User:Armin Reindl/sandbox3

Armin Reindl/sandbox3 Temporal range: Late Cretaceous: Maastrichtian,
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauromorpha
Clade:	Archosauriformes
Order:	Crocodilia
Superfamily:	Crocodyloidea
Genus:	†Jiangxisuchus Li et al., 2019
Species
†J. nankangensis Li et al., 2019 (type)

Jiangxisuchus is an extinct genus of crocodylian that lived during the Late Cretaceous in what is now China. It was described in 2019, and was proposed to be a basal member of Crocodyloidea. However, another concurrent 2019 study recovered Jiangxisuchus instead as a basal member of Alligatoroidea, within the newly named clade Orientalosuchina.

History and naming

Jiangxisuchus was described in 2019 by Chun Li, Xiao-Chun Wu and Scott James Rufolo as the first crocodilian from the Cretaceous Nanxiong Formation of Nankang, China. However, some other studies have proposed alternative stratigraphy for the Nanxiong Formation, suggesting the overarching name Guifeng Group to include the Hekou Formation, Tangbian Formation, and Lianhe Formation. Following this scheme, Jiangxisuchus would have been from the Hekou Formation. The type and only known specimen of the animal, consisting of the skull, parts of the lower jaw, some partial vertebrae, ribs and osteoderms, is held at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. Though it was initially interpreted as an early-diverging crocodyloid, a study by Tobias Massonne and colleagues published that same year instead argued for it to be a member of the newly named clade Orientalosuchina.

The name Jiangxisuchus derives from the Jiangxi Province of eastern China where Nankang is located, combined with the Greek suffix -suchus which translates to crocodile. The species name is also derived from the place of origin, specifically referencing the Nankang District.

Description

Like other orientalosuchins Jiangxisuchus possessed a short and blunt snout, which was initially likened to that of Stangerochampsa. Like in other orientalosuchins and modern crocodiles, a prominent notch is present where the premaxilla, which form the tip of the snout, meet the maxilla, which contains the majority of the toothrow of the upper jaw. As in modern crocodiles, this notch functions primarily to receive the enlarged fourth tooth of the dentary when the jaws are closed, separating the premaxillary from the maxillary teeth. This feature is among the characteristics that complicate the phylogenetic placement of orientalosuchins amongst crocodilians, as such a notch is generally thought to be a feature not seen in alligatoroids. This notch also contributes to the pronounced festooning of the jaw, the sinuous appearance of the dental margin created by convex and concave regions across the premaxilla and maxilla. While this festooning is not very visible when looking at the maxilla from above, a sideview reveals two bulging regions of the maxilla that peak with the fifth and eleventh tooth respectively.

The external nares are described as oval and face anterodorsally (meaning they open upwards and towards the front) with a slightly elevated rim. This opening is almost entirely surrounded by the premaxillae except for a small section at the back where the strap-like nasal bones insert themselves between the two halves to make their own contribution to the edge of the nares, even entering the opening slightly. Instead of joining one-another, each premaxilla forms a so-called premaxillary process that runs alongside the outer edges of the nasals until the level of the third maxillary tooth. The nasals stretch across the entire snout back of the nares until they meet the frontal bone, which separates them and causes them to diverge into two pointed processes that are wedged between the frontal and the rhomboid prefrontals to either side. The maxillae form the outer edges of the snout lateral to the nasals, maintaining a fairly consistent width from their contact with the premaxillae all the way back to where they contact the lacrimals. There each maxilla forms a small process that briefly slides into the space between lacrimal and nasal while on the edge of the skull they narrow significantly until they contact the jugal.

The eyesockets of Jiangxisuchus were large, larger than any other of the skull openings, and separated by the singular frontal bone. This bone can broadly be divided into the pointed anterior process that splits the nasals and a posterior region that contributes to the skull table, effectively briding the regions before and behind the eyes. Overall the surface of the frontal is described as flat, meaning that the edge of the eyesockets are not raised as is seen in some other crocodilians. The skull table is a raised section of the skull located just behind the eyes and in addition to the frontal typically consists of the paired postorbitals, squamosals and a single parietal bone. Two additional hallmarks of the skull table are the supratemporal fossae, two openings fairly central on the structure separated by the parietal. The postorbitals form the front corners of the skull table and form the upper part of the postorbital bar, a peg-like bone that separates the eyesocket and the infratemporal fenestra.

Looking at the skull from below reveals that the incisive foramen, a hole close to the tip of the snout, is located noticeably further back from the tip than the external nares on the other side of the skull.

Jiangxisuchus had large eyes and oval external nares.

