Varanosuchus sakonnakhonensis Pochat-Cottilloux & Lauprasert & Chanthasit & Manitkoon & Adrien & Lachambre & Amiot & Martin 2024, gen. nov., sp. nov.
Creators: name: Pochat-Cottilloux, Yohan
name: Lauprasert, Komsorn
name: Chanthasit, Phornphen
name: Manitkoon, Sita
name: Adrien, Jérôme
name: Lachambre, Joël
name: Amiot, Romain
name: Martin, Jeremy E.
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Atoposauridae
Varanosuchus
Varanosuchus sakonnakhonensis

description: Varanosuchus sakonnakhonensis gen. nov., sp. nov.
(Figs 2–15)
Zoobank registration: urn:lsid:zoobank.org:act: D05B36B9- FC50-4011-9478-A70123B33CD5
Derivation of name: After the province of Sakon Nakhon, Thailand, where the holotype and referred specimens were found.
Holotype: SM-2021-1-97/101, a three-dimensionally preserved, almost complete skull, lacking its anteriormost part (anterior part of premaxilla and nasal), with associated mostly complete postcranial skeleton, involving the axial column and osteoderms. The specimen lacks most of the pectoral girdle (except for the coracoid) and all the forelimb elements. The pubis, tibia, fibula, and digit are the only elements known from the pelvic girdle and the hindlimbs.
Type locality: Phu Sung locality, near Mueang Sakon Nakhon, Sakon Nakhon district, Thailand.
Stratigraphic horizon and range: The geological strata from which the new taxon originates belong to the Early Cretaceous Sao Khua Formation (Khorat Group) of Thailand (Chanthasit et al. 2019, Ditbanjong et al. 2019).
Referred specimens: SM-2023-1-16, a three-dimensionally preserved nearly complete skull. SM-2023-1-17, a three-dimensionally preserved partial skull table. Both specimens were found in the same strata as the holotype.
Diagnosis: a crocodylomorph characterized by the following unique combination of features: the dorsal part of the postorbital has an anterolaterally facing edge; the quadrate has no fenestrae; there are two waves of enlarged maxillary teeth; the quadratojugal has no ornamentation; the outer surface of the squamosal is dorsolaterally oriented, reduced, and sculpted; and there is a depression on the posterolateral surface of the maxilla.
Description
Cranial openings
The sutures between the parietal, the supraoccipital and the otoccipital are thin anteroposteriorly, but there is no clear opening in this region (i.e. no ‘true’ posttemporal fenestrae). The internal choanae are contained by the pterygoids (and maybe the palatines in SM- 2023-1-16) and are situated immediately anterior to the contact with the palatine, anteriorly. No septum can be seen. The orbit is large, about half the length of the skull table. It is D-shaped in dorsal and lateral views. This opening is surrounded by the jugal, the maxilla, the lacrimal, the frontal, the prefrontal, and the postorbital. The margins are not upturned.
As in all diapsids, the skull possesses two pairs of temporal fenestrae: the supratemporal and infratemporal ones. The supratemporal fenestra is ovoid, with its longest axis directed anteroposteriorly and almost the same length as the parietal. The supratemporal fossa has steep vertical walls in SM-2021-1- 97/101. The frontoparietal fossa is not apparent in SM-2021- 1-97/101 (owing to vertical walls) and not very wide, quickly disappearing ventrally in SM- 2023-1-16 and SM- 2023-1-17. The infratemporal fenestra is preserved on the left side in the two most complete specimens: it is not complete but appears triangular; it is bordered by the jugal laterally, the postorbital anteriorly, and the quadratojugal posteromedially. No spikelike projection inside the fenestra can be seen. In SM- 2023-1- 16, the antorbital fenestra is present near the triple junction between the maxilla, the lacrimal, and the jugal in lateral view (Figs 11D, 13D).
The cranioquadrate canal appears open as a groove in lateral view, immediately ventral to the squamosal and posterior to the tympanic region. It is formed solely of the otoccipital in SM-2021-1-97/101 and probably also in SM- 2023-1-16, although it is difficult to assess because of the poor state of preservation.
The foramen magnum is triangular in shape and ~ 1 cm at its widest part.
Cranium (Figs 2–4, 11–14)
The specimens are rather short-snouted; as a result, the rostrum makes up about half the size of the whole skull. The nasal is long, reaching the anterior limit of the skull. The braincase occupies the posterior one-third of the skull. The cranial table is ornamented with circular ovoid pits dorsally.
