https://orcid.org/0000-0001-8262-6960
Abstract
The Anteosauridae (Dinocephalia) were the largest predators of Guadalupian (Middle Permian) land communities. The, thus far, only known South American anteosaurid is the medium-sized syodontine Pampaphoneus biccai. This taxon was, until now, known only by its holotype, recovered from an outcrop of the Rio do Rasto Formation, Southern Brazil. Here we describe in detail an almost complete Pampaphoneus biccai skull associated with a few postcranial remains (UNIPAMPA 759) recently recovered from the type locality. The novel morphological information provided by this novel, better-preserved, specimen revealed that most characters previously proposed to distinguish Pampaphoneus biccai from Syodon biarmicum are probably ontogenetically constrained. We identify, however, new features that clearly distinguish these two taxa, and provide an updated diagnosis for Pampaphoneus biccai. Our phylogenetic reassessment of the taxon echoes previous propositions in which Pampaphoneus biccai was recovered as an early-divergent Syodontinae. Additionally, we tentatively indicate that a larger fragmentary specimen (UFRGS-PV-0249-P) may represent the mature size of the species, with the similar-sized holotype and UNIPAMPA 759 probably representing subadult individuals.
Introduction
Dinocephalians were a short-lived lineage of early therapsids, and included some of the largest known land predators of their time (Rubidge 1991, Day et al 2015). The clade was temporally restricted to the Guadalupian (Permian) but globally distributed, with specimens reported from both Laurasia and Gondwana (e.g. Kammerer 2011). Laurasian dinocephalians are reported from China (Liu 2013), Kazakhstan (Tchudinov 1968), and Russia (e.g. Kammerer 2011), whereas the Gondwanan records come from Brazil (Langer 2000, Cisneros et al. 2012, Boos et al. 2015), South Africa (e.g. Day and Rubidge 2020, Rubidge and Day 2020), Tanzania (Simon et al. 2010), Zimbabwe (Lepper et al. 2000), and Zambia (Sidor et al. 2014).
Dinocephalia is divided into two major clades: the carnivorous Anteosauria and the herbivorous Tapinocephalia (Rubidge and van den Heever 1997, Rubidge and Sidor 2001, Kammerer 2011, Boos et al. 2015). The Anteosauria comprise a single family—the Anteosauridae—which includes the subfamilies Syodontinae and Anteosaurinae (Kammerer 2011). In contrast to the Anteosaurinae, which usually present larger body mass and exceptional skull pachyostosis probably befitting a preference for bigger prey, the Syodontinae were smaller and mostly lacked skull reinforcements, indicating a diet limited to smaller animals (Kammerer 2011). The Syodontinae are formed by five species: Notosyodon guseviTchudinov 1968 (Kazakhstan), Syodon biarmicumKutorga 1838,Microsyodon orloviIvakhnenko 1995 (Russia). Australosyodon nyaphuliRubidge 1994 (South Africa), and the Brazilian taxon Pampaphoneus biccaiCisneros et al. 2012 (South America).
In South America, Pampaphoneus biccai is not only the single known anteosaurid, but also the sole dinocephalian with a species-rank attribution (Cisneros et al. 2012). It was recovered as the earliest-branching Syodontinae, being a medium-sized taxon with a moderately pachyostotic skull (Cisneros et al. 2012, Liu 2013). Pampaphoneus biccai was known only by its holotype, a well-preserved 320-mm long cranium, recovered in 2008 from the Boqueirão Farm (São Gabriel municipality, Southern Brazil) in a typical Rio do Rasto Formation outcrop. More recently, a second, almost complete skull was recovered from the same locality, together with some sparse (and mostly uninformative) postcranial remains. Here we present a detailed comparative description of this new specimen, UNIPAMPA 759, contributing to an anatomic revision of Pampaphoneus biccai and improvement of its diagnosis and phylogenetic affinity.
Institutional abbreviations:
UFRGS, Laboratório de Paleovertebrados, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil; UNIPAMPA, Laboratório de Paleobiologia, Universidade Federal do Pampa, São Gabriel, Brazil; NMQR: National Museum, Bloemfontein, South Africa; PIN, Borissiak Paleontological Institute of the Russian Academy of Sciences, Moscow, Russia; SAM: Iziko, South African Museum, Cape Town, South Africa.
Anatomical abbreviations:
bo, basioccipital; b.ptpp, broken pterygoid posterior process; bs, basisphenoid; b.tpt, broken transverse process of the pterygoid; b.te, broken teeth; ca, canine; cap, capitulum; cc, carotid canals; ce, cervix; co, occipital condyle d, depression; de, dentary; de.ca, dentary canine; de.te, dentary teeth; dex, dentary expansion; en external naris; exo, exoccipitals; f, frontal; fc, frontal crest; fe, temporal fenestra; fm, foramen magnum; g, groove; h, heel; i, incisor; i1, first incisor; i2, second incisor; i3, third incisor; i4, fourth incisor; j, jugal; k, keel; la, lacrimal; lc, lateral condyle; L.de, left dentary; mc, medial condyle; mk, meckelian fossa; mx, maxilla; n, nasal; nf, nutritive foramina; o, orbit; oc, occipital condyle; op, opisthotic; pa, parietal; pf, postfrontal; pfo, pineal foramen; pl, palatine; plb, palatine boss; pl.te, palatine tooth; ph, phalanx; pmx, premaxilla; po, postorbital; ppa, postparietal; prf, prefrontal; pro, prootic; pt, pterygoid; ptmr, pterygoid medial ridge; pt.te, pterygoid tooth; ptpp, pterygoid posterior process; qpr, quadrate pterygoid ramus; r, ridge; R.de, right dentary; s, striations; s.ca, slot for canine; se, serration; smx, septomaxilla; so, supraoccipital; sp, splenial; sq, squamosal;sy, symphysis; tab, tabular; tpt, transverse process of the pterygoid; tr, tuberculum v, vomers; vc, vidian canal.
Geological settings
The Paraná Basin is an intracratonic sedimentary basin that encompasses an area of approximately 1 500 000 km², covering parts of Brazil, Argentina, Uruguay, and Paraguay (Milani et al 2007). In Brazil, the Paraná Basin Permian record is restricted to the Gondwana I Supersequence (Milani et al. 2007). Among the Permian formations, the Rio do Rasto Formation is the only one to yield terrestrial tetrapod fossils, comprising rocks deposited between the Roadian and the Wuchiapingian (Dias-da-Silva 2012). The Rio do Rasto Formation was divided into two distinct members by Gordon Jr (1947), the lower Serrinha Member and the upper Morro Pelado Member where terrestrial tetrapod fossils occur.
The Morro Pelado Member is characterized by reddish siltstones and mudstones, intercalated by lenticular bodies of fine sandstones (Schneider et al. 1974, Holz et al. 2010). Its depositional environment is interpreted as ranging from meandering fluvial channels, lacustrine, proximal deltaic, and aeolian layers (Warren et al. 2008, Holz et al. 2010, Simões and Matos 2015). At the top of the Morro Pelado Member, there is a trend towards the predominance of sandstone layers, indicating an increased continentalization and aridification, which evolves towards the desert environments of the Piramboia Formation (Lavina 1991, Warren et al. 2008, Holz et al. 2010, Alessandretti et al. 2016, Boos et al. 2016). The tetrapod record from the Morro Pelado Member extends through the Rio Grande do Sul, Santa Catarina, and Paraná states (Southern Brazil), and includes pareiasaurs (e.g. Cisneros et al. 2005, 2021), temnospondyls (e.g. Strapasson et al. 2015, Pacheco et al. 2017, Dias et al. 2020), dicynodonts (e.g. Boos et al. 2016), and dinocephalians (e.g. Cisneros et al. 2012, Boos et al. 2015). The specimen described herein (Fig. 1A, B) is from the Boqueirão Farm Locality (Fig. 1C, D), Catuçaba District, São Gabriel Municipality, Rio Grande do Sul State, and was found along the East section of the outcrop (as defined by Boos et al. 2016).
A, right lateral view of UNIPAMPA 759 still embedded in the rock matrix; B, interpretative drawing. C, map of South America depicting the Paraná Basin. D, map of Boqueirão Farm Locality, Catuçaba District, São Gabriel Municipality, Rio Grande do Sul State. Map produced with QGIS2021 software (QGIS Development Team 2021) by Arielli Machado. E, the Boqueirão Farm East outcrop showing the reddish siltstones and sandstones. The white arrow indicates the place where the specimen was found. The brown areas at the interpretative drawing represent sedimentary remains or fractures filled with polyethylene glycol.