The region between the two fenestra is described as narrow, only about half the width of the space that separates the eyesockets. The fossa are circular

maxilla does not contribute to incisive foramen the palatines are shorter than the suborbital fenestra waist of the ectopterygoid broader than the narrowest part of the palatines between the fenestrae choana divided by septum

supraoccipital exposed

squamosal and quadrate enter dorsal edge of the triangular infratemporal

splenial not participating in symphysis

third premax the largest, fifth maxillary largest first two confluent diastema between premax and max, seventh and eight max as well as ninth-tenth dentary

Size

Jiangxisuchus is described as small to medium sized, something that appears to have been a trend among orientalosuchins.

Phylogeny

The classification of Jiangxisuchus has changed repeatedly since its description, with studies disagreeing over whether or not Jiangxisuchus is a crocodyloid or an alligatoroid and its precise relationship with orientalosuchins. The type description in 2019 grouped alligatoroids and crocodyloids (including tomistomines, which are now more commonly regarded as gavialoids) in the clade Brevirostres and found Jiangxisuchus in a basal position of the crocodyloid branch. In this position, the animal was furthermore recovered as the sister taxon to the early Paleocene Eoalligator chunyii. These results do reflect the beginnings of Orientalosuchina being recognized as a phylogenetic grouping, as they not only reject Eoalligator being synonymized with Asiatosuchus nanlingensis (as proposed by Yan-Yin Wang and colleagues in 2016) but further establish ties between it and Jiangxisuchus.

Orientalosuchina would come to be recognized later that same year with the description of Orientalosuchus by Tobias Massonne and colleagues. This study not only recognized the link between Eoalligator and Jiangxisuchus as was established by Li et al. but furthermore found them to clade with the then newly named Orientalosuchus, Protoalligator (named three years prior by Wang et al.) and Krabisuchus from Thailand. This grouping was at first only partially resolved, but clarified across future studies with the description of additional orientalosuchins such as Dongnanosuchus and Eurycephalosuchus. By the description of the latter relationships had shifted slightly, with Krabisuchus and Protoalligator in basal positions, Orientalosuchus, Dongnanosuchus and Eurycephalosuchus in a polytomy and finally Jiangxisuchus and Eoalligator still as sister taxa. A notable difference between the initial interpretation of Li and colleagues and the phylogenetic trees that recover a monophyletic Orientalosuchina concerns the wider relations of the group among crocodilians. While the Eoalligator-Jiangxisuchus relationship is consistently retained, these later studies generally find members of Orientalosuchina to be basal alligatoroids, not crocodyloids as was hypothesized by Li and colleagues. Both the results of Li et al. 2019 as well as those of Wu et al. 2022 are shown below.

Crocodilia Gavialoidea †Borealosuchus †Planocraniidae Brevirostres Alligatoroidea Crocodyloidea †"Asiatosuchus" germanicus †Albertosuchus †Prodiplocynodon †Eoalligator †Jiangxisuchus †Crocodylus depressifrons †Crocodylus affinis †Asiatosuchus grangeri †Brachyuranochampsa †Crocodylus acer Tomistominae Crocodylinae

Alligatoroidea †Leidyosuchus †Deinosuchus †Diplocynodon Globidonta †Orientalosuchina †Krabisuchus siamogallicus †Protoalligator huiningensis †Orientalosuchus naduongenis †Dongnanosuchus hsui †Eurycephalosuchus gannanensis †Jiangxisuchus nankangensis †Eoalligator chunyii Alligatoridae

However while multiple papers have recovered Jiangxisuchus as a member of a monophyletic Orientalosuchina at the base of alligatoroidea, this view is not uncontested. In addition to the type description, multiple subsequent studies have also come out in favour of Jiangxisuchus being an early crocodyloid and possibly entirely unrelated to Orientalosuchina. One example for this a study published by Jonathan P. Rio and Phillip D. Manion in 2021, in which Jiangxisuchus is an early branching crocodyloid diverging just before the split between Crocodylidae and Mekosuchinae. The two authors do however note that their study did not extensively sample other reported members of Orientalosuchina. The same does not apply to the 2024 osteology of Crocodylus palaeindicus, penned by Nils Chabrol and colleagues. In this work, orientalosuchines were given much more consideration and in fact did form a monophyletic group, however to the exclusion of Jiangxisuchus. Regardless of the precise plament of Orientalosuchina (once as basal longirostrans and once as early alligatoroids), Jiangxisuchus was recovered as being closer to Longirostres (crocodiles and gharials) than alligatoroids in both instances.

