基金Acknowledgements: This study was supported by the National Nature Science Foundation of China (No. 30270166), the Ministry of Science and Technology of the People's Republic of China (No. 2006FY110500) and the Foundation of Experimental Centre of Shenyang Normal University (No. SY200609). We are very grateful to Mr D.M. Ackland, Oxford University Museum of Natural History, Parks Road, Oxford OX 1 3PW, UK, for his reading of this manuscript and valuable suggestions and to WANG Ming-fu for providing the examined materials.