Siamosaurus

Siamosaurus (meaning "Siamese lizard") is a genus of spinosaurid theropod dinosaurs from the Early Cretaceous Period of what is now northeastern Thailand. It is the first spinosaurid reported from Asia and is known only from tooth fossils; the first of which were found in the Sao Khua Formation, with further material later recovered from the younger Khok Kruat Formation. The type species Siamosaurus suteethorni, whose name honours the Thai palaeontologist Varavudh Suteethorn, was formally described in 1986. One tooth possibly belonging to the genus was found in the Sebayashi Formation of Japan in 1994. Four teeth from China's Xinlong Formation were assigned to a new species of spinosaurid called "Sinopliosaurus" fusuiensis, which may be the same animal as Siamosaurus. Most of the aforementioned teeth were originally attributed to marine reptiles. A partial spinosaurid skeleton from the Khok Kruat Formation was described in 2007, but whether it belongs to Siamosaurus or not is unclear. Since it is based on very fragmentary remains, Siamosaurus 's body size and weight are unknown. The holotype tooth is 62.5 millimetres (2.46 in) long and 16.6 by 12.5 millimetres (0.65 by 0.49 in) wide at the base. Siamosaurus 's teeth were straight, oval to circular in cross section, and lined with well-marked lengthwise ridges. They also had wrinkled enamel, similar to that of teeth from the related genus Baryonyx. Siamosaurus 's identity has been questioned by many palaeontologists, who argue that its teeth are too similar to those of contemporaneous aquatic reptiles for it to be confidently classed as a dinosaur. Other researchers, who agree that the teeth are spinosaur in origin, deem them too similar to those of other Early Cretaceous spinosaurs to unequivocally raise a new genus. Like all spinosaurids, Siamosaurus 's teeth were conical, with reduced or absent serrations. This made them more suitable for impaling rather than tearing flesh, a trait typically seen in piscivorous (fish-eating) animals. Its relatives are also known to have consumed pterosaurs and small to medium-sized dinosaurs, and there is fossil evidence of Siamosaurus itself eating sauropod dinosaurs, either via scavenging or active hunting. Siamosaurus lived in a semi-arid habitat of floodplains and meandering rivers, where it coexisted with other theropod, sauropod, and iguanodontian dinosaurs. Isotope analysis indicates it may have had semiaquatic habits, and its possible role as a piscivorous predator might have reduced the prominence of long-snouted crocodilians living in the same time and place.

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, 130 million years ago, though it was formerly thought to be Late Jurassic in age. Out of the other five Late Triassic to Early Cretaceous formations in northeastern Thailand, the Sao Khua is the most abundant and diversified in vertebrate fossil discoveries. The Khorat Group exhibits taxa only of continental origin, with no definitive evidence for marine fossils or sedimentary structures found so far. In 1963, Yoshitsugu Kobayashi of the Hokkaido University reported ichthyosaur and plesiosaur teeth from the Sao Khua Formation, but these have now been identified as belonging to Siamosaurus and a crocodilian respectively. Bivalves unearthed at the formation were also previously considered marine by chemist Lothar Hahn.

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 Khon Kaen Province. They reached no conclusion 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, Siamosaurus suteethorni. The generic name was chosen after the ancient name of Thailand, "Siam", while the specific name honoured Thai geologist and palaeontologist Varavudh Suteethorn, and his contributions to vertebrate palaeontology discoveries in Thailand.

The best-preserved specimen within the series, an isolated tooth designated DMR TF 2043a, was chosen as the holotype of Siamosaurus. The paratypes comprised eight other well-preserved teeth catalogued as DMR TF 2043b to i. These fossils are currently housed in the palaeontological collection of the Department of Mineral Resources, Bangkok. Other fossils later referred to the genus include including two well preserved teeth (SM2016-1-147 and SM2016-1-165) and one cervical (neck) vertebra (SM2016-1-067) are stored in the Sirindhorn Museum, Kalasin Province. Isolated Siamosaurus teeth are common in the Sao Khua Formation.

Life-sized sculptures of the animal have been erected in various places across northeast Thailand, including Si Wiang Dinosaur Park, the Siam Paragon shopping mall in Bangkok, and the Phu Wiang Dinosaur Museum.undefined The animal was also illustrated on Thai postage stamps released in 1997, along with fellow Thai dinosaurs Phuwiangosaurus sirindhornae, Siamotyrannus isanensis, and Psittacosaurus sattayaraki.