Material and methods
The specimen UNIPAMPA 759 (Fig. 1) is formed by articulated skull and lower jaws, as well as postcranial remains comprising ribs, a phalanx, and some unidentified bones. The skull was preserved split into two halves, almost along the sagittal plane, exposing the internal surfaces of the bone elements. The palate and braincase were also separated from the remainder of the skull. Both lateral sides of the skull are well preserved. The right lateral portion lacks the postorbital, parts of the squamosal, and the dorsalmost portion of the right quadrate (Figs 2, 3). The skull roof is damaged, and parts of the nasals and the frontals are missing. On the left side, the quadrate is missing, as well as parts of the squamosal. On the right side of the braincase, some portions of the tabular and the opisthotic are damaged. On the left side, most of the tabular is missing and the opisthotic is damaged. The palate lacks the anteriormost portion of the vomers. Parts of the pterygoid are damaged. The stapes are missing. The lower jaws lack the post-dentary elements, most of the dentary incisors, and some post-canines. Overall, the skull is not significantly distorted, but a slight lateromedial compression is present.
Left lateral view of the skull of Pampaphoneus biccai UNIPAMPA 759 (A) and interpretative drawing (B). The brown areas at the interpretative drawing represent sedimentary remains or fractures filled with polyethylene glycol.
Right lateral view of the skull of Pampaphoneus biccai UNIPAMPA 759 (A) and interpretative drawing (B). The brown areas at the interpretative drawing represent sedimentary remains or fractures filled with polyethylene glycol.
All bones are covered by a dark oxidized manganese crust, as in Rastodon procurvidens and other fossils found in this locality (Boos et al. 2016). Some ferruginous concretions were also present covering most of the external surface of the skull and teeth. The surrounding rock, a pinkish fine sandstone, and oxidized crust were removed using pneumatic air scribers (PaleoAro 3 and Microjack 1 of Paleotools®). The acryloid Paraloid-B-72 and cyanoacrylate were used to consolidate the fossils and to glue together the fragmentary portions. Polyethylene glycol was used to fill missing portions for storage stability.
Comparative description
The present description of UNIPAMPA 759 was conducted primarily by comparison with the type specimen of Pampaphoneus biccai (UFRGS-PV-0386-P) (Fig. 4A–D) and key anteosaurians based on the literature and photographs of the specimens (see Supporting Information, Table S1).
UFRGS-PV-0386-P, the holotype of Pampaphoneus biccai. A, right lateral view. B, left lateral view. C, palatal view. D, dorsal view. E, dorsal view of UFRGS-PV-249-P. F, lateral view of UFRGS-PV-0249-P.
Phylogenetic analysis
The new anatomical information drawn out from the description of UNIPAMPA 759 allowed us to revise and update Pampaphoneus biccai scorings for seven morphological characters from the dataset of Liu (2013), which in turn is an update of the original data matrix of Kammerer (2011) (see the Appendix). This resulted in 38 out of 40 scored characters for this particular operational taxonomic unit (OTU), i.e. only 5% of missing data for Pampaphoneus biccai in our new scoring. In comparison, the previous analysis had 35 of 40 scored characters, resulting in 12.5% of missing data.
The dataset was edited using the software MESQUITE v.3.70 (Maddison and Maddison 2018), whereas the phylogenetic analysis was performed in TNT (Tree analysis using New Technology) v.1.5. (Goloboff and Catalano 2016). The low number of OTUs (N = 14) allowed us to perform an exact search strategy through the Implicit Enumeration search algorithm of TNT. Additionally, Bremer node supports were calculated through the retention of suboptimal trees (TBR, 10 steps), also in TNT. We also used TNT to perform symmetric resampling as a proxy for clade support (Goloboff et al. 2003).
Systematic palaeontology
Synapsida Osborn 1903
Therapsida Broom 1905
Dinocephalia Seeley 1904
Anteosauridae Boonstra 1954
Syodontinae Ivakhnenko 1994
Pampaphoneus biccaiCisneros et al. 2012
Holotype:
UFRGS–PV-0386-P, a near complete skull with lower jaw.
Locality and horizon:
Boqueirão Farm (30° 00ʹ 11.38ʹʹ S, 54° 05ʹ 17.44ʹʹ W), Catuçaba District, São Gabriel Municipality, Rio Grande do Sul State, Brazil. The rocks exposed at this site belongs to the Morro Pelado Member of the Rio do Rasto Formation (Guadalupian) (Cisneros et al. 2012)
Referred specimens:
UNIPAMPA 759, a near complete skull with lower jaw and postcranial remains also collected from the type-locality; UFRGS-PV-0249-P, a fragmentary right dentary, collected from the ‘Fazenda Fagundes’ locality (30° 01ʹ S, 54° 09ʹ W).
Revised diagnosis:
Pampaphoneus biccai can be distinguished from all other anteosaurids by a squamosal (posterior) process of the jugal extending beyond the anteriormost margin of the temporal fenestra and by the presence of a poorly developed, elliptical boss at the angular. It can be distinguished from all other anteosaurids, except Syodon biarmicum, by the presence of nine upper postcanines, which bear mesial and distal serrations and have a bulbous shape. It can be distinguished from Syodon biarmicum by a crest that extends from the pineal boss until the orbital rim; the presence of a diastema separating the upper canine from the first postcanines; the presence of a marked depression on the posterior portion of the maxilla, postfrontal and the entire lacrimal (limited to the lacrimal in Syodon biarmicum); and in Pampaphoneus biccai the frontals do not divide the parietals. In addition, Pampaphoneus biccai, has a frontal depression anterior to the pineal boss that is deeper than that present in Syodon biarmicum.
Description
Skull
General skull morphology:
The skull of UNIPAMPA 759 (Figs 2, 3) is 358.30 mm long, as measured from the tip of the first premaxillary tooth to the posterior end of the squamosal, being 45 mm larger (14.06%) than the holotype (UFRGS PV 386P), which has a length of 320 mm. At its midpoint, UNIPAMPA 759 is 112.5 mm deep, as measured from the dorsalmost portion of the nasal (at the skull roof) to the ventralmost portion of the maxilla (see Supporting Information, Table S2). The deepest portion of the skull measures 148.27 mm, from the dorsalmost portion of the frontal (at the skull roof) to the ventralmost portion of the jugal (see Supporting Information, Table S2). The left side of the skull is well preserved and bears a conspicuous depression (Fig. 2, d) close to the anterior limit of the orbit, covering part of the maxilla, lacrimal, and prefrontal. This concavity has a slight ventral inclination towards its anterior portion, straightening up close to the anterior half of the maxilla. This feature is also present on the right side of the skull (Fig. 3, d), as well as on the left side of Pampaphoneus biccai type-material (UFRGS PV 386P) (see Discussion). Similar depressions are shared with other syodontines, such as Syodon biarmicum (specimen PIN 157/2), despite being limited to the lacrimal bone in the latter. As in most anteosaurs, the dorsal margin of the snout is convex, gradually sloping ventrally towards its anterior tip. This condition contrasts with the dorsally concave snout displayed by the anteosaurine Titanophoneus and Anteosaurus (e.g. Kammerer 2011).
The external nares are drop-shaped, with a rounded anterior border and an acute posterior margin (Fig. 3). They are located in a retracted, non-terminal position, as in most anteosaurs, like Titanophoneus potens Efremov (1938) (e.g. PIN 157/1; Kammerer 2011). Other dinocephalians, on the other hand, have external nares placed close to the anterior limits of the snout, such as in the anteosaurid Sinophoneus yumenensisLiu 2013 (Liu 2013). Some skull bones display a moderate degree of pachyostosis, such as those that form the posterodorsal rims of the orbits (Fig. 2). This is also the case for the Pampaphoneus biccai holotype (UFRGS–PV-0386-P, Cisneros et al. 2012), Sinophoneus yumenensis (Liu 2013), Notosyodon gusevi (PIN 2505/1), and the subadult individual of Titanophoneus potens (PIN 157/1). On the other hand, Archaeosyodon praeventorTchudinov 1960 lacks pachyostotic bones in the orbital region (PIN, 1758), while in the adult specimen of Titanophoneus potens (PIN 157/3) and Anteosaurus magnificusWatson 1921 (SAM-PK-11296) pachyostosis is present on the entire surface of the skull. The zygomatic arches of UNIPAMPA 759, although still in their natural positions, are incomplete, failing to reach the skull roof (Figs 2, 3). The left, better-preserved side of UNIPAMPA759 displays five incisors, four premaxillary and one maxillary, one large canine, and nine upper postcanines (Fig. 2). The postcanines of Pampaphoneus biccai have posteriorly directed crowns.
