Spinosaurid theropod dinosaur remains from the Hin Lat Yao locality on Phu Wiang Mountain in Khon Kaen, Thailand, with comments on the status of Siamosaurus suteethorni

Spinosauridae was a clade of large-bodied theropods with proportionally long and narrow skulls. They had an almost cosmopolitan distribution across Laurasia and Gondwana. Spinosaurid remains are especially abundant in the Sao Khua Formation at the Hin Lat Yao locality on Phu Wiang Mountain in Khon Kaen, northeastern Thailand. Following the discovery of Siamosaurus suteethorni—the first named dinosaur from Thailand—several spinosaurid tooth morphologies have been identified, with most resembling either the Siamosaurus or Siamosaurus-type dentition. Here, we study spinosaurid teeth recovered from Hin Lat Yao to clarify their affinities. We also comment on other skeletal remains, including caudal vertebrae found at Hin Lat Yao. We employed both phylogenetic and discriminant analyses to assess the status of S. suteethorni, which shares minute distal denticles and a possibly plesiomorphic reduced crown curvature with Spinosaurinae. The non-serrated carinae are another spinosaurine feature. We, therefore, tentatively classify the Hin Lat Yao spinosaurid as distinct from Siamosaurus, and recognize the Southeast Asian region as having played a potentially significant role in the palaeobiogeography and evolution of Spinosauridae. Kridsanupong Puntanon [[e-mail]], Biodiversity Program, Walai Rukhavej Botanical Research Institute, Mahasarakham University, Maha Sarakham, 44150, Thailand; Suravech Suteethorn [[e-mail]], Department of Biology, Faculty of Science and Palaeontological Research and Education Centre, Mahasarakham University, Maha Sarakham, 44150, Thailand; Adun Samathi* [[e-mail]], Biodiversity Program, Walai Rukhavej Botanical Research Institute and Dinosaur Research Unit, Mahasarakham University, Maha Sarakham, 44150, Thailand.