Turtle assemblages of the Khorat Group (Late Jurassic–Early Cretaceous) of NE Thailand and their palaeobiogeographical significance

Abstract: The turtle assemblages from the Khorat Group consist mainly of trionychoids. They include the primitive Trionychoidae Basilochelys and basal eucryptodiran turtles from the Phu Kradung Formation (?Late Jurassic); the adocid Isanemys srisuki, the carettochelyidKizylkumemys sp. and undetermined Trionychoidea from the Sao Khua Formation (Early Cretaceous); and the carettochelyid Kizylkumemys khoratensis and the adocid Shachemys sp. from the Khok Kruat For-mation (Aptian). Our study shows some faunal links between the turtle faunas from the Khorat Group and those from the peripheral regions of Asia during the time span of the Khorat Group. Thus the coastal regions of Asia, and more particularly SE Asia, may have been important places for the origin and early diversification of the trionychoids. The Mesozoic turtle record of Thailand ranges in age from the Late Triassic to the Early Cretaceous. It includes Proganochelys ruchae from the Late Triassic Huai Hin Lat Formation of NE Thailand (de Broin et al. 1982; de Broin 1984) and Siamo-chelys peninsularis from the Middle Jurassic of the southern peninsula (Tong et al. 2002). However, most turtle remains come from the Late Jurassic to Early Cretaceous Khorat Group, in th