Premasilla (Figs 2A, C–E, 4A–E, G, 11A–E, G, 12A, 13A–D): The premaxilla, together with the anterior part of the nasal, forms the anterior part of the snout. It is straight posteriorly, forming an oblique suture with the maxilla in lateral view. In dorsal view, the opening for the nares is mostly composed of the premaxilla, with participation of the nasal in its posterior part. On the ventral surface, the foramen incisivum is a single opening; it is unique and completely enclosed by the premaxillae. Although it is damaged, this opening is cylindrical, with the longest axis directed anteroposteriorly, but it does not abut the premaxillary toothrow (Supporting Information, Model S3). The contact between the two bones is straight anteroposteriorly. The contact between the maxilla and the premaxilla is straight mediolaterally in ventral view, oblique in lateral view, and straight mediolaterally in dorsal view, with two posteriorly projecting processes of the premaxilla in SM-2021-1-97: one in the maxilla and one medially at the contact with the nasal. Medially in dorsal view and posteriorly to the nares, the two premaxillae do not contact because they are separated by the nasals. At the contact with the maxilla, the posterior part of the premaxilla bears a notch to accommodate the corresponding large dentary tooth; this is especially visible in right lateral view. Some pits are present in the lateral surface. Each premaxilla contains at least five teeth (although none is preserved). The preserved alveoli are as large as most of the maxillary alveoli and separated, with the third and fourth being the largest ones in SM- 2023-1-16. The alveoli are circular, and the premaxillary toothrow is curved anteriorly.
Masilla (Figs 2A, C–E, 4A–E, G, 11A–E, G, 12A, 13A–D): The two maxillae contact ventrally in an anteroposteriorly straight suture, whereas they are separated dorsally by the nasals, also in an anteroposteriorly straight suture. The triple junction between the maxilla, the premaxilla, and the nasal is situated at the level of the first maxillary alveolus. The contact with the jugal is curved anteriorly to oblique in lateral view. Medially, there is a suture with the ectopterygoid, which is also curved anteriorly. In lateral view, a depression for the insertion of a dentary tooth can be seen at the level of the sixth–seventh maxillary alveoli, where the maxilla curves medially. Dorsally, the maxilla has two planes: one directed dorsomedially to ventrolaterally, and one oriented anteroposteriorly and flat dorsoventrally: the specimens have an altirostral skull. The contact between these two planes is directed posteromedially until the most developed caniniform maxillary tooth (fourth alveolus in SM-2021-1-97) or the very marked lateral bulge of the maxilla (sixth alveolus on SM- 2023-1-16), then it seems to widen again laterally. The lateral margin of the maxilla forms two convex waves, with the maximum curvature at the level of the fourth and penultimate maxillary alveoli. Ventrally, the contact with the palatine is anteriorly convex, and the maxilla constitutes the anterior and anterolateral margin of the suborbital fenestra. The ventral surface is smooth. In SM-2021-1-97/99, there are at least five anterior maxillary teeth that are preserved, with the largest one being the fourth. The fifth alveolus is smaller than the third or the fourth one. All those alveoli are circular in cross-section. From the better preserved parts of SM- 2023-1-16, it can be assessed that the maxilla contains at least 10 teeth. The largest anterior alveoli, although separated, are the third and the fourth ones, but the fifth one is almost the same size as the third one. A diastema is present between alveoli five and six, probably to accommodate the enlarged dentary tooth. Then the alveoli globally decrease in size again posteriorly; they are mediolaterally compressed and closely spaced. In ventral view, the alveolar rows globally diverge posterolaterally, with a concavity at the level of the fifth–sixth alveolus. The largest tooth is preserved on each side, in addition to some other fragments of other teeth. Those are caniniform teeth, with no particular carinae or ridges, and they are not compressed mediolaterally.
Nasal (Figs 2A, E, 4A, C, E, G, 11A, E, 13A, D): The nasals are paired and elongated anteroposteriorly (no more than 1.5–2 cm). The suture with the maxilla and the premaxilla is straight anteroposteriorly. The contact between the frontal and the nasals is V-shaped, with the frontal projecting anteriorly in the nasals. The anterior end of the nasal constitutes the posterior and medial margin of the internal nares. In SM- 2023-1-16, the nasals form a complete internarial bar. The posterior end of the nasals is squeezed between the contact with the frontal and the anterior projections of the prefrontals laterally in SM-2021- 1-97 or the anterior projection of the lacrimal in SM- 2023-1-16. The nasal does not contact the lacrimal in SM-2021-1-97, but it does in SM- 2023-1-16. The bone is not visible in ventral view.
Lacrimal (Figs 2A, C, D, 4A–E, G, 11A, D, 13A, C, D): The lacrimal is triangular in shape, with one tip directed anteromedially. The posterior curved margin forms a part of the anterior margin of the orbit. The medial side is bordered by the prefrontal, whereas the lateral side contacts the posterior part of the maxilla. The lacrimal is shorter anteroposteriorly than the prefrontal in SM-2021-1-97, whereas it is of the same size in SM- 2023-1-16. The ventral surface is too damaged to be described. In SM- 2023- 1-16, the lacrimal forms the dorsal border of the antorbital fenestra and connects with the maxilla and the jugal ventrally.
