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 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.

[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.

[21] 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.

[36] 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.

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.

[23] The blood grooves (tiny furrows in the gaps between each denticle) of GMNH-PV-999 have an oblique orientation of 45 degrees, as in Baryonyx and KDC-PV-0003, the second Sebayashi formation tooth, which consists of a slightly recurved crown fragment with an almost circular cross-section.

[27][28][32] The cervical vertebrae of SM-KK 14 had elongated centra (vertebral bodies) with articulating surfaces that were not offset, as well as prominent epipophyses (processes to which neck muscles attached) and strong ligament scars.

Buffetaut and Suteethorn concluded that the closest taxon in dentition to Siamosaurus was Spinosaurus aegyptiacus from Egypt, whose fragmentary fossils had been destroyed during World War II.

[1][29]Many palaeontologists later questioned Buffetaut and Ingavat's identification of Siamosaurus, given that spinosaurid teeth, including many from Asia, have often been mistaken for those of aquatic reptiles like crocodilians, plesiosaurs, and ichthyosaurs.

[44] 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.

[20] Later discoveries revealed that largely straight tooth crowns with flutes and a lack or reduction of serrations were unique characteristics of spinosaurid teeth.

[17] In 2012, French palaeontologist Ronan Allain and colleagues described a partial skeleton from the Grès supérieurs Formation of Laos, and used it to name the new spinosaurid genus and species Ichthyovenator laosensis.

[51] 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.

[25] Likewise, the Khok Kruat skeleton shares mixed characteristics between Baryonyx and Spinosaurus,[28] and its precise phylogenetic placement is uncertain pending a description of the material.

[21][29] Buffetaut and Ingavat suggested in 1986 that Siamosaurus probably led a heavily piscivorous (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.

[1][37] Such a dietary preference had been suggested for Baryonyx the same year by British palaeontologists Angela 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.

[54] 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.

[37][56] 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.

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.

He noted that this likely occurred 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.

[57] 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 the oxygen isotope ratios of remains from theropod and sauropod dinosaurs, crocodilians, turtles, and freshwater fish recovered from eight localities in northeastern Thailand.

The study revealed that Siamosaurus teeth had isotope ratios closer to those of crocodilians and freshwater turtles than other theropods, and so it may have had semiaquatic habits similar to these animals, spending much of its daily life near or in water.

[58] 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.

[22][59] 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.

[5][65] 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.

[2][3] This ecosystem included pterosaurs, sinamiid fish; carettochelyid and acocid turtles; ptychodontid, hybodontid, and thaiodontid sharks; and the crocodyliform Khoratosuchus, as well as goniopholidids.

[16][42] In 2007, Milner and colleagues suggested that spinosaurids and iguanodontians may have spread from western to eastern Laurasia—the northern supercontinent at the time—during the Aptian, based on their distribution and presence in the Khok Kruat Formation.

[78] 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.