Siamosaurus (meaning "Siam lizard") is a potentially dubious genus of spinosaurid dinosaur that lived in what is now Thailand and possibly China 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. Siamosauruss 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 in 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, Siamotyrannus, and Psittacosaurus.
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, in which spinosaurid teeth are frequently reported, though most of them are hard to differentiate from those of Japan or Thailand without more complete fossil material, such as a skull. Buffetaut and Suteethorn suggested that the Xinlong Formation could be geologically related to the Sao Khua or Khok Kruat Formation, since similar types of fossils have been recovered in all three regions.
In 1994, an isolated tooth (specimen GMNH-PV-999) was found by a fossil prospector in the Sebayashi Formation, Japan. The tooth was believed, until 2003, to belong to a marine reptile, when Japanese palaeontologist Yoshikazu Hasegawa and colleagues assigned it to ?Siamosaurus sp. The tooth came from rocks dated to the Barremian, similar in age to sediments that Siamosaurus teeth have been recovered from in Thailand. In 2015, a more incomplete tooth was recovered from the same formation by two local children. Kept under the specimen number KDC-PV-0003, the tooth was assigned to an indeterminate spinosaurid in 2017 by Japanese palaeontologist Kubota Katsuhiro and colleagues. Further spinosaurid teeth from unnamed and indeterminate forms have been discovered in central China and Malaysia.
In 2004, excavation began on a partial skeleton from an outcrop of the Khok Kruat Formation near the city of Khon Kaen. The specimen (SM-KK 14) consists of cervical (neck) and dorsal (back) vertebrae, a high neural spine (upwards-extending process from top of vertebra), pelvis (hip) fragments, a possible metacarpal (long bone of the hand), and a chevron from the tail. The cervical vertebrae and pelvic region resemble those of the European spinosaur Baryonyx walkeri, and the neural spine is elongated, similarly to those of other spinosaurids. A Siamosaurus tooth found nearby indicates the skeleton may belong to this genus, though this could also represent evidence of scavenging. The skeleton, as well as two well-preserved teeth—SM2016-1-147 and SM2016-1-165, also attributed to Siamosaurus—are currently stored in the vertebrate fossil collection of the Sirindhorn Museum, Kalasin Province. In 2019, a series of spinosaurid caudal (tail) vertebrae possibly belonging to S. suteethorni were recovered from the Sao Khua Formation, and described in a dissertation by Samathi. The fossils (SM-PW9B-11 to 17, SM-PW9B, SM-PW9A-unnumbered, SM-PW9-unnumbered, and SM 2017-1-176) were designated by Samathi as "Phuwiang spinosaurid B", and bear similarities to the possible spinosaurid Camarillasaurus and a Baryonyx specimen discovered in Portugal.
Description
In 2004, American dinosaur researcher Don Lessem estimated Siamosaurus at long. In 2005, British author Sussana Davidson and colleagues gave a lower estimate of in length and weighing . In a 2016 popular book, authors Rubén Molina-Pérez and Asier Larramendi estimated it at approximately long, tall at the hips, and weighing . However, reliable calculations on the weight and body size of fragmentary dinosaurs like Siamosaurus are hindered by the lack of good material, such as a skull or postcranial skeleton, and thus estimates are only tentative. "Phuwiang spinosaurid B" was calculated as approximately long by Samathi in 2019. As a spinosaurid, Siamosaurus would have had low, narrow, and elongated jaws; well-built forelimbs; relatively short hindlimbs; and elongated neural spines on the vertebrae forming a sail on its back.
Type specimens
Fossil theropod teeth are typically identified by attributes such as the proportions, size, and curvature of the crown, as well as the presence and/or shape of the denticles (serrations). The holotype of S. suteethorni (specimen DMR TF 2043a) is in total length, with the crown being long, and wide at its base. It is among the larger teeth discovered by Buffetaut and Ingavat. One much smaller specimen (DMR TF 2043b) measures in length. According to the authors, this dramatic size range suggests the teeth are from individuals of different ages. Among theropods, this may also indicate size variation along the tooth row in the jaws, which is observed to have been prevalent in spinosaurids.