The braincase is a robust and complex structure exposed in the posterior portion of the skull (Figs 5, 6). It is posterodorsally oriented, so that the occipital condyle faces posteroventrally. A conspicuous boss surrounds the pineal foramen (Fig. 5A). In dorsal view, the braincase is V-shaped, with a maximum width of 82.4 mm (see Supporting Information, Table S2). The sphenoid portion of the braincase encloses the ventralmost portion of the skull, contacting the palate anteriorly through the basisphenoid (Fig. 6). The maximum height of the braincase, as measured from the ventral surface of the basisphenoid until the dorsal surface of the pineal boss, is 129 mm (see Supporting Information, Table S2).
A, dorsal view of the braincase of UNIPAMPA 759. B, interpretative drawing of the dorsal view of the braincase. C, left lateral view of the braincase of Pampaphoneus biccai UNIPAMPA 759. D, interpretative drawing of the left lateral view of the braincase of Pampaphoneus biccai. The brown areas at the interpretative drawings represent sedimentary remains or fractures filled with polyethylene glycol.
Occipital view of the braincase of Pampaphoneus biccai UNIPAMPA 759 (A) and interpretative drawing (B). The brown areas at the interpretative drawing represent sedimentary remains.
Premaxilla:
The premaxilla (Figs 2, 3) is restricted to the anterior portion of the skull and has a trapezoidal main corpus that contributes to the anteroventral corner of the external naris. It contacts the maxilla posteroventrally, the nasal posterodorsally, and the septomaxilla posteroventrally. The contact with the maxilla curves posteriorly close to its ventral portion and is located between the fourth and fifth tooth (Fig. 2). The posterodorsal (nasal) process of the premaxilla is slender, extending posteriorly to compose the external border of the naris. It contacts the nasal posterolaterally, forming a suture that gradually curves posteromedially (Fig. 3). The posterior (septomaxillary) process forms a notch to receive the septomaxilla at the level of the anterior portion of the naris. This notch forms a forked structure that surrounds the septomaxilla anteriorly (Fig. 2). This condition differs from what is seen in Sinophoneus yumenensis (GMV1601) and Syodon biarmicum (PIN 157/2), where the septomaxilla does not contact the premaxilla (Liu 2013). The alveolar margin is sinuous and posterodorsally projected (Figs 2, 3), as in other anteosaurs, like Archaeosyodon praeventor, Sinophoneus yumenensis, Syodon biarmicum, and Titanophoneus potens (PIN 157/1). The premaxillary alveolar margin is angled about 34.5° from the main axis of the skull.
Maxilla:
The maxilla forms most of the pre-orbital portion of the skull (Figs 2, 3). Its lateral surface displays ornamentation in the shape of anteroventrally directed ridges (Figs 2, 3), as described for the Pampaphoneus biccai holotype (Cisneros et al. 2012). The maxilla contacts the premaxilla anteriorly, the septomaxilla anterodorsally, the nasal and the frontal dorsally, the jugal posteroventrally, the lacrimal posteriorly, and the prefrontal posterodorsally, below the contact with the frontal. The alveolar margin of the maxilla is convex, curving slightly anteroposteriorly (Figs 2, 3). Anteriorly, the maxilla curves dorsally, making its contact with the premaxilla ventrally arched (Figs 2, 3). In this respect, Pampaphoneus biccai differs from Syodon biarmicum, in which the contact between those bones is mostly straight. The anterior portion of the maxilla contacts the premaxilla and the septomaxilla dorsally, and its anterior edge reaches the posterior-third of the premaxilla (Fig. 2). The contact between the maxilla and septomaxilla excludes the maxilla from the ventral border of the external nares (Fig. 2). It is posterodorsally sloped, following the main body of the septomaxilla. The complete extension of this contact is unclear on the left side of the skull, as it is covered by an unidentified bone (Fig. 2). On the right side, this contact is obliterated by the poor preservation (Fig. 3), however, it extends for almost the entire anterior-third of the maxilla, as in Syodon biarmicum and Titanophoneus potens (PIN 157/1). In UNIPAMPA 759, the contact between the maxilla and the nasal extends for almost two-thirds of the maxilla. It forms a near horizontal outline, with a small posterodorsal deviation (Figs 2, 3), similar to that displayed by Syodon biarmicum. The most posterodorsal portion of the maxilla of UNIPAMPA 759 contacts the prefrontal, which is marked by an interdigitating suture (Fig. 2). The posterior (jugal) process of the maxilla is a nearly triangular element that contacts the lacrimal anterodorsally and the jugal posteriorly (Figs 2, 3). The contact with the lacrimal is tentatively identified as being positioned where the maxilla becomes gradually deeper (Figs 2, 3). This suture has a forked outline (Fig. 3), as in Syodon biarmicum. In the latter, however, this contact is marked by an abrupt depression of the lacrimal. A similar, but more discreet, depression is present in Pampaphoneus biccai. The contact with the jugal forms a dorsoventrally directed suture line, bounding a triangular process (Figs 2, 3). The contact with the prefrontal is here tentatively identified as a nearly vertically directed interdigitating suture line (Figs 2, 3).
Septomaxilla:
The septomaxilla is a narrow bone throughout its whole extension. It marks the ventral boundary of the external nares, being better discernible on the left side of the skull (Fig. 2). The septomaxilla has an anterior contact with the premaxilla, a ventral contact with the maxilla, and a posterior contact with the nasal. The anterior (premaxillary) process is blunt and rests within a notch in the premaxilla (Fig. 2). A comparable notch is absent in Syodon biarmicum and Sinophoneus yumenensis. The contact with the maxilla is almost straight, with a slight posterodorsal deviation (Fig. 2). The contact with the nasal is unclear due the presence of an unidentified bone fragment covering this region on the left, better preserved, side (Fig. 2).
Nasal:
In UNIPAMPA 759, the nasal is a slender element in lateral view, composing most of the pre-orbital region of the skull roof (Figs 2, 3). This anteroposteriorly elongated bone forms, in lateral view, a gently convex dorsal outline that shapes the skull roof (Figs 2, 3). The nasal is concave ventrally, where it contacts the maxilla throughout its whole extension (Figs 2, 3). It also contacts the premaxilla anteriorly, and the frontal posteriorly. Anteriorly, the nasal is excluded from the external naris by the premaxilla, but the precise point where the anterior process of the nasal reaches the premaxilla cannot be discerned on the left side of the skull (Fig. 2). On the right side, in lateral view, this contact forms a suture that gradually slopes posterodorsally (Fig. 3). The nasal contacts the septomaxilla close to the posterior edge of the naris, but the exact shape of this contact is unclear due to the presence of an unidentified bone covering this region on the left side, as mentioned above (Fig. 2), whereas the right side is broken in this same spot (Fig. 3). In its posteriormost portion, the nasal contacts the frontal through a posterodorsally inclined suture line (Fig. 2).
Lacrimal:
The lacrimal is a sub-rectangular bone. It is convex anteriorly and concave posteriorly, where it forms the anterior margin of the orbit (Figs 2, 3). The lacrimal has a large lateral exposure on the pre-orbital region of the skull, and makes contact with the maxilla anteriorly, the jugal ventrally, and the prefrontal dorsally. The lacrimal is robust and marked by a pronounced depression in the lateral surface, with slightly sloping margins (Figs 2D, 3D), different from the well-marked margins observed on the antorbital depression of Syodon biarmicum. The right lacrimal displays ornamentation in the shape of anteroventrally sloping ridges that extend from the orbital rim to the maxilla (Fig. 3), a pattern already described by Cisneros et al. (2012) for the Pampaphoneus biccai holotype. The rounded anterior edge of the lacrimal excavates the maxilla, with the latter bifurcating to receive it (Figs 2, 3). The contact with the prefrontal is tentatively recognized as a convex line at the dorsal third of the orbit, extending in an anterior direction, being limited by the maxilla anteriorly and the orbit posteriorly (Figs 2, 3). The contact with the jugal is located at the ventral portion of the lacrimal and is here, tentatively recognized as a suture that gradually slopes posterodorsally (Fig. 3).
