Redescription, taxonomic revaluation and phylogenetic affinities of Proterochampsa nodosa (Archosauriformes: Proterochampsidae) from the early Late Triassic of the Candelaria Sequence (Santa Maria Supersequence) Author Simão-Oliveira, Daniel De Author Pinheiro, Felipe Lima Author Andrade, Marco Brandalise De Author Pretto, Flávio Augusto text Zoological Journal of the Linnean Society 2022 2022-07-01 196 4 1310 1332 http://dx.doi.org/10.1093/zoolinnean/zlac048 journal article 10.1093/zoolinnean/zlac048 fd305070-026a-46ac-a323-dd4361897354 0024-4082 7390673 PROTEROCHAMPSA NODOSA BARBERENA, 1982 Holotype : MCP 1694 PV ( Fig. 2 ), well-preserved skull with articulated mandible missing most of the occiput, right temporal region and posterior right lower jaw ramus. Locality: The specimen comes from a now-gone outcrop located 6 to 8 km west of the Botucaraí hill, located at km 136 of the BR-287 highway, on the outskirts of Candelária municipality, Rio Grande do Sul State , Brazil ( Barberena, 1982 ; Abdala et al. , 2002 ). Age and horizon: Late Carnian–Earliest Norian ( Rogers et al. , 1993 ; Langer, 2005 ; Furin et al. , 2006 ; Martínez et al. , 2011 ) from the Upper Triassic of Candelaria Sequence, Hyperodapedon Assemblage Zone, Rosário do Sul Group, Paraná Basin ( Barberena, 1982 ; Langer et al. , 2007 ; Horn et al. , 2014 ). Emended diagnosis: Proterochampsid archosauriform with a skull similar in proportions to Proterochampsa barrionuevoi ; dorsoventrally flattened, highly ornamented skull; rostrum elongated with respect to the temporal region; external nares, antorbital fenestrae, orbits, supratemporal fenestrae and infratemporal fenestrae facing dorsally; elongated piriform nares with anteroposteriorly extended narial fossae; ornamentation consisting of dermal sculpturing in the shape of longitudinally disposed ridges along the dorsal face of the skull; a complex arrangement of ridges with interspersed nodules in the interorbital region of the prefrontals and frontals; curved ridges in the postorbitals, along the posterior border of the orbits; periodic and small nodular protrusions along the dorsal midline of the skull; well-developed nodular growths aligned along the dorsolateral portion of the premaxillae, maxillae, lateroventral corner of the jugals and lateral surface of the quadratojugals; jugal excluded from the infraorbital fenestra; prominent horizontal shelf on the surangular, dorsal to the mandibular fenestra; wide adductor fossa in the mandible; ventrally curved lamina on the angular; absence of retroarticular process; hook-shaped projection on the posterolateral side of the quadratojugal. Proterochampsa nodosa can be distinguished from Proterochampsa barrionuevoi by having overall fewer nodules and ridges, but larger nodular growths on the dorsolateral side of the skull; elevated medial rim of the orbits along the postorbital, frontal and prefrontal, shaping the orbit in an elliptic, almost subquadrangular manner; nasal pair wider than the minimum interorbital space (while, in Pr. barrionuevoi , the space between the orbits is virtually the same width as that of the nasal pair); elongated and laterally straighter frontal with contribution to the medial border of the orbit diminished by the prefrontal, which in comparison is lateromedially wider in dorsal view; rounded anterior margin of the antorbital fenestra and less angular than in Pr. barrionuevoi ; small supratemporal fenestra comprising one-fourth or less of the length of the infratemporal fenestra, compared to one-third in Pr. barrionuevoi ; postorbital posteriorly elongated past the posterior border of the supratemporal fenestra; presence of a quadrate foramen; occiput region proportionally higher and less depressed, with the quadrate and squamosal forming a nearly 90° angle; proportionally large (relative to Pr. barrionuevoi ) contribution of the pterygoid on the occipital region; opisthotic isolated from contacting the quadrate by the squamosal and the pterygoid. Comparative description Skull: The skull of Proterochampsa nodosa is overall triangular. It has a slender and elongated snout, forming approximately two-thirds of the total skull length, and a comparatively wide and robust temporal region. The rostral region gradually tapers anteriorly in dorsal view. The cranium is dorsoventrally flattened, and the temporal fenestrae, nasal openings and orbits face dorsally. All dermal bones present, to some extent, ornamentation in the form of nodular protrusions and complex ridges that vary in size and are commonly swollen in width ( Figs 2–5 ). These are notably large in the maxilla, jugal and quadratojugal, where they form a continuous single line of nodules. The overall cranial shape and proportions are similar to the Argentinian species, Proterochampsa barrionuevoi (see: Reig, 1959 ; Barberena, 1982 ; Dilkes & Arcucci, 2012 ; Trotteyn et al. , 2013 ). The external nares are located at the dorsal midline of the snout and are separated from each other by a thin ridge formed by the premaxillae and nasals. Although the external openings can be identified in the specimen, the segmentation of the nasal cavity was hampered by the quality of the CT slices. Therefore, most of the information on its morphology is based on first-hand observation of the holotype . The external nares are piriform and anteroposteriorly extended, with anterior depressions in the premaxilla and elongated posterior fossae in the nasals, a characteristic shared by all Proterochampsidae ( Sill, 1967 ; Bonaparte, 1971 ; Kischlat, 2000 ; Trotteyn et al. , 2013 ). The temporal region is wider and taller than the snout. The supratemporal fenestra is small and oval-shaped, being