内蒙古西部额济纳旗晚奥陶世生物地理和奥陶-志留系分界

LATE ORDOVICIAN BIOGEOGRAPHY AND ORDOVICIAN-SILURIAN BOUNDARY IN THE ZHUSILENHAIERHAN AREA, EJIN, WESTERN INNER MONGOLIA

通过最近对内蒙古额济纳旗东南珠斯楞海尔罕一带 (阿拉善地块 )奥陶 志留系界线地层和生物群的野外调查和室内研究 ,首次在上奥陶统上部巴丹吉林组中发现阿什极尔中期 (midAshgill)腕足动物化石巨大全嘴贝(HolorhynchusgiganteusKiaer) ,这是该属在我国奥陶系中的首次记录。根据未成年个体的切片所发现的腹壳顶端发育中隔板构造 ,修订该属定义 ,并证明研究小个体对识别物种特征、探讨个体发育和系统演化有重要意义。分析了全嘴贝属的群落生态、生物地理和演化意义后 ,发现它在奥陶纪末大灭绝过程中 ,因居群规模和分布范围极大地缩减 ,至今未发现其化石记录 ;但在志留纪初环境好转后继续存活 ,可暂视其为复活属。根据在拐子湖组近底部发现志留系最下部Akidograptusascensus带的重要分子NormalograptuslubricusChenetLin ,确定本区的奥陶 志留系分界。阿拉善地块因不发育Hirnantia动物群 ,表明奥陶纪末期它与扬子区、西藏、滇西等富含该动物群的地区属于不同的生物地理区系 ;在阿什极尔中期可能位于热带海域 ,与祁连山、中亚、乌拉尔等地有重要联系。文中描记H .giganteus种 ,评述正常笔石类 (normalograptid)的系统分类位置并描记N .lubricus种。

