【摘要】 <正> 研究简史吉林浑江地区下奥陶统发育良好,剖面连续,化石丰富,研究历史也早.1958-1960年,长春地质学院吉南区测大队于浑江地区进行1/20万区域地质调查,测制奥陶系剖面,并采集了笔石等大量化石.1979年,吉林省地质局根据南京地质古生物所对笔石的研究,在靖宇县(1/20万)区域地质调查报告中报道了 Adelograptus-Clonograptus 带在本区的存在.陈均远等(1983),周志毅等(1984)均指出本区冶里组底部有 Dictyonema flabelliforme-Staurograptus dichotomus 带.1983年,林尧坤将华北区下奥陶统冶里组分为5个笔石带,并指出吉林辉南、浑江等地的冶里组亦有这些笔石带的带更多还原

【Abstract】 The Xinchangian Yehli Formation in Hunjiand area, Jilin Province approximately cOrresponding to Tremadocian is well-exposed and yields a rich biota including graptolites, conodonts, trilobites, etc., with five graptolite zones or graptolite- bearing beds recognized in discending order as follows: Y₅ Adelograptus-Clonograptus Zone Y₄ Muenzhigraptus-Psigraptus Zone （2） Psigraptus Subzone （1） Muenzhigraptus-Neoclonograptus Subzone Y₃ Dendrograptus-bearing beds Y₂ Anisograptus richardsoni Zone Y₁ Dictyonema Staurograptus dichotomusflabelliforme- Zone （2） Dictyonema parabola Subzone （1） Dictyonema proparabota Subzone Up to now this graptolite sequence is the most complete one for the earliest Ordovician found in China. The Dictyonema flabetlliforme-Staurograptus dichotomus Zone （Y₁） may be divided, into two subzones; The lower one is the Dictyonema proparabola Subzone erected by Lin Yao-kun （1986）, with a new graptolite fauna comprising altogether 3genera and 9 species, namely Dictyonema flabelliforme （Eichwald）, D. ex. gr. flabelliforme （Eichwald）, D. f. eosociale Lin, D. Proparabola Lin, D. sp., Heterograptus antiquus Lin. Staurograptus cf. dichotomus Emmons, S. dichotomus priscus Lin and S. sp.. In all well-preserved specimens the presence of a distinct sicula with three-vaned floating structure indicates a probable primitive and planktonic behavior of this fauna. This subzone can be recognized as the earliest Ordovician graptolite-bearing bed, but it is still unknown to the world. The upper one is the Dictyonema parabola Subzone, which is characterized by the richest dictyonemids and contains most of the early Tremadocian species known in the world, such as D. flabelliforme （Eichwald）, D. parabola Bulman, D. sociale Salter and D. belgicura Bulman, in assocration with such graptodendroids as Siaurograptus dichotomus Emmons, Anisograptus minutus Mu and Heterograptus radiatus Zhao et Zhang, among which the genus Heterograptus erected by kun, 1986) is similar to Staurograptus,but differs from the latter in having sparse dissepiments. It should be noted that the abundance of discoidal specimens and apical tufts of threads （multistrand "sheafed" nema） are all in fair preservation, indicating a quiet water environment. Most important elements of this fauna have been obtained from W. Newfoundland, Yukon, Oslo, Sweden, Taimyr, indicating that this subzone may be Correlated with the equivalent beds of these regions. The Anisograptus richardsoni Zone （Y₂） is about 1m above the D. flabelliforme-Staurograptus dichotomus Zone, 1 m in thickness. Graptodendroids display a high diversity, with the presence of Anisograptus, Bryograptus, Clonograptus, Adelograptus, Kiaerograptus and Staurograptus, but dictyonemids are rare Among them, the zonal fossil Anisograptus richardsoni is the richest one. It is interesting to note that both Kiaerograptus and Adelograptus have been foaud in this zone, which appeared earlietr in China than in other regions in the world. This zone is equivalent to the La₁ of Australia, the Anisograptus richardsont Zone of Yukon, Canada and Taimyr, USSR, and 2eδof the Oslo region, Norway. Its equivalent zone also has been recognized in Liaoning Province and South China. The Dendrograptus-bearing beds （Y₃） in the middle part of the Yehli Formation, are rather rare in graptolites, with only dendroids having been collected, snclsl as Dendrograptus of. yangtzensis Mu, D. cf. viminalis Lin, Callograptus cf. sinicus Mu and Dictyonema sp. etc. Some of these species have been obtained from the Callograptus taitzehoensis Zone in North China or from the Acanthograptus sinensis Zone in Central China. Since the authors have paid great attention in vain to find graptodendroids in this part, it is difficut to establish a graptolite zone to represent the graptolite-bearing beds just uow. These beds may roughly correspond to X₂ because it directly overlies the Anisograptus richardsoni Zone （X₁） and is overlain by the Muenzhigraptus-Psigraptus Zone (X_2.5). The Muenzhigraptus Psigraptus Zone （Y₄） is a very important graptolite zone defined by Zhao Xiang-lin a and Zhang Sbun-xin （1985）. It is characterized by the appearance of reclined graptolites with isolated autothecae