one-quarter or less of the length of the infratemporal fenestra, while the infratemporal fenestra of Pr. barrionuevoi is approximately one-third of the length of the infratemporal opening ( Barberena, 1982 ; Dilkes & Arcucci, 2012 ). The orbital and temporal regions of MCP 1694 PV present complex arrangements of protuberant ridges, which make the orbits of Pr. nodosa appear less rounded than those of Pr. barrionuevoi in dorsal view. At the interorbital region, the prefrontals and frontals form a complex ornamentation around the anteromedial and posterior orbital rims. In the dorsal view, the ridges extend anterolaterally from the midline of the frontals towards the lacrimals, forming a tall and U- or V-shaped ridge from the medial to anteromedial region of the interorbital zone ( Fig. 4 ). Most Pr. barrionuevoi specimens show a smoother surface in this region ( Dilkes & Arcucci 2012 ), and while some individuals may present some degree of ornamentation surrounding the orbits, they are never as rough as in Pr. nodosa . A similar condition is also seen in Chanaresuchus ( Romer, 1971 ; Trotteyn et al. , 2012 ; Trotteyn & Ezcurra, 2020 ). A well-marked set of ridges surrounding the anteromedial margin of the orbit is reported for Rugarhynchus sixmilensis ( Wynd et al. , 2020 ) . Differing from that of Proterochampsa and Chanaresuchus , however, these ridges connect to a maxillary ridge in Ru. sixmilensis . Additionally, the ridge that connects the orbits in Pr. nodosa seems to be unique to the taxon, not occurring in other proterochampsids or doswelliids. Posteriorly, the postorbitals and frontals form tall and curved ridges around the posterior rims of the orbits and the anteromedial edge of the infratemporal fenestrae, anteriorly to the supratemporal fenestrae. Notably, the lateral borders of the skull present larger nodules extending from the posterolateral edge of the premaxillae, through the maxillae, jugals and quadratojugals ( Fig. 5 ), ending in a hook-like projection in the posterodorsal corner of the quadratojugal, in lateral view. Indeed, the general arrangement and size of the nodules is one of the most evident distinctions between Pr. nodosa and Pr. barrionuevoi . While the nodules in the Brazilian species are larger, mainly laterally arranged and evenly spaced between each other, the nodules in the skulls of the Argentinian species are comparatively smaller, more densely concentrated and evenly distributed through most of the skull ( Reig, 1959 ; Barberena, 1982 ; Dilkes &Arcucci, 2012 ). A hook-like projection in the quadratojugals is present in both Pr. nodosa and Pr. barrionuevoi , and it was suggested to be a shared feature of the genus Proterochampsa ( Dilkes & Arcucci, 2012 ) . Figure 3. Skull of Proterochampsa nodosa (MCP 1694-PV) in palatal view, after virtual removal of the mandible. A, 3D-model. B, interpretative line drawing overlapping 3D-model of the skull. Dashed lines indicate poorly preserved structures or interpreted contacts. Abbreviations: aof, antorbital fenestra; ch, choana; ept, ectopterygoid; fr, frontal; iptv, interpterygoid vacuity; itf, infratemporal fenestra; ju, jugal; la, lacrimal; mx, maxilla; o, orbit; op, opisthotic; pa, parietal; pfr, prefrontal; pl, palatine; pmx, premaxilla; po, postorbital; pt, pterygoid; ptf, pterygoid flange; qd, quadrate; qdj, quadratejugal; qpt, quedrate ramus of the pterygoid; sq, squamosal; t, tooth; vo, vomer. Scale: 50 mm. Premaxillae ( Figs 2–4 ): The premaxillae are rostrally rounded and extend from the tip of the snout to the posterior limit of the external nares. In dorsal view, the premaxillae contact each other medially, anteriorly to the nares. A smooth premaxillary depression can be seen anterior to each external naris. The premaxilla– maxilla suture is clear. In lateral view, it arises at the level of the midpoint of the premaxilla–maxilla diastema, running obliquely to reach the dorsal surface of the skull, passing through the height of the posterior end of the narial fossa, where it sharpens posteriorly. Posteromedially, the posterodorsal process of the premaxilla overlaps the nasal bone, excluding the maxilla from the margin of both the narial fossae and the posterior margin of the external nares. A few thin and curved longitudinal ridges are present in the dorsolateral surfaces of the premaxillae, as well as some subtle bosses in their anterolateral sides. The premaxillae do not display any nodular ornamentation in their dorsal surfaces, differing from Proterochampsa barrionuevoi . The palatal rami of the premaxillae contact the anterior and lateral surfaces of the vomers. There is no sign of interdental plates in the premaxilla. Figure 4. Proterochampsa nodosa (MCP 1694-PV) in dorsal view. A, fossil. B, interpretative line drawing. Abbreviations: aof, antorbital fenestra; ar, articular; dt, dentary; ept, ectopterygoid; fr, frontal; itf, infratemporal fenestra; ju, jugal; la, lacrimal; mx, maxilla; na, nasal; nf, narial fossa; nr, nares; or, orbit; pa, parietal; pl, palatine; pmx, premaxilla; po, postorbital; pra, pre-articular; prf, prefrontal; pt, pterygoid; qd, quadrate; qdf, quadrate foramen; qj, quadratojugal; sq, squamosal; sur, surangular; sus, surangular shelf. Grey area represents the plaster restoration. Scale: 50 mm. Figure 5. Skull of Proterochampsa nodosa (MCP 1694-PV) in left lateral view. A, specimen MCP 1694-PV (Grey area represents the mandible of Pr. nodosa ). B, digital model of the skull. C, interpretative line drawing of the skull. Abbreviations: aof, antorbital fenestra; fr, frontal; itf, infratemporal