宁夏、内蒙古的一些早白垩世珠蚌类

SOME EARLY CRETACEOUS UNIONIDS FROM NINGXIA AND INNER MONGOLIA, NORTH CHINA

宁夏六盘山群上部和内蒙古赛汉塔拉组产出的珠蚌类化石为首次描述,其面貌与邻区相当地层产出的蚌类相似,前人将它们鉴定为Unio。现据它们的壳顶区缺双带构造、前假主齿乳头状并不呈“多”形排列和前闭壳肌痕内具树枝状构造等特征建立Ningxiaconcha gen. nov.。本文还对Pseudelliptio属及其模式种的含义作了补充修订;通过这些珠蚌类化石的对比,宁夏六盘山群和内蒙古赛汉塔拉组可与新疆吐谷鲁群中上部、辽西沙海组和阜新组、山东蒙阴群中上部、浙江寿昌组上段以及苏联、蒙古、德国韦尔登阶进行对比,其时代应归入早白垩世早期。

Here described are some Unionids from the Liupanshan Group (Madongshan and Naijiahe Formations) of Ningxia and the Bayanhua Group (Saihantala Formation) of Inner Mongolia, including all together 2 genera and 6 species, among which Ningxiaconcha is a new genus, with a revised diagnosis of the genus Pseudelliptio. The Madongshan Formation yielded four species, namely, Ningxiaconcha colensis (Martinson), N. obruschewi (Martinson), N. elongata (Martinson) and N. jiamudariyaensis (Wei). The first three species were attributed to the genus Unio by Martinson (1956, 1961), while the last one was included in Inversidens by Wei gin-ruing (1984). The Naijiahe Formation yielded N. grabaui, which was attributed to the genus Unio by Grabau (1923), Gu Zhi-wei (1957, 1976) and Martinson (1956, 1961) and is classified to Ningxiaconcha here (as a new genus). The discovery of these fossils not only adds some new members to freshwater Lamellibranchs, but also provides new evidence for making comparisons of the Cretaceous strata between Ningxia and Inner Mongolia where these fossils come out, with the relevant strata in Xinjiang, Liaoning, Jilin, Shandong and those in USSR and Mongolia. In these strata, besides the 2 genera and 6 speecies described in this article, the associated nonmarine Lamellibranchs usually include: Ferganoconcka, Margaritifera or M. (Mengyinaia), Solenaia, Nakamuranaia, Neomiodonoides, Limnocyrena. etc., which are too many to be described in detail. These genera are distributed not only in the Zhunbayin Formation of Mongolia, but also in the Lianmuqin Formation of Xinjiang, the middle and upper parts of the Mengyin Group of Shandong, the Fuxin Formation of western Liaoning, and the Changcai Formation of Jilin. At present, stratigraphers and Palaeontologists have very different opinions on the implication (in the broad sense or the strict sense) of the Jehol fauna, and its strata, comparasion and geological age. The unionids described in this article should be included in the Jehol fauna (broad sense). The Lamellibranch researchers both at home and abroad also have different opinions on its age., e.g., Zhu Guo-xing (1976) classified the Caganlimenur Formation to the Upper Jurassic—Lower Cretaceous Series; Grabau (1923) classified the Menyin Formation to the Cretaceous system while Gu Zhiwei et al. (1976) and Chert Jin-hua (1984) attributed it to the Upper Jurassic Series; Martinson (1956, 1961) classified the Wulagjing and Cagancabu Formations to the Upper Jurassic—Lower Cretaceous Series, and the Torkiu-Vigim and Zhunbayin Formations to the Lower Cretaceous Series. In addition, in the shell shape, length to height ratio, and shell sculpture, Ningxiaconcha grabaui (Martinson) mentioned in this article is very similar to Unio menkei Dunker produced in Wealden, Germany (see the description below). The case of Wealden yielding a great deal of Neomiodon with a few elements of Eomiodon is also very similar to that of the strata in the Madongshan, Naijiahe Formation and Saihantale Formation which contain more Neomiodonoides Ma, 1985 [='Corbicula (Mesocorbicula)'=Neomiodon?] with less Limnccyrena (=Sphaerium); equally, the genus Neomiodonoides is very close to the Muscullopsis of North America which appears only in the Lower Cretaceous Series. In a word, the author considers it more reasonable to classify the upper strata producing the biota in a broad sense to the Lower Cretaceous Series. DESCRIPTION OF NEW GENUS Ningxiaconcha gen. nov. Type species Unio obrutshewi Martinson, 1956 Diagnosis Shell medium-sized, oval-orbicular, elliptic to elongate-quadrilateral. Umbonal region containing regular concentric plications or concentric striae. Hinge teeth strong, arranged in the formula of (5a), 3a,3b,5b/4a,2a,2b,4b, among which 2a and 3a are triangular, with irregular crenellation on surface; 4a is short-flaky, appearing smooth in appearance, sub-parallel to anterior dorsal margin; 5a is slender, sometimes absent; while 2b,3b,4b and 5b are long-flaky, smooth, parallel to posterior dorsal margin. In dimyarings, anterior adductor scar deep, with rough grooves on its inner side, but shallower and smooth on its outer side, with a small retractor scar on upper and middle parts of its inner side; posterior adductor scar shallow and smooth, also with a small retractor scar on upper part of its inner side; pallial line simple. Remarks The new genus bears some similarities to Unio, Inversidens and 'Psilunio' (or Potomida) of different geological ages, but they differ from each other in umbonal sculpture, hinge tooth structure and anterior adductor scar, as shown in the following table: Abbreviations: R. Right valve; L. Left valve. Distribution Later Jurassic(?)—Cretaceous; North China, Siberia of USSR, Mongolia and Europe.