Jugal:
The jugal is a robust and tetraradiate bone, projected posteroventrally well beyond the limit of the alveolar margin of the maxilla (Figs 2, 3). It contacts the maxilla anteroventrally, the lacrimal anterodorsally, the squamosal posteroventrally, and the postorbital posterodorsally. It contributes to the ventral and anteroventral limits of the orbit and to the anteroventral limit of the temporal fenestra (Figs 2, 3). The anterior (maxillary) process of the jugal is triangular, limited dorsally by the lacrimal and ventrally by the jugal process of the maxilla (Figs 2, 3). In this respect, Pampaphoneus biccai is similar to Syodon biarmicum, but differs from Sinophoneus yumenensis, the latter having a blunt maxillary process. The dorsal (lacrimal) process of the jugal is tentatively recognized as being short and blunt, extending until about one-third of the dorsoventral extension of the orbit. The postorbital (posterodorsal) process of the jugal is wide and short, curved posterodorsally, contributing to the ventral and posteroventral margin of the orbit and to the anterior margin of temporal fenestra, like in Sinophoneus yumenensis (Liu 2013) and Syodon biarmicum (Figs 2, 3). The postorbital (posterodorsal) process of the jugal is wide and short, curved posterodorsally, and contributes to the posteroventral margin of the orbit and to the anterior margin of temporal fenestra (Figs 2, 3), again similar to Sinophoneus yumenensis (Liu 2013) and Syodon biarmicum. The posteroventral (squamosal) process is bifurcated by the squamosal, with the dorsal portion contributing to the anteroventral edge of the temporal fenestra (Fig. 3). The ventral portion underlies the anteriormost portion of the squamosal, forming the dorsal edge of the zygomatic arch and temporal fenestra (Fig. 3).
Prefrontal:
The prefrontal is an approximately rectangular bone that contacts the maxilla anteriorly, the lacrimal ventrally, the nasal anterodorsally, and the frontal posterodorsally (Figs 2, 3). The main body of the prefrontal is partially covered by a rib fragment on the left side of the skull (Fig. 2). Its contact with the maxilla forms a near vertical interdigitated suture (Fig. 2). Ventrally, it makes contact with the dorsal portion of the lacrimal (Figs 2, 3) in a convex suture that is limited anteriorly by the maxilla and posteriorly by the orbit (Fig. 2). The contact with the nasal is a small posterodorsally inclined suture (Fig. 2). The posterior process of the prefrontal makes the external border and the internal portion of the orbit, where it contacts the frontal (Fig. 2).
Frontal:
In lateral view, the fused frontals compose a slender element that forms part of the skull roof and the dorsal edge of the orbit (Fig. 2). It is anteroposteriorly elongated and ventrally wide, where it contributes to the internal margin of the orbit. The frontal forms the gentle posteroventral curvature of the skull roof (Fig. 2). It contacts the nasal anteriorly, the postfrontal posteriorly, and the prefrontal laterally in its anterior extension (Fig. 2). Where the frontal invaginates to form the internal surface of the orbit, however, its contact with the prefrontal is ventrally deviated. The anterior and ventral processes contact the postfrontal posteroventrally, composing the dorsal portion of the orbit, although the limits between the frontal, the postfrontal, and the prefrontal are tentative (Fig. 2). The anterior extension of the frontal forms an anteroventrally inclined straight suture with the nasal (Fig. 2). The ventral process is here tentatively recognized as a round structure that extends medially in the internal portion of the orbit, being dorsally and ventrally convex, where it contacts the prefrontal and the postfrontal (Fig. 2). It forms parts of the external border of the skull roof and the dorsal border of the orbit (Fig. 2), similar to the condition displayed by Syodon biarmicum. In lateral view, the ornamentation presented by Syodon biarmicum, in the shape of transversal striations, is absent in Pampaphoneus biccai. In dorsal view, the frontals contact the parietals posteriorly (Fig. 5). As in the holotype of Pampaphoneus biccai (UFRGS PV 386P), the frontals of UNIPAMPA 759 contribute to the pineal boss. In this respect, Pampaphoneus biccai resembles anteosaurs like Syodon biarmicum, differing, for instance, from Titanophoneus potens (PIN 157/1). The well-developed ridge cited by Cisneros et al. (2012) is not preserved in UNIPAMPA 759. There are paired depressions present on the frontal, anterior to the pineal boss, that are separated by a median ridge (Fig. 5), less accentuated than observed in Syodon biarmicum, being comparable to the condition of the subadult of Titanophoneus potens (PIN 157/1). There is a ridge in the midline of the skull roof, at the depression anterior to the pineal boss, where the frontals meet each other. The frontal makes an arched contact with the postfrontal, where it overlies the latter, close to the pineal boss (Fig. 2). In this respect, Pampaphoneus biccai differs from Syodon biarmicum, where the frontal forms a near triangular process, surrounded by the anterior bifurcating process of the parietal.
Postfrontal:
In lateral view, the postfrontal is a thick element that forms the posterodorsal margin of the orbit, also extending medially to compose the internal portion of this opening (Fig. 2). It contacts the postorbital ventrally and the frontal anteriorly, but the details of the contact with the frontal are unclear. The contact with the postorbital is marked by posteromedially directed suture that runs over the outer surface of the thick orbital rim (Fig. 2). In lateral view, the postfrontal bears ornamentation in the shape of transversal striations (Fig. 2), like in Syodon biarmicum.
Postorbital:
In left lateral view, the postorbital is a semicircular and robust bone that delimits the posteroventral edge of the orbit (Fig. 2). It reveals a moderate degree of pachyostosis, also presenting ornamentations in the shape of transverse striations (Fig. 2), in this respect being similar to Syodon biarmicum. On the right side of UNIPAMPA 759, parts of the bone are missing and only the portion that forms the orbital rim is preserved (Fig. 3). It contacts the jugal anteroventrally and the postfrontal dorsally. The contact with the jugal forms a near horizontal suture slightly inclined anterodorsally and extending medially (Fig. 2). The contact with the postfrontal forms a posterodorsally directed suture (Fig. 2).
Squamosal:
As preserved in the right lateral view, the squamosal is robust anteriorly, gradually tapering posteriorly (Fig. 3). It is a semicircular element that forms the posteriormost portion of the temporal fenestra, contacting the jugal anterodorsally (Fig. 3). As preserved in UNIPAMPA 759, the squamosal is displaced from the braincase region, making it impossible to access its contact with this region (Figs 2, 3). The contact with the jugal forms a process that surpasses the anteriormost portion of the temporal fenestra (Fig. 3), as described for the holotype of Pampaphoneus biccai by Cisneros et al. (2012). This process is underlain by the posterior process of the jugal, being excluded from parts of the ventral margin of the zygomatic arch (Fig. 3). Dorsally, the squamosal forms the ventral edge of the temporal fenestra, following its posterodorsal curvature (Figs 2, 3). This is also the case ventrally.
Quadrate:
In UNIPAMPA 759, only the left quadrate is partially preserved, being disarticulated from the skull (Fig. 7). In posterior view, the quadrate is a large and anteroposteriorly compressed element (Fig. 7A). Its main corpus was broken in its lateral and medial extensions. Ventrally, it forms two robust condyles. The lateral process curves gently posterolaterally and is deeper than the medial process (Fig. 7A). On the other hand, the medial process curves slightly posteromedially, being larger than the lateral process (Fig. 7A). The lateral condyle is a bulbous and oval structure extending posterolaterally, while the medial condyle presents a groove that gradually expands in anteroposterior direction. The medial condyle extends ventrally, with the posterior portion projecting below the lateral condyle. The medial condyle base forms an angle between 30° and 45° with the quadrate pterygoid ramus. The ventral margin of the medial condyle curves gently dorsomedially (Fig. 7).
Posterior view of the quadrate of UNIPAMPA 759 (A) and interpretative drawing (B); ventral view of the quadrate of UNIPAMPA 759 (C) and interpretative drawing (D). The brown areas at the interpretative drawings represent sedimentary remains or fractures filled with polyethylene glycol.
Parietal:
In dorsal view, the left parietal of UNIPAMPA 759 lacks parts of its lateral surface (Fig. 5). The fused parietals are narrow elements surrounding the pineal foramen. The parietals contact the frontals anteriorly, the tabulars laterally, and the postparietal posterodorsally (Fig. 5). The contact with the frontals is just anterior to the pineal foramen (Fig. 5), in a similar condition to what is seen in other syodontines, like Syodon biarmicum, but differing from Titanophoneus potens (PIN 157/1) and Sinophoneus yumenensis, in which the anterior border of the pineal boss contacts the frontal, forming an interdigitated suture (Liu 2013), but not contributing to the pineal boss. The contact between the parietals and the frontals forms a suture that extends anterolaterally from the pineal boss towards the lateral surface of the skull, where the frontals lie directly above the parietals (Fig. 5). This condition differs from Syodon biarmicum where the parietals form two anterior processes that make a notch to receive the frontals. The contact with the tabular is poorly preserved for the left parietal and is missing for the right one. On the left, better-preserved side, the suture cannot be seen due to a breakage exposing parts of the internal contact between these bones (Fig. 5). The contact with the postparietal forms an M-shaped suture, similar to the condition in Sinophoneus yumenensis (Liu 2013) (Fig. 5).
