Orientalosuchina

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. 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. 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==

Orientalosuchins were small crocodilians with short snouts, Even less has been stated on the size of Eoalligator and Protoalligator, with the former simply being stated to have been "median-sized" by Wu and colleagues in 2018 and "of moderate size" by Young in 1964. The latter is depicted as being approximately long in the disseration of Tobias Massonne. The size of Krabisuchus is also kept rather vague in its type description, with Martin and Lauprasert simply stating that it likely didn't grow larger than . However, two sources provide additional information on the potential size of Krabisuchus. In the 2022 description of Eurycephalosuchus, the Chinese taxon is stated to possibly represent the smallest orientalosuchin and that its skull length is only slightly shorter than that of Krabisuchus. The same study also compares Eurycephalosuchus to Eoalligator, again noting that the former was smaller than the latter when comparing the width of the occiput. Secondly, the introduction to Tobias Massonne's disseration also provides rough estimates for Krabisuchus, suggesting that the animal may have reached a total body length of only . This in turn suggests Eurycephalosuchus would have been even smaller. ==Phylogeny==

External relationships The precise relationship between orientalosuchins and other crocodilians is a disputed subject with several different interpretations of how they would fit into the group. Traditionally, orientalosuchins have been interpreted as part of the Alligatoroidea clade, most commonly thought to consist of some basal taxa like Deinosuchus and Diplocynodon as well as the derived alligatorids, which are split into Alligatorinae and Caimaninae. Shown below are the phylogenetic results of Shan et al. (2021) The results of Chabrol and colleagues in part reflect previous results concerning Jiangxisuchus. While most orientalosuchins had been regarded as alligatoroids prior to the clade being named, Jiangxisuchus was initially described as a crocodyloid, Eoalligator, though initially described as an alligatoroid, has also been frequently recovered as a crocodyloid. This includes as a synonym of Asiatosuchus nanlingensis in 2016, Internal relationships When Orientalosuchina was described in 2019 it was specifically defined based on the inclusion of five species: Orientalosuchus naduongensis, Krabisuchus siamogallicus, Eoalligator chunyii, Jiangxisuchus nankangensis and Protoalligator huiningensis. However, the internal topology of this clade was initially poorly resolved, with most of these species being placed in a large polytomy and only Orientalosuchus and Krabisuchus being found to have a closer relationship to each other than to other orientalosuchins. Additional research in subsequent years also saw the description and inclusion of Dongnanosuchus hsui and Eurycephalosuchus gannanensis within Orientalosuchina. In both instances, the authors maintained the monophyly of the group, continuing to include all five of Massonne's defining species within the clade, but with a better resolved internal relationships. Both studies place Krabisuchus as the earliest diverging orientalosuchin, followed by Protoalligator as the next basalmost genus within the clade. In both studies Jiangxisuchus and Eoalligator are sister taxa. The close relationship between the two animals from Cretaceous China has previously also been found by Li and colleagues in 2019, though in said case they were regarded as crocodyloids. Orientalosuchus, Dongnanosuchus and Eurycephalosuchus are all part of a polytomy that further includes the Jiangxisuchus - Eoalligator clade following the results of Wu et al. (2022). Walter and colleagues present another alternative, finding Protoalligator as the basalmost genus in place of Krabisuchus, lending further support for the Jiangxisuchus - Eoalligator clade and recovering Dongnanosuchus as the sister taxon to Krabisuchus and Orientalosuchus. While originally defined around the presence of five specific taxa, some later studies could only recover significantly less inclusive versions of the clade. In the study that recovered Orientalosuchina within Mekosuchinae, the core monophyletic group is only formed by Orientalosuchus, Krabisuchus, Jiangxisuchus and Eoalligator, with the latter two once again each other's closest relatives. Protoalligator was recovered as part of a polytomy at the base of Crocodilia and though Dongnanosuchus was still closely allied with orientalosuchins in the study, it was found to be phylogenetically closer to small-bodied mekosuchines, splitting off after the divergence of the other traditional forms. Chabrol and colleagues were likewise unable to place all members originally used in the definition of Orientalosuchina within a monophyletic group. Both in the scenario that orientalosuchins were alligatoroids and the scenario that they were closer to Longirostres, the group was found to consist of only Orientalosuchus, Dongnanosuchus, Krabisuchus and Eurycephalosuchus. In both scenarios Dongnanosuchus and Orientalosuchus were found to be sister taxa, with the key difference being whether Krabisuchus and Eurycephalosuchus formed their own clade or were successively branching species of the clade. Ristevski et al. (2023) Chabrol et al. (2024) (implied weighting) }} Chabrol et al. (2024) (equal weighting) ==Origins and dispersal==

One aspect of Orientalosuchina that has remained a matter of discussion is their evolutionary origin, especially in regards to their distribution when regarded as alligatoroids. All members of this clade are exclusively known from Southeast and East Asia, with especially their early evolution centered around the eastern part of China. This stands in stark contrast to the fact that alligatoroids are a primarily American clade that rarely ventured beyond that continent, with the few exceptions including the lineage leading up to today's Chinese alligator (Alligator sinensis) which according to Massonne and colleagues would have furthermore profited from the lower eustatic sea levels of the Maastrichtian. Alternatively to the dispersal via Beringia, it has been proposed that orientalosuchins could have plausibly arrived in Asia not directly from America but through Europe, justified through the presence of the putative stem-alligatoroids like Diplocynodon in Paleogene Europe. However, this hypothesis was proposed specifically to explain the distribution of some Paleogene forms and predates the concept of Orientalosuchina, meaning it does not consider subsequent Cretaceous records. Both routes suggest a terrestrial dispersal for Orientalosuchina, which ties into the fact that unlike crocodyloids and gavialoids, alligatoroids are presumed to have lost their salt glands early in their evolution, greatly restricting their ability to traverse saltwater. ==Paleoecology==

Diet Relatively little has been published on the ecology of orientalosuchins, with only occasional speculation having been discussed throughout their research history. At least in terms of diet, evidence seems to suggest that orientalosuchins may have been more generalist predators. Lifestyle Though generally found in deposits prominently featuring aquatic environments like ponds, swamps and lakes, Paleoenvironment Given that orientalosuchins spanned from the latest Cretaceous all the way into the late Eocene, members of this group existed in a wide range of environmental contexts. The oldest forms appeared in China at a time when dinosaurs were still the dominant megafauna, represented by groups such as tyrannosaurids, hadrosaurs, titanosaurs and a wide variety of oviraptorosaurs. All Cretaceous forms come from strata which are at times grouped together as the Nanxiong Formation of Nanxiong Group, There is direct evidence for the survival of Eoalligator across the KPG-boundary thanks to its continued presence into the lower Paleocene sediments of the Shanghu Formation. Protoalligator is known from the same time period, its fossils having been found in the similarly aged Whangudun Formation. By the Eocene orientalosuchins had established multiple geographically separated taxa spread across Southeast Asia. The formations that yielded the Eocene genera, namely the Na Duong Formation of Vietnam, the Krabi Basin of Thailand and the Youganwo Formation of China, are all described as tropical to warm-subtropical in their climate with extensive closed forests biomes likened to those still present in parts of Asia today. to ponds, lakes, rivers and possibly brooks, During this time the mammal fauna of Asia had changed significantly from that of the Paleocene, now also featuring early primates, various types of artiodactyls in addition to the anthracotheres such as mouse deer, predatory miacids, colugos and perissodactyls such as basal rhinos. ==References==