and may be divided into two subzones. The lower one is the Muenzhigraptus-Neoclonograptus subzone, with reclined Muenzhigraptidae as the dominant element, which is characterized by the autothecae isolated only in the proximal portion of the rhabdosome and comprises three genera: Muenzhigraptus, Diphygraptus and Hunjiangograptus. In addition, reclined Clonograptus, Clonograptus （Neoclonograptus） and horizontal Clonograptus, Clonograptus （Clonograptus） also have been found. A new species Psigraptus jacksoni has been collected by Richards and Stait （1984） from the Pontoon Hill Siltstone Member in Tasmania, Australia. Obviously, it can hardly be referred to Psigraptus, but belongs to Muenzhigraptus because of the proximally isolated autothecae and multiramous rhabdosome. Besides Tasmania, this special graptolite fauna is still unknown to other regions outside China. The upper one is the Psigraptus Subzone characterized by the appearance of psigraptids, such as Psigraptus, Yukonograptus and Holopsigroptus. All forms are notably of small size, with stipes nearly upright and autothecae obviously isolated throughout the stipes. A few species of Muenzhigraptus appear in the lower part of this subzone; only pauciramous forms composed of 2 to 4 stipes with all the autothecae obviously isolated have been obtained from the upper part, The evolutionary lineage of Clonograpzus （Neoclonograptus）-Mu enzhigraptus-Psigraptus gives an outline of: 1) the decrease in size of rhabdosome; 2) the reduction of stipes; and 3) the increase in isolation of au tothecae. This is an evolutionary offshoot in tile earliest Ordovician. This zone is equivalent to the La 1.5 Psigraptus-Clonograptus Zone of Australia, the Pontoon Hill Sihstone Member of Tasmania and the lower part of the Kiaerograptus antiquus Zone of Yukon. The Adelograptus-Clonograptus Zone （Y₅） occurs above the Muenzhigraptus-Psigraptus Zone and may be the highest graptolite zone of the Yehli Formation. In this zone Adelograptus and Clonograptus are predominant, associated with a few dendroids, but the reclined muenzhigraptids and psigraptids are extinct. The species obtained from this zone include Clonograptus （Clonograptus） tenellus Linnarsson, C. （C.） uniforrnis （Mu）, C. （C.） sinensis （Mu）, C. （C.） spp., Adelograptus horizontalis Lin, A. asiaticus Mu, A. sinensts Mu, A. victoriae （T. S. Hall）, A. papilionaceous Lin, Kiaerograptus hengshanensis Lin, K. minutus Zhao et Zhang, K. sp., Dendrograptus sinensis Mu, D. sp., Airograptus cf. furciferus Ruedemann, etc., among which Adelograptus, Clonograptus and Kiaerograptus are characteristic forms. This zone is equivalent to the La 2 zone of Australia, the Kiaerograptus antiquus Zone of Yukon and the Adelograptus-Clonograptus Zone of Texas, U.S.A. It should be noted that Dendrograptus sinensis Mu, a common element of the Callograptus taitzehoensis Zone has been obtained from this zone. In addition, C. taitzehoensis also has been found from the same horizon by Lin Yao-kun, one of the authors, indica,ting’ that this zone may be equivalent to the Callograptus taitzehoensis Zone. Based on the study of the Ordovician gra ptolite sequence in North China, Lin has defined the Adelograpntus-Clonograptus Zone above the C. taitzehoensis Zone in Hebei and Shanxi This will need a more detailed study on the actual relationship between the Adelograptus-Clonograptus Zone and?the Callograptus taitzehoensis Zone. Cambrian-Ordovician Boundary It is of great importance that a newa graptolite-bearing bed has been distinguishd 6.0 m, below the Dictyonema parabola Subzone and a new Dictyonema proparabola Subzone has been erected by Lin and his colleagues in 1985. This new subzone falls within the Dictyonema flabelliforme Staurograptus dichotomous Zone and can be recognized as the earliest Ordovician graptolite-bearing bed Then, the base of this graptolite zone lies between the conodont Cordylodus intermedus and Cordylodus lindstromi Zones. More precisely, its base is 5.28 m above the base of Cordylodus intermedius Zone and 0.54 m below the base of Cordylodus lindstromi Zone. Since the CambrianOrdovician Boundary Meeting in Calgary, 1985 has recommended the Cambrian-Ordovician Boundary to be defined on the conodont zone at or just below the first appearance of nematophorous graptolites, the authors are inclined to place the Cambrian-Ordovician Boundary at the base of tile Cordylodus intermedius Zone （FAD） or at the base of the Cordylodus lindstromi Zone, the nearest conodont zone to the Dictyonema flabelliforme-Staurograptus dichotomus Zone更多还原