Platyognathus hsui Young, 1944

Platyognathus hsui Young, 1944

Holotype: Cenozoic Research Laboratory ( CRL) V71 , the anterior part of a mandible including a partial right caniniform, 12 right alveoli and 11 left alveoli. The CRL of the Geological Survey of China was the precursor to the Institute of Vertebrate Paleontology and Paleoanthropology ( IVPP). The holotype has not been located and is considered lost (see Wu et al. 1996).

Holotype locality and horizon: ‘about 1 kilometer north of Shawan, Lufeng, Yunnan’ ( Young 1944). Zhangjiawa Member of the Lufeng Formation ( Fang et al. 2000).

Neotype: IVPP V8266 , rostral portion of a skull missing the dorsal bones and with articulated incomplete mandible. Designated by Wu et al. (1996).

Neotype locality and horizon: ‘About 10 m west of the intersection of the roads from Zhangjiawa to Dadi and from Dawa Elementary School to Dahuangtian, Yunnan, China; stratum 6 (middle of Dark Red Beds) of Lower Lufeng Formation’ ( Wu et al. 1996).

Referred specimen: CVEB (Centre for Vertebrate Evolutionary Biology, Yunnan University) 21301, a nearly complete skull with articulated mandible, articulated with the anterior 17 vertebrae and associated dorsal osteoderms; other dorsal, ventral, and appendicular osteoderms; part of the right scapulocoracoid; the left humerus; the right femur and proximal tibia and fibula; most of the left tibia and fibula articulated with the tarsus and proximal metatarsals.

Referred specimen locality and horizon: Zhangjiawa Member of Lower Lufeng Formation, at the low cliffs near the cornfield near Dadi village, Lufeng County, Yunnan, China.

Age: The geochronological age of the Zhangjiawa Member is poorly constrained. Based on vertebrate biostratigraphy, Luo and Wu (1994) placed it as correlative with the Sinemurian stage in the Early Jurassic, and this has generally been followed by later authors, including the Paleobiology Database (2024).

Diagnosis: Paired lower caniniforms with longitudinal ridges and groovesgivingthemapolygonalcross-section; lowercaniniforms occlude outside of the premaxilla–maxilla notch and reach nearly to the level of the nasals; antorbital fossa walled medially such that the antorbital fenestra is small or absent (distribution in other early branching crocodyliforms poorly known); anterior edge of premaxilla not visible in dorsal view; narrow, elongate, longitudinal fenestra between the posteroventral edge of the dentary and anterodorsal edge of the angular. Phylogenetic analysis identifies the following autapomorphic homoplastic characters: 19(1), infratemporal fenestra elongated, approximately twice as long as deep; 20(1), infratemporal fenestra facing dorsolaterally; 73(0), nasal extensively contacts lacrimal; 186(0), quadratojugal dorsal process contact with postorbital narrow, contacting only small part of postorbital; 279(0), dorsal edge of dentary in lateral view straight; 305(0), tooth crown serrations present; 308(1), posterior premaxillary teeth apicobasal length 1.5 times or greater than anterior teeth; and 392(1), dorsal osteoderms present.

Description

The bones are well preserved, although many bones are cracked. Sculpturing is pronounced on most of the external surface of the skull and osteoderms, suggesting a mature age for the specimen. The skull and mandible are nearly complete, except that the posterior third of the left hemimandible is missing. The skull appears to be mostly uncrushed but is dorsoventrally compressed slightly and has been sheared slightly posterolaterally to the right such that the left maxilla is visible in dorsal view but not the right maxilla, and the right squamosal is further posterior than the left ( Fig. 1). The skull is articulated with eight vertebrae and their articulated ribs overlain by the corresponding pairs of dorsal osteoderms, and patches of ventral or appendicular osteoderms have been dislocated onto the sides and dorsal surface of the dorsal osteoderms. The right scapulocoracoid is preserved posteroventral to the left hemimandible exposed in medial view and is missing the distal parts of the coracoid and scapula. The proximal end of the left humerus is preserved on top of displaced ventral osteoderms dorsal to the fifth to sixth left dorsal osteoderms, and the distal end is preserved separately with associated osteoderms. The proximal end of the left ulna is preserved separately.

Two separate blocks preserve vertebrae in continuity with those on the skull piece. Four articulated cervicodorsals articulate with the skull block, and a piece of an unidentified limb bone is preserved with them. This block articulates posteriorly with a block comprising five dorsal vertebrae, five corresponding right dorsal osteoderms, parts of at least three ribs, and a few small, possibly ventral osteoderms.

In addition to the vertebral blocks and the distal left humerus, six other pieces are preserved separate from the skull. A large piece comprises two layers of osteoderms, one of them ventral and the other dorsal, although the latter are poorly exposed. A second large piece comprises parts of five right dorsal osteoderms, three left dorsal osteoderms, one possible sacral vertebra, and the proximal end of the right femur. This block is largely unprepared and might contain other elements. A third block fits onto the second one and includes the remainder of the right femur articulated with the proximal ends of the tibia and fibula; the shafts of the left tibia and fibula; and associated appendicular osteoderms. A fourth piece fits onto the distal end of the left tibia of the third block and includes the distal ends of the tibia and fibula, the tarsus, the proximal ends of five metatarsals, and part of another metatarsal-sized bone. Two small pieces comprise fragmentary dorsal osteoderms, each with parts of three osteoderms.

