| Siamosaurus | |
|---|---|
 | |
| Illustration of theholotype tooth, withBritish penny for scale | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Spinosauridae |
| Genus: | †Siamosaurus Buffetaut & Ingavat,1986 |
| Species: | †S. suteethorni |
| Binomial name | |
| †Siamosaurus suteethorni Buffetaut & Ingavat, 1986 | |
| Synonyms | |
| |
Siamosaurus (meaning "Siam lizard") is a potentiallydubiousgenus ofspinosauriddinosaur that lived in what is nowThailand and possiblyChina during theEarly Cretaceousperiod (Barremian toAptian) and is the first reported spinosaurid from Asia. It is confidently known only from toothfossils; the first were found in theSao Khua Formation, with more teeth later recovered from the youngerKhok Kruat Formation. Theonly speciesSiamosaurus suteethorni, whose name honours ThaipalaeontologistVaravudh Suteethorn, was formally describedin 1986. In 2009, four teeth from China previously attributed to apliosaur—under the species"Sinopliosaurus"fusuiensis—were identified as those of a spinosaurid, possiblySiamosaurus. It is yet to be determined if two partial spinosaurid skeletons from Thailand and an isolated tooth from Japan also belong toSiamosaurus.
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. Theholotype 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 wrinkledenamel, similar to teeth from the related genusBaryonyx. As a spinosaur it would have had a long, low snout and robust forelimbs, and one possible skeleton indicates the presence of a tallsail running down its back, another typical trait of this theropodfamily.Siamosaurus is considered by some palaeontologists to be adubious 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 thesubfamilySpinosaurinae.
Like in all spinosaurids,Siamosaurus's teeth were conical, with reduced or absentserrations. This made them suitable for impaling rather than tearing flesh, a trait typically seen in largelypiscivorous (fish-eating) animals. Spinosaurids are also known to have consumedpterosaurs and small dinosaurs, and there is fossil evidence ofSiamosaurus itself feeding onsauropod dinosaurs, either viascavenging 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 ofSiamosaurus and other spinosaurids indicatessemiaquatic habits.Siamosaurus lived in asemi-arid habitat offloodplains and meandering rivers, where it coexisted with other dinosaurs, as well as pterosaurs, fishes, turtles,crocodyliforms, and other aquatic animals.
TheSao Khua Formation, where the firstSiamosaurusfossils were discovered, is part of theKhorat Group.[1] The formation is dated to theBarremianstage of theEarly Cretaceousperiod, 129.4 to 125 million years ago.[2][3] In 1983, FrenchpalaeontologistÉric 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 thePhu Wiang area ofKhon Kaen Province. They did not conclude as to what animal they originated from, their opinion being that the specimens belonged "either to an unusualtheropoddinosaur or to some unknowncrocodilian". In1986, a reassessment of the remains by the same authors attributed them to a newgenus andspecies ofspinosaurid theropod, which they namedSiamosaurus suteethorni. The generic name alludes to the ancient name of Thailand, "Siam", and is combined with theAncient Greek wordσαῦρος (sauros), meaning "lizard" or "reptile". The specific name honours Thaigeologist and palaeontologistVaravudh Suteethorn, and his contributions tovertebrate palaeontologydiscoveries in Thailand.[1]
The best-preserved specimen from the teeth described, designated DMR TF 2043a, was chosen as theholotype ofSiamosaurus.[1] Theparatypes[4] comprise eight other well-preserved teeth catalogued as DMR TF 2043b to i. The original fossils are currently housed in the palaeontological collection of theDepartment of Mineral Resources,Bangkok.[1]Siamosaurus teeth are common in the Sao Khua Formation, and further isolated specimens were found later throughout the late 20th and early 21st centuries.[5][6] Sculptures of the animal have been erected in various places across northeastern Thailand, including Si Wiang Dinosaur Park,[7] theSiam Paragon shopping mall in Bangkok,[8][9] thePhu Wiang Dinosaur Museum,[10] and theSirindhorn Museum.[11]S.suteethorni was also illustrated on Thai postage stamps released in 1997, along with fellow Thai dinosaursPhuwiangosaurus,Siamotyrannus, andPsittacosaurus.[12]
