Mid‐Cretaceous rudists (Bivalvia: Hippuritida) from the Langshan Formation, Lhasa block, Tibet
Mid‐Cretaceous rudists (Bivalvia: Hippuritida) from the Langshan Formation, Lhasa block, Tibet
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DOI：
10.1002/spp2.1019
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发表时间：
2015-11
影响因子：
2.3
通讯作者：
Xinquan Rao;P. Skelton;J. Sha;H. Cai;Yasuhiro Iba
中科院分区：
文献类型：
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作者：
Xinquan Rao;P. Skelton;J. Sha;H. Cai;Yasuhiro Iba
白垩纪狼山组出露沿着拉萨地块北方，主要由灰色或灰黑色细粒石灰岩、泥质石灰岩以及少量薄层粉砂岩和粉砂质泥岩组成。根据野外采集和/或拍摄的标本，结合对狼山组化石分类群的订正，确认了4个rudist分类群：（1）Auroradioptergen. nov.; (2)Eoradiaccf. davidsoni;（3）鲁托马盖兰叶蝉Magallanopyrutogensis sp. nov.;和（4）Sellaea sp.我们建议将亚洲西南部至太平洋的放射石种归为Auroradiopsis gen. nov.，以前归属于Eoradiopsis，其特征是外壳层致密（非纤维棱柱）。狼山组中的新属为A.双凸面（Yang等人，1982），包括其他几个synonymized类群。文献中的分类混乱是由于这些形式的强烈发育的韧带内折叠的不同外观造成的：在右瓣膜中，它有一个分叉的内部末端，但通过两个瓣膜的共同横截面显示后者与左瓣膜中的宽圆形对应物并列，使组合末端呈球形外观。相关的Eoradiescf. Davidsoni具有较小的尺寸，并且相比之下，显示出外壳层的纤维棱柱结构。最近描述的有沟的多孢子虫属，Magallaniti佐野et al.，2014年，由M. rutogensis sp. nov.，其与模式种的不同之处在于在左瓣中具有更多的和腹侧延伸的通道。可能属于后者的物种是孤立的右瓣，类似于Horiopleura，但含有tabulae。一个可能的Sellaea sp.是由保存不佳的右瓣代表，但显示出典型的特征，包括一个非常薄的外壳层和一个直立的后肌层壁，分离出一个广泛的后外肌层腔。根据Magallanidae的存在和orbitolinid的记录，这个rudist动物群可能不早于晚Albian，但不能排除更年轻的年龄。Magallanoptera和Auroradioptera都是亚洲西南部/太平洋中部动物区系的特征，而其他两个属分布更广。
The Cretaceous Langshan Formation, which crops out along the northern portion of the Lhasa block, is composed mainly of grey or grey‐black fine‐grained limestones, argillaceous limestones and a few thin‐bedded siltstones and silty mudstones. From specimens collected and/or photographed in the field, combined with a revision of taxa previously recorded from the Langshan Formation, four rudist taxa are recognized: (1) Auroradiolites gen. nov.; (2) Eoradiolites cf. davidsoni; (3) Magallanesia rutogensis sp. nov.; and (4) Sellaea sp. We propose Auroradiolites gen. nov. for the grouping of SW Asian to Pacific radiolitid species formerly attributed to Eoradiolites and characterized by a compact (non‐celluloprismatic) outer shell layer. The new genus is represented in the Langshan Formation by A. biconvexus (Yang et al., 1982), including several other synonymized taxa. Taxonomic confusion in the literature has resulted from the varied appearance of the strongly developed ligamentary infolding in such forms: in the right valve, it has a forked inner termination, but common transverse sections through both valves show the latter juxtaposed against its broad, rounded counterpart in the left valve, giving the combined terminations a bulb‐shaped appearance. Associated Eoradiolites cf. davidsoni is of smaller size and, by contrast, shows the celluloprismatic structure of the outer shell layer. The recently described canaliculate polyconitid genus, Magallanesia Sano et al., 2014, is represented by M. rutogensis sp. nov., which differs from the type species in possessing more numerous and ventrally extended canals in the left valve. Possibly belonging with the latter species are isolated right valves, similar to Horiopleura, but containing tabulae. A probable Sellaea sp. is represented by poorly preserved right valves nevertheless showing typical features, including a very thin outer shell layer and an erect posterior myophoral wall, separating off a broad posterior ectomyophoral cavity. Based on the presence of Magallanesia and the orbitolinid record, this rudist fauna is probably no older than late Albian, while a younger age cannot be excluded. Both Magallanesia and Auroradiolites are characteristic of a SW Asian/central Pacific faunal province, while the other two genera are more widely distributed.