Siamosaurus, meaning “Siamese lizard,” is a genus of theropod dinosaur from the Early Cretaceous Period of what is now Thailand. Little is known about this Cretaceous carnivore, but it is known from teeth that closely resemble the Spinosaurus, that it may have been a fish eater. The type species, S. suteethorni, was described by Buffetaut and Ingavat in 1986.
In a publication printed in 2010, Romain Amiot and colleagues found that oxygen isotope ratios of the bones of the specimen indicated a semi-aquatic lifestyle. The study found that isotopes of Siamosaurus teeth were much closer to those of turtles and crocodilians, than to those of other theropods. Based on their findings, the study authors concluded that Siamosaurus spent much of its daily life in water.
Siamosaurus, having different feeding habits and living in different habitats than other theropods, would most likely have been outside of direct competition with them.