A lost world in red stone
The Khorat Plateau of northeastern Thailand holds a thick sequence of Mesozoic rocks known as the Khorat Group. Its lowest member, the Phu Kradung Formation, is a distinctive series of reddish-brown siltstones and sandstones up to 1,200 meters thick. These sediments were laid down by ancient river systems between the Late Jurassic and the earliest Cretaceous period, roughly 152 to 142 million years ago. The environment was a lake-dominated floodplain cut by meandering rivers, existing under a semi-arid climate.
Within these layers, paleontologists have uncovered a diverse assortment of prehistoric life. The formation contains many vertebrate fossils, containing the remains of freshwater sharks, turtles like Phunoichelys thirakupti, and ancient crocodiles such as Sunosuchus thailandicus. The most important discoveries belong to the dinosaurs. The red beds have yielded teeth from predatory theropods and bones from armored stegosaurs.
The main story belongs to the sauropods, the long-necked, plant-eating giants. For decades, isolated bones and teeth hinted at their presence. Recently, more complete remains have allowed for the formal identification of new species. One is Uragasaurus kalasinensis, a member of the Mamenchisauridae family, a group known for their extremely long necks. The discovery of this mamenchisaurid, a group primarily known from China, links the dinosaur faunas of Southeast Asia and East Asia during this period.
Rewriting dinosaur distribution
The presence of Uragasaurus and other mamenchisaurids in Thailand shows dinosaur biogeography. Previously, this family of sauropods was thought to be largely restricted to China. The Thai fossils demonstrate that their range was much wider, extending deep into Southeast Asia. This suggests that a land connection existed, allowing these massive animals to migrate between the regions.
This finding challenges older ideas about geographic barriers and supports a hypothesis that Central, Eastern, and Southeast Asia formed a relatively isolated continent during the Late Jurassic. The dinosaur communities here evolved a distinct character compared to those on other continents like North America or Gondwana. The Phu Kradung Formation reveals that Southeast Asia was an important area of mamenchisaurid diversity.
Other dinosaurs from the region, like the 15-meter-long Phuwiangosaurus sirindhornae (named after Princess Sirindhorn) and the early tyrannosaur relative Siamotyrannus isanensis, further define this unique prehistoric ecosystem. The ongoing excavation at sites like Phu Noi in Kalasin Province produces new fossils, showing more about the lost world of Thailand's dinosaurs.