中国科学院南京地质古生物研究所机构知识库

The end Ordovician mass extinctions have been closely related to the formation and melting of Gondwanan ice masses. It is also suggested that the beginning of the extinctions may have been marked by the appearance of the cool-water-adapted Hirnantia brachiopod fauna; but investigation of new brachiopod material from the uppermost Katian and lower Hirnantian strata in the Yangtze Region, South China may modify the latter point of view. In the Yangtze Sea, a microcosm of the global perturbations in the Late Ordovician, there occur mixed-facies strata between the Wufeng Formation (black shale with graptolitic facies) and the Kuanyinchiao Beds (shelly facies with the Hirnantia-Mucronaspis Fauna) in the Upper Yangtze Region and within the Xinkailing Beds in the Lower Yangtze Region. These strata may represent a transitional stage of major changes in depositional environments and provide crucial evidence of the initial impact of the formation of the Gondwanan continental ice sheet on the South China Palaeoplate. However, they have been often neglected due to a thin development of sediments and inadequate research on their faunas. Based on collections accumulated during the last half a century, the present authors describe and revise systematically the genus Manosia Zeng, 1983, an overwhelmingly dominant, opportunistic taxon, associated rarely with the rhynchonellide brachiopods (Thebesia) and the trilobites (Triarthrus) in the mixed-facies strata. Its type species is assessed, concluding that the type species, Manosia yichangensis Zeng, 1983 is synonomised with Oxoplecia? inconstanta Xu et al., 1974 since both are conspecific. In the light of rare, well-preserved specimens with spiralia directed centro-dorsally, it is confirmed that Manosia belongs to the atrypides, and is provisionally assigned to the Family Atrypidae. The basic features of the Manosia Assemblage and its spatial and temporal distributions are discussed. Manosia failed to migrate out from the South China Palaeoplate, which palaeogeographically, may have been in an isolated position; and the dispersal ability of larvae of M. inconstanta may have been poor. Nevertheless, Manosia spread all over the Yangtze Sea from northeastern Yunnan to southern Jiangsu ( east-west), and from southern Shaanxi to northern Guizhou (north-south), inhabiting relatively deep-water, low energy and oxygen-deficient conditions (BA 4-5). Manosia ranges from the Diceratograptus mirus Subbiozone (= uppermost Paraorthograptus pacificus Biozone), uppermost Katian to the lower Normalograptus extraordinarius Biozone of the Hirnantian, Upper Ordovician. This short range may imply the diachronous onset of the effects of global perturbations in different areas in the Yangtze Sea. There was a process of "shallow water first and deep water later" at the start of this major change. In the Upper Yangtze Region, the Manosia Assemblage ranges from relatively shallower water in the D. mirus Subbiozone and to deeper water regimes in the interval of D. mirus Subbiozone-early N. extraordinarius Biozone. Whereas in the Lower Yangtze Region, the Manosia Assemblage migrated to deep waters and became extinct during the early and middle N. extraordinarius Biozone respectively. Study of community ecology, environments and its symbolic significance suggests that the signal of the onset of the end-Ordovician brachiopod extinctions in South China may have been not the Hirnantia Fauna itself, but the occurrence of the Manosia Assemblage; the extinctions may not have commenced in early Hirnantian, but earlier in the latest Katian. The end Ordovician mass extinctions coincided with global changes of climate and ocean environments and were the single, major biotic event in the Phanerozoic closely associated with glacigene activities.