fenestra; ju, jugal; lc, lacrimal; mx, maxilla; na, nasal; nod, nodules; op, ophistotic; or, orbit; pa, parietal; pmx, premaxilla; po, postorbital; prf, prefrontal; pt, pterygoid; qd, quadrate; qj, quadratojugal; so, supraoccipital; sq, squamosal. Scale bars: 50 mm. Maxilla ( Figs 3–5 ): The maxilla is large and comprises most of the lateral surface of the snout, extending from its contact with the premaxilla, level with the midportion of the nasal openings, until beyond the centre of the antorbital fenestra. The maxilla forms the anterior-third part of the border of the antorbital fenestra; however, it slightly differs in the left antorbital fenestra, as the anteromedial process of the left jugal forms most of its lateral border. Medially to the antorbital fenestra, in dorsal view, the maxilla of Proterochampsa nodosa has a sharp posterodorsal process that extends between the nasal and prefrontal but fails to reach the frontal. There is no maxillary antorbital fossa in the specimen. A shallow depression extending forwards from the anterior rim of the antorbital fenestra can be seen in the dorsal surface of the right maxilla, though it is not suggestive of a marked fossa, as seen in Pr. barrionuevoi ( Dilkes & Arcucci, 2012 ) . Furthermore, the anterior margin of the antorbital fenestra of Pr. nodosa is rounded and gently curved, differing from the sharp and angular condition in most Pr. barrionuevoi (e.g. PVL-2063 and PVSJ- 77). The dorsal contact between the maxilla and the jugal, in lateral view, is posteroventrally curved and goes from the anterior rim of the antorbital fenestra, across the lateral side of the skull and down until the end of the alveolar margin of the maxilla. The palatal process of the maxilla can be seen through the anterior half of the antorbital fenestra, in dorsal view ( Fig. 4 ) and contacts the palatine posteriorly and medially, forming an interdigitated suture, parallel to the anterior margin of the choana ( Fig. 3 ). Nasal ( Figs 3–5 ): The nasal forms the posterior border of the external nares and is better observed in the dorsal view. Its internarial process is thin and contacts the premaxilla anteriorly, in the midportion of the anteroposterior span of the external narial openings. There, the nasal and premaxilla form a point contact creating together an internarial bar. Anterolaterally the nasal contacts the premaxilla, excluding it entirely from the narial fossa, which is entirely composed by an excavation on the nasal surface. Laterally, the nasal contacts the dorsal facet of the maxilla for most of its anteroposterior length, extending from the posterior tip of the premaxilla and posteriorly surpassing the level of the posterior border of the antorbital fenestra. Posteriorly, the nasal inserts between the prefrontal and the frontal, and presents a posteromedial serrated suture with the frontal. Though some slices allow part of the nasal cavity to be seen ( Fig. 7 ), this is restricted to a small portion of the specimen. Indeed, most of the inner surface of the nasal cavity was poorly sampled in the tomograms, hampering a complete reconstruction. The nasal presents two longitudinal ridges: one along the medial suture, where either pair contacts each other; and one along the centre of the bone, extending anteriorly from the nares to reach the level of the antorbital fenestra. A few dispersed small nodules can also be seen, mostly concentrated in the posterior part of the left nasal. The posterior contact of the nasals with the prefrontal and frontal differs from most Pr. barrionuevoi specimens. While in Pr. barrionuevoi the nasal–frontal suture is almost transversally straight ( Reig, 1959 ; Dilkes & Arcucci, 2012 ), in Pr. nodosa both nasals form an anterior embayment and the frontals extend anteriorly between them. Another distinctive feature between both species is that, in the Brazilian form, the nasal pair is wider than the minimum interorbital space, while in Pr. barrionuevoi , the space between the orbits is virtually the same width as that of the nasal pair ( Dilkes & Arcucci, 2012 ). Prefrontal ( Figs 4–6 ): In dorsal view, the prefrontal has an overall trapezoid shape, from the anteromedial border of the antorbital fenestra until the posteromedial border of the orbit. Its lateral contact with the lacrimal is anteroposteriorly straight, levelled roughly at the lateromedial midpoint both of the antorbital fenestra and the orbit. The prefrontal forms most of the posteromedial border of the antorbital fenestra. Anteriorly, it has a small and orthogonal contact with the posteromedial end of the maxilla. The posterior region of the prefrontal, by its turn, forms the anteromedial border of the orbit. Medially and posteromedially, the prefrontal contacts the frontal, and this suture is marked by a small, longitudinal ridge. There is a small medial contact between the prefrontal and the nasal, which prevents the posterior tip of maxilla from reaching the frontal. Together with the frontal, the prefrontal forms a tall and well-developed interorbital ridge, which extends bordering the medial and anteromedial rim of the orbits, forming a deep and marked U-shaped depression, with its concavity facing anteriorly. This feature is markedly distinct from Pr. barrionuevoi , where the interorbital region is comparatively smoother and less elevated. Barberena (1982) also mentions that the prefrontal part of this ridge branches out into approximately three smaller ridges in Pr. nodosa , while Dilkes & Arcucci (2012) indicate the presence of only two branches. The left prefrontal counterpart could be interpreted as having a ridge branching into three smaller ridges, but only two of these branches can be pointed out with certainty. Frontal ( Figs 3–5 ): In dorsal view, the frontal is an elongate