Size: The specimen is relatively small for a crocodyliform but similar in size to ‘protosuchians’, such as Protosuchus Brown, 1934 (Colbert 1952, Clark 1986, Gow 2000), Orthosuchus ( Nash 1975, Dollman et al. 2019), and Sichuanosuchus Peng, 1995 ( Wu et al. 1997). Midline skull length is 5.6 cm, and width across the quadrates is 3.7 cm, the latter suggesting a body length of 63.35 cm ( O’Brien et al. 2019).

Skull shape: The rostrum is tall, as in Protosuchus , rather than dorsoventrally compressed as in most extant crocodylians ( Figs 2, 3). The maxilla is vertical, and its dorsal part does not curve medially as in Fruitachampsa Clark, 2011 or Zosuchus Pol & Norell, 2004a . In lateral view, the rostrum becomes lower anteriorly, but not as low as in the extant Paleosuchus Gray, 1862 to which it is otherwise similar in outline. In dorsal view, the rostrum is narrower than in Fruitachampsa but broader than in Protosuchus or Orthosuchus , and posteriorly it broadens less abruptly than in the latter two taxa. The mandible mirrors this, expanding in breadth across rami gradually rather than abruptly. The posterior part of the skull forms a broad, flat skull table as in other crocodyliforms. In dorsal view, the occiput is concave posteriorly, with the lateral part of the squamosal and paroccipital process extending further posteriorly.

Openings of the skull: The nares ( Figs 1A, 2A, B) face anteriorly and might have been divided but the internarial bar is mostly not preserved. The broken ventral part of the bar is preserved on the premaxillae along the anterior midline, and the anterior end of the nasals also appears broken. The premaxillae form nearly the entire narial border, except for the small dorsal midline portion formed by the nasals. The nares are anteroposteriorly short, hence the confluent opening is much wider than long.

The antorbital fossa ( Fig. 3) is roughly circular and comparable in size to the antorbital fenestra of Protosuchus , about one-third the length and one-half the height of the orbit. The fossae are not completely exposed, but there is a medial wall formed by a lamina from the lacrimal posteriorly and the maxilla anteriorly and ventrally, best preserved on the right side. This wall is similar to the antorbital fossa of Dibothrosuchus Simmons, 1965 ( Ruebenstahl et al. 2022) and might be present in other taxa in which the fossa is not well exposed. An antorbital fenestra is not evident within the fossa, but the area is not well exposed ventrally and appears to be complex. The anterior, anterodorsal, and ventral edges of the fossa are formed by the maxilla. The lacrimal forms most of the dorsal edge and all of the posterior edge, and the jugal approaches but does not appear to enter the fossa.

The orbits face laterally and are covered dorsally by two large palpebrals.They are sub-circular,slightly longer anteroposteriorly than high. The jugal forms the ventral border and the ventral half of the postorbital bar, the postorbital forms the dorsal half of the bar and the posterior edge of the orbit, the frontal forms the dorsomedial edge, and the lacrimal forms the anterior edge. The prefrontal appears to form a small part of the orbital border between the frontal and lacrimal, but this is poorly exposed.

The supratemporal fenestrae, best preserved on the left side, are obliquely oriented, directed anterolaterally. They are slightly more than half the length of the orbit and are about twice as long as they are wide. They are broadly separated by the parietal. The fenestra is bordered medially by the parietal, laterally by the squamosal and quadrate, and anteriorly by the postorbital; the frontal enters the anteromedial edge, but it is unclear how much it forms, because the anterior end of the parietal is poorly preserved.

The infratemporal fenestra is best exposed on the right side but has been slightly damaged. It is small, but larger than that of Protosuchus , and faces dorsolaterally. It is about twice as long as it is high and rises anteriorly. It is bordered dorsally by the quadratojugal, and anteriorly and ventrally by the jugal; it is not clear whether the postorbital enters the border anteriorly.

The choana is not exposed, but the pterygoids do not form a secondary palate, and a palatine secondary palate is not evident, although only the posterior end of the left palatine is exposed. A small suborbital fenestra is evident on the left side, anterolateral to the pterygoid and medial to the maxilla, but it is not clear whether the palatine bordered it.

Premaxilla: The facial process of the premaxilla is vertical and faces laterally. It contacts the nasal dorsally and the maxilla posteriorly, and posteriorly it forms with the maxilla a mediolaterally constricted pocket into which the lower caniniforms occlude, but the contact is covered by the teeth. The constriction does not appear to be roofed dorsally, but this area is not well exposed, and the caniniforms extend dorsally to above the notch. The premaxillae are narrow beneath the nares, contributing to the anterior orientation of the nares. The palatal portion of the premaxilla is not exposed.