Thailand'sKhok Kruat Formation is dated to theAptian age (between 125 and 113 million years ago), younger than the Sao Khua Formation.[3] The Khok Kruat Formation has provided many spinosaurid teeth, including ones fromSiamosaurus and closely allied forms.[14][15][16] Given the varied size andmorphology of the teeth found, the presence of multiple spinosaurtaxa in the region is likely.[6][17] 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 mainmorphotypes: the Khok Kruat morphotype, which is found only in the Khok Kruat Formation, and theSiamosaurus morphotype, which includes forms widely recovered from both the Sao Khua and Khok Kruat Formations.[6]
Siamosaurus is the first reported spinosaurid dinosaur from Asia,[1][18] and subsequently to its naming, material resembling or possibly belonging to the genus has been found across the continent. In1975, Chinese palaeontologist Hou Lian-Hai and colleagues described five teeth as a new species of thepliosauroidSinopliosaurus, which they namedS. fusuiensis, the specific name is in reference toFusui County inGuangxi, China, from which the fossils were collected.[19] 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.[20] In 2019, "S." fusuiensis was referred to by Thai palaeontologist Wongko Kamonrak and colleagues asSiamosaurus sp. (of uncertain species).[6] Later in 2019, Thai palaeontologist Adun Samathi and colleagues considered the teeth as belonging to an indeterminate spinosaurid.[21] The specimens were retrieved from China's Early CretaceousXinlong 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.[22] 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.[5]
In 1994, an isolated tooth (specimenGMNH-PV-999) was found by a fossil prospector in theSebayashi Formation, Japan. The tooth was believed, until 2003, to belong to amarine reptile, when Japanese palaeontologist Yoshikazu Hasegawa and colleagues assigned it to ?Siamosaurus sp.[23] The tooth came from rocks dated to the Barremian, similar in age tosediments thatSiamosaurus teeth have been recovered from in Thailand.[23][24] 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.[24] Further spinosaurid teeth from unnamed and indeterminate forms have been discovered in central China[25] andMalaysia.[26]
In 2004, excavation began on a partial skeleton from an outcrop of the Khok Kruat Formation near the city ofKhon Kaen.[27] The specimen (SM-KK 14) consists ofcervical (neck) anddorsal (back)vertebrae, a 60-centimetre (24-inch) highneural spine (upwards-extendingprocess from top of vertebra),pelvis (hip) fragments, a possiblemetacarpal (long bone of the hand), and achevron from the tail. The cervical vertebrae and pelvic region resemble those of the European spinosaurBaryonyx walkeri, and the neural spine is elongated, similarly to those of other spinosaurids.[16][21][28][29] ASiamosaurus tooth found nearby indicates the skeleton may belong to this genus, though this could also represent evidence ofscavenging.[28][27] The skeleton, as well as two well-preserved teeth—SM2016-1-147 and SM2016-1-165, also attributed toSiamosaurus—are currently stored in the vertebrate fossil collection of the Sirindhorn Museum,Kalasin Province.[30][31][32] In 2019, a series of spinosauridcaudal (tail) vertebrae possibly belonging toS.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 spinosauridCamarillasaurus and aBaryonyx specimen discovered in Portugal.[33]
In 2004, American dinosaur researcherDon Lessem estimatedSiamosaurus at 9.1 metres (30 feet) long.[34] In 2005, British author Sussana Davidson and colleagues gave a lower estimate of 8 m (26 ft) in length and weighing 1tonne (1.1short tons; 0.98long tons).[35] In a 2016 popular book, authors Rubén Molina-Pérez and Asier Larramendi estimated it at approximately 5.1 m (17 ft) long, 1.45 m (4.8 ft) tall at the hips, and weighing 255 kilograms (562 pounds).[36] However, reliable calculations on the weight and body size of fragmentary dinosaurs likeSiamosaurus are hindered by the lack of good material, such as a skull orpostcranial skeleton, and thus estimates are only tentative.[37] "Phuwiang spinosaurid B" was calculated as approximately 5 m (16 ft) long by Samathi in 2019.[33] 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 asail on its back.[37][38]
