中文摘要：

采用PCR方法对两爪鳖（Carettochelys insculpta）和山瑞鳖（Palea steindachneri）mtDNA的细胞色素b基因（Cytb）进行了扩增和测序,并结合GenBank中已公布的其它15种龟鳖类和两种鳄类的同源序列,进行了序列变异比较和系统发生分析.以凯门鳄和扬子鳄为外群,采用邻接（neighbor joining,NJ）法、最大简约（maximum likelihood,ML）法和最大似然（maximum parsimony,MP）法重建分子系统树.结果均支持将龟鳖目分为3支：侧颈龟科（Pelomedusidae）（非洲侧颈龟Pelomedusa subrufa）,两爪鳖科（Carettochelyidae）（两爪鳖Carettochelys insculpta）和一支由其它15种现生龟鳖类组成的进化支.在这里两爪鳖科被认为可能代表一新的亚目;而山瑞鳖聚于鳖科（Trionychidae）中,与中华鳖（Pelodiscus sinensis）最先聚成一支,再依次与马来鳖（Dogania subplana）、非洲鳖（Trionyx triunguis）、印度箱鳖（Lissemys punctata）聚成一组.

英文摘要：

The complete sequences of Cytochrome b（Cytb）genes in Carettochelys insculpta and Palea steindachneri were amplified using PCR technology.Meanwhile,the homologous sequences of other 15 turtle species were obtained from the GenBank.Based on these data,the sequence variation and the phylogenetic relationship of these groups were analyzed.Employing Caiman crocodilus and Alligator sinensis as outgroups,the phylogenetic trees were reconstructed using neighbor joining （NJ）,maximum parsimony （MP）and maximum likelihood （ML）methods.All of the three methods suggest the earliest phylogenetic tree splits separating into three basal branches：the Pelomedusidae（P.subrufa）,the Carettochelyidae （C.insculpta）and an assemblage of 15 cryptodiran turtle species.It suggested that C.insculpta may be a representation of distinct suborder.In the Trionychidae,Palea steindachne and Pelodiscus sinensis firstly gather into one clade,and then form a group with Dogania subplana,Trionyx triunguis and Lissemys punctata.