Orientalosuchina is an extinct clade of alligatoroid crocodylians from Southeast and East Asia that lived from the Maastrichtian to the Eocene. Orientalosuchina are generally small in stature, with even the larger genera being described as "medium-sized" at most and possibly reaching a length of only around . Members of the clade tend to have blunt, rounded snouts and more bulbous cheek teeth.
Though some members of the group have been known since the mid 20th century, the clade has only been recognized as such in 2019 and even then some authors have disputed the inclusion of certain taxa like Jiangxisuchus and Eoalligator. The exact relationship between orientalosuchins and other crocodylians has likewise been debated. They are conventionally regarded as a basal offshoot of Alligatoroidea, though some researchers have argued that they could be more closely related to crocodyloids or even mekosuchines. This confusion stems in part from the unique mix of anatomical features seen in members of the group, which combine the globular cheek teeth traditionally associated with alligatoroids with partially interlocking dentition as seen in crocodyloids.
The ecology of orientalosuchins has received little attention in literature. While it was once suggested that some of them may have been more terrestrial, it is now generally assumed that orientalosuchins were semi-aquatic. Their posterior teeth may suggest a more durophagous diet and fossils from the Na Duong Basin may be evidence of Orientalosuchus preying on turtles. By the Eocene orientalosuchins are known to have coexisted with the narrow-snouted and likely piscivorous gavialoid Maomingosuchus and at least in some regions with a large generalist crocodilian similar to Asiatosuchus. This suggests that the smaller orientalosuchins filled a different niche from these contemporary species.
History of discovery While Orientalosuchina have only been recognized as a clade relatively recently, the first discoveries assigned to this clade date back to 1964 when Yang Zhongjian (also known as C.C. Young) discovered crocodilian remains in the Guangdong Province of China. While much of the material discovered by Young was fragmentary, he nevertheless established two species based on it, the crocodyloid Asiatosuchus nanlingensis and the alligatoroid Eoalligator chunyii. Shortly after this discovery more material was uncovered in the Anhui Province, though these fossils would remain undescribed for close to 20 years before Young eventually coined the name Eoalligator huiningensis on the basis of skull remains.
Research on what are now recognized as orientalosuchins would pick up again in the 2010s, beginning with the description of Krabisuchus siamogallicus. The fossil material of Krabisuchus had been collected by the joint efforts of Thai and French paleontologists under Varavudh Suteethorn and Eric Buffetaut during the late 80s and early 90s from lignite pits within the Krabi Basin of Thailand. While preliminary observations had regarded the Krabi material as a species of Allognathosuchus, by 2010 the genus was regarded as a wastebasket taxon with several former species placed in their own genera. Due to this Jeremy E. Martin and Komsorn Lauprasert established the genus Krabisuchus. In 2014, Pavel P. Skutschas and colleagues reported the discovery of another broad-snouted crocodilian, this time from the Maoming Basin of southern China. While the initial findings were too incomplete for a detailed description, the team noted that the fossil appeared to represent an alligatoroid, simply referred to as the "Maoming alligator" at the time.
In 2016, a revision of Eoalligator was published by Yan-Yin Wang, Corwin Sullivan and Jun Liu, who took note of several issues with Young's description of it and Asiatosuchus nanlingensis. The team highlighted that Young did not provide a detailed description for many of the specimen he referred to Eoalligator, nor fully prepared all the fossil material. One of their main points however was that Young never properly distinguished his two taxa, which they argued share some key features in their anatomy. Wang and colleagues came to the conclusion that Eoalligator chunyii was actually a junior synonym of Asiatosuchus nanlingensis while Eoalligator huiningensis should be placed in its own genus which they dubbed Protoalligator. This further had the side effect that the team regarded Eoalligator chunyii to represent a crocodyloid, while Protoalligator remained an alligatoroid. While the distinction between Eoalligator and Protoalligator was readily accepted, Wang eventually contributed to a second study published in 2018 that reinstated Eoalligator as a valid genus after continued study revealed anatomical differences between it and Asiatosuchus nanlingensis.
Two major studies were published in 2019, first among these the description of Jiangxisuchus nankangensis from the Late Cretaceous of China, which was interpreted as a crocodyloid closely related to Eoalligator. The second major study was the description of Orientalosuchus naduongensis, which was based on a plethora of specimen including multiple skulls, lower jaws and even postcranial material, discovered in the late 2000s and early 2010s in the Na Duong Basin in Northeastern Vietnam, close to the border with China. The description of Orientalosuchus allowed Tobias Massonne and colleagues to furthermore establish a monophyletic clade consisting of multiple South-East Asian crocodilians. This clade, which they dubbed Orientalosuchina, was defined as the most inclusive clade to contain to contain Orientalosuchus, Krabisuchus, Jiangxisuchus, Protoalligator and Eoalligator but not Brachychampsa, Stangerochampsa, Bottosaurus, Diplocynodon, Leidyosuchus or any modern crocodilian.