Maxilla: The facial process of the maxilla faces laterally ( Figs 2, 3), such that the rostrum is taller than in living crocodylians, except Paleosuchus . The maxilla contacts the premaxilla anteriorly, the nasal dorsally, the lacrimal posterodorsally above the middle of the antorbital fenestra, and the jugal posteriorly. The maxilla forms a medial wall to the antorbital fossa along with the lacrimal, the maxilla forming the anterior part, but the ventral part of the fossa is poorly exposed and appears to be complex. The jugal appears to separate the maxilla from the lacrimal posteroventrally, but there might be some small contact near the posteroventral edge of the antorbital fenestra. The anterior part of the facial process is only lightly sculpted, and the portion ventral to the antorbital fossa is smooth and slightly concave. The ventral edge of the maxilla is nearly straight but rises at its anterior end as it forms the posterior part of a notch. Posteriorly, the jugal contact rises anteriorly as it approaches the antorbital fenestra and extends above the posterior five teeth. The medial part of the maxilla, including the palatal portion, is not exposed. Neurovascular openings for sensory dome organs ( Lessner et al. 2023) are not evident, but preservation of the surface is poor.

Jugal: The jugal is heavily sculpted laterally, including the postorbital bar ( Fig. 2). It contacts the maxilla and lacrimal anteriorly, the postorbital dorsally on the postorbital bar, and the quadratojugal posterodorsally; it is not clear whether it contacts the quadrate. The anterior and posterior processes of the jugal are nearly equal in length. In lateral view, the anterior process is concave dorsally and convex ventrally, and the posterior process is nearly straight, and together these combine to give a gentle dorsal arch to the bone beneath the postorbital bar, as in the Platyognathus hsui neotype. The anterior end is relatively flat ventrolaterally where it overlaps the maxilla laterally, but near the posterior part of the orbit a longitudinal ridge forms and becomes more prominent posteriorly, although less prominent than in Zaraasuchus . The postorbital process is gently inset from this ridge, flattened mediolaterally, and tapers dorsally. The contact with the postorbital is not exposed. The posterior process becomes dorsoventrally flattened posteriorly.

Quadratojugal: The quadratojugal is broader than in extant crocodylians but narrower than in Protosuchus . It is relatively flat and extends anterodorsally to contact the postorbital, and anterodorsally it does not broaden mediolaterally. The anterior process overlaps the posterior half of the posterior process of the jugal. Posteriorly, it overlaps the quadrate dorsally and does not enter the mandibular articulation surface. No part of the quadratojugal is sculpted.

Nasal: The paired nasals extend from the nares posteriorly to meet the frontals opposite the anterior end of the orbit. The nasal contacts the premaxilla and maxilla laterally, the lacrimal and prefrontal posterolaterally, and the frontal posteromedially. The nasal faces dorsally and is nearly flat. Its anterior end is mediolaterally narrow, and the bone broadens posteriorly above the antorbital fossa about three times its anterior breadth, then narrows posteriorly. The lateral edge of the nasal is gently laterally concave anterior to the maxilla. The frontals project anteriorly between the posterior end of the nasals.

Frontal: The paired frontals contact the nasal anteriorly, the prefrontal anterolaterally, the postorbital posterolaterally and the parietal posteriorly. The suture between the frontals is obscure posteriorly owing to fragmentation. Its dorsal surface is relatively flat but turns up slightly at the orbital border. It is heavily sculpted. The frontals extend a short distance anteriorly between the nasals, have a brief contact with the prefrontal, border the middle of the dorsal edge of the orbit and its posterodorsomedial corner, and form the medial half of the bar between the supratemporal fenestra and orbit. The contact with the postorbital is exposed on the right side, where the postorbital fits into a slot on the lateral edge of the frontal. The frontal extends posteriorly to border the anterior part of the supratemporal fenestra, but its contact with the parietal is obscured by poor preservation of the parietal; the posterior end of the frontals might be intact as preserved above the anterior end of the parietal. Its ventral contact with the laterosphenoid might be visible on the left side.

Parietal: The fused parietals are fragmented ( Fig. 1), especially anteriorly. It broadly separates the supratemporal fenestrae, but this region is poorly preserved. It contacts the frontals anteriorly and the squamosal laterally on the skull surface. Its mediolateral breadth is about three-quarters of the greatest breadth of the squamosal. Its dorsal surface is flat and sculpted, and there is a small indentation on the midline of its occipital edge, which is otherwise transversely oriented. The contact with the squamosal is anteroposteriorly broad and longitudinally aligned, and the squamosal appears slightly to overlap the parietal dorsally, although the squamosal has been medially dislocated slightly on the left side. Only a small part of its occipital surface is exposed.