Fossil theropod teeth are typically identified by attributes such as the proportions, size, and curvature of thecrown, as well as the presence and/or shape of thedenticles (serrations).[39] The holotype ofS.suteethorni (specimen DMR TF 2043a) is 62.5 millimetres (2.46 in) in total length, with the crown being 47.7 mm (1.88 in) long, and 16.6 by 12.5 mm (0.65 by 0.49 in) wide at its base. It is among the larger teeth discovered by Buffetaut and Ingavat. One much smaller specimen (DMR TF 2043b) measures 24.3 mm (0.96 in) in length. According to the authors, this dramatic size range suggests the teeth are from individuals of different ages.[1] Among theropods, this may also indicate size variation along the tooth row in the jaws, which is observed to have been prevalent in spinosaurids.[37][40]
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, thecarinae (cutting edges) ofSiamosaurus teeth lack well-defined serrations,[1] though unworn teeth do exhibit very fine denticles.[21] Some teeth (including the holotype) have a wave-like double recurvature when viewed from the front or back, which Buffetaut and Ingavat compared to that seen incarnosaur teeth from the same formation and oneDeinonychus tooth described byJohn Ostrom in 1969. TheS.suteethorni holotype is symmetrically concave front to back, and bears 15 flutes (lengthwise grooves) on itslingual (inward facing) andlabial (outward facing) surfaces. These flutes run from the base of the crown before stopping 5 mm (0.20 in) from the rounded tooth tip. A region of the holotype where theenamel (outer layer of the teeth) has weathered away reveals that these flutes extend down to thedentin (second layer of the teeth).[1] The enamel also has a granular (finely wrinkled) texture, as seen in teeth from the spinosauridBaryonyx.[41] Some of theroot is preserved in the holotype and, as in all theropods, there is a large pocket for the toothpulp, which would have housed blood vessels and nerves.[1]
Of the two Khok Kruat Formation tooth morphotypes assigned by Kamonrak and colleagues in 2019, morphotype I, the Khok Kruat morphotype, is on average 46.35 mm (1.825 in) in total length, of which the crown takes up 43.77 mm (1.723 in), with a 18.40 by 13.5 mm (0.724 by 0.531 in) wide base. They are oval in cross-section, have well-defined carinae, and a smooth enamel surface, which becomes wrinkled at the base of the crown. They bear fine, sharply defined flutes, of which there are about 21 to 32 on each side. Morphotype II, theSiamosaurus morphotype, is on average 51.25 mm (2.018 in) long, with a 48.30 mm (1.902 in) tall crown that is 17.30 by 14.65 mm (0.681 by 0.577 in) wide at the base. They are also oval in cross-section and have distinct carinae, but unlike the Khok Kruat morphotype, the entire length of the crown has wrinkled enamel, and the flutes are coarser and fewer in number, with 11 to 16 on each side. TheSiamosaurus morphotype also shares withS.suteethorni, GMNH-PV-999, and IVPP V 4793 a wrinkled enamel surface and between 12 and 15 flutes on each side.[6]
The first Sebayashi Formation specimen (GMNH-PV-999) is an isolated tooth crown with a partially intact root. It is not known in which jaw the tooth was positioned or 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. Besides having a broader, 20 by 14 mm (0.79 by 0.55 in) wide base and being slightly smaller at 51 mm (2.0 in) in length, GMNH-PV-999 has a very similar morphology to theS.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 flutes on the crown surface, the Japanese specimen having 12 on each side. The teeth also share a crown surface with numerous small granular structures oriented parallel to their lengths. Because of these resemblances, Hasegawa and colleagues regarded GMNH-PV-999 as nearly identical to theS.suteethorni holotype tooth.[23] Theblood grooves (tiny furrows in the gaps between each denticle) of GMNH-PV-999 have anoblique orientation of 45 degrees, as inBaryonyx and KDC-PV-0003, the second Sebayashi formation tooth, which consists of a slightly recurved crown fragment with an almost circular cross-section. It has better preservation of small details than the former specimens, such as visible, though poorly defined serrations, with two to three denticles per mm (0.039 in). Like GMNH-PV-999, it has a granular texture and at least 12 flutes on its surface, not all of which stretch to the crown's full length.[24]