Jugal (Figs 2A, C, E, F, 3A–C, G, 4, 11A, C–E, G, 13A, C, D): The jugal is elongate and plate-like (taller than wide). Anteriorly, it is directed straight anteroposteriorly, whereas posteriorly it curves mediolaterally (near the contact with the quadratojugal). The jugal sends a convex process to connect with the maxilla on the lateral side, at the same level as the lacrimal, extending to the penultimate alveolus in SM- 2023-1-16. In both specimens, a crest followed by a depression can be seen, with both being directed anteroposteriorly. Posteriorly, the bone connects medially with the quadratojugal almost all the way to the posterior extremity of the skull and almost participates in the articulation with the mandible. The jugal also forms most of the lateral edge of the orbit; however, it does not form a dorsal bulge in this region. Behind the orbit, the jugal has a dorsomedially angled process, which sutures with the postorbital in an anterolaterally curved suture. The posterior margin of this contact, together with the preserved posterior part of the jugal, forms the lateral and anterolateral margins of the infratemporal fenestra. The contact with the quadratojugal is straight, and no foramina or special ornamentation can be seen in dorsal view.
Prefrontal (Figs 2A, C, D, 4A–E, G, 11A, C–E, G, 13A, C, D): The prefrontal is squeezed between the lacrimal laterally and the frontal and the nasal medially, both with straight sutures. Its anteromedialmost part connects with the posterolateralmost part of the nasal. It also connects anteriorly with the maxilla in a mediolaterally straight suture. It is wider anteriorly than posteriorly in SM-2021-1-97, whereas it wider posteriorly than anteriorly in SM- 2023-1-16. It makes most of the medial margin of the orbit. Ventrally, it is damaged but connects with the palatine through a transversely expanded prefrontal pillar. Those pillars meet at the midline, although this might be attributable to taphonomic deformation.
Frontal (Figs 2A, D, 4A, C–E, G, 11A, C–E, 13A, C, D, 14A, F): The frontal forms a bridge between the rostrum and the postorbital region of the skull and makes part of the posteromedial margin of both orbits. Those margins are dorsally raised. Anteriorly, it connects with the nasals in a V-shaped suture, with an anteriorly projected process of the frontal. Posteriorly, it sutures with the parietal and the postorbitals in zigzagged sutural surfaces that are straight overall. Laterally, it does not contact with the lacrimal but with the prefrontal anteromedially. It forms the anteromedial margin of the supratemporal fenestra. The dorsal surface of the frontal is slightly concave and bears a sagittal crest. It is not thick dorsoventrally. In SM-2021-1-97/99, the ventral surface bears cristae cranii frontales, which delimit the olfactory tract (Iordansky 1973; Fig. 2B).
Postorbital (Figs 2A, C, D, 4A–E, G, 11A, C–E, 12B, C, 13A, C, D, 14A, C, D, F): The postorbital forms the posterior margin of the orbit, the anterolateral margin of the supratemporal fenestra, and the anterior margin of the infratemporal fenestra. It has a T-shape, with one branch directed medially, another one posteriorly, and the final one ventrally. Medially, it contacts only the frontal. Posteriorly, the suture with the squamosal is straight (curved posteriorly in SM- 2023-1-17) and is situated rather anteriorly, at the level of the middle of the supratemporal fenestra. Laterally, the postorbital sends a long projection connecting with the jugal. As a result, it forms most of the anterior margin of the infratemporal fenestra.
Parietal (Figs 2A, F, 4A, F, G, 11A, E, 13A, E, 14A, E): This single bone is part of the skull roof. It has a rectangular shape with depressed sides, like an hourglass. It is less wide mediolaterally than the posterior part of the frontal in SM-2021-1-97 but as wide as in SM- 2023-1-16. Its anterior edge is smaller than its posterior one in SM-2021-1-97 and SM- 2023-1-17 as well because it widens abruptly posteriorly to the supratemporal fenestrae, but those edges are the same size in SM- 2023-1-16. Laterally, the parietal forms the medial margin of both supratemporal fenestrae, extending ventrolaterally. Those margins are elevated dorsally. The parietal also bears a sagittal crest on its posterior part, aligned with the one of the frontal. On the posteromedial corner of the supratemporal fenestra, the anterior temporal foramen (or orbitotemporal foramen; Kuzmin et al. 2021) can be seen on each side in SM- 2023-1-16. The parietal contacts the frontal anteriorly, the supraoccipital posteriorly, and the squamosal laterally. Within the supratemporal fossa, the parietal contacts the quadrate and the laterosphenoid. In SM- 2023-1-17, the suture with the squamosal is raised, making a continuous ridge with the raised medial margin of the supratemporal fenestra.
Squamosal (Figs 2A, C, D, F, 4A, C, D, G, 11A, D, F, 12B, C, 13A, D, E, 14A, C–E): The squamosal is T-shaped and forms the posterolateral corner of the skull roof. It contacts the quadrate anteriorly to the external auditory meatus and the otoccipital posteriorly in SM-2021-1-97 and SM- 2023-1-16. This suture is straight anteroposteriorly. The anterior process connecting with the postorbital is long and forms up to two-thirds of the posterolateral margin of the supratemporal fenestra. Posteriorly, the contact with the parietal is straight anteroposteriorly (curved laterally in SM- 2023-1-17) and raised dorsally, forming a crest. In the posteriormost part, the squamosal contacts the otoccipital ventrally; it forms the dorsolateral part of the skull. Posterolaterally, the squamosal forms an elongated process that is directed posterolaterally and in the same plane as the one of the cranial table in SM-2021-1-97 or ventrally in SM- 2023-1-16, but this is probably a taphonomic artefact. The squamosal has a longitudinal groove for the muscles of the external ear flap located dorsally to the external auditory meatus in lateral view.
