| Large freshwater gastropods of the family Viviparidae are today distributed on allcontinents except South America (where they are recorded as fossils，and Antarctica).They usually occur in lakes, ponds, and lentic rivers of temperate to tropical regions.Currently, approximately150species in30genera are recognized in the Viviparidae,several of which have been assigned to the three subfamilies Viviparinae, Lioplacinae,and Bellamyinae. For more than half of the genera, however, the exact systematicposition and phylogenetic relationships are unknown. With a few exceptions, theViviparidae are characterized by medium high-spired shells with moderately tostrongly inflated whorls that are often more or less circular in cross-section andseparated by deep sutures. The large, thin, horny operculum lacks calcareouscarbonate and is usually not preserved in fossilized specimens. In most species, theshells are thin, often stabilized by spiral ribs or bands, and relatively large, rangingbetween25and70mm in height. Based on their near-global distribution andabundance, the Viviparidae are thought to have evolved before the break-up of thePangaea supercontinent, and thus at least during the Jurassic. The highest diversity ofextant Viviparidae is found in East and Southeast Asia, comprising several widespreadas well as endemic genera and species. The rich fossil record of Viviparidae in Europe,however, provides several examples of Pliocene endemic lake radiations in this group,leading to an extraordinary high diversity of viviparids. Literature on fossil Cenozoicfreshwater gastropods from continental East Asia is scarce. Fossil Viviparidae have,however, been described from several regions, and seem to have been widespread inEast Asia since the Palaeogene. Eocene and Miocene viviparids from Thailand havebeen used in stable isotope studies and several Miocene mollusc assemblagesdominated by viviparids were reported from northern Thailand. Additionally,Oligocene to Pleistocene mollusk assemblages that yielded viviparid gastropods havebeen described from Myanmar and China. Scattered additional records are listed indetailed compilations of fossil freshwater molluscs of China and East Asia.However, rich freshwater gastropods fauna are currently known near the city of Santang，where a good outcrops were chosen as the studied section in this thesis. It islocated ca2km to the northeast of Nanning, exposed a somewhat more than6m thicksuccession of the Gongkang Formation which consisted of well-bedded mud-andsiltstones, yielding abundant fossils including gastropods, bivalves and somevertebrate fossils. The new fossil collections with a remarkably high diversity andextraordinary preservation allow to carry on the investigations as following5parts:(1) Rich freshwater gastropods fauna from the Lower Oligocene Gongkang Formationof the Nanning Basin (Guangxi Province, SE China) has yielded shells of16speciesof gastropods and two species of bivalves most of which been thought to be endemicto the Pleistocene to Recent freshwater lakes of Yunnan Province (SW China), and isfirst described from the Oligocene of Guangxi herein. One new genus and7newspecies are erected in this thesis.(2) Component and Distant analysis were done for the6giant gastropods taxa.It shows that Taxonomy can be arranged by programs of computer and it willbe fewer mistakes compared with checking pictures only.(3) Scars of the gastropods, caused by predators were calculated.The result shows that the gastropods which are more compacted rounded in shape and the gastropods are thicker and with more decoration can defend the predator efficiently.(4) The microstructure of the gastropod shell are analyzed using SEM. More than400photos were taken. The microstructure can be used in Taxonomy for gastropods.(5) Analysis of stable isotope samples more than200are used to reconstructthe paleoenvironment, it is documented by the study that the early Oligocenemight be a warm and humid time-interval in the Naning basin during the Paleogene. |
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