Out of the four teeth attributed to "S."fusuiensis, specimenIVPP V 4793 is the most intact, although still somewhat deformed. The crown, which is missing its tip, is 69 mm (2.7 in) long and 16.5 by 13 mm (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, though their serrations have been heavily eroded, similar to those of KDC-PV-0003. Like the Thai and Japanese teeth, the "S."fusuiensis specimens bear developed flutes and a granular surface. As in both Sebayashi Formation teeth, there are 12 flutes on each face of the "S."fusuiensis teeth. Like in KDC-PV-0003, these flutes vary in length. Buffetaut and colleagues found the "S."fusuiensis teeth most similar to those ofSiamosaurus, given their identical crown shape, fluting, and granular enamel.[20]
Though no skeletal elements were associated with the originalSiamosaurus teeth, the Khok Kruat skeleton, SM-KK 14, may be attributable to the genus.[27][28][32] The cervical vertebrae of SM-KK 14 had elongatedcentra (vertebral bodies) witharticulating surfaces that were not offset, as well as prominentepipophyses (processes to which neck muscles attached) and strongligament scars. All of these characteristics were also present inBaryonyx.[16][27] The cervicals also became longer towards the front of the neck and—based on comparison withBaryonyx—may represent the fourth, sixth, seventh, and tenth vertebrae. The dorsal vertebrae had enlarged infraprezygapophyseal fossae—depressions under theprezygapophyses, which connect adjacent vertebrae[21]—and their neural spines were elongated similarly to those of other spinosaurids, indicating the presence of a sail on the animal's back; like in the Asian spinosauridIchthyovenator.[21][28][37] One of the neural spines of SM-KK 14 measured at least 60 centimetres (24 inches) in height.[29] The chevron lacked a process on its front end, as in other spinosaurids. Viewed distally (towards the centre of attachment), the lower end of thepubis had an L-shape, similar to that ofIchthyovenator and the AfricanSuchomimus.[21] Also as inIchthyovenator, the hind rim of the pubis had a notch-likeobturator foramen.[21][42] 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 inIchthyovenator.[21]
In 1986, Buffetaut and Ingavat classifiedSiamosaurus as a theropod because of the straight, tall crown and double sideways 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 inSiamosaurus's teeth to those ofceratosaurian tooth crowns, some of which also have longitudinal flutes. However, this identification was ruled out, since ceratosaur teeth are more narrow and blade-like in cross-section, bear far fewer dental flutes, and have distinct serrations. Buffetaut and Suteethorn concluded that the closest taxon indentition toSiamosaurus wasSpinosaurus aegyptiacus from Egypt, whose fragmentary fossils had been destroyed duringWorld War II. LikeSiamosaurus, this African taxon had straight and unserrated conical teeth. ThoughSpinosaurus lacked the developed flutes seen inSiamosaurus, Buffetaut and Ingavat noted that both smooth and fluted spinosaur teeth had been reported from Africa. Therefore, they tentatively placedSiamosaurus in thefamily Spinosauridae, based on the close similarities in dentition toS.aegyptiacus.[1][29]
Many palaeontologists later questioned Buffetaut and Ingavat's identification ofSiamosaurus, given that spinosaurid teeth, including many from Asia, have often been mistaken for those of aquatic reptiles like crocodilians,plesiosaurs, andichthyosaurs.[43] In view of this, the German palaeontologistHans-Dieter Sues and colleagues in 2002 asserted that there is not enough material to confidently identifySiamosaurus as a dinosaur.[44] In 2004, American palaeontologistThomas Holtz and colleagues considered it adubious name, stating that the teeth might instead belong to a contemporaneous fish such as asaurodontid or anichthyodectidteleost.[45] The same year, American palaeontologistDavid Weishampel and colleagues consideredSiamosaurus an indeterminate theropod.[46] 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.[47] This problem is especially common with spinosaurids, given that skull and skeletal fossils from the group are rare.[37]
