Siamosaurus

''Siamosaurus'' (meaning "Siam reptile") 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'' body size is uncertain, though it has been estimated at between in length. The holotype tooth is 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'' 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'' 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.

History of discovery

The Sao Khua Formation, where the first ''Siamosaurus'' fossils were discovered, is part of the Khorat Group. The formation is dated to the Barremian stage of the Early Cretaceous period, 129.4 to 125 million years ago. In 1983, French palaeontologist Éric Buffetaut and his Thai colleague Rucha Ingavat described a set of fossil teeth recovered from the Phu Pratu Teema locality of the Sao Khua Formation, in the Phu Wiang area of Khon Kaen Province. They did not conclude as to what animal they originated from, their opinion being that the specimens belonged "either to an unusual theropod dinosaur or to some unknown crocodilian". In 1986, a reassessment of the remains by the same authors attributed them to a new genus and species of spinosaurid theropod, which they named ''Siamosaurus suteethorni''. The generic name alludes to the ancient name of Thailand, "Siam", and is combined with the Ancient Greek word ('), meaning "lizard" or "reptile". The specific name honours Thai geologist and palaeontologist Varavudh Suteethorn, and his contributions to vertebrate palaeontology discoveries in Thailand.

The best-preserved specimen from the teeth described, designated DMR TF 2043a, was chosen as the holotype of ''Siamosaurus.'' The paratypes comprise eight other well-preserved teeth catalogued as DMR TF 2043b to i. The original fossils are currently housed in the palaeontological collection of the Department of Mineral Resources, Bangkok. ''Siamosaurus'' teeth are common in the Sao Khua Formation, and further isolated specimens were found later throughout the late 20th and early 21st centuries. Sculptures of the animal have been erected in various places across northeastern Thailand, including Si Wiang Dinosaur Park, the Siam Paragon shopping mall in Bangkok, the Phu Wiang Dinosaur Museum, and the Sirindhorn Museum. ''S''. ''suteethorni'' was also illustrated on Thai postage stamps released in 1997, along with fellow Thai dinosaurs ''Phuwiangosaurus sirindhornae,'' ''Siamotyrannus isanensis'', and '' Psittacosaurus sattayaraki''.

Thailand's Khok Kruat Formation is dated to the Aptian age (between 125 and 113 million years ago), younger than the Sao Khua Formation. The Khok Kruat Formation has provided many spinosaurid teeth, including ones from ''Siamosaurus'' and closely allied forms. Given the varied size and morphology of the teeth found, the presence of multiple spinosaur taxa in the region is likely.'''' Nearly 60 fossil teeth were recovered from the formation during fieldwork by Thai-French palaeontological teams between 2003 and 2008, including specimens from the Sam Ran, Khok Pha Suam, and Lam Pao Dam localities. Eight of these teeth were described in detail by Kamonrak and colleagues in 2019, and classified into two main morphotypes: the Khok Kruat morphotype, which is found only in the Khok Kruat Formation, and the ''Siamosaurus'' morphotype, which includes forms widely recovered from both the Sao Khua and Khok Kruat Formations.''''

''Siamosaurus'' is the first reported spinosaurid dinosaur from Asia, and subsequently to its naming, material resembling or possibly belonging to the genus has been found across the continent. In 1975, Chinese palaeontologist Hou Lian-Hai and colleagues described five teeth as a new species of the pliosauroid '' Sinopliosaurus'', which they named ''S. fusuiensis'', the specific name is in reference to Fusui County in Guangxi, China, from which the fossils were collected. Four of these teeth—one was not found in the museum collection—were reassigned by Buffetaut and colleagues in 2008 to a spinosaurid theropod and referred to as "''Sinopliosaurus''" ''fusuiensis.'' The researchers deemed it as "closely related to, if not identical with", ''S''. ''suteethorni.'' In 2019, "''S''." ''fusuiensis'' was referred to by Thai palaeontologist Wongko Kamonrak and colleagues as ''Siamosaurus'' sp. (of uncertain species).'''' Later in 2019, Thai palaeontologist Adun Samathi and colleagues considered the teeth as belonging to an indeterminate spinosaurid. The specimens were retrieved from China's Early Cretaceous Xinlong Formation

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