A Metriacanthosaurid Tooth (Dinosauria: Theropoda) from the Phu Kradung Formation (Upper Jurassic–Lower Cretaceous) of Mukdahan, Thailand

Metriacanthosauridae is a clade of Laurasian allosauroid theropods closely related to Allosaurus. They appeared during the Middle Jurassic to the Late Jurassic of Europe and Asia and possibly survived to the Early Cretaceous. Here, we report an isolated tooth of a theropod dinosaur from Dan Luang locality, Mukdahan Province, northeastern Thailand. The tooth is studied to establish its taxonomic affinities. A discriminant analysis and cladistic analyses of dentition-based characters and supermatrix of theropods were performed and the tooth was compared with theropod teeth from the Late Jurassic to Early Cretaceous. The analyses and synapomorphies present on the Dan Luang material indicate that the tooth can be referred to the clade Metriacanthosauridae. The synapomorphic characters present in the material include (1) asymmetric D-shaped cross-section at the crown base, (2) a longitudinal groove adjacent to the mesial carina on the lingual surface, and (3) a concave surface adjacent to the distal carina on the lingual surface. This study is the third report of metriacanthosaurids in Thailand and Southeast Asia. The result expands the distribution of this clade in the Late Jurassic–Early Cretaceous of Thailand. This study also shows that a single tooth of theropod dinosaurs has potential to offer important information that we should not overlook.