Squamosal: The squamosals are large, forming nearly all of the dorsal roof of the lateral temporal region. They are roughly triangular in dorsal view, with a temporal side along the lateral edge of the skull roof, a side along the lateral edge of the supratemporal fenestra and articulating with the parietal, and an occipital side along the occipital margin. The occipital edge is slightly shorter than the other two edges, which are subequal. The occipital margin is obliquely oriented, facing posteromedially. The dorsal surface descends posterolaterally, as in Fruitachampsa and Gobiosuchus , to approach the mandibular condyle of the quadrate, but the squamosal does not appear to contact the quadrate or close the cranioquadrate passage laterally, which might be attributable, in part, to dorsoventral crushing of the posterolateral part of the bone. Unlike in Fruitachampsa and Gobiosuchus , the descending portion is not sculpted. The lateral edge of the bone is relatively thin dorsoventrally, as in Protosuchus . The posterolateral corner descends ventrally. The lateral edge has a brief ventral lappet anteriorly, and a thin groove for the earflap muscle extends along the entire lateral edge. The contact with the postorbital is poorly preserved, but a fragment of the anterior end of the squamosal on the left overlaps the postorbital. The dorsal surface is heavily sculpted, but the posterolateral corner of the bone is smooth. The anterodorsal process of the quadrate is evident on the left side and would have contacted the squamosal along the entire lateral edge of the supratemporal fenestra but is dislocated medially. The occipital margin is partly exposed on the right side and is vertically oriented.

Prefrontal: The prefrontals are poorly exposed but are partly visible on the skull roof. The right element is better exposed. It borders the orbit posterolaterally, the frontal posteromedially, the nasal anteromedially, and the lacrimal anterolaterally. It does not extend as far anteriorly as the lacrimal. It is too poorly exposed to determine its shape. Its dorsal surface is sculpted, and the right element has a short longitudinal ridge on the left side of its dorsal surface.

Lacrimal: The lacrimal is about twice as broad dorsally as the prefrontal. It extends anteriorly nearly the length of the dorsal edge of the antorbital fenestra, where it contacts the maxilla. The posterior end of its dorsal part is covered by a palpebral, but the ventral part extends posteroventrally about one-third the length of the orbit. Its dorsal surface is sculpted. It forms the anterior edge of the orbit and posterior edge of the antorbital fenestra and is relatively narrow between them. Ventrally, it curves posteroventrally along the anteroventral part of the orbit medial to the jugal. The lacrimal foramen is not exposed.

Postorbital: The postorbitals are small, forming the dorsal part of the postorbital bar and the lateral part of the bar between the orbit and supratemporal fenestra. The dorsal portion has a small exposure between the squamosal posteriorly and the frontal medially and borders the anterior corner of the supratemporal fenestra. It is sculpted dorsally, except in a smooth area anteriorly where the posterior palpebral overlaps it. It is not exposed well ventrally, but this area appears complex on both sides, not a simple lap joint with the jugal. The posteroventral contact with the quadratojugal is exposed on the left side, but it is difficult to define the edge of the contact.

Quadrate: The quadrates are not well exposed,except for the mandibular articulation. The articulation surface is mediolaterally broad and dorsoventally narrow, as in Protosuchus , and extends laterally slightly beyond the lateral edge of the squamosal. It is nearly horizontal, projecting only slightly posteroventrally, and faces slightly posterolaterally. Lateral and medial condyles are not well developed, but there are slight swellings that might correspond to them. The ventral surface of the quadrate extends anteromedially, but it is difficult to define the contact with the basisphenoid, and the left element is very fragmentary in this region. The dorsal part of the quadrate body is not well exposed, and no quadrate fenestrae are exposed.

Parabasisphenoid: The parabasisphenoid is well exposed ventrally but is fragmented on the left side ( Fig. 1). It is relatively broad and ventrally convex, comparable to that of Protosuchus , and presumably housed pneumatic cavities. It extends anteriorly a short distance between the pterygoids, and its anterior and posterior edges are obliquely oriented, aligned anteromedially to posterolaterally. Its anterior contact with the pterygoid is well exposed on the right side, but its lateral contact with the quadrate is not well preserved or exposed, although on the left side parts of the quadrate are well preserved next to fragments of the basisphenoid. A basipterygoid process is absent, as in all crocodyliforms and Almadasuchus Pol et al., 2013 ( Leardi et al. 2020). The posterior contact with the basioccipital is not well exposed.

Basioccipital, otoccipital, supraoccipital, prootic, and laterosphenoid: These bones are all poorly exposed. Part of the supraoccipital might be visible ventral to the parietal. The paroccipital process of the otoccipital might be visible on the right side, and the anterodorsal part of the left laterosphenoid is visible within the supratemporal fenestra.

Pterygoid: The pterygoids are separate and have an unsculpted ventral surface. The quadrate ramus is broad and is convex anterolaterally, suggesting that it housed pneumatic cavities. The ramus contacts the basisphenoid along its posteromedial edge and the quadrate posterolaterally, but the latter contact is poorly preserved or exposed. The lateral flanges are moderately developed, comparable to Protosuchus , but their lateral surface (torus transiliens) is not exposed. The lateral contact with the ectopterygoid is not exposed, and the left flange is covered ventrally by what appears to be an osteoderm or the coronoid. The pterygoid appears to extend anteriorly medial to the suborbital fenestra, but this area is poorly preserved. The medial edge of the pterygoid in this region has a slight longitudinal ridge near the midline, extending ventrally.

