution series.In the evolution of the Periodon species,the diagnostic Pa and Pb elements indicate an increase in the amount of free denticles on the anterior process.The M element bears more denticles on the anterior basal margin of the cusp,and the number of small denticles between the cusp and the largest denticle on the posterior process of the S element has a similar trend,but this feature is instable.In practice,the quantity of denticle rows of the Pa element of Pygodus species and the denticle numbers on the anterior process of the Pa and Pb elements of Periodon species are the most significant diagnosis of these two genera.Based on this,conodont taxonomy and biostratigraphy of the uppermost Darriwilian strata from the marginal facies of the carbonate platform on the western margin of the Ordos Basin were carried out for a purpose to check the evolutionary and biostratigraphical issues of these two genera.A total of 527 conodonts were obtained from 15 samples collected continuously from the Bei'anzhuang and Machiakou formations of Core Qitan-9.These conodonts can be assigned into 13 genera,20 species,in which Pygodus and Periodon are dominated,and consequently two biozones(Pygodus serra zone and Pygodus anserinus zone)are recognized.The present conodont fauna is suitable to be used to test the understanding of the evolutionary series of Pygodus and Periodon,and the global comparability of biozones,subbiozones and superbiozones previously established on the basis of them.The present study demonstrates that the biozones and subbiozones based on Pygodus species can be well-correlated worldwide,but the diachronism of the biostratigraphic units on Periodon species grounds is remarkable.