bone that extends from level with the posterior border of the antorbital fenestra until the posterior border of the orbit. The medial suture between the two frontals can be seen along most of its extent. In dorsal view, the frontal can be divided into an anterior and a posterior ramus, which are separated by an interorbital ridge, with the posterior ramus standing significantly higher than the anterior one ( Fig. 4 ) The anterior ramus is narrower and elongated, comprising nearly two-thirds of the total anteroposterior length of the frontal. While in Pr. barrionuevoi the anterior portion of the frontal contributes to the medial rim of the orbit, in Pr. nodosa the anterior ramus of the frontal is isolated in the middle of the skull by the prefrontal and only its posterior ramus contributes to the orbit, forming its posteromedial rim. Barberena (1982) highlights the frontal morphology and the frontal–nasal suture as important features in distinguishing Pr. nodosa from Pr. barrionuevoi . Dilkes & Arcucci (2012) noted that the discovery of new specimens of Pr.barrionuevoi diminishes this difference between the two species, given the intraspecific variability in the Argentinean species. However, its anterior and lateral sutures are still sufficiently distinct to be denoted as species-specific features. While the frontal in specimens of Pr. barrionuevoi is nearly rectangular, forming lateromedially straight sutures with the nasal and parietal, the sutures in Pr. nodosa are more complex, displaying mainly oblique frontal–nasal and frontal–parietal contacts, with its anterolateral border delimited by a posterior process of the nasal bone. Lacrimal ( Figs 3–5 ): The lacrimal is a small element with an irregular shape. In dorsal view, this bone is limited to the space between the posterior border of the antorbital fenestra and the anterolateral border of the orbit, but the morphology of both lacrimals in Pr. nodosa is inconsistent. While the right lacrimal is restricted in-between the antorbital fenestra and the orbit, its left counterpart seems to extend posteriorly, nearly contacting the anterolateral edge of the postorbital. While in Pr. barrionuevoi the lacrimal is restricted between the posterior margin of the antorbital fenestra and the anteromedial margin of the orbit, in Pr. nodosa the lacrimal is comparatively expanded and forms a greater portion of the anterior and anterolateral rim of the orbit. Its overall shape is semicircular. Laterally, the contact with the jugal bone is rounded, while its medial contact with the prefrontal is anteroposteriorly straight. A subtle longitudinal ridge is present at the anterolateral end of both lacrimals. Figure 6. Proterochampsa nodosa (MCP 1694-PV) in occipital view. A, fossil. B, interpretative line drawing. Thicker lines indicate the separation of the skull and mandible. Grey area represents the plaster restoration. Abbreviations: ang, angular; ar, articular; ept, ectopterygoid; op, opisthotic; pa, parietal; par, pre-articular; pt, pterygoid; qd, quadrate; qdf, quadrate foramen; qj, quadratojugal; so, supraoccipital; sp, splenial; sq, squamosal; sur, surangular. Scale: 50 mm. Jugal ( Figs 3–5 ): The jugal is triradiate and formed by an anterior, a dorsal and a posterior ramus. While the left jugal element is completely preserved, the right counterpart has a fractured posterior ramus. The jugal would be best described in lateral view as an upside-down ‘T’ ( Fig. 5 ), sprawling anteroposteriorly and, to a lesser extent, dorsally. This bone comprises a large part of the lateral side of the skull, forming most of the lateral rim of the antorbital fenestra, a small portion of the lateral border of the orbit, and most of the anterior and lateral rim of the infratemporal fenestra. Anteriorly, it forms a dorsoventrally oblique contact with the maxilla, roughly at the middle to anterolateral point of the antorbital fenestra. Its left counterpart contacts the lacrimal in a near U-shaped suture when seen dorsally, while the same contact in its right counterpart appears to be nearly longitudinally straight. The posterior ramus of the jugal, in lateral view, extends forming an oblique contact that overlaps the anterior ramus of the quadratojugal. The dorsal ramus of the jugal contacts the postorbital bone, with the former element composing most of the anterior margin of the infratemporal fenestra. In dorsal view, the anterior ramus of the left jugal forms the lateral margin of the antorbital fenestra, up to its anterior margin. However, the suture of the anterior contact between the right jugal and right maxilla is not as clear, and the anterior ramus of the jugal seems to form only the posterolateral margin of the antorbital fenestra. In ventral view, the contact between the jugal and the ectopterygoid can be observed ( Fig. 3 ). It is similar to the condition depicted in Pr. barrionuevoi , in which the lateral margin of the ectopterygoid overlaps the medial margin of the jugal. Figure 7. Tomographic slices of Proterochampsa nodosa (MCP 1694-PV). Red stripe on the skull model indicates the approximate location of the slice. Numbers at the bottom-left reference the slice number on the tomographic image stack. Abbreviations: aof, antorbital fenestra; ca, canals on the nasal cavity; ch, choana; dt, dentary; fr, frontal; itf, infraorbital fenestra; mx, maxilla; na, nasal; nc, nasal cavity; or, orbit; pmx, premaxilla; po, postorbital; pt, pterygoid; qd, quadrate; sp, splenial; sq, squamosal; sur, surangular; surs, surangular shelf; t, tooth. Postorbital ( Figs 3–5 ): In dorsal view, the postorbital is a triradiate, nearly Y-shaped structure, located posterior to the orbit and between the