Postparietal:
The postparietal contributes to the posterodorsal edge of the braincase. It contacts the parietals anteriorly, the tabulars laterally, and the supraoccipital ventrally (Fig. 6). The main corpus of the postparietal presents a well-developed dorsoventrally nuchal crest that extends from the contact with the parietals towards the ventral end of the bone (Fig. 6). This dorsoventrally elongate shape of the postparietal differs from what is displayed by Sinophoneus yumenensis, in which the maximum width of the bone is similar to its height (Liu 2013). The suture between the postparietal and the parietals is an M-shaped suture (Figs 5, 6), similar to what is described for Sinophoneus yumenensis (Liu 2013). The contacts with the tabulars occur on the lateral edges of the postparietal, being better preserved in the right side of the bone. In UNIPAMPA 759, however, this contact is tentatively identified as being close to a groove resulting from a breakage between both bones (Fig. 6). The ventral limit of the postparietal, where it contacts the supraoccipital, is hidden by preservation. Here, this contact is tentatively identified as occurring level with the ventral end of the postparietal ridge.
Tabular:
The paired tabulars form two posterolaterally extending surfaces that make for a considerable portion of the posterior wall of the braincase. Each tabular contacts the parietal dorsally, the postparietal medially, the opisthotic ventrally, and the squamosal laterally (Fig. 6). The ventromedial contact with the supraoccipital is unclear. The bone surface where the contact with the parietal would be located is lost in UNIPAMPA 759, but parts of the internal contact are exposed. The contact with the postparietal is unclear, but it is probably marked by a groove resulting from damage between these bones (Fig. 6). The precise shape of the contact with the opisthotic is also indiscernible, as sutures are unclear. It is, however, probably located level with the post-temporal fenestra (Fig. 6).
Supraoccipital:
The supraoccipital is a nearly rectangular bone, forming the dorsal edge of the foramen magnum (Fig. 6). It contacts the postparietal dorsally, the tabular laterally, and the exoccipitals ventrally. The bone surface is mostly hidden by preservation, but the contact with the postparietal can be inferred as being located ventrally to the nuchal crest (Fig. 6). The contact with the tabular is also obscured by a dark oxidized crust, and the contacts with the exoccipitals are damaged.
Exoccipitals:
The exoccipitals are paired elements that form the dorsal surface of the occipital condyle, also contributing to the lateral edges of the foramen magnum. They contact the supraoccipital dorsally, the tabular dorsolaterally, the opisthotics laterally, and the basioccipital ventrally (Fig. 6). The exoccipitals are rounded in their ventral margin. Dorsally, however, they project as nearly rectangular bone plates that contribute to the lateral edge of the foramen magnum (Fig. 6). The exoccipitals contact each other in the dorsal surface of the occipital condyle, with no contact on the dorsal edge of the foramen magnum (Fig. 6), as in Titanophoneus potens (PIN 157/1) and in Sinophoneus yumenensis (Orlov 1958, Liu 2013). The contact with the supraoccipital is broken and cannot be described. The jugular foramen observed in Sinophoneus yumenensis by Liu (2013), is not visible in UNIPAMPA 759 due to poor preservation. The contact between exoccipitals and the basioccipital is marked by ventrolaterally directed sutures (Fig. 6).
Basioccipital:
The basioccipital is the main component of the occipital condyle, contacting the exoccipitals through rectilinear, ventrolaterally directed sutures at the dorsal half of the condyle (Fig. 6). The contribution of the basioccipital to the ventral edge of the occipital condyle is robust and semicircular (Fig. 6, co). This bone also articulates with the opisthotic laterally and the basisphenoid ventrally, but the exact limit of these contacts is still hidden by the typical manganese oxide crust that covers most Rio do Rasto Fm. tetrapod fossils.
Opisthotic:
The opisthotic projects posterolaterally, contacting the tabular dorsally, the exoccipitals anterolaterally, and the basioccipital ventromedially. Neither of the two opisthotics is entirely preserved in the specimen, but they appear to be nearly rectangular in shape in posterior view (Fig. 6). Both opisthotics display poorly preserved medial projections in their ventralmost portions, where these bones contact the basioccipital, but the exact morphology of this contact is hidden by preservation (Fig. 6). Each opisthotic contacts its corresponding tabular dorsally, level with the post-temporal fenestra, and a dorsoposteriorly directed suture between these bones is tentatively recognized at the right side (Fig. 6). There is an anteromedial contact with the exoccipital, but its precise shape is unclear in occipital view.
Basisphenoid:
The basisphenoid is a large element that forms the posterior portion of the skull in palatal view. It diverges in two posterolateral processes and a near squared posterior process (Fig. 8), contacting the pterygoid anterolaterally and the basioccipital posteriorly. The basisphenoid has an anterior process forming a medial ridge that diverges in two lateral processes posteriorly (Fig. 8). The anterior process contacts the pterygoid, forming a W-shaped suture located just anterior to the carotid canal (Fig. 8). The carotids are separated by a medial ridge, in a similar condition to that displayed by Notosyodon gusevi. The vidian canals are present in the UNIPAMPA 759 basisphenoid, just anterior to the carotid canals (Fig. 8). The posterolateral process of the basisphenoid follows a ridge, and becomes bulbous posteriorly at the basal tuber, projecting laterally to, presumably, contact the stapes (Fig. 8). The contact with the basioccipital would be located at the end portion of the posterior process, but it is hidden by oxidized crust.
Palate and basicranium in ventral view of Pampaphoneus biccai UNIPAMPA 759 (A) and interpretative drawing (B). The brown areas at the interpretative drawing represent sedimentary remains.
Palate—general morphology
The palate is laterally compressed and anteroposteriorly elongated (Fig. 8). It lacks lateral exposure, except for the transverse process of the pterygoid, easily discernible in lateral view and extending beyond the ventral edge of the lower jaw (Figs 2, 3, ptvp). The lateral compression of the palate has mainly a natural source, although some degree of taphonomic compression happened, it’s not clear if it significantly distorts the width of the palate. The vomerine portion of the palate is slender, and the palate becomes thicker posteriorly, culminating in a strong palatine boss in its mid-portion (Fig. 8). The length of the preserved portion of the palate is 254.84 mm (see Supporting Information, Table S2), as measured from occipital condyle until the preserved anteriormost portion of the vomers. Its maximum width is 48.08 mm at the level of the palatine boss (see Supporting Information, Table S2).
Vomer:
The vomers are long and slender. The two counterparts contact each other medially, where they form two anteroposteriorly directed ridges (Fig. 8). The ‘scroll-like’ shape of these ridges makes them similar to what is characteristic of other anteosaurs, such as Australosyodon nyaphuli, Titanophoneus potens (PIN 157/1), and Syodon biarmicum. The anterior tips of the vomers were not preserved. Posteriorly, however, these bones contact the palatines, visibly overlapping them (Fig. 8). The vomerine ridges are anteriorly sinuous, maybe as an artefact of preservation. Posteriorly, however, they are straight and similar to the condition observed in Archaeosyodon praeventor and Australosyodon nyaphuli (Rubidge 1994).
Palatine:
Both palatines are preserved. Each one is roughly semicircular, contacting its counterpart medially (Fig. 8). Together, the palatines form strong bosses, where 14 peg-like teeth can be counted. On the holotype, the exact number of teeth was not given by Cisneros et al. (2012), and due to preservation, it cannot be observed. The palatines of UNIPAMPA 759 are similar to what is seen in Syodon biarmicum and Titanophoneus potens (PIN 157/1), except for the fact that Titanophoneus potens displays fewer teeth. The palatine bosses are prominent, and they join together level with the midline of the palate (Fig. 8), similar to what is seen in Syodon biarmicum and Australosyodon nyaphuli (Cisneros et al. 2012). The palatines are dorsally overlapped by the vomers, having a posterior contact with the pterygoid (Fig. 8). The contact with the vomers lies above the palatine bosses and cannot be described with precision.
Pterygoid:
Each pterygoid can be divided into three portions: an anterior process that contacts the palatine, a wing-like, ventrolaterally projected transverse process and a posterior process that diverges posterolaterally to contact the basisphenoid and the quadrate (the quadrate ramus) (Figs 2, 3, 8). In palatal view, the palatine process of the pterygoid diverges anterolaterally to contact the posteromedial portion of the palatine at the midline (Fig. 8). Posterior to this, the pterygoids form a boss where two tooth series are present, being five teeth inserted on the left pterygoid and four on the right one (Fig. 8). The pterygoid bosses are connected to the anterior processes and, as preserved, present a marked pit between them (Fig. 8). A concave surface separates the pterygoid boss from the transverse process. The transverse process of the pterygoid is a bulged element, extending ventrolaterally, where it displays a slight posterior curvature (Fig. 8). In palatal view, it is possible to observe a series of teeth on the transverse process. This process bifurcates posteromedially, forming a round edge that is confluent with the anterolateral portion of the quadrate ramus and contacts its counterpart posteromedially through a bulbous ridge (Fig. 8). The quadrate ramus is, in palatal view, a posterolaterally divergent structure that contacts the basisphenoid posteromedially and the quadrate posterolaterally (Fig. 8). In UNIPAMPA 759, the exact limit of the contact with the quadrate was not preserved, and the suture with the basisphenoid forms a W-shaped outline, just anterior to the carotid canal (Fig. 8). The quadrate rami of the pterygoid present a well-developed longitudinal ridges, as in Syodon biarmicum, and in the subadult of Titanophoneus potens (PIN 157/1). These ridges persists towards the basisphenoids (Fig. 8). The quadrate rami also present two posterolateral ridges following their deviation.