Quadratojugal (Figs 2A, C, D, 4A, C–E, G, 11A, 12B, C, 13A, D): This bone extends posteriorly from the dorsomedial corner of the infratemporal fenestra. The posterior part is thin and squeezed between the quadrate medially and the jugal laterally. It extends all the way to the posterolateralmost part of the skull and participates in the articulation with the mandible. The suture with the jugal is straight in SM- 2023-1-16 and curved medially in SM-2021-1-97, whereas the suture with the quadrate is curved laterally. The bone is smooth dorsally, thin, and plate-like, and of the same width all the way; it is also higher dorsomedially than ventrolaterally.
Quadrate (Figs 2A–D, F, G, 3G, 4, 11A, D, F, 12B, C, 13, 14B): The quadrate has a complex shape. Anteriorly, it does not reach the infratemporal fenestra, but it reaches the ventral margin of the supratemporal fenestra and contacts the parietal. Dorsally, it contacts the squamosal anteriorly to the external auditory meatus in SM-2021-1-97/99 and in SM- 2023-1-16. The suture with the quadratojugal is straight anteromedially to posterolaterally in SM-2021-1-97/99, whereas it is curved laterally in SM- 2023- 1-16. The quadrate should contact the pterygoid ventrally, but the area is too damaged on all specimens to describe. Posteriorly, the contact with the otoccipital is straight, situated posteriorly to the external auditory meatus, and posteroventrally the quadrate is also connected to the basioccipital in SM-2021-1-97/99 (although this is probably a taphonomic artefact) but not in SM- 2023-1-16. The subtympanic foramen, the oval opening leading into the cavity within the quadrate, can be observed at the anteromedial margin of the quadrate on SM-2021-1-97/99. The foramen aërum can be seen on the left quadrate of SM-2021- 1-97/99; it is located dorsolaterally to the medial condyle. Crest B (Iordansky 1973) is extremely developed ventrally, extending closely to the quadrate–quadratojugal suture posteriorly and curving anteromedially to medially. Posteriorly, the quadrate articulates with the mandible. Its surface is divided into two hemicondyles laterally and medially, equal in size. The quadrate is bordered almost all the way by the squamosal and the otoccipital on the medial side and by the quadratojugal on the lateral side. Medially, it sutures with the pterygoid and probably the other bones of the braincase, but the area is too damaged in all specimens to be described. This bone is 1–1.5 cm tall in cross-section and curved dorsally.
Supraoccipital (Figs 2A, F, 4A, F, 11A, F, 13A, E, 14E): This bone is triangular in shape in posterior view.It is exposed in dorsal view. Laterally, it connects with the otoccipital. It also does not participate in the formation of the foramen magnum in SM-2021-1-97. It is more difficult to assess in SM- 2023-1-16 and SM- 2023-1-17 because those areas are covered by other fragments.
Otoccipital (Figs 2F, 4A, C, D, F, G, 11F, 13E, 14E): This bone connects laterally with the quadrate, medially with the supraoccipital, and dorsally with the squamosal in a straight to curved contact. In posterior view, it extends laterally to the lateral edge of the skull (not in SM- 2023-1-17). On SM-2021- 1-97 there are three foramina: the lateralmost one is for the internal carotid artery, and the two others are the two exits for cranial nerves XII. On SM- 2023-1-16, a foramen can be seen dorsolaterally from the occipital condyle; it is the foramen for the cranial nerves IX–XI. The otoccipital also connects with the basioccipital ventrally. The whole bone is plate-like and gradually protrudes caudally with an angle of ~30° to the vertical axis, showing a small ridge directed mediolaterally.
Basioccipital (Figs 2B, D, F, G, 4A, B, F, 11B, F, 13B, E): The basioccipital sutures laterally with the otoccipital (in posterior view), the quadrate (in lateral view), and maybe the pterygoid posteroventrally (in SM-2021-1-97/99). In this specimen, the separation between the parabasisphenoid and the basioccipital is very difficult to assess; therefore, it might also be that the basioccipital is separated from the pterygoid by the parabasisphenoid. It bears a median crest and two lateral tuberosities, and the median pharyngeal foramen is clearly visible ventrally to the central crest. The basioccipital is only preserved posteriorly; it has a plate-like shape directed anterolaterally to posteromedially. The occipital condyle is directed posteroventrally.
