新疆寒武纪早期单壳类软体动物化石分类学与生物地层学

TAXONOMY AND BIOSTRATIGRAPHY OF THE EARLY CAMBRIAN UNIVALVED MOLLUSC FOSSILS FROM XINJIANG

作者系统描述了新疆阿克苏—乌什地区下寒武统玉尔吐斯组中上部单壳类软体动物化石10属10种,其中包括3个新种、3个老种和4个未定种,它们是Shabaktiella multiformis sp . nov .,Ilsanella acuta sp . nov .,Xianfengella yuertusiensis sp .nov .,Parcaconus xinjiangensis , Protostenotheca xinjiangensis , Aldanella attlebo-rensis , Obtusoconus sp .,Yochelcionella sp .,Bemella sp .和Mellopegma sp .。上述10个属种基本上代表了玉尔吐斯组单壳类软体动物的面貌特征。文中介绍了本研究区10个属和属级以上分类系统在国际学术界的意见分歧后,重申了以壳的对称性和有无肌痕确定纲级界线的最佳分类原则,提出了应用壳型划分目级分类单元和口缘形状、壳顶与口缘的相对位置划分科级分类单元的可行性和有效性,但是不能接受把所有单板状软体动物化石通常被归入单板纲的做法,因为寒武纪早期一些单板状软体动物化石不一定符合单板纲的真实含义。文中还回顾了近二十年来建立的始单板纲(Eomonoplacophora)( Missarzhevsky ,1989) ,太阳女神螺纲( Helcionelloida) ,背壳肌纲(Tergomya)(Peel ,1991a ,b)和似腹足纲(Paragastropoda)(Linsley & Kier ,1984)等纲级的新概念。还有一些早期软体动物专家将许多单板状(包括帽状和螺旋状)化石归入单板纲(Runnegar et al .,1976 ,1985) ,而另有一些中青年软体动物专家继承传统理念,将它们归入了腹足纲(Parkhaev in Alexander et al .,2001)。钱逸和本格森将早期单壳类软体动物化石分成了五大形态类别,未确定科级以上分类单元名称( Qian &Bengtson,1989)。总之,目前纲级分类单元界线不清,目级以下分类单元十分混乱,在没有全面清理早期单壳类软体动物百余个属和属级以上分类单元之前,是难以统一本文研究的早期单壳类软体动物10个属和属级以上分类系统。我们认为在确定属级以上分类系统及其与之密切相关的演化谱系关系时,不是利用理论前提,而是在不间断的剖面上对单壳类软体动物化石进行逐层采集并对其构造要素的详细研究,才能使得出的结论和提出的新分类系统发生错误的几率最小。文中还归纳和总结了本研究区玉尔吐斯组中上部单壳类软体动物群面貌的六大特点,其中二点最重要:1)该动物群与梅树村期微软体动物群的面貌有明显的差别,它缺失了梅树村期微软体动物群中最原始、最特征、最丰富的类群,如笠帽状的马哈螺类化石Maikhanella, Purella等和螺旋状的始旋螺类化石Archaeospira等。但是该动物群面貌却与邻区的哈萨克斯坦Atdabanian期的微软体动物和我国峡区西蒿坪动物群面貌接近,出现了许多相同或相似的属种,如Shabaktiella,Mellopegma,Aldanella,Yochelcionella等。2)该动物群共生有全球分布的相当于筇竹寺期的标准化石,如原牙形类Gapparodus,Amphigeisina,Hagionella等,有口唇并有背腹分异和复杂口盖的软舌螺类Microconus,Pachytheca等,具有肉茎孔和铰合面的有铰腕足类和像Obolus那样的原始无铰腕足类,有各种形态类型的金臂虫Dabashanella,Liangshanella等,还有可疑的三叶虫颊刺等。上述信息足以说明玉尔吐斯组中上部地层时代应属筇竹寺期而非梅树村期,从而解决了长期以来有关玉尔吐斯组时代归属的争议。玉尔吐斯组单壳类软体动物群的研究,再次证实了寒武纪生命大爆发期间,早期单壳类软体动物与小壳动物群一起经历了三次大发展和三次大绝灭事件。这在早期单壳类软体动物化石分类学、生物地层学和早期演化史上均有较高的科学价值。

This paper describes univalved mollusc fossils from the upper part of the Yuertusi Formation of the Early Cambrian at Aksu-Wushi region, Xinjiang. The fossils comprise 10 genera and 10 species, including 3 new species, 3 known species and 4 undetermined species. These species are Shabaletiella multiformis sp. nov. , Ilsanella acuta sp. nov. , Xianfengella yuertusiensis sp. nov. , Parcaconus xinjiangensis, Protostenotheca xinjiangensis, Aldanella attleborensis, Obtusoconus sp. , Yochelcionella sp. , Bernella sp. , Mellopegma sp. These 10 genera and species basically reflect the aspect of univalved molluscs of the Yuertusi Formation. After introducing different opinions on classification, this paper preferrs to determine Class＇ partition based on symmetrical characteristic and muscle, and proposes to divide Order according to shell morphology and to partition Family based on aperture aspect and position between shell apex and aperture. This paper cannot accept the idea of ascribing all univalves under Monoplaeophora because that some Early Cambrian univalved mollusc fossils do not belong to Monoplacophora. The present paper reviews new concepts about Class Eomonoplacophora （Missarzhevsky, 1989）, Helcionelloida, Tergomya （Peel, 1991） and Paragastropoda （Linskey and Kier, 1983）. Some early studies assigned many monoplacophoran forms （including cap or spiral forms） to Monoplacophora （Runnegar et al. , 1976, 1985） while some other younger scholars yet regarded them as Gastropoda （Parkhaev in A.lxamder et al. , 2001）. Qian and Bengtson （1989） divided early univalves into five morphological groups and did not determine classification units of Family or above family. So present division of class unit is not clear. Order or below order unit is in disorder. Before classification on more than hundred genera of early univalved mollusks is put in order, it is relatively difficult in uniting classification system of genera and above genera in this paper. We think that determining classification system of above genera and their relative lineage needs to collect successively univalved molluskan fossils in continuous sections and to investigate construction elements in detail. This paper also summarizes the six characters of univalved mollusc fauna of the upper Yuertusi Formation in Aksu-Wushi region. The most important are： 1. This fauna is obviously different from the Meishueunian mieromollusks because the lack of the most primitive, characteristic and abundant group, such as Maikhanella, Purella and Archaeospira and so on. However, this fauna is similar to micromolluscs of the Atdabanian Stage from the adjacent Kazakhstan and Xihaoping fauna in the Yangtze Gorge area of China. There are some same or similar genera and species, such as Shabaktiella, Mellopegma, Aldanella and Yochelcionella. 2. This fauna contains some standard fossils that are distributed globally and correspond to the Qiongzhusian Stage, such as protoconodonts （Gapparodus, Arnphigeisina and Hagionella）, Hyoliths with opertural lip and dorsal and ventral differentiation as well as complex opereulum （Microconus and Pachytheca）, primitive inarticular brachiopods （Obolus）, a variety of morphological Bradoriids （Dabashanella and Liangshanella） and problematic genal spine of trilobite. All the above suggests that age of the middle and upper part of the Yuertusi Formation should be assigned to the Qiongzhusian Stage instead of the Meishucunian Stage. Research on the univalved molluscs in the Yuertusi Formation confirms again that early univalved molluscs went through three great developments and three great extinctions during the Cambrian explosion together with the small shelly fauna, This is of great significance in the taxonomy and biostratigraphy of early univalved mollusc fossils.