Upper Ordovician-lowest Silurian strata crop out near Danmianshan, about 10 km west of Zhusilenghaierhan Mt., about 55 km south of Yagan, 135 km east of Ejin Banner town, western Inner Mongolia (Alxa block). The strata along with their related major fossil groups (such as graptolites, corals and trilobites) have been studied by Zheng et al. (1982), Zhu et al. (1987), and Ge et al. (1990). Recent field examination in this area was carried out by the authors in the summer of 2002. Our subsequent investigation on collections has led to the discovery Holorhynchus (virgianids, Brachiopoda) from the Late Ordovician rocks, to make sure that there occur no latest Ordovician Hirnantia fauna, and to define the Ordovician-Silurian boundary on the basis of the occurrence of Normalograptus lubricus. The Ashgill brachiopod biogeography is discussed.Holorhynchus giganteus Kiaer, 1902 has been found from the Badanjilin Fomation (mid Ashgill) at the No. 3 section of Zheng et al. (1987) in this area for the first time. This is also the first record of the genus Holorhynchus in Late Ordovician of China. Holorhynchus has been known to be characterized by the total absence of a ventral median septum, together with smooth shell exterior and short, separated outer brachial plates (e.g. St. Joseph, 1938; Cocks, 1982). Through sectioning, however, the discovery of an apical median setum in ventral valve of immature specimens (less than 8mm wide) of H. giganteus led to a revised generic diagnosis for the genus (see Rong et al. in preparation). Actually, this structure is also known from an internal mold material and a transverse section of the identical species in South Norway (Cocks, 1982) and Zeravshano-Gissar Mountains, Tadzhikistan (Nikiforova and Sapel'nikov, 1973) respectively. Holorhynchus from the Badanjilin Formation at the No.3 section of Zheng et al. (1987) is associated with the brachiopods Altaethyrella, Ovalospira, Leptellina, Sowerbyella, Pectenospira, and the Agetolites tabulate coral fauna. These associates, with the exception of Petenospira, occur also from the Zhejiang-Jiangxi border area, East China (Chen et al., 1987; Rong and Zhan, 1995; Zhan and Cocks, 1998; Zhan et al., 2002), suggesting an assignment of BA 3 position for this association. Biogeographycally, Alxa block may have been linked with Qaidam (Lin in Lai et al., 1982), Kazakhstan (Borissiak, 1956; Rukavishnikova and Sapel'nikov, 1973; Sapel'nikov and Rukavishnikova, 1975), southwestern Tien Shan (Nikiforova and Sapel'nikov, 1973), the Urals (Modzalevskaya and Sapelnikov, 1973), Taimyr (Nikiforova, 1989), Kolyma (Nikolaev and Sapelnikov, 1973; Oradovskaya in Koren et al., 1983), Alaska (Blodgett et al., 1987), and the Baltica (Sapel'nikov and Beznosova, 1980; Paskevicius, 1982; Jaanusson, 1982) based on the occurrences of Holorhynchus fauna, together with the Agetolites fauna. It is remarkable that the two faunas have not been known from both North America-Siberia and Gondwana- peri-Gondwana regions during the mid Ashgill time. Moreover, Holorhynchus occurs from many places in the mid Ashgill (Text-figure 2), disappeared from the fossil record in the late Ashgill (Hirnantian, a crisis interval) due to the greatly decreasing of its population size and distribution areas, and recovered from Kazakhstan and North China in the later Rhuddanian or early Aeronian. At the present, Holorhynchus can be provisionally considered a Lazarus taxon of brachiopods through the Ordovician and Silurian transition (Rong et al., in preparation). Throuth an assessment of the composition of the early Hirnantian brachiopods of the Danmianshan bed, it is confirmed that there occurs no typical, cool water Hirnantia fauna in the area studied. Dalmanella, Cliftonia, Triplesia, and Leptaena that are recorded from this bed are almost cosmopolitan taxa from many places including both the warm water province possessing the Mid Continent fauna and the cool water province yielding the Hirnantia fauna in the latest Ordovician. But the lack of those charactaristic forms, such as Hirnantia and Kinnella in the present area indicates that the Alxa block may have been located in warm water regions (mainly tropical zone) during the contemporary time of glacio-eustatic lowering of sea level. Thus, this block is biogeographically differentiated from South China, Tibet, Sibumasu, and other related blocks where the cool water Hirnantia fauna developed.The Ordovician and Silurian boundary biostratigraphy of this area is reviewed herein. Zheng et al. (1982) defined this boundary between the Danmianshan bed (limestone with Ordovician shelly fossils) and the Guaizihu Formation (shale and silty mudstone with Silurian graptolites) at Danmianshan. Subsequently, Zheng et al. (1984) and Ge et al. (1990) found Parakidograptus sp. and P. acuminatus(Nicholson)respectively at a level about 5m above the base of the Guaizihu Formation and correlated it with the P. acuminatus Biozone. The present authors remeasured the Guaizihu Formation at the No. 1 section(102°21.552′, 41°47.445′)of Zheng et al. (1984) and collected the following graptolites in descending order:AFF 308 (11-12 m above the base of the Guaizihu Formation)Normalograptus mirnyensis (Obut et Sobo-levskaya) Neodiplograptus diminutus (Elles et Wood)AFF 307 (5.5 m above the base of the Guaizihu Formation)N. mirnyensis (Obut et Sobolevskaya) N. skeliphrus Koren et Melchin Metaclimacograptus sp. Sudburigraptus sp. Dimorphograptus sp.At the same layer, Ge et al.(1990)described P. acuminatus (Nicholson), N. angustus (Perner) (=Glyptograptus elegans Packham, Ge et al.,1990), N. mirnyensis (Obut et Sobolevskaya) (=Climacograptus linearis Packham, Ge et al., 1990), N. cf. normalis (Lapworth) (=C. transgrediens Waern, Ge et al., 1990), N. skeliphrus Koren et Melchin (=C. linearis Packham, Ge et al., 1990, and C. tamariscoides Packham, Ge et al., 1990).AFF 306 (1.87 m from the base of the Guaizihu Formation)N. laciniosus (Churkin et Carter) N. lubricus (Chen et Lin) N. mirnyensis (Obut et Sobolevskaya) N. skeliphrus Koren et Melchin N. ex gr. normalis (Lapworth)The A. ascensus Biozone is recognized from this area for the first time in the light of the discovery of N. lubricus Chen and Lin from the basal part of the Guaizihu Formation, which is described in this paper. N. lubricus has been found from AFF 306 (see Text-figure 3) and P. acuminatus from AFF 307 in the lower part of the Guaizihu Formation. The former as a leading species of the A. ascensus Biozone, although it ranges from the A. ascensus Biozone to P. acuminatus Biozone, is associated with N. laciniosus (Churkin and Carter), N. mirnyensis (Obut and Sobolevskaya), N. skeliphrus Koren and Melchin, and N. ex gr. normalis (Lapworth). The graptolites in these two layers (AFF 306 and 307) can be correlated with the A. ascensus and P. acuminatus biozones respectively. The Ordovician and Silurian boundary, therefore, is now defined at the base of the layer AFF 306 in the basal part of the Guaizihu Formation, which is 1.87 m above the base of the formation. The basal Guaizihu Formation may be considered to be of late Hirnantian (=N. persculptus Biozone), i.e. the top of the Ordovician rocks.