| Siamosaurus | |
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| Illustration of the holotype tooth, with British penny for scale | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Spinosauridae |
| Genus: | †Siamosaurus Buffetaut & Ingavat, 1986 |
| Species: | †S. suteethorni
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| Binomial name | |
| †Siamosaurus suteethorni Buffetaut & Ingavat, 1986
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| Synonyms | |
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Siamosaurus (meaning "Siam lizard") is a genus of spinosaurid dinosaur that lived in what is now known as China and Thailand during the Early Cretaceous period (Barremian to Aptian) and is the first reported spinosaurid from Asia. It is confidently known only from tooth fossils; the first were found in the Sao Khua Formation, with more teeth later recovered from the younger Khok Kruat Formation. The only species Siamosaurus suteethorni, whose name honours Thai palaeontologist Varavudh Suteethorn, was formally described in 1986. In 2009, four teeth from China previously attributed to a pliosaur—under the species "Sinopliosaurus" fusuiensis—were identified as those of a spinosaurid, possibly Siamosaurus. It is yet to be determined if two partial spinosaurid skeletons from Thailand and an isolated tooth from Japan also belong to Siamosaurus.
Since it is based only on teeth, Siamosaurus's body size is uncertain, though it has been estimated at between 5.1 to 9.1 metres (17 to 30 feet) in length. The holotype tooth is 62.5 millimetres (2.46 inches) long. Siamosaurus's teeth were straight, oval to circular in cross-section, and lined with distinct lengthwise grooves. Its teeth had wrinkled enamel, similar to teeth from the related genus Baryonyx. As a spinosaur it would have had a long, low snout and robust forelimbs, and one possible skeleton indicates the presence of a tall sail running down its back, another typical trait of this theropod family. Siamosaurus is considered by some palaeontologists to be a dubious name, with some arguing that its teeth are hard to differentiate from those of other Early Cretaceous spinosaurids, and others that it may not be a dinosaur at all. Based on dental traits, Siamosaurus and "S." fusuiensis have been placed in the subfamily Spinosaurinae.
Like in all spinosaurids, Siamosaurus's teeth were conical, with reduced or absent serrations. This made them suitable for impaling rather than tearing flesh, a trait typically seen in largely piscivorous (fish-eating) animals. Spinosaurids are also known to have consumed pterosaurs and small dinosaurs, and there is fossil evidence of Siamosaurus itself feeding on sauropod dinosaurs, either via scavenging or active hunting. Siamosaurus's role as a partially piscivorous predator may have reduced the prominence of some contemporaneous crocodilians competing for the same food sources. Isotope analysis of the teeth of Siamosaurus and other spinosaurids indicates semiaquatic habits. Siamosaurus lived in a semi-arid habitat of floodplains and meandering rivers, where it coexisted with other dinosaurs, as well as pterosaurs, fishes, turtles, crocodyliforms, and other aquatic animals.