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Authors such as Buffetaut and Ingavat in 1986, and Hasegawa and colleagues in 2003, have noted that since crocodilian teeth are usually more strongly recurved than spinosaur teeth, they can be distinguished from each other.[1][23] Crocodilians also lack the lateral double recurvature ofSiamosaurus's tooth crowns, which, based on their shape, were vertically inserted into the jaw, whereas long-snouted crocodilian teeth are usually angled outwards from the mouth. ThoughSiamosaurus and plesiosaur teeth are similar in overall shape, Buffetaut and Ingavat pointed out that plesiosaur teeth were significantly more recurved.[1] Other researchers also noted that compared to plesiosaurs, Asian spinosaurid teeth also have coarser and more numerous flutes that extend almost the whole length of the crown.[1][20][23] In 2008, Buffetaut and colleagues stated that the "S."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 ofSiamosaurus, were also brought up by the researchers as further evidence ofSiamosaurus's spinosaurid classification.[20] Later discoveries revealed that largely straight tooth crowns with flutes and a lack or reduction of serrations were unique characteristics of spinosaurid teeth.[37][48]
In 2014, Italian palaeontologist Federico Fanti and colleagues considered the various spinosaurid teeth from East Asia, including those ofS.suteethorni, as identical to those ofSpinosaurus.[49] In 2017, Brazilian palaeontologists Marcos Sales and Cesar Schultz suggested that the various Asian teeth might eventually be attributed toIchthyovenator-like forms. The researchers acceptedSiamosaurus 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.[48] Carrano and colleagues noted that the Khok Kruat skeleton may provide answers to their identification.[47] Authors such as Milner and colleagues in 2007,[28] Bertin Tor in 2010,[29] Holtz in 2011,[50] and Kamonrak and colleagues in 2019 regarded the Khok Kruat skeleton as first definitive evidence of spinosaurs in Asia.[17] In 2012, French palaeontologist Ronan Allain and colleagues described a partial skeleton from theGrès supérieurs Formation of Laos, and used it to name the new spinosaurid genus and speciesIchthyovenator laosensis. They considered it the first definitive evidence of spinosaurids in Asia, in light of the debated identity ofSiamosaurus and "S."fusuiensis.[42] In a 2014 abstract, Allain announced that furtherIchthyovenator material, including three teeth, had been excavated. Typically of spinosaurines,Ichthyovenator's teeth bore straight and unserrated crowns, though no comparison was made to the other Asian teeth.[51]
Thetaxonomic andphylogenetic 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.[37] Traditionally, the group is split into thesubfamilies Spinosaurinae (unserrated, straight teeth with well marked flutes and more circular cross-sections) and Baryonychinae (finely serrated, somewhat recurved teeth with weaker flutes and a more oval cross-section). Since spinosaurines were, on average, larger animals than baryonychines, their teeth were also generally larger.[37][25] The morphological variation seen in spinosaurid teeth, however, has shown that the aforementioned characteristics are not always consistent within the subfamilies.[25] Likewise, the Khok Kruat skeleton shares mixed characteristics betweenBaryonyx andSpinosaurus,[28] and its precise phylogenetic placement is uncertain pending a description of the material.[52] The possibility that Baryonychinae is aparaphyletic (unnatural) grouping has been suggested by researchers such as Sales and Schultz, on the basis that genera such asIrritator andAngaturama (the two are possible synonyms) may represent intermediate forms between baryonychines and spinosaurines.[48] As it is definitively known only from teeth,Siamosaurus's exact position within the Spinosauridae is difficult to determine.[23][24][14] In 2004, Brazilian palaeontologists Elaine Machado andAlexander Kellner suggested it as a possible spinosaurine, given its lack of dental serrations.[52] Likewise in 2010, British palaeontologist David Hone and colleagues placedSiamosaurus and "S."fusuiensis in the Spinosaurinae.[25] British palaeontologist Thomas Arden and colleagues identifiedSiamosaurus as abasal (early diverging or "primitive") member of this subfamily in 2019; theircladogram can be seen below:[38]
| Spinosauridae |
