晚二叠世非海相腹足类在我国的首次发现

DISCOVERY OF NON-MARINE GASTROPODS FROM UPPER PERMIAN XIAOL ONGKOU FORMATION OF JIMSAR, XINJIANG

一、前言1982年,中国科学院南京地质古生物研究所新疆地层古生物研究队在准噶尔盆地东南缘吉木萨尔地区考察时,笔者之一(朱祥根)曾在大龙口剖面小龙口组采到一批腹足类化石。1983年和1985年又进行了化石的补充采集和详细的地层工作,采获丰富的腹足类化石,计得2属、3种(包括1新属、2新种),它们是Xinjiangospira rotundata gen. et sp. nov., Xinjiangospira cf. gondwanica (Cox), Bernicia? jimsarensis sp. nov.。

The non-marine fossil gastropods dealt with in the present paper were collected from the Xiao-longkou Formation in the Jimsar district of southeastern Junggar Basin, Xinjiang by the Northern Xinjiang Stratigraphicat Working Team, Nanjing Institute of Geology and Palaeontology, Academia Sinica, including one of the writers (Zhu) in 1982, 1983 and 1985 successively. The Xiaolongkou Formation is chiefly composed of brown, greenish grey mudstone, intercalated with greyish green fine sandstone and argillacequs siltstone, with thin-bedded marlite, about 160m in thickness. The gastropods were mainly obtained from the marlite, generally well-preserved, with three species in two genera, including one new genus and two new species, namely, Xinjiangospira rotundata gen. et sp. nov., Xinjiangospira cf. gondwanica (Cox), and Bernicia? jimsarensis sp. nov.So far as known, the Late Paleozoic marine gastropods are widely distributed in many parts of the world, with aboundant genera and species and under extensive study, but the Late Paleozoic nonmarine gastropods are known to be very rare although they have been separately reviewed and summarized by T. C. Yen (1949) and A. Solem and E. L. Yochelson (1979) in the past. In China, these rare fossils have been mentioned from the southern areas; unfortunately, neither descriptions nor illustrations of them have been published. Therefore, the writers consider that the non-marine gastropods are discovered for the first time from the Upper Permian Xiaolongkou Formation of Xinjiang in China, representing a most important developmental stage in their systematic evolution and providing the best substantial evidences available for the division and correlation of the Upper Permian, the discussion on sedimentary environments, zoogeographical provinces and the exploration of their evolution and development.The characteristic forms of gastropods described in this paper include Xinjiangospira rotundata gen. et sp. nov., X. cf. gondwanica (Cox), associated with Palaeomutella, Palaeoanodonta, Paracuneatochara, Striodon, Jimsaria, etc. Among them, Xinjiangospira gondwanica was originally described from the Madumabisa Shale of the Karroo System in southern Rhodesia, while Palaeomutella and Palaeonanodonta were also yielded from the same bed of the Karroo System. This indicates that the gastropod-bearing bed of the Xiaolongkou Formation may be attributed to the Late Permian.Fossil description Genus Xinjiangospira gen. nov.Typespecies: Xinjiangospira rotundata gen. et sp. nov.Diagnosis: Shell small, elongated-conical or conical, higher than wide. Whorls 4 to 7 in number, roundly convex on outer side. Apex obtusely rounded. Protoconch submerged or deviated. Suture well-impressed. Aperture subcircular, holostomatous; peristome thin. Umbilicus wide and deep. Surface ornamented with growth lines and costae.Discussion: In the general form of the shell, this genus is similar to Hydrobia Hartman, 1821, but differs from the latter in the submerged or deviated protoconch, the obtusely rounded apex and the presence of costae.Xinjiangospira rotundata gen. et sp. nov. (Pl. Ⅰ, figs. 10, 11; Pl. Ⅱ, figs. 1—10; Text-fig. 1)Material: Ten well-preserved internal moulds and four external moulds.Description: Shell very small, conical, higher than wide. Apex obtusely rounded; pleural angle approximately 58°. Protoconch submerged and deviated (Pl. Ⅱ, figs. 3—5, 7). Whorls 4—5 in number, increasing slowly and regularly in the first 3—4 whorls and rapidly in the last one; last whorl ventricose, occupying about 2/5 the height of the shell. Exposed surface of early whorls roundly convex, while that of the last one ventricose. Suture well-impressed, slightly oblique; sutural angle approximately 13°.Aperture large, subcircular in shape; peristome thin; parietal lip oblique, while columellar lip arcuated. Umbilicus rather wide and deep.Surface ornamented with few and scattered costae (Pl. Ⅱ, figs. 1, 2, 6), but with dense and slightly curved growth lines.Comparison: This form is closely related to Xinjiangospira cf. gondwanica (Cox) in the general outline of the shell, but differs from the latter in the conical shell, the larger apical angle and the wider umbilicus.Xinjiangospira cf. gondwanica (Cox) (Pl. Ⅰ, figs. 1—9)cf. 1953 hydrobia gondwanica Cox, p. 204, Text-fig. 1.Material: Three well-preserved internal moulds in the collection.Description: Shell small in size, elongated-conical in shape. Apex obtusely rounded; pleural angle approximately 38°—45°. Protoconch submerged or deviated. Whorls six in number, increasing regularly in early ones and rapidly in the last one. Exposed surface of whorls roundly convex, constricted near the suture. Suture well impressed, slightly oblique, with sutural angle approximately 16°. Aperture not well-preserved. Umbilicus moderately wide. Surface ornamented with growth lines and some obscure costae.Comparison: In general form of the shell and the submerged protoconch, these specimens resemble Hydrobia gondwanica Cox from the Madumabisa Shale of the Karroo System in southern Rhodesia, but differs from the latter in the more obtusely rounded apex.Genus Bernieia Cox, 1927 Bernicia? jimsarensis sp. nov. (Pl. Ⅱ, fig. 11; Text-fig. 2)Material: Only a single specimen.Description: Shell small-sized, globose. Apex obtuse; pleural angle approximately 70°. Whorls 3 in number, increasing regularly in the first two, but abruptly in the last one; last whorl strongly ventricose, occupying about 3/5 the height of the shell. Exposed surface of whorls broadly rounded and separated by impressed sutures; rampnarrow and distinct. Surface smooth, covered with fine growth lines and some costae.Comparison: The generic reference of this species is uncertain, but the general outline of the shell suggests an affinity to Bernicia Cox, 1927, especially to the type species B. praecursor Cox. However, it differs from the latter in the more ventricose last whorl and in the presence of some strong costae.ACKNOWLEDGMENTSThe authors want to thank E. L. Yochelson of National Museum of Natural History, Washington, D. C., U. S. A. for giving comments and valuable suggestions to improve this article.