Indosinosuchus
| Indosinosuchus Temporal range: Late Jurassic,
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| Holotype skull of I. potamosiamensis | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Archosauria |
| Clade: | Pseudosuchia |
| Clade: | Crocodylomorpha |
| Clade: | Crocodyliformes |
| Suborder: | †Thalattosuchia |
| Family: | †Teleosauridae |
| Genus: | †Indosinosuchus Martin et al., 2019[1] |
| Type species | |
| †Indosinosuchus potamosiamensis Martin et al., 2019
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| Species | |
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Indosinosuchus is a genus of
History and naming
Fossil material of Indosinosuchus is only found in the
The name Indosinosuchus is derived from the Indochinese
Description
Generally, all teleosaurs display a longirostrine skull shape, meaning their snouts are elongated and slender. However the degree of this elongation varies within the group, and with a rostrum to skull length ratio of 0.66 Indosinosuchus potamosiamensis falls into a group of teleosaurs with relatively short snouts compared to their relatives. This makes it similar to
The two species can be differentiated by several subtle differences in the skull as well as a few
The vertebral column shows no difference to that of other teleosauroids and although no specimen preserves both a femur and a humerus, the likely ratio between these two limb bones could be estimated based on their size proportional to the skull. Here too Indosinosuchus follows what is seen in its relatives, with a humerus to femur ratio of 0.63.[1]
The osteoderm armor of Indosinosuchus follows the patterns typical for teleosaurs, with wider than long osteoderms in the trunk region and longer than wide, subrectangular osteoderms on the tail, all of which preserve a keel that runs from the front to the back. The osteoderm on the underside of the body are also longer than wide, but lack the keel and subsequently form a flat surface.[1]
Phylogeny
Initially, two phylogenies were conducted on Indosinosuchus, both based on prior work by Wilberg. The first analysis found Indosinosuchus to be nested deep inside of Teleosauroidea as the sister taxon to Machimosaurus hugii. While this tree was well resolved, Martin and colleagues noted a variety of problems with the
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Only a year later, Johnson, Young and Brusatte published a major revision of Teleosauroidea with particular focus on the genus Steneosaurus, which had previously been a wastebasket taxon containing a wide array of unrelated teleosaurs. Although it was in this paper that I. kalasinensis was named as a second species of Indosinosuchus, none of the phylogenetic trees that were part of the study recovered the two species as actual sister taxa as would be expected. In the most parsimonious tree using the extended implied weights method, I. kalasinensis was found to be the basalmost member of Teleosauridae while I. potamosinensis was notably more derived. Unlike in Martin's previous study, Indosinosuchus was not found to clade anywhere near Machimosaurus, with the two genera falling into entirely different families, the teleosaurids and the machimosaurids. The Bayesian analysis mirrors the split into teleosaurids and machimosaurids, but shows a much poorer resolved internal phylogeny for the former. Here teleosaurids are split into multiple clades that together form a broad polytomy. Here too the Indosinosuchus species do not directly clade with one another, as I. potamosiamensis nested with the Chinese teleosauroid and Mystriosaurus, while I. kalasinensis stood on its own. Part of the reason for this lack of a recovered relationship could be the lack of material for I. kalasinensis and the fact that the two species do not preserve as much of the postcranial skeleton as their relatives.[2]
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Yet another phylogeny was recovered by Wilberg et al. (2023) in the description of the basal thalattosuchian Turnersuchus. Here Indosinosuchus potamosiamensis clades directly with the Chinese teleosauroid within a clade also containing Macrospondylus and Platysuchus. Since the phylogenetic analysis in this particular publication was based on Wilberg's 2019 dataset, Indosinosuchus kalasinensis was not included.[4]
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Paleobiology
Paleoecology
Traditionally, teleosaurs were split into up to six different ecological groups based on their general anatomy, including the overall shape of the cranium, tooth shape and at times anatomy of the postcrania. This included various degrees of specialisation within longirostrines, terrestrial ecomorphotypes, pelagic species and macrophages or durophages.[5]
In 2022 Johnson and colleagues conducted a large scale
Although their precise ecology remains generally unresolved, Johnson and colleagues conclude that Indosinosuchus was most likely a mesorostrine generalist species, showing no particular adaptations for going after large or armored prey like derived machimosaurs.[5]
Paleoenvironment
While teleosaurs are generally considered to be marine animals, with the vast majority known from the waters of the western
While the environment is well understood, the age of the Phu Kradung Formation is not as well known due to the lack of marine foraminifera or volcanic ash that could help in dating. To this come two contradictory hypothesis based on different methods. On the one hand, the general fauna of the formation better matches the
Mass death
The discovery of 10 Indosinosuchus specimen at Phu Noi, which cover different ages from juvenile to full mature, raises the question how the animals of that particular assemblage died. The material is not complete enough to indicate conditions similar to those that preserved the thalattosuchians of Holzmaden, which are known in their entirety thanks to the anoxic preservational environment. Limited transport and scavenging following their death would account for the partial but not full disarticulation of the bones. One specimen in particular, a nearly complete and still articulated vertebral column, could have been preserved when the bloated corpse of the animal floated on the water surface and lost its head, before undergoing rapid burial of the remaining body. Another hypothesis proposes that rather than rapid burial, it was the drying out of the carcass that caused its preservation. This is supported by marks left from the activity of dermestid beetles on some of the fossils, which would only be active in the dry season and require at least 8 weeks of dry conditions for their larvae to fully develop. Martin and colleagues suggest that the Phu Noi Indosinosuchus specimen congregated in a body of water during a major drought, something still seen in modern species of crocodilians. An ongoing drought could have depleted resources to the point that at least some of the now trapped teleosaurs died, eventually preserving them when their surroundings dried up entirely and allowing scavenging arthropods to access the carcasses. Alternatively, the animals may have died in a flash flood, their decomposition beginning while the waters were still receding but eventually leaving the carcasses stranded on dry land.[1]