Indosinosuchus is a genus of teleosaurid crocodyliform that lived during the Late Jurassic or Early Cretaceous in what is now Thailand. There are at least two recognized species, the type species I. potamosiamensis from the lower Phu Kradung Formation and I. peninsularensis from the Khlong Min Formation. A third species has also previously been proposed based on a single skull from the Phu Kradung Formation, initially named Indosinosuchus kalasinensis, but a 2026 study has argued that this taxon was synonymous with Indosinosuchus potamosiamensis. It is unique among teleosauroids as the type species is the only named genus known from a freshwater environment, while most other members including I. peninsularensis are marine. Indosinosuchus is placed in the family Teleosauridae, but has a relatively robust skull that bears resemblance to members of the Machimosauridae. Biomechanical analysis of its mandible and teeth suggest that it would have had a substantial bite force comparable to animals like Lemmysuchus. Most known specimens of this genus were recovered from a single locality, which has been interpreted as a mass death site, possibly caused by a drought or flash flood. The precise age of Indosinosuchus is unclear, as the vertebrate fossils of the Phu Kradung Formation support a Late Jurassic age, while palynological data suggests an Early Cretaceous (Berriasian) age. The oldest remains from southern Thailand on the other hand suggest that the genus first appeared during the late Middle or early Late Jurassic.
History and naming Fossil discoveries attributed to Indosinosuchus date as far back as the mid to late 2000s, with material having been collected from the Ban Nam Pun site of Thailand's Khlong Min Formation in 2004 and other remains having been found in the sediments of the Phu Noi locality, part of the Phu Kradung Formation, in 2008. By 2019, when Martin and colleagues named Indosinosuchus potamosiamensis, ten individuals known from skull remains and various assorted postcranial fossils were known from the Phu Kradung Formation, all collected from Phu Noi. Of the material, specimen PRC-11 (a complete skull and mandible) was chosen as the holotype. Much of the referred material consists of skulls and mandibles, but also features a specimen that preserve femurs, vertebrae, osteoderms, parts of the shoulder girdle and pelvic girdle and even the fossils of a juvenile individual. While describing the Thai material, Martin and colleagues also considered the possibility that the remains belonged to the previously described Chinese teleosaur Peipehsuchus. However, they concluded that the holotype was said taxon was undiagnostic and the genus thus a nomen dubium, while the referred skull didn't match the fossils from Thailand. This cemented the decision that Indosinosuchus was a distinct genus of Asian teleosaur. In their 2020 revision of teleosauroids, Johnson, Young and Brusatte also examined specimen PRC-239, which was among the various specimen referred to I. potamosiamensis. After comparing the fossil with the other material assigned to the species, they concluded that PRC-239 was anatomically distinct in several areas, enough to be the basis for a second species they named Indosinosuchus kalasinensis. However, this conclusion was eventually questioned in a 2026 study by Komsorn Lauprasert and colleagues as they described the teleosauroid material from Ban Nam Pun. By that point specimen PRC-239 had been completely prepared and removed from the plaster jacket that had previously encased part of the fossil. Following this, the team concluded that many of the features previously thought to distinguish Indosinosuchus kalasinensis from Indosinosuchus potamosiamensis were actually the result of post-mortem deformation caused by the process of preservation. Accordingly, Lauprasert and colleagues proposed that the former species was merely a synonym of the latter. Despite this, the study did not alter the number of Indosinosuchus species, as the team also found evidence that the Ban Nam Pun fossils represented a form referrable to Indosinosuchus but distinct on a species level, which they named Indosinosuchus peninsularensis. The name Indosinosuchus is derived from the Indochinese tectonic block, which includes the area that is now Thailand, and "soukhos" (σοῦχος), Ancient Greek for crocodile. The species name of the type species, potamosiamensis, references the fact that the animal was found in freshwater deposits through the use of the Greek word for river. The species name also specifies its origin in Thailand, using the historical name Siam which is common in scientific names. The name of the possibly synonymous I. kalasinensis meanwhile comes from the Kalasin Province. Indosinosuchus peninsularensis was named for the fact that its remains come from the southern peninsular of Thailand rather than the country's north, where the other species has been found.
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