Lower jaw
General morphology: The left and right mandibular rami are disarticulated in UNIPAMPA 759. In lateral view, each ramus is considerably deep and anteriorly convex at the symphyseal portion (Figs 9–12). The dorsal margin of the mandibular ramus is concave, and the ventral margin slightly convex (Figs 9, 11). Both counterparts bear well-preserved teeth (Fig. 13) and, on the left side, there are some nutritive foramina close to the tooth row, being visible in medial view. These structures could also be resorption pits associated with tooth replacement, so that the attribution as nutritive foramina is tentative and will be evaluated further. Some portions of the medial surface of each mandibular ramus are missing, exposing the Meckelian fossa (Figs 10, 12). When articulated, both rami form a V-shaped structure, gradually diverging posteriorly (Fig. 13).
Left lateral view of the lower jaw of Pampaphoneus biccai UNIPAMPA 759 (A) and interpretative drawing (B). Ventral view of the left rami of the lower jaw of Pampaphoneus biccai UNIPAMPA 759 (C) and interpretative drawing (D). The brown areas at the interpretative drawings represent sedimentary remains.
A, left medial view of the lower jaw of Pampaphoneus biccai UNIPAMPA 759. B, interpretative drawing. C, dorsal view of the left rami of the lower jaw of Pampaphoneus biccai UNIPAMPA 759. D, interpretative drawing. The brown areas at the interpretative drawings represent sedimentary remains.
Right lateral view of the lower jaw of Pampaphoneus biccai UNIPAMPA 759 (A) and interpretative drawing (B). The brown areas at the interpretative drawing represent sedimentary remains.
Right medial view of the lower jaw of Pampaphoneus biccai UNIPAMPA 759 (A) and interpretative drawing (B). The brown areas at the interpretative drawing represent sedimentary remains or fractures filled with polyethylene glycol.
Articulated lower jaws and the preserved teeth (A) and interpretative drawings (B). The brown areas at the interpretative drawing represent sedimentary remains.
Dentary:
The dentary is a strong, nearly rectangular element that composes most of the lateral surface of the lower jaw, since the postdentary elements are not preserved in UNIPAMPA 759. The symphyseal region is anteroventrally rounded, but forms close to a 90° angle between its anterior and dorsal edges (Fig. 9). In lateral view, the alveolar margin is slightly concave, so that the posterior-third of the dentary is dorsally projected (Figs 9, 11). The ventral border is nearly rectilinear for most of its extension, but gradually slopes dorsally after the posterior-third of the bone (Fig. 9). The dentary presumably contacts the angular posteriorly, since this latter is absent in UNIPAMPA 759. In ventral view, the dentary contacts the splenial medially, forming an anteroposteriorly rectilinear suture (Fig. 9C). The medial surface of the dentary is exposed in the right mandibular ramus, as most of the splenial is absent (Fig. 12). Here, the Meckelian fossa is visible as an anteroposteriorly elongated depression that extends from the mandibular symphysis to the posterior process of the dentary. Dorsal to the Meckelian fossa, the dentary is medially projected, forming a sub-rectangular surface (Fig. 12). In medial view, the alveolar margin displays large nutritional foramina between each alveolus (Figs 10, 12). Ventral to these, a marked longitudinal ridge is present.
Splenial:
Although both the left and right splenials are present, the left one is better preserved, being almost complete (Fig. 10). Together with the dentary, the splenial forms most of the anterior part of the lower jaw, being a nearly rectangular, slender element that forms the ventral half of the mandibular rami in medial view (Fig. 10). It contacts the dentary dorsally and ventrally, medially enclosing the Meckelian fossa. The splenial probably participates in the mandibular symphysis. The ventral margin of the splenial is nearly horizontal throughout most of its extension, slightly sloping dorsally in its posterior end (Fig. 10). A well-marked suture, where the splenial contacts the dentary, is visible in ventral view (Fig. 9C). This suture is nearly straight, gradually deviating laterally towards its posterior end.
Dentition
General morphology: UNIPAMPA 759 displays five upper incisors in each premaxilla (Fig. 2). A large posteriorly recurved canine is present on both maxillae, being followed by nine postcanines, those being either completely preserved or demonstrable through the recognition of alveoli (Figs 2, 3). The right dentary displays a well-preserved incisor with its root partially exposed (Fig. 12 i). Both dentary canines lack their crown apices. In Pampaphoneus biccai, the postcanines are posteriorly directed and present two distinct morphologies, with the mesial teeth differing from the last three distal teeth (Fig. 2). They all are, however, robust and bulbous at the bases of their crowns, as in Syodon biarmicum and the subadult of Titanophoneus potens (PIN 157/1). As preserved in UNIPAMPA 759, both pterygoids display at least nine poorly preserved peg-like teeth, lacking most of their crowns (Fig. 8). Fourteen teeth are present on the surface of the palatine boss (Fig. 8).
Incisors:
Specimen UNIPAMPA 759 displays five upper incisors in each premaxilla (Fig. 2). This differs from the description of the holotype by Cisneros et al. (2012), where four premaxillary teeth were reported, including the one we identify as belonging to the maxilla. There is no diastema [diastema recess sensuRubidge (1994)] between the incisors and the canine (Fig. 2), as in Syodon biarmicum but contrasting with Sinophoneus yumenensis (Liu 2013) and Australosyodon nyaphuli. The incisors are long, robust, and gently curved posteriorly (Figs 2, 3). The typical anteosaurid-like lingual heels are not visible, probably due to poor preservation. The first premaxillary incisor pair presents a distinct morphology from what is displayed by the second and the third (Figs 2, 3). In labial view, the first incisor has almost the same basal–apical width, and has a nearly cylindrical shape, differing from the second and the third incisors by being strongly labio-lingually compressed (Fig. 14A). In labial view, the second and the third incisors have almost the same basal–apical width for their whole extension, with a near cylindrical shape (Fig. 14A). This condition differs from Australosyodon nyaphuli, where the third incisor is larger than the second and the first ones (Rubidge 1994). As preserved in the left side of the skull, the crown apices of these incisors are broken, while, in the right labial view, the third incisor was lost (Figs 2, 3). In labial view, the fourth premaxillary incisor is shorter than the first three ones (Fig. 2). This tooth gradually becomes acute in the apical direction, contrasting with the three first teeth, those becoming abruptly acute apically. It lacks serrations, and its crown apex is blunt, probably due to wear (Fig. 14C, D, E). This tooth presents longitudinal striations in lingual view and is flattened labially (Fig. 14C). The fifth premaxillary incisor is, in labial view, a cylindrical, recurved tooth, with a large base that tapers towards its apical portion (Figs 2, 3). It presents fine serrations in a conspicuous distal carina, level with the crown base (Figs 3, 13B). A groove is present mesially to the serrations and the crown apex is blunt (Fig. 13B).
Dentition of Pampaphoneus biccai. A, anterior view of the skull, not in scale; B, lateral view of the right fifth upper incisor; C, lingual view of the right fourth upper incisor; D, distal view of the right fourth upper incisor; E, apical view of the right fourth upper incisor; F, basal view of the right fourth upper incisor; G, the three distalmost postcanines of the left maxilla; H, anterior view of the lower jaw of UNIPAMPA 759; I, labial view of the left sixth upper postcanines; J, lingual view of the dentary incisor of UNIPAMPA 759; K, mesial view of the dentary incisor of UNIPAMPA 759; L, lingual view of the tenth postcanine of UNIPAMPA 759; M, labial view of the second, third, and fourth right dentary postcanines; N, lingual view of the second, third, and fourth right dentary postcanines. O, close up of the second dentary postcanines in lingual view, showing the serration in details, not in scale. The scale of 5 mm applies to B-N.
