User:Armin Reindl/sandbox3

Armin Reindl/sandbox3 Temporal range: Eocene
Different skulls of Orientalosuchus naduongensis in dorsal (upper line) and ventral (lower line) views
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauromorpha
Clade:	Archosauriformes
Order:	Crocodilia
Superfamily:	Alligatoroidea
Clade:	Globidonta
Clade:	†Orientalosuchina
Genus:	†Orientalosuchus Massonne et al., 2019
Type species
†Orientalosuchus naduongensis Massonne et al., 2019

Orientalosuchus is an extinct genus of alligatoroid crocodilian from the Late Eocene that was found in the Na Duong Formation in Vietnam.

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

notch ridge between nasal and max acute frontal extends between nasals supratemporals small supraoccipital largely exposed, baring parietal from posterior postition foramen aerum of quadrate on the dorsomedial angle of the bone

suborbitals large, reach 7th to 8th max palatine max suture obtuse angle and not extended beyond suborbitals palatine pterygoid between suborbitals pterygoid neck around choanae

mand. fenestra small foramen aerum lingual margin of the retroarticular process

5th max largest 16 dentary teeth post laterally compressed

Postcrania

axis hypapophysis near central centrum large glenoid of coracoid iliac blade post. rectangle, indented dorsally

no to modest ridge on osteoderms

Phylogeny

When Orientalosuchus was named in the 2019 study by Massonne et al., the team also examined some other extinct alligatoroid taxa from Asia. Phylogenetic analysis at the time found that they were all closely related and together formed a monophyletic clade the team dubbed Orientalosuchina, which they recovered at the base of Alligatoroidea. Orientalosuchina, as defined in 2019, included Orientalosuchus itself, Eoalligator (previously proposed to be a synonym of Asiatosuchus nanlingensis), Protoalligator, Krabisuchus and Jiangxisuchus. Initially Orientalosuchus was regarded as the sister taxon to Krabisuchus, with the other three positined in a polytomy. The monophyly of Orientalosuchina would see some support by future studies, especially the descriptions of Dongnanosuchus and Eurycephalosuchus, both of which replicate the same placement within Crocodilia and composition as the study by Massonne and colleagues, sans the obvious inclusion of the new forms. For instance in the description of Eurycephalosuchus, Wu and colleagues find Krabisuchus and Protoalligator in basal positions, Orientalosuchus in a polytomy with Dongnanosuchus and Eurycephalosuchus and finally the clade composed of Eoalligator and Jiangxisuchus.

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

However not all studies agree with the placement of Orientalosuchina at the base of Alligatoroidea. While Massonne and colleagues have argued that certain features that could be used to argue for crocodyloid-affinities are the result of the basally branching position of Orientalosuchina, others have instead argued that they are indeed evidence that the clade should be placed closer to crocodyloids than to alligatoroids. Nils Chabrol et al. 2024 managed to recover both hypothesis, with Orientalosuchina composed of Orientalosuchus, Krabisuchus, Eurycephalosuchus and Dongnanosuchus, but lacking the remaining members included by Massonne and colleagues. The results that recovered the clade closer to Crocodyloids, specifically as a basal branch of Longirostres outside of the crocodyloid-gavialoid split, places Orientalosuchus as the sister taxon to Dongnanosuchus. The other alternative, with orientalosuchins remaining alligatoroids as originally envisioned, sees the same sister taxon relationship, but with Krabisuchus as their next closest relative and Eurycephalosuchus as the basalmost member (in the Longirostres interpretation these later two are each others sister taxa).

Longirostres †Eurycephalosuchus gannanensis †Krabisuchus siamogallicus †Orientalosuchus naduongenis †Dongnanosuchus hsui Crocodyloidea Gavialoidea

†Qianshanosuchus †Asiatosuchus depressifrons †Asiatosuchus germanicus †Crocodylus affinis †Planocraniidae Alligatoroidea †Eurycephalosuchus gannanensis †Krabisuchus siamogallicus †Orientalosuchus naduongenis †Dongnanosuchus hsui other alligatoroids

Another study recovering Orientalosuchina closely to Longirostres was that of Jorgo Ristevksi and colleagues. In their 2023 revision of Australasian crocodylomorphs, two out of their eight phylogenetic trees recover orientalosuchins as being deeply nested within Mekosuchinae between the large-bodied forms like Baru and the smaller dwarf forms like Trilophosuchus. This represents yet another notably different internal topology, retaining the close relationship between Jiangxisuchus and Eoalligator found by some with Orientalosuchus as their immediate sister taxon, Krabisuchus more basal and Dongnanosuchus closer to dwarf mekosuchines than any of the other traditional orientalosuchins. However the remaining six analysis of the same study all recover more traditional results with Orientalosuchina absent from Mekosuchinae and support is generally regarded as weak, although worth of being researched further.

