西沙群岛石岛晚更新世碳酸盐土壤层陆栖蜗牛化石

TERRESTRIAL SNAIL FOSSILS IN SOIL LAYERS OF LATE PLEISTOCENE EOLIAN BIOCALCARENITE IN THE SHIDAO ISLAND OF THE XISHA ISLANDS

西沙石岛四层化石土壤层含有17个种,全系陆栖蜗牛类。但各化石土壤层所含化石种类和个数不尽相同。第三化石土壤层所含种类最多,第四化石土壤层种类最为稀少,但居群个体数量却极为丰富。这个蜗牛类化石动物群以现今分布于热带、亚热带的种类为主,也有少量出现较早的化石种。其中,在各成土期都十分发育的Bradybaena phragmitum (Heude)曾产于上更新统下蜀组地层。因此,认为石岛风成石灰岩可能形成于晚更新世。

16 terrestrial snail species, collected from outcrops and drilling holes in four fossil soil layers interspersed among late Pleistocene eolian biocalcaren-ite in the Shidao Island of the Xisha Islands, are analysed and described. The fossil fauna has the following characteristics: 1. It is fully composed of terrestrial snails which are different in shapes and sizes, and rich in species and individuals. Dominant species are Bradybaena phragmitum (Heude) and Pleuropoma (Sphaerocomia) hungerfordianum hungerfordianum (Moellendorff). 2. The terrestrial snail species increase obviously in number from the first fossil soil layer to the third, but very few can be seen in the fourth layer. 3. Because most of the species are distributed in the Philippeans, Indonesia and in Hongkong, Guangdong and Yunnan of China, this fauna has more shows of tropical terrestrial snails. 4. Bradybaena phragmitum (Heude) is dominant in the fauna, occurs in all the four layers, and becomes an absolute occupant in the fourth layer. B. phragmitum (Heude) exists in Xiashu Formation of upper Pleistocene. Therefore, this indicates that the eolian biocalcarenite in the Shidao Island belongs to late Pleistocene in age.