Palatine (Figs 2B, G, 3D, E, 4B, G, 11B, 13B): The palatine connects with the maxilla in an anteriorly convex suture in SM-2021-1-97/99 and SM- 2023-1-16. The posterior region is heavily damaged and remodelled, but it does not include the choanae in SM-2021-1-97/99 (Fig. 3G). However, this could be the case in SM- 2023-1-16, where the palatine would form its anterior margin, but because of the poor state of preservation it is difficult to assess. The suture with the pterygoid is also anteroposteriorly straight immediately anteriorly to the internal choanae. The paired palatines are flat when they meet at the midline. In ventral view, the lateral margins of the palatines are parallel and straight in SM-2021-1-97/99. This is not the case in SM- 2023-1-16, but might more probably be attributable to a deformation induced by the humerus. The ventral surface is smooth and raised dorsally in SM- 2023-1-16. The vomer cannot be seen.
Pterygoid (Figs 2B, D, F, G, 3C–F, 4B–D, F, G, 11C, 12B, C, 13B, C): The pterygoid is sutured to the palatine anteriorly, the ectopterygoid laterally, and the basioccipital posteriorly. Each pterygoid makes a straight connection with the corresponding ectopterygoid and tends to be more curved ventrally at that point. The internal choanae are ovoid, bordered by prominent anterior margins in ventral view, and have no midline process. They are totally enclosed by the pterygoids in SM-2021-1-97/99 (their status is unknown in SM- 2023-1-16) and are situated anteriorly to the posterior margin of the suborbital fenestra. Anteriorly, a median process of the pterygoid extends to contact the palatine and forms the ventral edge of the interorbital septum. The pterygoid is smooth on all surfaces except on the lateralmost sides.
Ectopterygoid (Figs 2B, D, 3C, D–F, 4B, C, F, G, 11D, 12B, C, 13B, D): This bone contacts the jugal and the maxilla laterally and the pterygoid medially (forming the pterygoid flange). The triple junction between the maxilla, the jugal, and the ectopterygoid is situated at the middle of the anterior process of the ectopterygoid in SM-2021-1-97/99. The ectopterygoid could constitute the posterolateral margin of the suborbital fenestra, but it cannot be assessed with certainty owing to the poor preservation of this area in this specimen. However, it does not ascend on the medial margin of the postorbital bar. In SM- 2023-1-16, the ectopterygoid forms the posterolateral margin of the suborbital fenestra, and its anterior process contacts the posteriormost maxillary alveolus. This bone is curved ventrally and laterally. In both specimens, the ectopterygoid extends up to the preserved posteriormost part of the pterygoid wing.
Laterosphenoid (Fig. 12B, C): The dorsoposterior ridge of the laterosphenoid connects with the parietal. The ventralmost part of the bone contacts the quadrate, forming the foramen for cranial nerve V.
Parabasisphenoid (Figs 11C, D, 12B, C, 13C, D): The parabasisphenoid is complex. Anteriorly, the parabasisphenoid rostrum is short and not dorsoventrally elevated. It does not contact the laterosphenoid; however, it does contact the prootic and the pterygoid. Posteriorly, it has a plate-like shape directed anterolaterally to posteromedially. It contacts the pterygoid ventrally, the quadrate dorsally, and the basioccipital posteriorly, encircling the pharyngeal foramen. This part also bears a crest directed anteroventrally to posterodorsally.
Prootic (Fig. 12B, C): This small bone sutures with the laterosphenoid posteriorly on the posterior margin of the foramen for cranial nerve V and the parabasisphenoid rostrum anteriorly.
Mandible (Figs 2B, G, 3, 4)
There is no external mandibular fenestra. Both coronoids are missing. Overall, the mandible is ornamented with circular to ovoid pits and grooves.
Dentary (Figs 2B–E, G, 3B, C, 4B–G): This bone is the only tooth-bearing element of the mandible, with at least 13 alveoli. The two rami separate at the level of the fifth dentary tooth and are firmly sutured anteriorly. The symphyseal region remains wide anteriorly and is U-shaped at its anteriormost point. In medial view, the dentary is thin dorsoventrally and forms an acute angle anteriorly. Ventrally and dorsally, the medial suture with the splenial is oblique, directed posterolaterally to anteromedially. The dentary remains uniform in width up to the point of divergence, where it begins to taper off. Posteriorly, the dentary connects dorsally with the surangular, from the end of the toothrow, and ventrally with the angular at its posteriormost part. The ventral surface is smooth, and anteriorly it is curved dorsally. The lateral side shows no groove ventrally to the tooth row. In lateral view, the dorsal margin is sinusoidal, marked by two sets of waves that culminate at the level of the 3rd/4th and 11th/12fth dentary alveoli. There is also a medial depression to accommodate the largest caniniform maxillary tooth at the level of the 9th dentary alveolus, especially visible on the left side. In terms of size, alveoli 4–10 and 13 are relatively small, whereas the 3rd one is the largest, and the 11th and 12th are also large. Alveoli 4–7 and 8–10 are closely spaced, whereas there is a larger space between alveoli 7 and 8. The ventral margin of the dentary is straight anteroposteriorly.