The first dentary incisor is a poorly preserved tooth on the left ramus, being displaced labially in anterior view (Fig. 14H), it gradually tapers apically. The better-preserved dentary incisor is a roughly triangular tooth, slightly recurved apically, and presenting the typical anteosaurid-like lingual heels (Fig. 14J, K). The apex of the crown is blunt, while its base presents a cervix. The tooth has serrations in both its mesial carinae, and these are slightly labially displaced (Fig. 14J, K). There are basal–apical marked striations in the lingual view of the dentary incisor, forming a ‘scalloped’ surface (Fig. 14J, K), similar to some teeth of Archaeosyodon praeventor. As preserved, this tooth has a partially exposed root, which is rounded and presents basal–apical striations. It is also comparatively smaller than the dentary incisors of Australosyodon nyaphuli.
Canines:
The canines are large and strongly recurved teeth (Figs 2, 3), comparable to what is seen in Syodon biarmicum and Titanophoneus potens (PIN 157/1). They differ, however, from Archaeosyodon praeventor, by being more slender. The canines in the Russian taxon are much larger at their bases, tapering apically to form strong fangs. Longitudinal striations are evident in their labial surfaces (Figs 2, 3), similar to what is seen in Archaeosyodon praeventor and Syodon biarmicum. This character is not preserved in PIN 157/1, a subadult of Titanophoneus potens. The canines of Pampaphoneus biccai lack serrations, a similar condition to what is seen in other dinocephalians such as Syodon biarmicum and Archaeosyodon praeventor. The apicalmost portion of the left canine is missing (Fig. 2), but the right canine is slightly peaked (Fig. 3). In anterior view, the canine is strongly recurved labially (Fig. 14A).
In the left mandibular ramus of UNIPAMPA 759 lower jaw, as preserved, the dentary canine lacks its apical portion, with only the crown base present (Fig. 12). In lingual view, the tooth surface is covered by manganese oxide, and details cannot be described. In labial view, parts of the tooth are covered by the same crust, but a portion of the crown surface with preserved enamel is visible (Fig. 11). The apicalmost portion of this tooth is occluded with the fifth right premaxillary incisor, being covered by it (Fig. 3). Its visible portion shows well-preserved enamel along the entire labial surface, and its apical portion gradually sharpens towards the crown apex.
Postcanines:
The left maxillary postcanines are relatively well preserved, with one empty alveolus and eight nearly complete teeth (Fig. 2). At the right side, there are six well preserved teeth (Fig. 3) and three broken postcanine crowns (Fig. 11). Nine teeth (or alveoli) were already recorded in the Pampaphoneus biccai holotype (Cisneros et al. 2012), differing from the 12 to 13 reported for Australosyodon nyaphuli (Rubidge 1994). In Pampaphoneus biccai, a diastema is present between the canine and the first postcanine (Figs 2, 3), differing from what is seen in Syodon biarmicum and Australosyodon nyaphuli (Rubidge 1994), where a diastema is absent. The postcanines show three distinct morphologies. They are similar to each other in being low and robust (Fig. 2), as those of Syodon biarmicum (Cisneros et al. 2012), and posteriorly curved, differing from Australosyodon nyaphuli, in which the first two postcanines are anteriorly curved (Rubidge 1994). The postcanines of Pampaphoneus biccai bear distal serrations, with only the two distalmost teeth bearing both mesial and distal carinae (Fig. 14G), a feature not noticed by Cisneros et al. (2012). The second maxillary postcanine has a bulbous crown apex. Its preserved apical portion is just slightly thinner than its base, and the tooth presents distal serrations (Figs 2, 11). The sixth left maxillary postcanine is strongly distally curved, with a bulbous base and a thinner apex (Fig. 2). Possibly due to wear, the crown apex is blunt, and serrations are present in the distal portion of the tooth. Mesial serrations are not visible. The eighth and ninth left maxillary postcanines are both well preserved, being smaller than the mesial most postcanine (Fig. 2). On both sides, the eighth maxillary tooth is smaller than the ninth. Besides this size difference, the general morphology of these two teeth is similar, being bulbous at their bases, with a trend to thinner towards a sharp crown apex (Figs 2, 3). These teeth are curved, but not as curved as the sixth postcanine, being more similar to the pattern presented by the second tooth pair. Both the eighth and ninth maxillary postcanines bear mesial and distal serrations, and the carina is slightly displaced meso-labially (Fig. 2, 13G).
In the dentary, there are at least ten postcanines. These show two different morphologies, the mesial teeth differing from the distal ones (Fig. 12). The mesial postcanines are low and robust, with their bases being slightly larger than their apices. The teeth curve slightly distally and lack the ‘scalloped-like’ striations (Fig. 14N) seen in the dentary incisors (Fig. 12) and in some teeth of other anteosaurs, such as Archaeosyodon praeventor (PIN 1758/95). The apices of the crowns are blunt, probably due to biological wear, and a cervix is present at the bases, carinae are visible in the mesial surface (Figs 12, 14N, O). In UNIPAMPA 759, the 10th dentary postcanine is almost complete, with parts of its base missing (Fig. 14L). In lingual view, this tooth is bulbous and robust, having a ‘talon and heel’ morphology. The apex is blunt, but serrations are present both in mesial and distal positions (Fig. 14L). Its carinae are displaced lingually, as in Syodon biarmicum (PIN 157/2), and the tooth lacks the ‘scalloped-like’ striations present in the dentary incisors (Fig. 14L).
Palatine boss dentition:
The palatine boss of UNIPAMPA 759 has a total of 14 teeth, being six on the right ramus and eight on the left one, whereas three small teeth have a parasagittal alignment (Fig. 8). The crowns are broken in all but three teeth, a large one from the left palatine and two of the small parasagittal ones. The best-preserved tooth from the right palatine is a mesially oblique element, presenting a thicker base compared to its apical portion (Fig. 8, pl.te), similar to some teeth of Syodon biarmicum (PIN 157/2). Its crown apex is broken apically. Both parasagittal teeth are similar, except by their inclination. While the most mesial one curves slightly distally, the most distal one curves mesially (Fig. 8). They are rounded and smaller than the other well-preserved teeth of the palatine boss, with their bases larger than their apexes, and slightly tapering apically (Fig. 8).
Pterygoid dentition:
At least nine poorly preserved teeth are present on the pterygoid boss, five of those on the left side and four on the right side (Fig. 8). Due to damage in the pterygoid dentition, the morphology of only one tooth can be confidently addressed. This tooth, on the right pterygoid, is a small, blunt element, with an ankylosed implantation (Fig. 8).
Postcranial elements
The postcranial elements presented here consists of identifiable bones and some fragmentary material cannot be identified, so, this description will omit these latter.
Ribs:
Several fragmentary ribs are preserved in UNIPAMPA 759, but only one of those is fairly complete (Fig. 15). This element allows the description of its posterior surface, as it is still embedded within the matrix. The rib is arched in posterior view, slightly curving posteriorly (Fig. 15). Its tuberculum is displaced laterally relative to the capitulum and has a subtriangular articulation surface. The capitulum is rounded in the posterior view. A groove is present throughout the posterior surface of the rib shaft, following the curvature of the bone. The distal most portion of this element is missing (Fig. 15).
A, postcranial remains of UNIPAMPA 759; B, interpretative drawing. The brown area at the interpretative drawing represents sedimentary remains.
Phalanx:
UNIPAMPA 759 preserves a small, disarticulated phalanx, exposed on its dorsal surface (Fig. 15). It is spool-shaped, with a distal and a proximal condyle separated by a groove (Fig. 15). As this phalanx lacks context, it is not possible to infer if it is a proximal or a medial element.
Phylogenetic analysis
Our analysis resulted in a single most parsimonious tree (MPT) of 84 steps (Consistence Index = 0.59, Retention Index = 0.73) (Fig. 16B) (see Appendix for the matrix and characters’ list). Despite the somewhat elevated number of revised scores for Pampaphoneus biccai (seven out of 40 morphological characters), the recovered MPT is identical to the one recovered by Liu (2013)—except for the fact that Liu’s (2013) MPT has a length of 83 steps. Our MPT recovers Pampaphoneus biccai as an early-diverging Syodontinae, in a sister-taxon relationship with the clade formed by Notosyodon gusevi + (Syodon biarmicum + Australosyodon nyaphuli) (Fig. 16B). The position of Pampaphoneus biccai is consistent with the original proposition by Cisneros et al. (2012). In our MPT, however, Syodontinae (as well as all the clades within it) have low Bremer support values (1). In this particular topology, clades such as Tapinocephalia and Anteosaurinae are much better supported (Bremer support values of 3 and 5, respectively).
A, reconstruction of the skull of Pampaphoneus biccai UNIPAMPA 759 (A), white indicates unpreserved elements and grey represents the preserved elements. B, the phylogenetic relationships of Anteosauria, the values above the nodes indicate the Bremer index and below the symmetric resampling.