Siamosaurus is the first report of a spinosaurid dinosaur from Asian fossils, and subsequently to its naming, material resembling or possibly belonging to the taxon has been found across the continent. In 1975, the Chinese palaeontologist Hou Lian-Hai and colleagues described five teeth as a new species of the pliosauroid Sinopliosaurus they named S. fusuiensis. Four of these teeth (the fifth was lost) were reassigned by Buffetaut and colleagues in 2008 to a Siamosaurus-like taxon and referred to as "Sinopliosaurus" fusuiensis. The researchers deemed "Sinopliosaurus" fusuiensis as "closely related to, if not identical with" Siamosaurus suteethorni. Mickey Mortimer informally placed "S." fusuiensis under the latter genus as a tentative additional species, S. fusuiensis. The original specimens were collected from China's Early Cretaceous Xinlong Formation, in which indeterminate spinosaurs are frequently reported, although most of them are hard to differentiate from those of Japan or Thailand without better material than teeth. Buffetaut and Suteethorn consider it possible that the Xinlong Formation could be related to the Sao Khua or Khok Kruat Formations, since similar types of fossils have been recovered in these regions. Spinosaurid teeth are also known from central China and Malaysia. In 1994, an isolated tooth (specimen GMNH-PV-999) was found by a fossil prospector in the Sebayashi Formation in Japan. The tooth was believed to be that of a marine reptile until 2003, when the Japanese palaeontologist Yoshikazu Hasegawa and colleagues referred it to ?Siamosaurus sp. (of uncertain species).The tooth comes from rocks dated to the Barremian, similar in age to the sediments that Siamosaurus teeth have been recovered from in Thailand. In 2015, a more incomplete tooth was recovered from the same formation by local children. Kept under the specimen number KDC-PV-0003, the tooth was assigned to an indeterminate spinosaurid in 2017 by Kubota Katsuhiro and colleagues. Thailand's Khok Kruat Formation assumes a younger (Aptian) age than the Sao Khua Formation, and has provided many spinosaurid teeth, including those of Siamosaurus. Given the varied size and morphology of the teeth found, the presence of multiple spinosaur taxa in the region is likely. In 2004, excavation began on a partial skeleton from an outcrop of the formation near the city of Khon Kaen. The material was described by Angela Milner, Buffetaut, and Suteethorn in 2007, and consists of cervical (neck) and dorsal (back) vertebrae, one 60-centimetre (24 in) high neural spine, pelvis (hip bone) fragments, and limb bones. The cervical vertebrae and pelvic region resemble those of the European spinosaur Baryonyx, and the neural spine is elongated similarly to, although not as much as, those of the African genus Spinosaurus. A Siamosaurus tooth found in close proximity to the skeleton indicates that it may belong to this genus, though this could also represent scavenging.

Description
Some authors have published size estimates of Siamosaurus ranging from 8 to 9.1 metres (26 to 30 ft) in length and weighing 1 tonne (1.1 short tons). However, reliable calculations on the weight and body size of fragmentary spinosaurs like Siamosaurus are hindered by the lack of good material, such as a skull or body fossil.

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 Siamosaurus suteethorni (specimen DMR TF 2043a) is a tooth measuring 62.5 millimetres (2.46 in) in total length, with the crown being 47.7 millimetres (1.88 in) long, and 16.6 by 12.5 millimetres (0.65 by 0.49 in) wide at its base. The 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 (edges) of Siamosaurus 's teeth lack well-defined serrations. Some teeth (including the holotype) have a wave-like double recurvature when viewed front to back, which Buffetaut and Ingavat compared to that seen in carnosaur teeth from the same formation and one Deinonychus tooth described by John Ostrom in 1969. The Siamosaurus suteethorni holotype is symmetrically concave front to back, and bears 15 flutes (ridges) on its lingual (the side that faces the tongue) and labial (outward facing) surfaces. These flutes run lengthwise from the base of the crown before stopping 5 millimetres (0.20 in) from the rounded tooth tip. A region of the holotype where the enamel (outer layer of the teeth) has weathered away reveals that these flutes extend down to the dentine (second layer of the teeth). The enamel also has a rugose (roughly wrinkled) texture, as seen in teeth from Baryonyx. 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. The holotype is among the larger teeth discovered by Buffetaut and Ingavat, one much smaller specimen (DMR TF 2043b) measures 24.3 millimetres (0.96 in) in length. According to the authors, this dramatic size range suggests the teeth are from individuals of different ages.

