Сиамозавр: Неустойчивый спинозаврид из Азии

Potentially dubious genus of spinosaurid theropod dinosaur

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 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.

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 σαῦρος (sauros), meaning "lizard" or "reptile". The specific name honours Thai geologist and palaeontologist Varavudh Suteethorn, and his contributions to vertebrate palaeontology discoveries in Thailand. 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. 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. 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. 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). 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. 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. 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. 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.

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. 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. The holotype tooth is relatively straight, with only minor front to back curvature. It is oval in cross section while other specimens are nearly circular in this aspect. Unlike in most theropods, the carinae (cutting edges) of Siamosaurus teeth lack well defined serrations, Some of the root is preserved in the holotype and, as in all theropods, there is a large pocket for the tooth pulp, which would have housed blood vessels and nerves. However, in SM KK 14 the front of the pubis was concave and the chevrons were curved backwards, in contrast to the straight condition these bones had in Ichthyovenator. In view of this, the German palaeontologist Hans Dieter Sues and colleagues in 2002 asserted that there is not enough material to confidently identify Siamosaurus as a dinosaur. In 2004, American palaeontologist Thomas Holtz and colleagues considered it a dubious name, stating that the teeth might instead belong to a contemporaneous fish such as a saurodontid or an ichthyodectid teleost. The same year, American palaeontologist David Weishampel and colleagues considered Siamosaurus an indeterminate theropod. In 2012, an analysis by American palaeontologist Matthew Carrano and colleagues agreed with the possibility of confusion with other reptiles, and regarded the genus as a possible indeterminate spinosaurid. They noted that oftentimes, isolated teeth are an unstable foundation for naming new theropod taxa, and most species based on them turn out to be invalid. This problem is especially common with spinosaurids, given that skull and skeletal fossils from the group are rare. [[File:Ichthyovenator laosensis by PaleoGeek. png|alt=Reconstruction of the spinosaurid genus Ichthyovenator in a left facing walking pose|thumb|Restoration of Ichthyovenator, a spinosaurine from what is now Laos and one of the closest known relatives of Siamosaurus. In 2017, Brazilian palaeontologists Marcos Sales and Cesar Schultz suggested that the various Asian teeth might eventually be attributed to Ichthyovenator like forms. The researchers accepted Siamosaurus as a spinosaurid, but stated that its teeth and those of "S." fusuiensis are too similar to those of other Early Cretaceous spinosaurids to erect new taxa unequivocally; and thus considered both taxa as dubious. and Kamonrak and colleagues in 2019 regarded the Khok Kruat skeleton as first definitive evidence of spinosaurs in Asia. The taxonomic and phylogenetic affinities of the Spinosauridae are subject to active research and debate, given that in comparison to other theropod groups, many of the family's taxa are based on poor fossil material. The possibility that Baryonychinae is a paraphyletic (unnatural) grouping has been suggested by researchers such as Sales and Schultz, on the basis that genera such as Irritator and Angaturama (the two are possible synonyms) may represent intermediate forms between baryonychines and spinosaurines. upright=0.60|thumb|alt=Fossil vertebra of a spinosaurid inside a display case|Vertebra from specimen SM KK14, which may belong to Siamosaurus

Later in 2019, the Khok Kruat Formation teeth were also referred to the Spinosaurinae by Kamonrak and colleagues, on the basis that both the Khok Kruat and Siamosaurus morphotypes lack characteristics seen in baryonychines, such as long and slender roots, 0–10 flutes on each side, no well defined carinae, a sculptured surface of the crown base, and 45 degree orientation of the blood grooves. But they share with spinosaurines a sub circular to oval cross section, fluted tooth crowns, well defined front and rear carinae, distinct striations on the crown, varying denticle size, and a wrinkled surface of the crown base. The authors also noted that unlike spinosaurines such as Irritator and Spinosaurus, Asian spinosaurines usually have more laterally compressed tooth crowns, and wrinkles across more of the enamel surface. Due to new discoveries and research on spinosaurid teeth since Siamosaurus was named in 1986, a reassessment of the genus' validity is currently being prepared by Buffetaut. The elongate, interlocking jaws of spinosaurids also had snout tips that fanned out into a rosette like shape—a trait also observed in highly piscivorous crocodilians such as gharials—which made them well adapted to catching and feeding on fish. Fossil evidence has shown that besides aquatic prey, spinosaurids also consumed other dinosaurs and pterosaurs. In the Sao Khua Formation, localities such as Wat Sakawan have yielded sauropod remains in association with tooth crowns from Siamosaurus, documenting either predation or scavenging on part of the latter. A similar scenario was proposed for spinosaurids by Hone and colleagues in 2010, who also noted that compared to large crocodilians and obligate aquatic predators, they could more easily travel from one body of water to another in search of prey. In 2010, Amiot and colleagues published another oxygen isotope study on turtle, crocodilian, spinosaurid, other theropod remains, this time including fossils from Thailand, China, England, Brazil, Tunisia, and Morocco. The analysis showed that Thai spinosaurid teeth tended to have the largest difference from the ratios of other, more terrestrial theropods, while those of Spinosaurus from Tunisia and Morocco tended to have the least difference, despite the advanced piscivorous adaptations in the skull observed for this genus. The authors suggested that piscivory and semiaquatic habits may explain how spinosaurids coexisted with other large theropods. By feeding on different prey items and occupying a distinct ecological role, a phenomenon that is known as niche partitioning, the different types of theropods would have been out of direct competition. Further lines of evidence have since demonstrated that spinosaurids, especially those within the Spinosaurinae, developed strong adaptations for aquatic environments, such as dense limb bones for buoyancy control; reduction of the pelvic girdle; and elongated neural spines on the tail, likely used for underwater propulsion.

thumb|alt=Sketch of a spinosaur feeding on a sauropod carcass| Restoration of a generic spinosaur feeding on a sauropod carcass in the Sao Khua Formation environment, a behavior hypothesized based on teeth belonging to Siamosaurus. The ornithomimosaurs in the background are Kinnareemimus.|left

Of all the Mesozoic formations in northeastern Thailand, the Sao Khua is the most abundant and diverse in vertebrate fossil discoveries. The sediments of the Sao Khua Formation, which comprise red clays, mudstones, sandstones, siltstones, and conglomerate rocks, record a fluvial environment dominated by lakes, floodplains, and meandering low energy rivers. This is consistent with the types of vertebrate fauna present in the formation, which comprise only terrestrial or freshwater animals. the ornithomimosaur Kinnareemimus khonkaenensis, the megaraptoran Phuwiangvenator yaemniyomi, the basal coelurosaur Vayuraptor nongbualamphuensis, a compsognathid theropod, and indeterminate birds. Theropod eggs with embryos have also been recovered from the formation.'' There were also sauropods like the titanosauriform Phuwiangosaurus sirindhornae, mamenchisaurids, and indeterminate forms. and Theriosuchus grandinaris. The Sao Khua and Khok Kruat Formations had a more semi arid climate than the older, more humid Phu Kradung Formation, dated to the Berriasian. iguanodontians like Sirindhorna khoratensis, and Siamodon nimngami; a titanosauriform sauropod similar to Phuwiangosaurus; and various indeterminate theropods. In 2012, Allain and colleagues suggested such a global distribution may have occurred earlier across Pangaea before the Late Jurassic, even if Asia became separated from the supercontinent first. In 2019, Spanish palaeontologist Elisabete Malafaia and colleagues also indicated a complex biogeographical pattern for spinosaurs during the Early Cretaceous, based on anatomical similarities between Ichthyovenator and the European genus Vallibonavenatrix''.