Palatine: A bone extending posteriorly from the matrix on the left side and underlying the left pterygoid is presumed to be the palatine. It lies medial to the skull midline, suggesting that there is no palatine secondary palate, at least posteriorly. Two small bones project ventrally from the matrix in this region, but it is unclear what they are.

Ectopterygoid and vomer: These bones are not exposed but are likely to be present.

Palpebrals: Two palpebrals were preserved above each orbit, an anterior and a posterior one, but the right one was subsequently separated. They were loosely attached to the skull, not sutured. The anterior palpebral is the larger one. It is about twice as long as it is broad and broader posteriorly than anteriorly. It overlaps the prefrontal and lacrimal anteriorly and abutted the frontal laterally. The posterior palpebral is subcircular and overlapped the postorbital posteriorly; it was displaced to be nearly vertically oriented on both sides. The contact between the two palepebrals is not well exposed but was not sutural, and together they would have covered the entire dorsal surface of the orbit.

Dentary: The dentaries are unusually long, more than three-quarters the length of the mandible, longer than in Protosuchus and Orthosuchus . They are completely fused at the symphysis ( Fig. 4). Although the ventral surface of the symphysis is fragmented, a large piece along the midline posteriorly and a smaller one anterior to it show no suture. The symphysis comprises approximately one-quarter the length of the mandible. The posterior part of the ventral surface of the symphysis has a smooth depression along the midline, as in the holotype and neotype, but the symphysis lacks the transverse groove described for the holotype. The symphysis bulges laterally slightly beneath the two caniniforms. The anterior part of the symphysis rises gently, and the anterior end is rounded. The lateral and ventral surfaces of the dentary are sculpted, except for the posterior-most part, preserved only on the left side. The dentary extends posteriorly to opposite the middle of the infratemporal fenestra. Posteriorly, it expands dorsally beneath the arch of the jugal, where it is overlain by the surangular. The dentary extends further posterodorsally than posteroventrally, and the posteroventral end does not extend posteriorly as a process opposite the posterodorsal end. The splenial covers its medial surface from the symphysis posteriorly to the mandibular fossa. It contacts the angular posteroventrally, starting beneath the middle of the orbit.

Posterior to the dentary the surangular contacts the angular, occluding most of the typical position of an external mandibular fenestra ( Fig. 5). However, the bones are fragmented and might be compressed together dorsoventrally, with a small fenestra being possible in the dorsal part of the posterior edge of the dentary, and two bone fragments on the lateral surface near the posterior end of the dentary are obscuring some of the anatomy. A narrow, longitudinally elongate fenestra is present ventral to the posterior end of the dentary between it and the angular, like one in the neotype mandible. The slender anterior end of the surangular contacts the posterodorsal edge of the dentary, and there is no opening between them.

Splenial: The splenial is a dorsoventrally broad, unsculpted bone on the internal surface of the mandible. It extends anteriorly into the symphysis ( Fig. 5), but it does not appear to meet its opposite to form a part of the symphysis and is not visible on the ventral surface of the symphysis. Its anterior end is not completely exposed, and the forking evident on the neotype, for the mandibular nerve, is not visible. Posteriorly, it extends to opposite the postorbital bar, where it has a posteroventrally concave posterior end, as seen briefly on the left side. Its posterior end contacts the angular ventrally. The splenial does not appear to have much exposure, if any, ventrally on the mandible. Angular: The angular forms the posteroventral part of the mandible. It extends anteriorly to opposite the middle of the orbit and posteriorly almost to the end of the mandible. It is sculpted laterally and, possibly, along its thin ventral edge. Anteriorly, it forms a thin process wedged between the dentary and splenial, and posteriorly it gradually expands dorsoventrally and posterior to the dentary forms the ventral half of the mandible. It contacts the articular posteriorly and extends medially along with the articular. Its dorsal contact with the surangular is preserved only on the right side and is poorly preserved. It is separated by a narrow, elongate fenestra from the posterior end of the dentary. Its ventral edge is relatively straight in lateral view until its posterior end, where it descends ventrally.

Surangular: The surangular extends from opposite the postorbital bar posteriorly to contact the articular at the posterior end of the mandible, but this contact is not clear. Anteriorly, it forms a slender, dorsally arched process over the dorsal edge of the dentary. At the posterior end of the dentary, it expands ventrally to cover the posterior edge of the dentary and meet the angular. The area where a dentary–surangular groove would be is not exposed. The lateral surface is sculpted posterior to the dentary.

Articular: The articular is preserved only on the left side. It is mediolaterally wide, like the quadrate condyle, and sends a process medially, but this process does not appear to contact the braincase as in Protosuchus . A retroarticular process is lacking, and the posterior surface of the articular is gently convex, like Sichuanosuchus ( Wu et al. 1997) , rather than having the complex surface found in Protosuchus .