infra- and supratemporal fenestrae. Anteriorly, it forms the posterior border of the orbit with a well-developed and curved ridge ornamentation. Medially, it forms the anterolateral border of the supratemporal fenestra, while its lateral surface forms the medial border of the infratemporal fenestra. The postorbital forms a contact with three distinct bones, these being the jugal, the parietal and the squamosal. Its anterolateral ramus makes a rounded contact with the dorsal ramus of the jugal bone. Anteromedially, it contacts the parietal obliquely, without forming a contact with the frontal (as is present in Pr. barrionuevoi ). Posteriorly, it contacts the squamosal. The posterior ramus of the postorbital bone is especially elongated when compared to that in other representatives of Proterochampsia, such as Pr. barrionuevoi and Pr. ischigualastensis ( Dilkes & Arcucci, 2012 ; Trotteyn et al. , 2012 ; Trotteyn & Ezcurra, 2014 ). However, it resembles the condition described in Chanaresuchus bonapartei ( Romer, 1971 ; Trotteyn & Ezcurra, 2020 ). Parietal ( Figs 3–6 ): In dorsal view, the parietal is a large and robust irregular element that occupies most of the posterior dorsomedial portion of the cranium. The medial contact between the parietals is completely fused, with no evidence of suture line. The parietal has a small contribution to the posteromedial border of the orbit, but forms the entire anterior and medial borders of the supratemporal fenestra. Most of the posterolateral portion of the right parietal is fractured and, while the anteromedial border of the supratemporal fenestra is preserved, it is covered with the plaster and can only be visualized in the CT-scan (Supporting Information, Fig. S1 ; de SimãoOliveira et al. , 2022). The anterior contact of the parietals with the frontal forms an oblique suture and, medially, it inserts itself in a small process between the frontal posteriormost tips. Between the orbit and the supratemporal fenestra, the anterolateral ramus of the parietal has a short and oblique contact with the postorbital, while, posterolaterally, the parietal– squamosal contact is much longer and extended. In agreement with the original description by Barberena (1982) , only a few nodules seem to be present in the dorsal surface of the parietals. However, the bone does have ridges that run along the longer axis of the bone and are interspaced by large dorsal fossae: one between the orbit and the supratemporal fenestra; and another one on the posteromedial border of the supratemporal fenestra. Medially, in between the supratemporal fenestrae, there is a medial convex depression in the parietals, although it is not clear if this is a taphonomical condition or not, given the entire skull roof may have suffered a slight collapse. The parietal has a small dorsal exposure when seen in occipital view. A large dorsal nodule is present, projecting outwards from the posterodorsal surface of the skull. Dorsolaterally, it forms a slightly curved contact with the squamosal. Ventromedially, the parietal–supraoccipital contact is oblique and slightly curved, nearly forming an S-shaped suture. Meanwhile, most of its ventral surface forms the contact with the opisthotic bone, which overlaps the parietal. Squamosal ( Figs 3–6 ): The squamosal composes the posterior corner of the dorsal skull table and the dorsolateral portion of the occipital margin. In dorsal view, the anterior process of the squamosal is overlapped by the posterior ramus of the postorbital bone, while forming the dorsal half of the posterior margin of the infratemporal fenestra. Medially, it contacts the lateral side of the parietal and forms the posterolateral margin of the supratemporal fenestra. Its suture with the parietal is simple, but slightly inclined posterolaterally from the medial axis of the skull. The posterodorsal corner of the squamosal is projected posterolaterally in dorsal view, outwards from the occipital margin of the skull. In occipital view, the ventromedial surface of the squamosal contacts the parietal and the opisthotic, as well as the dorsolateral extremity of the occipital process of the pterygoid. The ventrolateral surface of the squamosal makes a sinuous contact with the dorsal portion of the quadrate and part of the quadratojugal. Quadratojugal ( Figs 3–6 ): The quadratojugal comprises the posteroventral portion and lower corner of the skull of Proterochampsa nodosa . In lateral view, it contacts the squamosal dorsally, while also composing the posteroventral margin of the infratemporal fenestra ( Fig. 5 ). Anteriorly, the quadratojugal makes an oblique contact with the posterior ramus of the jugal. The quadratojugal extends posteriorly, well beyond the occipital portion of the cranium. The quadratojugal is heavily ornamented by tubercular growths and lateral nodular projections. A hook-like projection is particularly present in the lateral end of the bone and it is characteristic of the genus Proterochampsa , as denoted by Dilkes & Arcucci (2012) . Its occipital surface contacts the quadrate ventrally and medially, forming an almost right angle, in the corner of which a shallow embayment constitutes the quadratojugal contribution to the quadrate foramen. Quadrate ( Figs 3–6 ): The quadrate is a robust and large bone that is most notably exposed in posterior view. In dorsal view, its corner is squared, forming a near 90° angle, contrasting with Proterochampsa barrionuevoi , where it is gently rounded. Furthermore, contrary to Pr. barrionuevoi , the quadrate of Pr. nodosa does not contact the opisthotic, being isolated from it by the squamosal and the pterygoid. The quadrate comprises a large portion of the