While some subsequent studies would come to question Massonne's interpretation of Orientalosuchina, especially the reported alligatoroid affinities, others would expand on it. In 2021 Hsi-yin Shan and colleagues would describe additional material from the Maoming Basin, allowing them to describe the short-snouted crocodilian from these sediments as Dongnanosuchus hsui. Only a year later a team led by Xiao-Chun Wu described yet another orientalosuchin, the incredibly short snouted Eurycephalosuchus gannanensis from the Maastrichtian Hekou Formation of China.
Species +GenusSpeciesAgeLocationNotesImageDongnanosuchusD. hsuiEoceneDongnanosuchus is the youngest orientalosuchin from China and known from fossil remains recovered from the oil shale beds of the Maoming Basin (Youganwo Formation).EoalligatorE. chunyiiMaastrichtian - PaleoceneEoalligator was the first orientalosuchin to be named, having been described by Yang Zhongjian in 1964. Fossils have been recovered from the Late Cretaceous Zhenshui Formation as well as the Paleocene Shanghu Formation, suggesting that Eoalligator survived the meteor impact that was responsible for the extinction of the dinosaurs. Eoalligator was once thought to be synonymous with Asiatosuchus nanlingensis, but research has since then rejected this hypothesis.EurycephalosuchusE. gannanensisMaastrichtianEurycephalosuchus from the Late Cretaceous Hekou Formation is regarded as one of the most unusually proportioned orientalosuchins. While most members of the clade have a snout described as short and blunt, that of Eurycephalosuchus is extreme even by these standards. It is also possibly the smallest member of Orientalosuchina.JiangxisuchusJ. nankangensisMaastrichtianJiangxisuchus is among the more debated members of Orientalosuchina, having originally been classified as a crocodyloid closely related to Eoalligator. Some subsequent research has continued to argue for it being more closely related to modern crocodiles than to alligators, but even studies placing it within Orientalosuchina generally do so as a close relative to Eoalligator. Fossils of Jiangxisuchus were initially described to have come from the Late Cretaceous Nanxiong Formation, though later papers have attributed the specific sediments to the Hekou Formation.KrabisuchusK. siamogallicusEocene (Priabonian)Krabisuchus is exclusively known from the Krabi Basin of southern Thailand, where it lived during the latest Eocene. It was initially interpreted as a relative of Allognathosuchus before being placed in the clade Orientalosuchina.OrientalosuchusO. naduongensisEocene (Late Bartonian - Priabonian)Orientalosuchus is the namesake of the clade Orientalosuchina and was described in 2019 from several specimen recovered from the Na Duong Basin of northern Vietnam. Due to its broad skull and blunt posterior teeth, it has been interpreted as a generalist whose diet might have included turtles.ProtoalligatorP. huiningensisMiddle PaleoceneProtoalligator was originally named Eoalligator huiningensis, but moved to its own genus in 2016 after Eoalligator was briefly synonymized with the crocodyloid Asiatosuchus nanlingensis. Since Protoalligator was still regarded as an alligatoroid, its name was specifically chosen to carry on the meaning of Eoalligator, though now both are considered to be valid species and potential relatives. The fossils are known from the Paleocene Wanghudun Formation.
Description Orientalosuchins were small crocodilians with short snouts, though there was some variation in the specific morphotypes displayed by members of this group. Some taxa, such as Jiangxisuchus and Dongnanosuchus, are described as more platyrostral, emphasizing the fact that their snouts like those of many modern crocodilians were flattened. Others like Orientalosuchus and Krabisuchus meanwhile tend to be described as brevirostral, highlighting the proportional shortness of their snouts. Krabisuchus in particular is noted for appearing to have had a wedge-shaped head similar to that of Hassiacosuchus, described as altirostral by Martin and Lauprasert. The most extreme case of brevirostry is seen in Eurycephalosuchus, literally named for its unusual proportions. The skull of Eurycephalosuchus is nearly as wide as it is long and even the skull table is described as "abnormally" short.
Orientalosuchins have been described as displaying a mix of features traditionally associated with either crocodyloids or alligatoroids, contributing to the uncertainties surrounding their classification. For example, several forms in this clade, such as Orientalosuchus, Dongnanosuchus, Krabisuchus and Eurycephalosuchus, possess a notch situated at the contact between the premaxilla and the maxilla. This notch, which receives the enlarged fourth dentary tooth, is typically associated with modern crocodyloids and not seen in alligators. The exact shape and size of the notch can vary and is described as more of a fossa or pit in Eoalligator, while in Dongnanosuchus the notch itself contains a pit in its roof. In both cases this causes the fourth dentary to be at least partially obscured when the jaws were closed rather than fully exposed. In Krabisuchus the notch is fully formed, but it has been speculated that the fourth dentary tooth might have slid into a medial pit regardless.
This resemblance to crocodyloids is contrasted by the heterodont dentition of several forms which features globular teeth in the back of the jaw, a trait that is otherwise regarded as characteristic for alligatoroids within the clade Globidonta. Orientalosuchus for example had pointed anterior teeth that eventually transition into blunt, conical cheek teeth, though they are not as globular as in animals like Hassiacosuchus. Blunt to bulbous posterior teeth are also known from Krabisuchus and Protoalligator and those of Jiangxisuchus are described as having had a constricted base and bean-shaped crowns.