Coronoid and prearticular: Coronoids are not exposed, although a bone on the left pterygoid lateral flange might be one. A prearticular is not evident on the right side where the mandibular fossa is well exposed.

Dentition: Four teeth are preserved in each premaxilla, but the first teeth are broken at the alveoli. The teeth increase in size posteriorly, especially between the second and third tooth. The teeth are slender, and at least the third and fourth are slightly compressed mediolaterally and slightly recurved. The base of the fourth tooth is exposed more dorsally than the other teeth, such that although it ends only slightly ventral to the third tooth it has much more exposure dorsally.

Positions for 11 maxillary teeth are preserved. An alveolus for a 12th appears to be present on the right side, but this area is poorly preserved on the left side. The second and third teeth are the largest. The teeth gradually become smaller posteriorly, and the first is comparable in size to the most posterior teeth. All but the first tooth are collinear; the first tooth is further dorsal and projects slightly anteriorly. The teeth are slightly compressed mediolaterally, and the third is slightly recurved. The mesial edge of the second left tooth is serrated; the serrations are similar in relative size to those on other basal crocodyliform teeth. Serrations might be present on other teeth, but the edges are poorly preserved.

Most of the dentary teeth are not visible, covered by the premaxillary and maxillary teeth. The only lower teeth that are well exposed are two large caniniform teeth on the right side and the distal caniniform and alveolus for the mesial one on the left side ( Fig. 6), corresponding to the fifth and sixth dentary teeth on the holotype and neotype ( Wu and Sues 1996). The caniniforms occlude with a notch between the premaxilla and maxilla but extend dorsal to the notch rather than occluding within it as in most other ‘protosuchians’. The right caniniforms are similar in length and size. The mesial one appears to be slightly smaller in diameter than the distal one, but this is attributable to the greater fragmentation and spread of the distal tooth. The right caniniforms as preserved are both nearly circular in cross-section, taking fragmentation into account, and the CT scans show that this is true of the unexposed portions, hence it is not attributable to damage.The left distal caniniform is polygonal in cross-section, similar to the holotype and neotype, with longitudinal ridges and flattened areas on the sides of the tooth. In cross-section ( Fig. 6B), a labial ridge is bordered by small grooves and mesiolabial and distolabial flat surfaces, and lingually a large ridge is bordered by large grooves. However, the lingual surface is poorly preserved and might not accurately reflect the morphology. The left caniniform reaches dorsally to the midline height of the rostrum, but the right caniniforms are slightly shorter. The caniniforms are slightly recurved at their tips. Post-caniniform dentary teeth are not visible, except for parts of two teeth between the left fourth to fifth and sixth to seventh maxillary teeth.

Cervical vertebrae: The first eight vertebrae are preserved in articulation with the skull ( Fig. 2C), and the next four are preserved separately. Based on the dorsal migration of the rib articulations in vertebra 10, there are nine cervicals. The atlas and axis are obscured by matrix. The ventral surface of the cervical vertebrae are gently keeled but lack hypapophyses. All the exposed vertebral centra are slightly constricted in the middle. Separate rib articulations (parapophysis and diapophysis) are present, and double-headed ribs are preserved on the right side. Prezygapophyses are exposed on several vertebrae and are obliquely oriented, not horizontal. Articulations between centra are not well exposed, but there is no indication of a posterior condyle. The neural spine on the 10th vertebra is moderately tall, anteroposteriorly elongate as in basal crocodyliforms rather than rod-like as in eusuchians, and lacks a distal expansion.

Dorsal vertebrae: The block with the last cervical vertebra connects with a block with five dorsal vertebrae ( Fig. 7), hence there are eight articulated dorsals. By at least the 14th vertebra (fifth dorsal) the transverse process is long, at least twice as long as the width of the centrum. The dorsal centra become narrower posteriorly in the column and are constricted in the middle. The zygapophyses are not well exposed on the 10th–12th vertebrae (first to third dorsals), but by the 13th they are nearly horizontal. Neural spines are not well exposed, but the 15th and 16th vertebrae seem to have short spines. The centra are not well exposed anteriorly and posteriorly, but they appear to be amphicoelous or amphiplatyan, because no condyle is evident.

?Sacral vertebra: A single vertebra is preserved on the block with the right femoral head, and its large size, breadth, dorsoventral flattening, and position near the femur suggest that it might be a sacral.It is mediolaterally broader than the 12th-17th vertebrae (third to seventh dorsals), and the dorsoventral height of the centrum is about three-quarters its breadth. However, one side is exposed, and there is no evidence of a transverse process, nor is there evidence of fusion to another sacral vertebra. The lack of a transverse process is enigmatic no matter what the position of the vertebra.

Scapulocoracoid: The right scapulocoracoid is preserved behind the right side of the skull ( Fig. 8A), with its medial surface exposed. The dorsal part of the scapula and most of the coracoid are missing. The scapula is broken where it starts to expand dorsally, and it might have had a broad dorsal edge, but the preserved portion is relatively narrow anteroposteriorly. The scapula and coracoid are tightly appressed, and their suture is not clear. Both are oriented obliquely relative to the glenoid fossa, with the scapula extending posterodorsally from it and the coracoid posteroventrally. The glenoid is not well exposed, but the scapular portion indicates that it might not have been as laterally oriented as in extant crocodylians. The coracoid is broken where the bone constricts distal to the glenoid.