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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 andSiamosaurus 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 asIrritator andSpinosaurus, Asian spinosaurines usually have more laterally compressed tooth crowns, and wrinkles across more of the enamel surface.[6] In 2020, a paper by British palaeontologist Robert Smyth and colleagues consideredS.suteethorni a dubious name and attributed its teeth to an indeterminate spinosaurine, given the uncertainties of classifying spinosaurid teeth at the genus or species level, as well as the degree ofheterodonty (variation within the tooth row) that spinosaurines apparently exhibited.[53] Due to new discoveries and research on spinosaurid teeth sinceSiamosaurus was named in 1986, a reassessment of the genus' validity is currently being prepared by Buffetaut.[21][29] In a 2025 paper describing indeterminate spinosaurid teeth from the Sao Khua Formation which likely belongs to a distinct taxon, Puntanon, Suteethorn and Samathi supported the validity ofSiamosaurus based on possibly diagnostic, plesiomorphic traits shared with members of Spinosaurinae, including small distal denticles and reduced tooth crown curvature.[54]
Buffetaut and Ingavat suggested in 1986 thatSiamosaurus probably led a heavilypiscivorous (fish-eating) lifestyle, since its dentition—like those of other spinosaurids—had a highly specialised morphology better suited for piercing rather than tearing flesh, due to the long, straight conical tooth crowns with reduced or absent serrations. The authors noted that this dental morphology is also seen in other piscivorous predators like plesiosaurs and long-snouted crocodilians.[1][37] Such a dietary preference had been suggested forBaryonyx the same year by British palaeontologistsAngela Milner and Alan Charig, and was later confirmed in 1997 with the discovery of acid-etched fish scales inside the body cavity of its holotype skeleton.[55] The elongate, interlocking jaws of spinosaurids also had snout tips that fanned out into arosette-like shape—a trait also observed in highly piscivorous crocodilians such asgharials—which made them well-adapted to catching and feeding on fish.[38][56] Fossil evidence has shown that besides aquatic prey, spinosaurids also consumed other dinosaurs andpterosaurs.[37][57] In the Sao Khua Formation, localities such as Wat Sakawan have yieldedsauropod remains in association with tooth crowns fromSiamosaurus, documenting either predation or scavenging on part of the latter.[5]
In 2006, Thai biologist Komsorn Lauprasert examined fossils collected from the Phu Kradung, Sao Khua, and Khok Kruat Formations. In this study, the teeth ofSiamosaurus and a Moroccan spinosaurid were compared to those of crocodilians usingscanning electron microscopy. Lauprasert found that spinosaurids and crocodilians may have employed similar feeding tactics and been under comparablemechanical constraints, based on resemblances in themicrostructure of their tooth enamel. Therefore, Lauprasert suggested thatSiamosaurus—as a piscivorous predator—could have replaced theecological niche of contemporaneous long-snouted crocodilians. He noted that this likely occurred in correlation with the rising aridity of the Sao Khua and Khok Kruat Formations during the Early Cretaceous, sinceSiamosaurus had better mobility in a dry environment than crocodilians did. This might explain the absence of long-snouted crocodilian fossils from that time and place.Goniopholidid crocodilians were prevalent, however, and since this group had broader, shorter snouts and thus more varied diets, Lauprasert suggested that this would have kept them fromcompeting withSiamosaurus.[58] 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.[25]
In 2008, French palaeontologist Romain Amiot and colleagues compared theoxygen isotope ratios of remains from theropod and sauropod dinosaurs, crocodilians, turtles, and freshwater fish recovered from eight localities in northeastern Thailand. The study revealed thatSiamosaurus teeth had isotope ratios closer to those of crocodilians and freshwater turtles than other theropods, and so it may have hadsemiaquatic habits similar to these animals, spending much of its daily life near or in water. Discrepancies between the ratios of sauropods,Siamosaurus, and other theropods also indicate these dinosaurs drank from different sources, whether river, pond, or plant water.[59] 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 ofSpinosaurus 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 asniche partitioning, the different types of theropods would have been out of direct competition.[22][60] 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 forbuoyancy control; reduction of the pelvic girdle; and elongated neural spines on the tail, likely used for underwater propulsion.[38][61][62][63]