Possible material


The first Sebayashi Formation specimen (GMNH-PV-999) is an isolated tooth crown with a partially intact root. It is unknown on which jaw the tooth was positioned nor which surface faced the inside or outside of the mouth. The tooth's front and back carinae are well-defined, though the former is not well preserved. The crown's surface is, like the Siamosaurus suteethorni holotype, rugose, with numerous small granular structures oriented parallel to its length. Besides having a broader, 20-by-14-millimetre (0.79 by 0.55 in) wide base and being slightly smaller at 51 millimetres (2.0 in) in length, GMNH-PV-999 has a very similar morphology to the Siamosaurus suteethorni holotype. Features shared between the two specimens include: a straight and only slightly compressed shape; a somewhat oval cross section; no serrations on the carinae (possibly due to bad preservation); and a flutes on the crown surface, the Japan specimen having 12 on each side. Because of these resemblances, Hasegawa and colleagues regarded GMNH-PV-999 as nearly identical to DMR TF 2043a.

KDC-PV-0003, the second tooth found in the Sebayashi Formation, consists of a slightly recurved crown fragment 10 millimetres (0.39 in) long. At its base, it is 7.4 millimetres (0.29 in) long and 6.8 millimetres (0.27 in) wide, with an almost circular cross section. Though less complete than the former specimens, KDC-PV-0003 exhibits better preservation of small details such as visible serrations, although poorly defined ones. The denticles, of which there are two to three every 1-millimetre (0.039 in), are visible only on the front carinae, due to the rear of the tooth being obscured by rock matrix. A 5.5-millimetre (0.22 in) long gap at the fossil's base exposes the dentine, which is covered in a 1.6-millimetre (0.063 in) thick layer of enamel. The little that is preserved of KDC-PV-0003 shows that like the first Sebayashi Formation tooth, it has a rugose texture and a minimum of 12 flutes on its surface. The granular strucutre of KDC-PV-0003 are sharper than in GMNH-PV-999. The interdenticle sulci (tiny wrinkles near the gaps between each denticle) of KDC-PV-0003 have an oblique orientation of 45 degrees, exactly like those seen in GMNH-PV-999. Out of the four teeth referred to "Sinopliosaurus" fusuiensis, specimen IVPP V 4793 is the most intact, although still somewhat deformed. The crown, which is missing its tip, is 69 millimetres (2.7 in) long and 16.5 by 13 millimetres (0.65 by 0.51 in) wide at the base. The tooth is straight, only slightly recurved, and has an oval cross section. The front and rear carinae are distinct, yet their serrations have been heavily eroded. Like the Thailand and Japan teeth, the "Sinopliosaurus" fusuiensis specimens bear developed flutes and a wrinkled surface. As in both Sebayashi Formation teeth, there are 12 flutes on each face of "Sinopliosaurus" fusuiensis's teeth. These flutes are of varied lengths; not all of them stretch to the full length of the crown. Buffetaut and colleagues likened them most to those of Siamosaurus, with respect to their identical crown shape, ribbing, and wrinkled enamel.

Classification


In 1986, Buffetaut and Ingavat classed Siamosaurus as a theropod because of the straight tall crown and double lateral recurvature of its teeth. At the time, Siamosaurus 's particular combination of dental characteristics, especially the longitudinal fluting and lack of serrations, had not been observed in other theropods. The authors noted similarities in Siamosaurus 's teeth to those of ceratosaurian tooth crowns, some of which also have longitudinal flutes. However, this identification was ruled out, since ceratosaur teeth differ in cross section, bear far fewer dental flutes, and have distinct serrations. Buffetaut and Suteethorn concluded that the closest taxon in dentition to Siamosaurus was Spinosaurus. Like Siamosaurus, this African genus had straight and unserrated conical teeth, and although Spinosaurus lacked the developed flutes seen in Siamosaurus, Buffetaut and Ingavat noted that both smooth and fluted spinosaur teeth have been reported from Africa. Therefore, they tentatively placed Siamosaurus in the Spinosauridae family, based on close similarities in dentition to Spinosaurus aegyptiacus. Many palaeontologists have expressed skepticism towards Buffetaut and Ingavat's identification of Siamosaurus, given that spinosaurid teeth (including many from Asia) have been time and again mistaken for those of aquatic reptiles like crocodilians, plesiosaurs, and ichthyosaurs. In view of this, the German palaeontologist Hans-Dieter Sues and colleagues in 2002 argued that there is not enough material to confidently identify Siamosaurus as a dinosaur. In 2004, American palaeontologist Thomas Holtz and colleagues considered it a nomen dubium (dubious name), stating that its teeth might instead belong to contemporaneous fish such as saurodontid or ichthyodectid teleosts.[40] The same year, American palaeontologist David Weishampel and colleagues placed Siamosaurus as an indeterminate theropod.[41] And in 2012, an analysis by American palaeontologist Matthew Carrano and colleagues agreed with the danger of confusion with other reptiles, and regarded the genus as a possible indeterminate spinosaur. They point out that oftentimes, isolated teeth are an unstable foundation for naming theropod taxa, and most species based on them turn out to be invalid. This problem is especially common with spinosaurs, given that cranial or skeletal fossils from the group are rare.