Humerus: The right humerus is preserved dorsal to the fifth to sixth dorsal osteoderms ( Fig. 8B, C) and is nearly complete, but the proximal end is not well exposed, and the distal end is preserved separately. The medial and ventral surfaces of the proximal end are mostly exposed, and the distal part is completely exposed, except where it is covered by osteoderms. The proximal condyle extends posteriorly further than in extant crocodylians, but possibly not as far as in Junggarsuchus Clark et al., 2004 , and it is not possible to see if it is hooked. The proximal articulating surface is not exposed. The ventral part of the deltopectoral process is visible, but proximally it is covered by matrix. The ventral surface of the proximal end is distinctly concave, but the dorsal surface is flatter. Distally, the medial condyle is larger than the lateral one, making the distal end oblique relative to the shaft. The diaphysis is gently bowed dorsally.

Ulna: The isolated proximal end of the right ulna ( Fig.8D) is identified due to its size and its sub-triangular articulating surface that is not compressed like a radius. The shaft is strongly arched such that the articulating surface faces obliquely proximomedially, as in most crocodyliforms, including Protosuchus richardsoni (Brown, 1933) ( Colbert and Mook 1951) and Cassissuchus ( Buscalioni 2017) . An olecranon process is not developed; the proximal articulating surface is relatively flat.

Femur: The right femur is nearly complete and partly covered in osteoderms ( Fig. 9A). The diaphysis is bowed dorsally, more strongly than the humerus. In lateral view, the head bulges anteriorly slightly, but a distinct inturned head is not evident. The fourth trochanter is not exposed. On the distal end, the lateral condyle is distinctly larger anteroposteriorly than the medial condyle, and the distal end is oblique to the shaft, as on the humerus but in the opposite direction. An intercondylar groove is well developed ventrally but not dorsally.

Tibia: The proximal end of the right tibia is preserved articulated with the femur, and most of the left tibia except its proximal end is preserved on the same block and connects to a piece with its distal end articulated with the astragalus ( Fig. 9A). The tibial shaft is about twice the diameter of the fibular shaft and is round in cross-section. The proximal end is expanded anteroposteriorly and mediolaterally, and it articulates with the lateral condyle of the femur. The distal end articulates with the astragalus and abuts the medial part of the distal end of the fibula. The articulation with the astragalus appears to be less oblique than in, e.g. Sebecus icaeorhinus Simpson, 1937 ( Pol et al. 2012), but not as horizontal as in neosuchians, although the exposed surface of the astragalus is not well preserved.

Fibula: The proximal end of the right fibula is preserved in articulation with the femur ( Fig. 9A), and most of the left fibula is preserved, except for its proximal end and a segment near its distal end. The shaft is mediolaterally flattened. The proximal part of the shaft is gently bowed anteriorly. The shaft expands slightly at its distal end, and a small posterior process is present at its distal end. The contact with the tarsals is obscure, because the surface of the tarsals is not well defined.

Astragalus and calcaneum: The left astragalus and calcaneum are completely preserved and articulated ( Fig. 9B), but the surface of the bone is not well preserved, and the articulation between them is not evident. The astragalus appears to be flatter proximodistally than in extant crocodylians. The calcaneal tuber is well preserved, and it is oriented obliquely posterolaterally, as in extant crocodylians, rather than anteroposteriorly, as in many basal crocodylomorphs. Distal tarsals are not evident.

Metatarsals: The proximal ends of all five left metatarsals are preserved, articulated to the tarsals ( Fig. 9B). The proximal ends of the first three metatarsals are subequal in breadth, the fourth is slightly larger than the others, and the first three overlap the adjacent metatarsals dorsolaterally. The fifth metatarsal is not well preserved, but a broken distal end indicates that it was longer than in extant crocodylians, and it is thicker proximally than the fifth metatarsal of extant crocodylians. The identity of a sixth bone, preserved ventral to the fourth metatarsal, is unclear.

Dorsal osteoderms: Eight pairs of dorsal osteoderms are in articulation behind the skull ( Fig. 1A), five right elements are in articulation on the piece with dorsal vertebrae ( Fig. 7), two left elements and parts of several others on the piece with the proximal right femur, and pieces of several others are also preserved separately. All dorsal osteoderms are sculpted dorsally, mainly as pits rather than ridges and grooves and unlike the fleur-de-lys pattern of gobiosuchids. The dorsal osteoderms are wider than long and are imbricated, with the posterior edge overlapping the anterior edge of the following osteoderm. There does not appear to be a smooth area on the dorsal surface of the osteoderms where the more anterior osteoderm overlaps it, and an anterior process is not evident but almost all osteoderms do not expose where one would be. The first pair of osteoderms is small, about half the width and length of the third pair. They are also flat, whereas more posterior osteoderms are dorsally convex. The more posterior cervical osteoderms develop a ventral bend laterally, and the posterior part of this bend forms a slight keel on more posterior osteoderms. The osteoderms in the dorsal region become flatter and lack the ventral bend, but the two osteoderms on the block with the right femur are relatively flat but have a sharp ventral bend near their lateral edge.