Discussion
The carnivorous anteosaurid Pampaphoneus biccai was first described by Cisneros et al. (2012), representing the single occurrence of the clade in South America. In that contribution, only a brief description of the skull was presented, and little information was given about the species dentition and overall details of its bony anatomy. The newly described specimen UNIPAMPA 759 not only represents a second individual referred to Pampaphoneus biccai, but in many aspects extends our understanding of its anatomy. For instance, Cisneros et al. (2012) described the holotype (UFRGS–PV-0386-P) as bearing eight teeth on the right maxilla and nine on the left, but in UNIPAMPA 759 both left and right maxilla have nine postcaniness, revealing the presence of eight teeth on the holotype’s right maxilla as a preservation artefact, while in Syodon biarmicum there are 10 maxillary postcanines. The holotype was described as presenting four premaxillary teeth and, accordingly, if Cisneros et al. (2012) were right, UFRGS-PV-386-P would differ from UNIPAMPA 759 in this condition. However, we argue here that this variation results from the poor preservation of the holotype’s incisors, and thus is not an unequivocal anatomical feature.
The new specimen also shows a depression anterior to the orbit, excavating the maxillae, the lacrimals, and parts of the prefrontals, a feature not described for UFGRS-PV-0386-P. This feature is unlikely to be a result of diagenetic deformation of UNIPAMPA 759, as it is also present in the left side of the type-material (Fig. 4B). Also, a similar feature is displayed by Syodon biarmicum (PIN 157/2) and by other early-diverging anteosaurids: Sinophoneus yumenensis and Archeosyodon praeventor. It is absent or less conspicuous in all anteosaurines (e.g. Anteosaurus magnificus SAM-PK-11296, and in the syodontine anteosaurid Australosyodon nyaphuli NMQR 3153).On the necessity of a revised diagnosis for Pampaphoneus biccai, here, we consider as invalid some characters from the original diagnosis of Pampaphoneus biccai (Cisneros et al. 2012), especially those that differentiate this taxon from Syodon biarmicum. According to Cisneros et al. (2012), Pampaphoneus biccai can be distinguished from other anteosaurids by a premaxilla that bears four teeth, a squamosal jugal process that surpasses the anteriormost margin of the temporal fenestra (Fig. 4A), and by the presence of a shallow, elliptical, angular boss (Fig. 4A). All of these are here regarded as valid—except the number of premaxillary teeth, here identified as being five—as their combination is absent in all other anteosaurs. Cisneros et al. (2012) also state that Pampaphoneus biccai can be distinguished from all anteosaurids except Syodon biarmicum by the presence of at least eight postcanines, those being bulbous and bearing fore and aft serrations. Here we confirm the presence of serrations in the mesialmost teeth, but absence in the distalmost is probably a preservational artefact. Furthermore, nine postcanines are present on both sides of UNIPAMPA 759 skull, while Cisneros et al. (2012) count eight postcanines on the holotype’s right maxilla and nine on the left, which is also a consequence of preservation.
According to Cisneros et al. (2012), Pampaphoneus biccai would differ from Syodon biarmicum by displaying: a larger size; more robust snout; thickened postorbital that forms an orbital boss; and well-developed crests that extend from the pineal boss to the orbital rim. These features, however, can relate to ontogeny and body size increase; more robust snouts and thicker postorbitals were identified as ontogenetic variation in Sinophoneus yumenensis and Anteosaurus magnificus (Liu 2013, Kruger et al. 2018). From the features cited above, only the well-developed crest that extends from the pineal boss to the orbital rim remains valid, being poorly preserved at UNIPAMPA 759 (Fig. 5B) and visible at UFRGS–PV-0386-P (Fig. 4D).
According to our observations, other unambiguous characters can be used to distinguish Pampaphoneus biccai from Syodon biarmicum: (i) Pampaphoneus biccai displays a diastema between the canine and the postcanines, while in Syodon biarmicum this diastema is absent. This character, however, should be regarded with caution, as its distribution throughout dinocephalian ontogeny is still unclear. (ii) Pampaphoneus biccai bears an antorbital depression that gradually becomes deeper, excavating portions of the posterior part of the maxilla, the postfrontal and in the lacrimal, while in Syodon biarmicum this depression is abrupt and limited to the lacrimal (Figs 2, 3, 4B). (iii) Another feature that differentiates Syodon biarmicum from Pampaphoneus biccai is the presence of bifurcating anterior processes in the parietals of Syodon biarmicum. These processes in Syodon biarmicum form a notch to receive the frontals while, in Pampaphoneus biccai, the anterior process is not bifurcated and forms a more rounded contact with the frontal (Fig. 5). (iv) In Pampaphoneus biccai the frontal depression anterior to the pineal boss is abrupt and deeper than in Syodon biarmicum (Fig. 5).
Ontogenetical implications
A third specimen may be referable to Pampaphoneus biccai. UFRGS PV-0249-P (Fig. 4E, F) is a tooth-bearing dentary fragment described by Langer (2000) and also addressed by Kammerer (2011). This fossil features four postcanine teeth. The mesialmost one is well complete and well preserved, the second and third teeth have their apexes missing, and the fourth one is only represented by a broken root and alveolus. The preserved morphology of the mesialmost tooth in this dentary is entirely comparable with the mesialmost postcanines of Pampaphoneus biccai, being short, very bulbous, labiolingually compressed, with a cervix, and a conical apex that is slightly pointed distally. This tooth also features distinct carinae both on the mesial and distal apical facets. This dentary is ~33 mm in labiolingual dimension and the biggest tooth, the second one in the preserved series, measures ~12.5 mm in mesodistal length. These dimensions considerably surpass those of both the holotype and the slightly larger UNIPAMPA 759, the latter with a dentary that has less than 18mm in labiolingual dimension on its postcanine region and postcanines that do not reach 10 mm in mesodistal length.
The size discrepancy cannot be explained by the lateral compression seen in both UNIPAMPA 759 and UFRGS PV-0386-P, and indicates that UFRGS PV-0249-P was a much larger individual. If we accept the taxonomic identification of the latter as Pampaphoneus biccai, based on the overall similarity of dental characters and also taking into consideration that this specimen was collected merely 5 km W from the type locality (see Cisneros et al. 2021: fig. 1, localities 4 and 5, respectively), it is valid to suggest that UFRGS PV-249-P may represent an older individual. In the case that both the holotype and UNIPAMPA 759 were subadults, we must consider that some morphological characters may change during ontogenetic development, especially those affected by pachyostosis, as it is known to happen in other anteosaurids (Kammerer 2011). This hypothesis obviously requires more fossils of the Brazilian anteosaurid in order to be tested.
Conclusion
The new specimen of Pampaphoneus biccai sheds light on the anatomy of the species, improving our knowledge of the Brazilian anteosaurid. The diagnosis presented by Cisneros et al. 2012 was not adequate to distinguish Pampaphoneus biccai from Syodon biarmicum, since the features used to distinguish both taxa are mostly ontogenetic or ambiguous. The new diagnosis, presented here, reveals new characters previously unknown for the species, allowing the separation of these two syodontines. Results increase confidence in the position of Pampaphoneus biccai as an early-divergent Syodontinae, as the phylogenetic analysis shows the same results presented by Liu (2013). We also raise the possibility that UFRGS-PV-0249-P represents an adult individual of Pampaphoneus biccai based on the tooth morphology and the geographic proximity of both exposures. We treat this, however, as a hypothesis that needs more samples to be tested.
Supplementary data
Supplementary data is available at Zoological Journal of the Linnean Society Journal online.
Table S1. Dataset of specimens used in anatomical comparison with UNIPAMPA 759.
Table S2.? Table of measurements for UNIPAMPA 759.
Acknowledgements
We thank C. Kammerer for sharing photographs and important comments that improved the quality of this manuscript and Arielli F. Machado for creating one of the figures illustrated in this paper (Fig. 12).
Funding
This study was supported by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES) - Finance Code 001, the Harvard Lemann Brazil Research Fund and the Conselho Nacional de Desenvolvimento Científico e Tecnológico [CNPq 316811/2021–1 to FLP.
Conflict of interest
The authors declare that they have no conflict of interest.
Data availability
All the necessary data for replicating the work is available in the manuscript. For access to any additional data, please contact the corresponding author. We have provided the Table S1 and Table S2 as a supplementary information.
Appendix
1. Updated scorings of Pampaphoneus biccai in the dataset of Liu (2013):
0110101111?111101011 0011221011 111010110?
2. List of modifications on Pampaphoneus biccai scorings from the original dataset of Liu (2013):
Character 4: 1 -> 0
Character 5: 0 -> 1
Character 17: 0 -> 1
Character 19:? -> 1
Character 22: 1 -> 0
Character 29:? -> 1
Character 34:? -> 0
