The age of the Phu Kradung Formation (Khorat Group, NE Thailand): Indications from the turtle faunas

The Mesozoic Khorat Group (NE Thailand) contains five formations (from bottom to top, the Phu Kradung, Phra Wihan, Sao Khua, Phu Phan and Khok Kruat formations; Racey 2009). It is now generally accepted that most formations of the Khorat Group are of Early Cretaceous age, while the age of its basal unit, the Phu Kradung Formation, is still uncertain. The evidence from vertebrate palaeontology, notably dinosaurs, supports a Late Jurassic age for that formation; while palynology and detrital zircon thermochronology, suggest an Early Cretaceous age. Composed of sandstones, siltstones and mudstones of mainly fluvial origin, the Phu Kradung Formation is rich in vertebrate remains, including freshwater sharks, bony fishes, temnospondyl amphibians, turtles, crocodiles, pterosaurs and various dinosaurs (sauropods, theropods, stegosaurs and ornithopods). Turtle remains are abundant. In the lower part of the formation, two sites (Phu Noi and Ban Khok Sanam) in Kalasin Province have yielded xinjiangchelyids. Phunoichelys thirakhupti is represented by several incomplete shells and isolated shell elements. This small turtle may be related to some primitive xinjiangchelyids from the Sichuan Basin, China (Tong et al. 2015). Kalasinemys prasarttongosothi , a more advanced xinjiangchelyid consists of several shells and a well preserved skull. It has a more heavily built shell and is distinct from Phunoichelys in the smooth shell surface and other shell features. The structure of the arterial system on the skull is characteristic of xinjiangchelyids, and the skull outline is close to Annemys (Tong et al. in press). At Ban Khok Sanam, a few fragmentary shell elements with fine radiating ridges on the carapace are reminiscent of some xinjiangchelyids and macrobaenids from China, although the fragmentary nature of the material prevents a precise systematic assignment. In the upper part of the formation, remains of a basal trionychoid turtle, Basilochelys macrobios , are common (Tong et al. 2009). Several localities in Mukdahan Province (Kham Phok, Huai Sai, Dan Luang, Huai Pai, Dan Kaeng) have yielded abundant material. Basilochelys was also found recently in Kalasin Province, together with a large incomplete primitive eucryptodiran turtle shell, which has strong radiating ridges on the carapace. The small and strip-shaped epiplastra and reduced leaf-shaped entoplastron are reminiscent of Macrobaenidae. The Phu Kradung Formation thus appears to have two distinct turtle faunas. That from the lower part of the formation consists of diverse xinjiangchelyids and has close affinities with those from the Late Jurassic of China and Mongolia, which seems to support a similar age for that part of the formation. The upper part of the formation has yielded mainly basal trionychoids and has no clear equivalent in mainland Asia, where the Early Cretaceous turtle faunas are dominated by sinemydids/macrobaenids, with a few more advanced trionychoids. The recent discovery of a macrobaenid in Kalasin Province seems to support an Early Cretaceous age for the upper part of the Phu Kradung Formation. It is noteworthy that turtle faunas from the upper part of the Phu Kradung Formation, as well as from the overlying Sao Khua and Khok Kruat formations are more comparable with those from the Early Cretaceous Tetori Group of Japan in the presence of various primitive trionychoids and some sinemydids/macrobaenids, on the basis of which stratigraphical correlations can be made.