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

Paleoenvironment

The the Na Duong Formation of Vietnam is generally regarded as Eocene in age, corresponding to the Bartonian to Priabonian, though some studies have also suggested a sligthly younger Late Eocene to Oligocene age. The formation preserves a tropical to warm-subtropical swamp biome that across its deposition featured a variety of aquatic biomes ranging from brooks and rivulets to an evironment dominated by shallow ponds to even an annoxic lake. The main coal seam is surrounded by pond deposits, but itself represents an annoxic lake, with the transition from the former to the latter representing the primary fossil-bearing layer that also preserves the remains of Orientalosuchus. The annoxic lake would have recieved freshwater from rivers that brought with them sediments, leaves and large logs of driftwood. The terrestrial environment would have been represented by a waterlogged swamp forest. The main coal seam preserve abundant fossilized tree trunks, some of which measuring 1 m (3 ft 3 in) across, as well as the stems of royal ferns (Osmundaceae). The diameter of the trunks, root system and the distance between the stumps (standing generally 3–5 m (9.8–16.4 ft) apart), suggest a density of about 600 trees per hectare, comparable to the peat swamp forests of modern Sumatra and Kalimantan. The Na Duong swamp forest would have also resembled today's peat swamp forests of Indonesia in the height of the forest canopy, estimated at about 35 m (115 ft) in height.

In addition to the fossilized tree stumps, the upper parts of the seam also preserve what have been interpreted as deltaic beds containing the leaves of angiosperms and fragments of ferns, with the former preserving a toothed margin that is more characteristic of temperate climates. However, this can be explained with the environment of the formation, with wet conditions favoring leaves with toothed margins and the flora is indeed indicative of a high water table. Plant fossils include leaves similar to those of the legume Bauhinia (which includes several species of lianas) and resin collected is similar to that of Shorea rubriflora, indicating the presence of Dipterocarpaceae. Aquatic plants have been identified from slightly older layers of the Na Duong Formation and include lotus, which would have formed meadows at the same time that shallow ponds were present. Overall the flora superficially resembles that of Lower Oligocene Haselbach, Germany.

The mammal fauna broadly reflects this environmental reconstruction. The basal rhino Epiaceratherium naduongense was an entirely terrestrial animal, but has been interpreted as an obligate browser who's long and slender bones and tapir-like hands fit it being a forest dweller. The forests are also supported by the presence of an early primate, Anthradapis. Other mammals from the formation, the anthracotheres Bakalovia orientalis, Anthracokeryx naduongensis and the two species of Bothriogenys are a better match for the often submerged forests and annoxic lake of the Na Duong Basin, given that anthracotheres are often interpreted as semi-aquatic animals similar to modern water buffalos or hippopotamus, though the latter interpretation has been questioned. Fish fossils are common at Na Duong as makes sense given the depositional environment, but are generally disarticulated or isolated. Böhme and colleagues list two families as being present, bowfins and cyprinids, the former primarily found in the same layer as Orientalosuchus and the other more widespread across the lacustrine sediments, though only one is known to co-occur with Orientalosuchus. While the exact bowfin species cannot be determined, fossil remains indicate that they reached a maximum body length of 50 cm (20 in). The carp relative can be narrowed down as far as Barbinae.

Paleoecology

Massonne and colleagues have noted certain similarities in the faunal compositions of various East Asian fossil localities during the Eocene, namely Wai-Lek of the Krabi Province of Thailand, China's Maoming Basin and Vietnam's Na Duong Basin where Orientalosuchus was found. All three localities feature longirostrine species of the genus Maomingosuchus, a type of early gavialoid. The Krabi species remains unnamed, the Maoming Basin was home to Maomingosuchus petrolica and Na Duong yielded the remains of M. acutirostris. All three localities also share the presence of orientalosuchins, Krabisuchus, Dongnanosuchus and Orientalosuchus respectively and similar parallels exist in other reptile groups, specifically pan-geoemydid turtles. In addition to the widespread Maomingosuchus, Böhme and colleagues furthermore mention the presence of a generalist longirostrine taxa similar in appearance to today's Crocodylus or the extinct Asiatosuchus which may have obtained lengths of up to 6 m (20 ft) and possessing heterodont dentition not dissimilar to that of Orientalosuchus. Naturally, the differences in size and skull shape indicate that these animals would have occupied different niches. While Maomingosuchus acutirostris is thought to have been more piscivorous on account of its elongated snout, Orientalosuchus with its broader skull may have been much more of a generalist. At the same time, the robust back teeth of Orientalosuchus may represent adaptations to preying on hard-shelled or armoured prey, such as the local turtles. This finds some support in the clear presence of crocodilian bite marks on fossil carapace recovered from Na Duong. Bite marks are also found on the limb bones of mammals and even some crocodilian skulls, though those were more likely created by the large Crocodylus-like taxon rather than Orientalosuchus.

Coprolites and footprints

In a 2022 short communication, Kazim Halaclar and colleagues report numberous pieces of fossilized feces, better known as coprolites, from the Na Duong Formation. Particular focus of this report was a crocodilian coprolite bearing two prominent impressions. Analysis of the overall morphology and composition of the coprolite itself confirms it stems from a carnivorous animal and the limited amount of bone fragments suggests that the producer had highly effective stomach acid. Among the fauna known from Na Duong in 2022, this description best suits the local crocodilians. Aspects of the preservation, such as the lack of deformation, can also be explained by the originator having been a semi-aquatic animal, another point in favor of the crocodilian hypothesis, before being rapidly buried by sediment, which can be explained through being positioned at the edge of a river or even seasonal flooding.The impressions are regarded as being too slender to have come from the feet of a squamate, but would be a good fit for the fourth and fifth fingers of a crocodilian, especially given the absence of claw marks of webbing. This also suits the hypothesis that the producer of the coprolite and the track maker were the same animal or at least of the same species. Comparing print size with total body length in modern crocodilians suggests that the track macker was likely around 2 meters long. While this matches the estimated body size of Orientalosuchus, it could have also been made by any of the other Na Duong crocodilians.