华南奥陶纪末生物大灭绝的肇端标志——腕足动物稀少贝组合(Manosia Assemblage)及其穿时分布

An indicator of the onset of the end Ordovician mass extinction in South China:the Manosiabrachiopod assemblage and its diachronous distribution

奥陶纪末生物大灭绝与冈瓦纳大陆冰盖的形成与消融密切关联的认识已被广泛接受;同时,赫南特贝腕足动物群(Hirnantia Fauna)常被视作这次大灭绝的肇端标志。本文通过对扬子区-全球环境巨变的一个缩影-相关层段的腕足类的研究,改变了后一认识。上扬子区五峰组(笔石页岩相)与观音桥层(介壳相)之间,以及下扬子区新开岭层之中的"混合相地层"(既产笔石,又产介壳相生物),代表奥陶纪最晚期沉积-生物-环境发生重大变化的过渡阶段,记录了南方大陆冰盖的形成对华南各地影响的开始;只因厚度小、研究少,该段地层及其所含化石群长期未被重视。根据半个世纪积累的材料,本文系统记述了该混合相地层的优势化石腕足类稀少贝(Manosia);根据保存腕螺构造的珍贵标本,证实它是无洞贝目的成员,暂归于无洞贝科;通过识别稀少贝组合(Manosia Assemblage)的基本特征和时空分布,探讨它的群落生态、环境及其标志意义。奥陶纪末的华南板块处于一个相对孤立的古地理位置,且稀少贝幼虫的漂浮能力不强,故未能"飘洋过海"离开华南板块一步;尽管如此,东西向从滇东北到苏南,南北向从陕南到黔北,它成为一个机遇泛滥属种,遍布于整个扬子海域,栖居于深水、低能、贫氧的底域(BA4-5)。稀少贝组合的地层历程从上奥陶统凯迪阶顶部(Diceratograptus mirus亚带)到赫南特阶下部(Normalograptus extraordinarius带中下部);其短暂的历程反映全球气候和海洋环境扰动对扬子海域不同地区造成的影响之开始。这里有一个"先浅水、后深水"的穿时过程:①在上扬子区浅水海域,它仅限于凯迪末期,几乎同时迁移到较深水域并延续到赫南特初期;②侵入到下扬子深水海域则是到赫南特早期并在中期灭绝。因此,华南奥陶纪末大灭绝的起步标志不是赫南特贝动物群本身,而是稀少贝组合的出现;大灭绝的肇端并非始于赫南特初期,而是凯迪末期。同时可能也指示大陆冰盖的形成对世界其他板块不同水深海域的影响也不是等时的。赫南特早中期扬子海域生物群分布状况演变得非常复杂。奥陶纪末生物大灭绝与全球气候和海洋环境的剧变相伴发生,是显生宙唯一一次与冰川活动有密切关联的特大事件。

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.