Some crocodilians have teeth with flutes similar to those in Asian spinosaurid teeth. However, crocodilian teeth are usually more strongly recurved than spinosaur teeth. Crocodilians also lack the lateral double recurvature of Siamosaurus 's tooth crowns, which, based on their shape, were probably vertically inserted into the jaw, whereas long-snouted crocodilian teeth are usually jut out from the mouth at an angle. Though Siamosaurus &lt;nowiki&gt; and plesiosaur teeth are similar in overall shape, Buffetaut and Ingavat point out that the latter taxon has significantly more-recurved tooth crowns. Compared to plesiosaurs, Asian spinosaurid teeth also have coarser and more numerous flutes that extend almost the whole length of the crown.In 2008, Buffetaut and colleagues noted that "Sinopliosaurus" fusuiensis teeth bear carinae on the plane of the crown's curvature, a condition not observed in plesiosaur teeth. The discovery of the Khok Kruat skeleton and of baryonychine teeth with dental flutes similar to those of Siamosaurus, were also brought up by the researchers as further evidence of Siamosaurus 's spinosaurid classification. And indeed, later discoveries revealed that flutes and a lack of serrations were common characteristics of spinosaurid teeth.

In 2017, the Brazilian palaeontologists Marcos Sales and Cesar Schultz accepted Siamosaurus as a spinosaur, but stated that its teeth and those of "Sinopliosaurus" fusuiensis are too similar to those of other Cretaceous spinosaurids to unequivocally erect new taxa. However, Carrano and colleagues stated that the partial Khok Kruat skeleton may provide answers to their identification. Authors such as Milner and colleagues, Bertin Tor, Kamonlak Wongko and colleagues, and Thomas Holtz regarded the skeleton as first irrefutable evidence of spinosaurs in Asia. Nevertheless, in 2012 Ronan Allain and colleagues described the Laos spinosaurid Ichthyovenator and dubbed it "The first definitive Asian spinosaurid". Mickey Mortimer pointed out shortly after that even discounting teeth, the Khok Kruat skeleton was first mentioned eight years prior to Ichthyovenator 's description. Sales and Shultz suggested that the various Asian teeth might eventually be attributed to Ichthyovenator-like forms. In a 2014 abstract, Allain announced that three teeth from Ichthyovenator had been discovered, which typically of spinosaurines, bore straight and unserrated crowns. Though no comparison was made to the other Asian teeth. Spinosaurids were large bipedal carnivores with well-built forelimbs and elongated, crocodile-like skulls. The taxonomic and phylogenetic affinities of the group are subject to active research and debate, given that in comparison to other theropod groups, many of the family's taxa (including Cristatusaurus) are based on relatively poor fossil material. Traditionally, the group is split into the subfamilies Spinosaurinae (unserrated, straight teeth with well marked flutes and a circular cross section) and Baryonychinae (finely serrated, somewhat recurved teeth with weaker flutes and a more oval cross section). The morphological variation seen in spinosaurid teeth, however, shows that these characteristics are not always consistent within the subfamilies. The exact position of Siamosaurus within the Spinosauridae is difficult to determine because of its fragmentary nature, difficulty in classifying the many spinosaurid teeth from Asia arises from the lack of an associated skull or skeleton. Asian spinosaurid teeth show characteristics from both Spinosaurinae and Baryonychinae. KDC-PV-0003 has irregularly sized mesial serrations, just as in baryonychines. In 2010, the British palaeontologist David W. E. Hone and colleagues referred Siamosaurus and "Sinopliosaurus" fusuiensis to the Spinosaurinae. Likewise, British palaeontologist Thomas Arden and colleagues resolved Siamosaurus as a basal member of the Spinosaurinae in 2018, their cladogram can be seen below.

Diet and feeding
Buffetaut and Ingavat suggested in 1986 that Siamosaurus, like other spinosaurids, probably led a heavily piscivorous (fish-eating) lifestyle. Animals from this family, including Siamosaurus, had highly specialised teeth better suited for piercing rather than tearing flesh, thanks to their long, conical tooth crowns with reduced or absent serrations. This dental morphology is also seen in plesiosaurs and long-snouted crocodilians. Though skull material from Siamosaurus is lacking, spinosaurs are known to have had elongated, low and narrow jaws with snout tips that fanned out into a rosette-like shape, similar to that of the highly piscivorous modern Gharial. Fossil evidence shows that besides fish, spinosaurids also consumed 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.