Of all theMesozoic formations in northeastern Thailand, the Sao Khua is the most abundant and diverse in vertebrate fossil discoveries.[5][65] The Khorat Group yields fossil taxa only of continental origin, with no definitive evidence for marine fossils orsedimentary structures found so far. In 1963, Yoshitsugu Kobayashi ofHokkaido University reported ichthyosaur and plesiosaur teeth from the Sao Khua Formation, but these have now been identified as belonging toSiamosaurus and a crocodilian respectively.[5][66] The sediments of the Sao Khua Formation, which comprisered clays,mudstones,sandstones,siltstones, andconglomerate rocks, record afluvialenvironment dominated by lakes,floodplains, and meandering low-energy rivers.[3][65] This is consistent with the types of vertebrate fauna present in the formation, which comprise only terrestrial or freshwater animals.[5][65]
BesidesSiamosaurus, there were theropod dinosaurs like themetriacanthosauridSiamotyrannus,[67] theornithomimosaurKinnareemimus,[68] themegaraptoranPhuwiangvenator, the basalcoelurosaurVayuraptor,[69] acompsognathid theropod, and indeterminate birds. Theropodeggs withembryos have also been recovered from the formation.[70][71] There were also sauropods like thetitanosauriformPhuwiangosaurus,mamenchisaurids, and indeterminate forms.[5][70] Sauropod remains are some of the most abundant in the Sao Khua and Khok Kruat Formations.[5][16] Noornithischian (or "bird-hipped") dinosaur fossils have been found in the Sao Khua Formation, possibly suggesting that they were uncommon compared tosaurischian (or "lizard-hipped") dinosaurs.[5] The faunal assemblage also included indeterminate pterosaurs;carettochelyid,adocid, andsoftshell turtles;hybodont sharks likehybodontids,ptychodontids, andlonchidiids;pycnodontiform fish;ray-finned fishes such assinamiids andsemionotids;[5][65][70] and the goniopholididcrocodyliformsSunosuchus,[65]Siamosuchus,[72] andTheriosuchus.[73] The Sao Khua and Khok Kruat Formations had a moresemi-arid climate than the older, more humidPhu Kradung Formation, dated to theBerriasian.[58]
The Khok Kruat Formation is composed mostly of sandstones, conglomerates, siltstones, andshales. Similar to the Sao Khua Formation, the deposition of these sediments occurred in an arid to semi-arid floodplain environment of slow-moving, meandering rivers.[2][3] This ecosystem included pterosaurs, sinamiid fish; carettochelyid and acocid turtles; ptychodontid, hybodontid, andthaiodontid sharks; and the crocodyliformKhoratosuchus, as well as goniopholidids.[70] BesidesSiamosaurus, the dinosaur fauna of the Khok Kruat Formation included thecarcharodontosauridSiamraptor;[74]iguanodontians likeSirindhorna,[2]Ratchasimasaurus,[75] andSiamodon;[76] a titanosauriform sauropod similar toPhuwiangosaurus;[16] an indeterminateceratopsian;[77] and various indeterminate theropods.[15][16] The formation is probably equivalent to the Grès supérieurs Formation of Laos, since animals like spinosaurids, sauropods, andderived ("advanced") iguanodontians have also been found there.[16][42]
In 2007, Milner and colleagues suggested that spinosaurids and iguanodontians may have spread from western to easternLaurasia—the northernsupercontinent at the time—during the Aptian, based on their distribution and presence in the Khok Kruat Formation.[28] American palaeontologistStephen Brusatte and colleagues noted in 2010 that the discovery of spinosaurids in Asia, a family previously known only from Europe, Africa, and South America, suggests a faunal interchange between Laurasia andGondwana (in the south) during the early Late Cretaceous. Though it may also be possible that spinosaurids already had acosmopolitan distribution before the Middle Cretaceous, preceding the breakup of Laurasia from Gondwana. However, the authors noted that more evidence is needed to test this hypothesis.[78] In 2012, Allain and colleagues suggested such a global distribution may have occurred earlier acrossPangaea before the Late Jurassic, even if Asia became separated from the supercontinent first.[79] In 2019, Spanish palaeontologist Elisabete Malafaia and colleagues also indicated a complexbiogeographical pattern for spinosaurs during the Early Cretaceous, based on anatomical similarities betweenIchthyovenator and the European genusVallibonavenatrix.[80]
Media related toSiamosaurus at Wikimedia Commons
Data related toSiamosaurus at Wikispecies
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