Ventral osteoderms: The ventral osteoderms ( Fig. 7C) are not preserved in place, and the occurrence of some lateral to the dorsal osteoderms in the cervical region suggests that they might have covered the lateral surface in addition to the ventral surface. They are generally square, flat, do not appear to be imbricated, and are sculpted in a pattern similar to dorsal osteoderms.

Appendicular osteoderms: Small osteoderms are preserved with the right femur ( Fig. 9A) and the right humerus, indicating that the limbs were armoured. Those associated with the femur are particularly extensive and partly surround the limb, although some of those associated with the humerus appear to be ventral osteoderms. The presumed appendicular osteoderms are smaller and rounder than the ventral osteoderms, with less distinct sculpturing, and all are relatively flat.

Phylogenetic results

The TNT analysis using equal weights resulted in 42 trees of length 1760 (consistency index =.304, retention index =.631). Our analysis, like those by Dollman et al. (2021) and Young et al. (2024), finds a paraphyletic Protosuchia and the Thalattosuchia within rather than outside of Crocodyliformes ( Fig. 10A), unlike some previous analyses (e.g. Wilberg et al. 2019). Unlike Young et al. (2024), Thalattosuchia is within the Mesoeucrocodylia (following the definition of Ortega et al. 2000). It finds Orthosuchus forming a clade with Protosuchus , the Notochampsoidea ( Fig. 10A). Crocodyliform outgroups are resolved as paraphyletic to the group rather than with a monophyletic Sphenosuchia ( Ruebenstahl et al. 2022), and with Hallopodidae ( Hallopus Marsh, 1881 ; Macelognathus Marsh, 1884 ; and Almadasuchus ) sister to the Crocodyliformes. Relationships among early branching crocodyliforms are resolved but weakly supported, with some exceptions.

The strict consensus resolves Platyognathus as sister to the Gobiosuchidae ( Cassissuchus , Zaraasuchus , and Gobiosuchus ). This grouping has a Bremer support of two and is supported by the following synapomorphies: 6(0), external nares in dorsal view wider than long; 148(0), posterolateral projection of squamosal: not a descending ornamented process; 149(2), squamosal extends posterior to the quadrate condyle in lateral view; 274(1), dentary symphysis completely fused, no evidence of suture; and 399(1), appendicular osteoderms present.

The Cassissuchus + Zaraasuchus + Gobiosuchus clade has a Bremer support of two and is supported by the following derived features: 12(1), supratemporal fenestra reduced to a thin slit or absent; 44(2), premaxilla, proportion of total length posterior to the external nares: 36%–45%; 112(1), palpebrals contact with frontal extensively sutured to each other and to the lateral margin of the frontal; and 286(1), longitudinal ridge along the dorsolateral surface of surangular. Cassissuchus and Gobiosuchus also appear to share a lacrimal that lacks a dorsal roofing component unknown in Zaraasuchus ( Buscalioni 2017) .

The clade Platyognathus + gobiosuchids is closer to Nominosuchus , Shantungosuchus Young, 1961 , and most other crocodyliforms, with a Bremer support of one by the following synapomorphies, three of which (*) are scored in Platyognathus : 1(1)*, rostrum proportions: broad oreinirostral; 19(2)*, lateral temporal fenestra nearly absent or lost (reversed to state 1 in Platyognathus ); 57(1), morphology of ventrally open notch at premaxilla–maxilla contact in lateral view a broadly open notch; 68(0), large and aligned neurovascular foramina on lateral maxillary surface absent; 147(1)*, posterolateral edge of squamosal: with descending ornamented process as in Sichuanosuchus and Fruitachampsa ; and 366(1), pubis with markedly expanded distal end.

An analysis with implied weights, in which homoplastic characters are downweighted using concavity functions of 6 and 12, produced notochampsoid and gobiosuchoid monophyly, but Shantungosuchus moved up two nodes to group with Sichuanosuchus , Fruitachampsa and Nominosuchus move to group together, and the notochampsoids are altered to have Orthosuchus at the base and UCMP 97638 sister to Protosuchus ( Fig. 10B). When characters are run unweighted and unordered, gobiosuchoids are monophyletic, but Hemiprotosuchus Bonaparte, 1967 , is sister to other crocodyliforms and UCMP 97638 is moved up one node above gobiosuchoids, based, in part, on its more developed secondary palate (Supporting Information, Fig. S3). When the analysis is run with unweighted, ordered characters without Platyognathus , relationships are like the full analysis, except that the Sichuanosuchus and Shantungosuchus form a clade, Nominosuchus and Fruitachampsa form a clade, and the latter clade is in a trichotomy with Zosuchus (Supporting Information, Fig. S4). A detailed description of the complete analysis will be presented elsewhere.