†Planocraniidae †Asiatosuchus germanicus †Asiatosuchus depressifrons †Crocodylus affinis †Qianshanosuchus †Jiangxisuchus †Quinkana †Australosuchus †Orientalosuchina Gavialoidea Crocodyloidea

†Asiatosuchus germanicus †Crocodylus affinis †Asiatosuchus depressifrons †Qianshanosuchus †Planocraniidae †Orientalosuchina other alligatoroids †Australosuchus †Jiangxisuchus †Prodiplocynodon Gavialoidea Crocodyloidea

An intermediate between the crocodyloid interpretation and the Orientalosuchina classification has been recovered by Jorgo Ristevski and colleagues in 2023, though it simultaneously added yet another competing topology into the mix. In this study, two phylogenetic trees found orientalosuchins to be a group within the Australian clade Mekosuchinae, located between the large-bodied representatives of the group such as Baru and Paludirex and the smaller dwarf forms like Mekosuchus. These results bear close internal resemblance to the topology also seen in Wu et al. 2022, with Krabisuchus at the base and Jiangxisuchus once again assuming its position as the sister taxon to Eoalligator. The main divergence regard the positions of Dongnanosuchus, which was found closer to the small-bodied mekosuchines, and the absence of Protoalligator (which was recovered either in a basal crocodilian polytomy or outside of Crocodylia entirely). So while this matches the overall idea of Orientalosuchina as a group of Cretaceous to early Paleogene crocodilians as established by Massonne and colleagues, it simultaneously does not find the alligatoroid affinities and instead places them within a group that is closer to Longirostres.

However this view is likewise not without caveats and the support for Orientalosuchina being placed within Mekosuchinae remains weak. As highlighted in the same study, few elements truly unite the two groups and many of them are either present in only some of the members of Orientalosuchina and Mekosuchinae respectively or not limited to these groups and in fact also seen in a variety of other crocodilians.

Crocodilia

Alligatoroidea †Planocraniidae †Prodiplocynodon †"Crocodylus" affinis †"Crocodylus" depressifrons †"Crocodylus" acer †Brachyuranochampsa Longirostres Gavialoidea Crocodyloidea †Asiatosuchus germanicus Mekosuchinae †Kalthifrons †Quinkana meboldi †Quinkana fortirostrum †Quinkana timara †Paludirex gracilis †Paludirex vincenti †Baru wickeni †Baru darrowi Orientalosuchina †Krabisuchus †Orientalosuchus †Jiangxisuchus †Eoalligator †Dongnanosuchus †"Baru" huberi †Volia †Trilophosuchus †Mekosuchus sanderi †Mekosuchus whitehunterensis †Mekosuchus inexpectatus

Paleobiology

The Nanxiong Formation is well known for its preservation of Late Cretaceous dinosaurs, but the statigraphy of the region is poorly understood with several differing interpretations. Li and colleagues use the name Nanxiong Formation for sediments of the Ganzhou basin, which other authors refer to as the Guifeng Group, itself divided into the Hekou, Tangbian and Lianhe Formation. Studies using the latter terminology have assigned fossil remains traditionally stated to be from the Nanxiong Formation of Ganzhou as stemming from the Hekou Formation, which would include those of Jiangxisuchus. The age of the Hekou Formation is typically accepted to be Maastrichtian, though some studies have considered a Coniacian to Santonian age as well. The Hekou Formation may have been roughly contemporary with the Dafeng Formation of the Nanxiong Group (located in the nearby Nanxiong Basin).

The Hekou Formation is marked by the influence of rivers and is thought to have been deposited in a subhumid environment in the proximal parts of an alluvial fan. Streams would have spread out beyond the fan, stretching across a floodplain.

Though theropods are abundant in numbers, they are comparably low in clade-diversity. Two tyrannosaurs have been described from the Nanxiong Formation, the small but deep-snouted Asiatyrannus and the long-snouted Qianzhousaurus, which at twice the size of the former likely filled the nische of apex predator. Most of the remaining theropod fauna consists of oviraptorosaurs, with more than seven species having been reported, including Banji, Tongtianlong, Ganzhousaurus, Jiangxisaurus, Huanansaurus, Nankangia and Corythoraptor, though given the poor understanding of the formations stratigraphy they may not have necessarily coexisted. The native herbivore fauna would have included the sauropods Gannansaurus and Jiangxititan as well as hadrosaurs. The small fauna featured squamates like Conicodontosaurus, Chianghsia and Tianyusaurus, the turtles Jiangxichelys and Nanhsiungchelys as well as the mammal Erythrobaatar.