Splenial (Figs 2B, G, 3C, 4): The splenial is most exposed medially; it forms a vertical plate that sutures with the dentary and with the other splenial at the midline anteriorly. It reaches up to the fifth dentary tooth and is not exposed ventrally. Posteriorly, it becomes thin and plate-like along the medial surface of the dentary (even becoming the medial wall for the last posterior dentary alveoli, from the 11th one) until it disappears posteriorly without meeting the angular or the surangular. In posterior view, the circular foramen intermandibularis oralis is present at the point where the two splenials diverge (not on the medial sides) and is small. The mandibular symphysis completely involves the first eight alveoli. The dorsal exposure of the splenial is strictly triangular in shape and makes up more than one-third of the mandibular symphysis. The dorsal surface bears rugosities, whereas the medial surface is smooth and flat.
Angular (Figs 2B, D, F, G, 3A, B, H, 4B–D, 4F, G): The angular is the ventralmost mandibular element (approximately half of the total length of the mandible). In lateral view, it is elongated, and posteriorly it is curved dorsally. Medially, the angular sutures with the dentary, the surangular, and the articular to form a huge medial depression (adductor chamber; Iordansky 1973). Dorsally, it sutures with the dentary anteriorly and the surangular posteriorly, finishing as a sharp process laterally to the retroarticular process and curving inwards and upwards. Only the lateral surface is ornamented; the others are smooth. The area of insertion of the muscle pterygoideus is not very developed and is visible only in the posteroventral margin of the angular.
Surangular (Figs 2C, F, 3A–C, G, H, 4): This bone is robust and elongated. Anteriorly, its dorsal process might extend between the dentary and the splenial, but the preserved parts of the specimen show only a contact between the surangular and the dentary anteriorly. Posteriorly, it tends to curve dorsally and sutures with the angular for the rest of its length and medially with the articular. It also becomes more plate-like. This suture is linear anteriorly, and posteriorly it curves dorsally. The bone also curves medially and forms the lateral margin of the articular fossa. Laterally to this, there is a short ridge oriented anteroposteriorly, forming a depression. The dorsal surface is convex, not ornamented, and flattens before the glenoid surface. On the lateral surface, there is a ridge directed posterodorsally to anteroventrally.
Articular (Figs 2A, B, D, F, G, 3G, H, 4A, B, D–F): The articular is the posteriormost element of the mandible. It has two dorsal surfaces separated by a ridge oriented mediolaterally. The anterior surface articulates with the quadrate; the ridge helping to stabilize this articulation as its posterior wall is tall and dorsally edged. The articular fossa is divided into a lateral and a medial portion of equal size by a small ridge oriented anteroposteriorly. The posterior surface (retroarticular process) is concave overall and paddle shaped. It seems to taper posteriorly, but the posteriormost part is broken on each side. Ventromedially, the articular forms the posteromedial wall of the adductor chamber. It sutures with the surangular at its lateral margin and with the angular at its ventral margin.
Dentition (Figs 2C–E, H, I, 4C–E, G, 12A): A large maxillary caniniform tooth is preserved on each side of the skull. It is conical and slightly curved lingually. The base of the tooth crown is ovoid in cross-section, and the apex is pointed. There are no carinae or crenulations visible in SM-2021-1-97/99, whereas there are carinae but with no crenulations in SM- 2023-1-16. The enamel, although damaged, shows thin and basoapically directed striations. This morphotype corresponds to the ‘pseudocaniniform’ morphotype described previously (Schwarz and Salisbury 2005, Lauprasert et al. 2011, Tennant et al. 2016). The upper dentition also preserves a smaller tooth, on the left side immediately anteriorly to the big caniniform tooth. On the mandible, a lot more teeth are present, throughout the tooth row, and they belong to both the ‘pseudocaniniform’ and the ‘lanceolate-shaped’ morphotypes (Schwarz and Salisbury 2005, Lauprasert et al. 2011, Tennant et al. 2016). The anterior teeth are strongly procumbent, and the third one is well developed, as are the 11th and 12th. The third and fourth dentary alveoli are confluent.
Axial skeleton (Figs 5–8, 10, 15A–E)
Most crocodylians possess eight cervical vertebrae, 16 dorsals (lumbar included), two sacrals, and 30–40 caudals (Mook 1921, Gomes de Souza 2018). Furthermore, the cervical and the first three dorsal vertebrae exhibit a similar morphology, whereby the parapophysis and the diapophysis are separated. Then, from the fourth dorsal vertebra, the parapophysis and the diapophysis are fused, forming the transverse processes connecting with the ribs (Gomes de Souza 2018). In SM-2021- 1-97/101, there are at least 32 vertebrae. Given that the areas of transition between the cervicals and the anterior dorsals (SM-2021-1-98; Fig. 5) and between the posterior dorsals and the sacrals (SM-2021-1-100; Fig. 6) are preserved, we can establish the following: SM-2021-1-97/101 preserves 4 cervicals (5 th – 8th), 12 dorsals (1st–8th in the anterior part and 13 th –16th in the posterior part), 2 sacrals, and 14 caudals (only the 1st can be numbered). The proatlas/atlas and axis are missing. All the preserved vertebrae seem to be amphicoelous. All processes are described based on the nomenclature of Gomes de Souza (2018).