posterior surface of the skull. In occipital view, the quadrate outline has an overall hourglass shape ( Fig. 6 ). A marked notch at its lateral surface constitutes the medial margin of the quadrate foramen, which also has a contribution of the quadratojugal. The ventralmost portion of the quadrate is saddle-shaped forming an articular process that receives the articular bone of the mandible, to perform the jaw articulation. In the holotype , this structure is obscured by the jaw occlusion, but it was possible to observe some information on the articulation surface due to the segmentation of the tomographic scans. Despite the deformation caused by the contact with the lower jaw, it is possible to observe that the square-shaped posteromedial corner of the quadrate also extends to its articular surface. As a result, the posterior margin of the articular surface of the quadrate, which received the mandible, is perpendicular to the main longitudinal axis of the cranium. Opisthotic ( Figs 3 , 6 ): The opisthotic is the bestpreserved bone of Proterochampsa nodosa occiput and, as in Pr. barrionuevoi , seems to be the most prominent element around the foramen magnum, although the region is mostly missing in the Pr. nodosa holotype . The opisthotic can be divided in two distinct parts: an occipital and a ventral ramus. The ventral ramus of the opisthotic is blade-like, and slightly elevated when compared to the pterygoid, with which it loosely contacts its entire anteroventral surface.The opisthotic occipital surface is slightly concave in comparison to the bones to which it is connected. Most of its dorsal surface contacts the parietal bone. The paroccipital process of the opisthotic is offset relative to the rest of this element and is marked by some subtle ridges near the occiput. Dorsomedially, it forms a small and curved contact with the supraoccipital, which is fragmentary in the specimen. The ventromedial side of the opisthotic, where it would contact the basioccipital, is marked by a parallel to acute-angled sulcus. Supraoccipital ( Figs 3 , 6 ): Only a fragment of the left supraoccipital is preserved in Pr. nodosa , appearing as an elongated, plate-like structure, visible only in occipital view, with slightly curved dorsal and ventral edges. It contacts the parietal bone dorsally and the opisthotic ventrolaterally. The supraoccipital is a slightly convex bone, with its dorsal margin standing slightly higher than its centre. Palate ( Figs 3–7 ; Supporting Information, Fig. S2 ): Most of the palate of Proterochampsa nodosa is obscured by the jaw occlusion of the specimen.Although the jaw rami were virtually removed by segmentation, the overall interpretation of contacts, or even the morphology of some palatal elements, were affected by the poor resolution of the tomography in the jaw occlusion area. The dentary teeth are mostly crushed against the palate, which generates X-ray ‘flares’ in the scans, making it difficult to visualize the anterior palatal surface, around the vomer and palatine, and to determine its morphology. The exposed medial portion of the palatine contacts the vomer and pterygoid, and forms the posterolateral border of choana, similar to the description in Pr. barrionuevoi . This contrasts to the morphology of Chanaresuchus bonapartei , where the palatine forms the entire posterior border of the choana, as well as part of its posterolateral and posteromedial margins ( Trotteyn & Ezcurra, 2020 ). Most of the morphology of the pterygoids is exposed, although the right counterpart is mostly fragmented. Both pterygoid counterparts contact each other medially with their anterior rami. Anterolaterally, the pterygoid contacts the palatal process of the maxilla. Laterally, the pterygoid contacts the ectopterygoid, which extends from the anterior margin of the antorbital fenestra to the anterior margin of the infratemporal fenestra. The ventral ramus of the pterygoid is extended downwards and almost touches the pre-articular and angular bones, at the midline of the mandibular fenestra. This contact of the pterygoid and the mandible is most likely exaggerated by a taphonomic rotation of the mandible. This ventral process of the pterygoid contributes to form a bony secondary palate that may have helped the animal to maintain itself half-submerged under water margin. This potential adaptation is also seen in Proterochampsa barrionuevoi ( Dilkes & Arcucci, 2012 ; Ezcurra, 2016 ) and a similar morphology is displayed by other semi-aquatic archosauriforms, such as phytosaurs and crocodylians. A few palatal teeth are present in the ventral surface of the right pterygoid ( Fig. 3 ), but the manner by which the tooth rows are disposed is not completely clear. These were also too subtle to be in the CT-scan and resulting 3D-models. However, from what remains preserved, they appear to be disposed in a single row, extending from the posterior palatal ramus of the pterygoid towards the palatine. These teeth are small, round and blunt in shape. Judging by the positioning of these palatal teeth, this could be related to the single tooth row (tooth zone T3) observed in Chanaresuchus and Pr. barrionuevoi , which is exclusive to these taxa amongst archosauromorphs ( Dilkes & Arcucci, 2012 ; Ezcurra, 2016 ; Trotteyn & Ezcurra, 2020 ). However, the fragmentary condition in this specimen makes it unclear to determine this with certainty. Figure 8. Interpretative drawing of the left mandibular ramus of Proterochampsa nodosa (MCP 1694-PV) over its digital model. A, dorsal view. B, medial view. C, lateral view. D, ventral view. Abbreviations: ang, angular; ar, articular; dt, dentary; lang, lamina on the angular; mdf, mandibular