Orientalosuchus is an extinct genus of crocodilian from the Late Eocene that was found in the Na Duong Formation in Vietnam. The genus was described in 2019 based on the fossil remains of at least 29 individuals and was key in establishing the clade Orientalosuchina, initially interpreted as a group of early alligatoroids endemic to Asia, although later studies have argued for them actually being crocodyloids instead. Orientalosuchus was a comparably small crocodilian with a blunt and rounded snout and dentition that featured both pointed teeth towards the front of the jaw and blunt, conical teeth in the back. During the Late Eocene it would have inhabited the tropical to warm-subtropical freshwater biomes of the Na Duong Formation, which featured ponds, an anoxic lake and swamp forests as some of the primary habitats. Orientalosuchus would have shared these with the narrow-snouted gavialoid Maomingosuchus and a large taxon similar to Asiatosuchus. Compared to these, interpreted as a piscivore and a generalist respectively, Orientalosuchus would have been better equipped to deal with hard-shelled prey, such as the plethora of turtles found in the region.
History and naming The fossil remains of Orientalosuchus were discovered between 2009 and 2012 during systematic paleontological surveys of the Na Duong Basin in Northeastern Vietnam, near the Chinese border. All fossils come from the Eocene (late Bartonian to Priabonian, 39 to 35 Ma) Na Duong Formation and appear to represent a minimum of 29 distinct individuals. The holotype specimen, GPIT/RE/09761, consists of a partial skeleton featuring the skull, lower jaw and a plethora of postcranial bones including several vertebrae, ribs, various limb elements and over 50 osteoderms. The description of Orientalosuchus was accompanied by the recognition of an entire clade of Cretaceous to Eocene crocodilians from east Asia, dubbed Orientalosuchina. In addition to Orientalosuchus, the clade was created to include Krabisuchus, Jiangxisuchus, Protoalligator and Eoalligator, though more genera would be found later. The name Orientalosuchus is a combination of the Latin word "oriens" meaning "east" and the Greek "soukhos" meaning "crocodile", referencing the overall geographic range of the animal and other orientalosuchins. The species name of O. naduongensis meanwhile more specifically references the Na Duong coal mine where the material was found.
Description
Skull
The tip of the snout is formed by the premaxillae, whose contact with the maxillae coincides with a deep notch similar to that seen in true crocodiles and serves to receive an enlarged dentary tooth. Whether or not such a notch was also present in juveniles is however unknown. From this notch the suture extends backwards, creating an elongated premaxillary process that runs alongside the nasal bones. The premaxillae bulge out around the nares, which they almost entirely surround, and towards the side of said bulge a deep depression or notch can be seen. The nares face upwards (dorsally) and are roughly square-shaped, though with rounded edges and a small indentation formed by the premaxillae forming a process that extends into the opening from the front. As in other orientalosuchins, the nasals extend into the nares, preventing the premaxillae from meeting. The maxillae feature sinuous outer edges, a condition known as festooning, with alternating flaring and constricting of the bone. This means that the maxilla is convex at approximately the level of the 5th tooth, concave around the 7th to 8th and then flaring outwards towards its contact with the jugal. Another prominent feature of the maxilla is a prominent ridge that begins at the backmost part of the premaxillary process and proceeds to run parallel to the nasals, extending onto the lacrimal bone and prefrontal bone and ending at the orbital margin, the eyesocket. This ridge is accompanied by a groove that runs along the maxilla. The relative brevity of this ridge does help differentiate it from the related Dongnanosuchus, in which the preorbital ridge is just one element of a wider complex. The lacrimal itself has an elevated medial margin and is overall shaped like a slender triangle and the prefrontal is wedge-shaped. The frontal bone consists of a long and narrow anterior process and a wide posterior region. The anterior process runs between the paired lacrimals and frontals and forms a pointed peak that extends in-between the nasal bones. The frontal lies flush with the edge of the eyesockets, only upturning very little rather than forming an elevated rim. The posterior section of the frontal forms the very front of the skull table, where it contacts the boomerang-shaped postorbital bones and the large rectangular parietal, with all three bones connecting in a small triple junction. The small supratemporal fenestrae are located comparably close to the front of the skull table, with a longer stretch of bone separating them from the posterior edge and creating a long contact between parietal and the squamosals. In adult specimens of Orientalosuchus, the parietal never actually reaches the back of the skull table, being barred from the edge by the broadly-exposed supraoccipital.
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