Cervicals (Figs 5A–C, E, 8): The neural arches and spines are tall, narrow, and pointed dorsally. The diapophyseal processes are longer than the parapophyseal processes. The zygapophyses are large in comparison to the centra, with the prezygapophyses being larger than the postzygapophyses. These structures become horizontal posteriorly. All the centra are amphicoelous, and the lateral sides of the centra (between the diapophyseal and parapophyseal processes) are notably depressed. The last cervical vertebra has a tall neural spine (more than two times the height of the centrum). The hypapophyses are broken, but their areas of insertion on the centrum are still visible on all centra. The diapophyseal processes gradually increase in size posteriorly, and they also migrate from the lateral side of the centrum to the lateral side of the neural arch. The parapophyseal processes do not seem to increase in size, but they also migrate dorsally, going from the ventrolateral margin to the lateral side of the centrum posteriorly.
Dorsals (Figs 5B–D, 6B, C, 8): The neural spines are anteroposteriorly longer than those of the cervicals. The first dorsal vertebra has the tallest neural spine (more than two times the height of the centrum). The diapophyseal and parapophyseal processes are too damaged to be described. On the centrum of the first dorsal vertebra, the proximal part of the hypapophysis is preserved; it is half the size of the centrum. The ventral hypapophyses are either absent or too damaged to be seen from the second or third centra. The articular facets of the pre- and postzygapophyses are more horizontally oriented.
Sacrals (Figs 6, 8): There are two sacral vertebrae of the same size. The base of their neural spine is long anteroposteriorly for both. On the second vertebra, the transverse processes and their articular surfaces are more developed, probably because they are less damaged than on the first one. The costal caudalis is directed ventrally on the first sacral. The contact between the two centra is flat. The anterior articular surface of the first sacral and the posterior articular surface of the second sacral are both concave.
Caudals (Figs 6, 7B–D, 8): The first caudal vertebra is large, and its articular surfaces are concave to flat. The other ones are damaged. The second has a neural spine that is preserved; it is high dorsoventrally and long anteroposteriorly. Some of them have two horizontal ridges on the centrum, perhaps for the insertion of the chevron. A chevron is preserved; it is triangular in shape and open in the middle.
Ribs (Figs 5A, 7B): Some ribs are preserved, but it is difficult to identify to which vertebra they were attached. When those structures are preserved, the tuberculum is less developed than the capitulum. The ribs are slightly curved and are rod-like.
Pectoral girdle (Fig.9)
Coracoid (Fig. 9): The left coracoid is preserved. This bone is convex overall. The shaft is triangular, and both ends of the bone are extended. The coracoid foramen is visible in the most proximal part. The articular surface with the scapula is flat, and the ventral part of the glenoid fossa is saddle shaped and directed posteriorly. The distal end is more developed anteriorly than posteriorly.
Pelvic girdle (Figs 6B, C, 9A, B, D)
Pubis (Figs 6B, C, 9A, B, D): The two bones are flat. The most proximal part articulating with the rest of the pelvic girdle is missing in both. Overall, it has a round shape, with a convex and narrow dorsal margin.
Forelimb (Figs 11B, C, G, 13B, C)
Humerus (Figs 11B, C, G, 13B, C): The humerus is squeezed between the anterior margin of the suborbital fenestra and the right side of the parabasisphenoid rostrum. Its deltopectoral crest is slender and placed anteroproximally. On the proximomedial side, there is a ridge that connects the deltopectoral crest and the triangular-shaped head of the humerus. The humerus is straight and not very curved or twisted. The head of the humerus is very well developed and unusual among crocodyliforms, extending posterolaterally. The articular surface is flat and decreases anteromedially to articulate with the glenoid fossa. There is also a lateral projection where the humeral head reaches the shaft. The area of insertion for the muscle teres major is visible on the posterior side. Distally, on the bicondylar articulation with the zeugopod, the medial condyle for the ulna is larger than the lateral condyle for the radius.
Hindlimb (Figs 7A, B, 9A, BD)
Tibia(Figs 7A, B, 9A, B, D): The shaft of the left tibia, although deformed taphonomically, appears to be straight anteroposteriorly and curved laterally. However, both ends of the bone are on the same plane mediolaterally. The proximal articulation surface is flat, with a posterior concavity. The distal articulation surface is ovoid.
Tibia (Fig 9A, B, D): The fibula is straight and very slender. The proximal end is compressed mediolaterally. The distal articular surface comprises two articular surfaces that are flat to convex: one is posteromedial and would have articulated with the calcaneum, and the other one is distal and would have articulated with the astragalus.
Digits (Fig 9A, B, D): The preserved metatarsal is concave ventrally. The distal end is separated into two condyles, with a depression on each side laterally and medially to the hemicondyles. The preserved ungual is curved ventrally and pike shaped.