In 2006, the Thai biologist Komsorn Lauprasert examined fossils collected from the Phu Kradung, Sao Khua, and Khok Kruat Formations. In this study, the teeth of Siamosaurus and a Morrocan spinosaurid were compared to those of crocodilians using scanning electron microscopy. Lauprasert found that spinosaurids and crocodilians may have employed similar feeding tactics and been under comparable mechanical constraints, based on resemblances in the microstructure of their tooth enamel. Lauprasert suggests that Siamosaurus could have therefore replaced the ecological niche of contemporaneous long-snouted crocodilians as a piscivorous predator. He notes that this likely occured in correlation with the rising aridity of the Sao Khua and Khok Kruat Formations during the Early Cretaceous, since Siamosaurus had better mobility in a dry environment than crocodilians did. This could explain the absence of long-snouted crocodilian fossils from that time and place. A similar scenario was proposed for spinosaurids by Hone and colleagues in 2010.

Aquatic habits
A 2010 publication by French palaeontologist Romain Amiot and colleagues found that the oxygen isotope ratios of spinosaurid bones indicated semiaquatic lifestyles. Isotope ratios of teeth from the spinosaurids Baryonyx, Irritator, Siamosaurus, and Spinosaurus were compared with isotopic compositions from contemporaneous theropods, turtles, and crocodilians. The study revealed that, among theropods, spinosaurid isotope ratios were closer to those of turtles and crocodilians. Siamosaurus specimens tended to have the largest difference from the ratios of other theropods, and Spinosaurus tended to have the least difference. The authors concluded that spinosaurids, like modern crocodilians and hippopotamuses, spent much of their daily lives in or near water. The researchers also suggested that semiaquatic habits and piscivory may explain how spinosaurids coexisted with other large theropods; by feeding on different prey items and occupying a distinct ecological niche, the different types of theropods would have been out of direct competition.

Palaeoenvironment and palaeobiogeography


The sediments of the Sao Khua Formation, composed of 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, including dinosaurs, turtles, freshwater hybodont sharks and actinopterygian fishes, and the freshwater crocodilian Sunosuchus (previously referred to Goniopholis). Besides Siamosaurus, there were dinosaurs like the metriacanthosaurid Siamotyrannus isanensis (previously thought to be a tyrannosaurid), the ornithomimosaur Kinnareemimus khonkaenensis, a Compsognathus-like theropod, as well as sauropods like Phuwiangosaurus sirindhornae and two indeterminate forms. Sauropod remains are some of the most abundant in the Sao Khua and Khok Kruat Formations. No ornithischian dinosaur fossils have been found in the Sao Khua Formation, suggesting that they were uncommon compared to saurischian dinosaurs. Due to its early age, Buffetaut and Suteethorn consider the Sao Khua faunal assemblage important in understanding the evolution of dinosaurs and other terrestrial vertebrates in Asia. Geological evidence indicates that the Sao Khua and Khok Kruat Formations had a more semi-arid climate than the older, more humid Phu Kradung Formation. The Khok Kruat Formation is composed mostly of sandstones, conglomerates, siltstones, and shales. Similarly to the Sao Khua Formation, the deposition of these sediments occurred in an arid to semi-arid floodplain environment of slow-moving, meandering rivers. Vertebrates from the Khok Kruat assemblage include semionotiform fishes, hybodont sharks, turtles, dinosaurs, pterosaurs, and crocodilians. The dinosaur fauna of this ecosystem included iguanodontians like Sirindhorna khoratensis, Ratchasimasaurus suranaerae, and Siamodon nimngami; as well as a Phuwiangosaurus-like sauropod, an indeterminate ceratopsian, and at least two indeterminate theropods. The formation is probably equivalent to the Gres superieurs Formation of Laos, since animals like advanced iguanodontians, sauropods, and spinosaurids (including Ichthyovenator) have also been found there. The discovery of spinosaurids in Asia, a family previously known only from Europe, Africa, and South America, suggests faunal interchange between the supercontinents Laurasia (in the north) and Gondwana (in the south) during the early Late Cretaceous. Though it may also be possible that spinosaurids already had a cosmopolitan distribution before the Middle Cretaceous, and thus preceding the breakup of Laurasia from Gondwana. However, more evidence is needed to test this hypothesis. During the Early Cretaceous, Thailand's Sao Khua and Khok Kruat Formations probably neighboured what is now southern China, where "Sinopliosaurus" ''fusuiensis teeth are known from. ''However, the discovery of spinosaurids in Asia indicates that the origin and dispersal of the group was complex.