fenestra; sp, splenial; sur, surangular; surs, surangular shelf; sym, mandibular symphysis. Scale: 50 mm. Mandible ( Figs 3 , 6 , 7 , 8 ): The digital scan has allowed the reconstruction of most of the lower jaw, except for the anterior edge of the dentaries and their alveoli. However, sutures are not entirely clear in the superficial morphology of the bones due to the low resolution of the CT-scan in the surfaces where mandible occludes with the upper jaw, especially at the occlusal region around the dorsal face of the posterior contact of the surangular with the articular and the anterior contact of the surangular with the dentary. Therefore, the contacts among these mandibular bones (as depicted in Fig. 8 ) are in part tentative and based on the inner contact of the mandibular elements accessible via the tomographic scans. As a result of the dorsal crushing of the mandible, its left mandibular ramus is rotated, with the tooth row rotating laterally and the ventral portion artificially positioned medially. The mandible is Y-shaped in dorsal or ventral views and it is slightly shorter than the skull. The anterior region of the mandible is narrow and slender when compared to its posterior end, where it is relatively tall and robust. The mandibular symphysis is elongated and fused primarily along the dentaries, while the splenials contribute only with the edge of their anterior rami. The mandibular fenestra is large and its dorsal border there is a laterally wide shelf on the surangular bone ( Fig. 8 , surs). Some subtle protuberances occupy the ventral surface of the surangular and angular at the posterior flooring of the mandible. The presence of a coronoid element could not be verified. Dentary ( Figs 3 , 8 ): The dentary is a slender and long bone when compared to the remaining of the mandible. It forms more than half of the mandibular dorsal surface. The mandibular symphysis is restricted to the anterior portion of this element and extends from the anteromedial end of the mandible to level with the ninth or tenth alveolus, where the dentary also begins its anteromedial contact with the splenial, occupying approximately one-fourth of the dentary. The dentary forms a subtle curvature at its posterior half, where it sprawls laterally ( Fig. 8A ). The dentary–splenial contact is long. It occupies the medial side of the dentary and extends from the posterior end of the mandibular symphysis up to the anterior margin of the mandibular fenestra. The dentary, along with the splenial, forms the anterior half of the mandible ventral margin. Posteriorly, the dentary forms a slender extension that ascends dorsally and reaches the mandibular fenestra, contacting the angular ventrally (in lateral view), the surangular dorsally and forming a small contribution to the anteromedial margin of the mandibular fenestra. The dentaries preserve a total of 44 teeth, 18 of those in the left dentary and around 25 in its right counterpart. Although, as previously mentioned, most alveoli are obscured in the specimen and some of the teeth are either crushed or entirely missing, which hamper a precise estimation of the full tooth count. Splenial ( Figs 3 , 6 , 8 ): The splenial is lateromedially slender and forms much of the medial surface of the mandible. It contacts the medial side of the dentary from the mandibular symphysis to the anterior end of the mandibular fenestra. It contributes only a small portion of the posterior edge of the mandibular symphysis. In ventral view, it forms a curved contact with the anterior process of the angular. This potentially differs from Pr. barrionuevoi , in which the splenial is reported to taper posteriorly and interpose itself between the angular and pre-articular ( Dilkes & Arcucci, 2012 ). Angular ( Figs 3 , 6 , 8 ): The anterior process of the angular contributes only with a small portion of the adductor fossa of the mandible but composes the entirety of the ventral rim of the mandibular fenestra. Along with the dentary, it forms most of the ventral edge of the mandible, contacting the dentary anteriorly and laterally. In its medial side, the angular makes a curved contact with the ventral surface of the pre-articular by the midline of the adductor fossa, reaching the articular in medial view, though this contact is almost prevented by the pre-articular. Near the anterior end of this contact, the angular forms an ascending contact with the splenial in medial view. A lamina is present in the posteroventral surface of the angular ( Fig. 8 , lang). This feature is shared only with Pr. barrionuevoi ( Dilkes & Arcucci, 2012 ) and it is not seen in any other proterochampsian. Laterally, the contact with the surangular is simple and longitudinally straight. This element presents a series of small protuberances that extend from the ventrolateral surface of the angular, near the height of the anterior margin of the mandibular fenestra, up to its lateral surface where it contacts the surangular, posteriorly, and ventrally it presents an elongated and longitudinally transverse contact that extends from the height of the anterior margin of the mandibular fenestra, up to its contact with the dentary. Surangular ( Figs 3 , 6 , 8 ): The surangular forms most of the dorsal surface of the posterior-third of the mandible and the entire the dorsal margin of the mandibular fenestra. Its anterior ramus contacts the dentary. Medially, it contacts the pre-articular through a horizontal line that spans from the posterior portion of the mandibular fenestra and reaches the articular posteriorly. In lateral view, the ventral surface forms a longitudinal contact with the angular. Finally, the posterior end contacts the ventral