Osteoderms (Figs 5–7, 10)
The dermal armour consists of a dorsal shield composed of at least three rows of mediolaterally expanded osteoderms and a ventral shield composed of rectangular-shaped osteoderms sutured to each other. Paravertebral and accessory osteoderms could not be distinguished because of preservation issues and unknown position on the body. Given that an uneven number of osteoderm rows is not known in crocodylomorphs (Salisbury and Frey 2001, Puértolas-Pascual and Mateus 2020), Varanosuchus either had at least four rows of paravertebral osteoderms or at least two rows of paravertebral osteoderms and two rows of accessory osteoderms. Some osteoderms bear a longitudinal keel that does not extend on the whole surface, but it is difficult to situate them on the body. Appendicular osteoderms could not be determined with certainty.
Dorsal shield (Figs 5, 6A, B, 7A, C, 10A, B, D): The osteoderms are more expanded mediolaterally anteriorly than posteriorly, but their anteroposterior size remains the same. All osteoderms are flat or slightly arched dorsally and ornamented with circular pits, as is seen on the cranial table and the posterolateral side of the mandible. There are some spine- or peg-like processes for articulation with the more anteriorly situated osteoderm that are preserved (Fig. 10A). The articulation system is as follows: the anterior osteoderm overlaps the posterior osteoderm, and the left osteoderm tends to overlap the right one. The margins are straight.
Ventral shield (Figs 5, 6C, 7B, D, 10C, E): The ventral shield is more damaged; however, it consists of square osteoderms. There are at least two rows of osteoderms. The ornamentation is the same as the dorsal shield; it consists of flat circular pits. The ventral shield is missing in the sacral region.
Phylogenetic analyses
The analysis generated 43 most parsimonious trees and a consensus tree with a length of 1509 steps (Supporting Information, Material S6; consistency index = 0.26, retention index = 0.58). Although the support values are low (Supporting Information, Material S6), the main groups inside Neosuchia are retrieved and supported by numerous synapomorphies. Those results are also retrieved using the heuristic search procedure, with the same topology (1509 steps, consistency index = 0.26, retention index = 0.58), hinting at the relative robustness of our analysis.
BasedonACCTRANoptimization, weretrieveAtoposauridae sensu Schwarz et al. (2017) as a monophyletic group (including Varanosuchus sakonnakhonensis; Fig. 16, node 4) based on the following combination of characters: a broad altirostral skull (character 3); little participation of the premaxilla in the internarial bar (character 4); the quadrate, squamosal, and otoccipital do not meet to enclose the cranioquadrate passage (character 49); the antorbital fenestra is much smaller than the orbit (character 67); one wave of enlarged maxillary teeth (character 79); dorsal osteoderms with a well-developed process located anterolaterally in dorsal parasagittal osteoderms (character 96); two parallel rows of dorsal osteoderms (character 97); a symmetrically developed lateral compression on the maxillary teeth (character 140); and a lacrimal that tapers posteroventrally and does not contact or only slightly contacts the jugal (character 229).
Furthermore, Paralligatoridae sensu Rummy et al. (2022) forms a monophyletic group (Fig. 16, node 5) defined by the following synapomorphies: no vascular opening on the dorsal surface of the postorbital bar (character 27); the medial quadrate condyle expands ventrally, being separated from the lateral condyle by a deep intercondylar groove (character 170); a sharp ridge along the lateral surface of the angular (character 219); the ulna has a wide and rounded olecranon process (character 260); and a foramen located in palatal view on the premaxilla–maxilla suture near the alveolar border (character 320).
Eusuchians (including Bernissartia, Atoposauridae, Paralligatoridae, Hylaeochampsidae, and Crocodylia; Fig. 16, node 3) are also retrieved as a monophyletic group, with the following synapomorphies: the choanal groove is undivided (character 69); the cervical vertebrae are procoelous (character 92); the dorsal osteoderms have a discrete convexity on the anterior margin (character 96); there are more than two rows of dorsal primary osteoderms (character 97); and the supraoccipital is exposed in the skull roof (character 171).
Finally, the two most complete specimens from Phu Sung are retrieved as a monophyletic group (Fig. 16, node 8), with the following synapomorphies: the dorsal part of the postorbital has an anterolaterally facing edge (character 29); the quadrate has no fenestrae (character 45); two waves of enlarged maxillary teeth (festooned; character 79); a quadratojugal with no ornamentation (character 145); the outer surface of the squamosal dorsolaterally oriented is reduced and sculpted (character 168); and there is a depression on the posterolateral surface of the maxilla (character 207).
description: Published as part of Pochat-Cottilloux, Yohan, Lauprasert, Komsorn, Chanthasit, Phornphen, Manitkoon, Sita, Adrien, Jérôme, Lachambre, Joël, Amiot, Romain & Martin, Jeremy E., 2024, New Cretaceous neosuchians (Crocodylomorpha) from Thailand bridge the evolutionary history of atoposaurids and paralligatorids, pp. 1-27 in Zoological Journal of the Linnean Society 202 on pages 5-17, DOI: 10.1093/zoolinnean/zlad195, http://zenodo.org/record/14335145