side of the articular, which is better observed in dorsal and lateral views. Indeed, in lateral view, the surangular interposes itself between the angular and articular, preventing these bones from contacting each other. Like the angular, a series of protuberances mark the lateral surface of the surangular, but one of its most striking features is a wide and well-developed shelf ( Fig. 8 , surs) that is projected laterally in the mandible. This feature is shared by all proterochampsids, but it is especially prominent in the genus Proterochampsa ( Dilkes & Arcucci, 2012 ; Trotteyn et al. , 2013 ). Pre-articular ( Fig. 8 ): The pre-articular is a slender and curved bone only seen in the medial side of the mandible. Posteriorly, it contacts the articular by an almost vertical suture. The pre-articular contacts the angular in a curved manner and anteriorly its ventral facet articulates orthogonally with the angular. The pre-articular comprises the entire ventral rim of the mandibular adductor fossa, bounded by its dorsally concave surface. Although the pre-articular contacts the posterolateral edge of the splenial, the nature of this contact may differ to that described in Pr. barrionuevoi , as previously discussed. Articular ( Figs 6 , 8 ): The articular is small and overall square-shaped when visualized in dorsal view. Most of this bone is covered by the quadrate in the specimen but, as observed in the CT slices, dorsally it forms two curved depressions that articulate with the articular process of the quadrate. Ventrally, it contacts the pre-articular and the angular, while contacting the surangular anteroventrally. There is no retroarticular process in the posterior end of Proterochampsa nodosa . The absence of a retroarticular process is diagnostic for the genus Proterochampsa , being shared with the Rhadinosuchinae proterochampsid Pseudochampsa ischigualastensis ( Trotteyn et al. , 2012 ; Trotteyn & Ezcurra, 2014 ). In contrast, all other proterochampsids possess this structure, although underdeveloped ( Trotteyn et al. , 2013 ). Dentition ( Figs 3 , 5 , 8 ): Proterochampsa nodosa preserves a total of 82 teeth, positioned as follows: ten in the premaxillae; 22 in the maxillae; and 50 in the dentaries. The teeth of Pr. nodosa are posteriorly curved, generally large with an oval root and base that taper apically. No serrations are visible in any teeth. The premaxillary teeth of Pr. nodosa are slenderer and smaller when compared to the rest of the dentition. The maxillary teeth intercalate in size, varying between larger and smaller teeth along the alveolar shelf, tending to decrease in size towards its posterior end, near the height of the anterior border of the antorbital fenestra and the jugal. Some of the larger maxillary teeth of Pr. nodosa are distorted and form an inconsistent shape ( Fig. 5B, C ). PHYLOGENETIC ANALYSIS The phylogenetic analysis applied to the modified data matrix and parameters set by Trotteyn & Ezcurra (2020) recovered a similar consensus tree ( Fig. 9 ; Supporting Information, Fig. S3 ) to the one presented by those authors. The analysis resulted in 27 most parsimonious trees with 3754 steps each, with a consistency index of 0.253 and a retention index of 0.650. The recovered topology for proterochampsids is the same as that reported by previous works ( Butler et al. , 2019 ; Trotteyn & Ezcurra, 2020 ), with both species of Proterochampsa forming a group as the basalmost proterochampsids, which has Doswelliidae as a sister-taxon. Cerritosaurus and Tropidosuchus are recovered as successive taxa leading to a polytomy composed of Chanaresuchus , Gualosuchus , Pseudochampsa and Rhadinosuchus . The phylogenetic analysis employing the dataset of Wynd et al. (2020) , using the ‘new technology’ search parameters, also recovered a similar consensus tree ( Fig. 10A ; Supporting Information, Fig. S4 ). Analysis resulted in 432 most parsimonious trees of 3440 steps with a consistency index of 0.255 and a retention index of 0.621. The recovered topology for proterochampsids is the same as that described for the previous analysis, with Proterochampsa as the basalmost proterochampsids and Doswelliidae as a sister-taxon to Proterochampsidae . Different topologies were recovered when analysed with varying weights against homoplasy. With a value of k = 4, Dosweliidae nests with Proterochampsa within Proterochampsidae , as a sister-group to all other proterochampsids ( Fig 10B ; Supporting Information, Fig. S5 ). The clade composed of Litorosuchus + Vancleavea is nested outside Proterochampsia (which groups the same OTUs as Proterochampsidae in this scenario). Additionally, Proterochampsia is recovered basal to Dorosuchus noetus , which in turn is the sister-taxon of Euparkeria capensis . The latter OTU is recovered as the sistertaxon to Archosauria in this iteration of the analysis. Milder concavities ( k = 7; k = 16) each recover the sametopology for Proterochampsia ( Fig. 10C, D ; Supporting Information, Figs S6 , S 7 ). In these iterations, Litorosuchus + Vancleavea are nested in Proterochampsia, as a sister-taxon to Proterochampsidae . Doswelliidae are recovered within Proterochampsidae , with Proterochampsa as a sister-taxon. Proterochampsa is now recovered as the sister-taxon to Archosauria, as in the analysis with equal weights employing both datasets (Trotteyn & Ezcura, 2020; Wynd et al. , 2020 ). Proterochampsian topology recovered for k = 7 and k = 16 is similar to that recovered by the implied-weighting ( k values ranging from 4 to 18) analyses performed by Trotteyn & Ezcurra (2020) . This is also the same topology (for Proterochampsia) recovered by Wynd et al. (2020) using Bayesian analysis.