The skull of the Upper Cretaceous baurusuchid crocodile Baurusuchus albertoi Nascimento & Zaher 2010, and its phylogenetic affinities

Authors

  • PAULO MIRANDA NASCIMENTO,

    1. Museu de Zoologia da Universidade de São Paulo, Serviço de Vertebrados, Avenida Nazaré 481, Ipiranga, São Paulo, 04263-000 SP, Brazil
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  • HUSSAM ZAHER

    Corresponding author
    1. Museu de Zoologia da Universidade de São Paulo, Serviço de Vertebrados, Avenida Nazaré 481, Ipiranga, São Paulo, 04263-000 SP, Brazil
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E-mail: hzaher@usp.br

Abstract

We here describe the cranial remains of Baurusuchus albertoi Nascimento & Zaher, 2010, a Baurusuchidae from the Upper Cretaceous of Brazil that is known from a partially complete and articulated skeleton. The cranial anatomy provided important new information that allowed a more accurate analysis of its phylogenetic affinities. Phylogenetic results suggest that B. albertoi is the sister-group of a clade formed by Baurusuchus pachecoi and Baurusuchus salgadoensis. Characters that place B. albertoi within the family Baurusuchidae are: antorbital portion of the jugal more than two times higher than the infratemporal part; rod-shaped infratemporal bar of the jugal; dorsal process of the quadratojugal extensively contacting the postorbital; posterolateral process of squamosal totally deflected ventrally; cervical neural spines anteroposteriorly large; spool-shaped vertebral centra; cranial table as wide as ventral portion. Baurusuchus albertoi differs from the remaining two species of the genus by having a more excavated triangular depression of the jugal suborbital portion, an area that is also more ventrally developed; a vertical and lateromedially thin retroarticular process; and descending expansion of the lateral edge of the squamosal anteriorly concave.

© 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 163, S116–S131.

INTRODUCTION

The Notosuchia was originally defined by Gasparini (1971) to accommodate a number of small-sized fossil crocodyliforms (Notosuchus terrestrisWoodward, 1896; Uruguaysuchus aznareziRusconi, 1933; and Araripesuchus gomesiPrice, 1959) unearthed in Cretaceous continental sediments of South America. The most important features of the group are the short snout, reduction in number of teeth, heterodonty, and the frontal position of the nares, indicating a probable terrestrial way of life (Gasparini, 1971, 1981). More recently, the concept of Notosuchia was broadened to include almost all non-neosuchian mesoeucrocodylians (Ortega et al., 2000; Pol, 2003; Sereno et al., 2003; Pol & Norell, 2004; Gasparini, Pol & Spalletti, 2005; Pol & Apesteguía, 2005; Zaher et al., 2006; Andrade & Bertini, 2008).

One of the least inclusive clades included in Notosuchia by these authors is Sebecosuchia, a clade of large terrestrial crocodiles that comprises the Sebecidae and the Baurusuchidae families, characterized by a high skull, long snout, and ziphodont teeth (Price, 1945; Colbert, 1946; Gasparini, 1984; Gasparini, Chiappe & Fernandez, 1991; Bertini, Manzini & Brandt Neto, 1999).

The monophyly of the Sebecosuchia clade, including both Sebecidae and Baurusuchidae, is still controversial and, although corroborated in several recent phylogenetic analyses (Gasparini et al., 1991; Buckley & Brochu, 1999; Ortega et al., 2000; Sereno et al., 2003; Carvalho, Ribeiro & Avilla, 2004; Turner, 2004; Turner & Calvo, 2005), was not retrieved in the analyses of Benton & Clark (1988), Clark (1994), Larsson & Sues (2007), Riff (2007), and Andrade & Bertini (2008).

The family Baurusuchidae was erected by Price (1945) to allocate Baurusuchus pachecoiPrice, 1945, known from a partially preserved skull from the Upper Cretaceous Adamantina Formation. The second species of the genus, Baurusuchus salgadoensisCarvalho, Campos & Nobre 2005, was found in the vicinity of the city of General Salgado, known only from an almost complete skull (Carvalho et al., 2005). This same locality yielded the specimen described in the present work.

Other fossil genera have been recognized as baurusuchids: Cynodontosuchus rothiWoodward, 1896, known just from the tip of a snout with mandible, and a referred incomplete skull discovered in El Molino Formation, Bolivia (Buffetaut, 1980), and the allocation of which to the Baurusuchidae was first suggested by Price (1945) and posteriorly corroborated by Gasparini (1981); Stratiotosuchus maxhechtiCampos et al. 2001, a large-size, long-snouted baurusuchid known from two well-preserved skulls (Campos et al., 2001; Pinheiro et al., 2008); Pabwehshi pakistanensisWilson, Malkani & Gingerich, 2001, a species known from the tip of a snout with mandible and an isolated mandibular symphysis (Wilson et al., 2001); and Wargosuchus australisMartinelli & Pais, 2008, a very fragmentary specimen from Argentina, composed of just a skull roof and half of the tip of a snout (Martinelli & Pais, 2008).

Almost all features erected by Price (1945) to define the family Baurusuchidae turned out to be either homoplastic or plesiomorphic. However, recent information made available on new baurusuchid taxa allows us to provide a more accurate diagnosis for the family, which can now be characterized by the following derived characters: wide contact between the postorbital and the quadratojugal (Clark, 1994; Carvalho et al., 2005); infratemporal bar of the jugal with a cylindrical shape (Clark, 1994; Carvalho et al., 2005); lateral region of the ectopterygoids expanded anteroposteriorly, inserted in the transversely expanded ventral surface of the jugal and contacting the maxilla anteriorly (Riff, 2003); maxilla with a reduced number of teeth (five) (Riff, 2003); posterior portion of the squamosal entirely vertical (ventrodorsally directed) (Riff, 2003); presence of a deep concavity on the lateral surface of the quadrate; anterior process of the jugal with a large, laterally deflected groove (Riff, 2003); the mandible as high as the neurocranium at the level of the orbits; the external mandibular fenestra significantly larger than the temporal fenestrae (at least 50% larger than the supratemporal fenestra and 60% larger than the infratemporal fenestra); presence of a large depression in the lateral surface of the posterior portion of the angular and surangular that receives the M. pterygoideous posterior; peculiar shape of the retroarticular process, with a short rounded dorsal projection and a tiny, anteriorly deflected ventral projection. Despite the extended diagnosis above, both Wargosuchus and Pabwehshi are very fragmentary and still difficult to assign to the family with certainty. However, Pabwehshi was placed near the family Peirosauridae by Larsson & Sues (2007) and Riff (2007).

Contrary to the family, the genus Baurusuchus still lacks an accurate diagnosis. According to Carvalho et al. (2005), B. salgadoensis belongs to the genus because of its similar ‘cranial pattern’ to B. pachecoi. However, the diagnosis given by Carvalho et al. (2005) overlaps widely with Price's (1945) definition of the family, lacking unambiguous features with which to characterize the species. Although Carvalho et al.'s (2005) definition leaves room for uncertainty, suggesting that both species are conspecific, we found at least four unique characters that distinguish B. salgadoensis from B. pachecoi, which are: the more posterior position of the ascending bar of the jugal; anterior portion of the articular lacking a marked crest that divides the dorsal surface from the medial one; ventral outline of the maxillae anteriorly convex and posteriorly concave; ventral projection of the retroarticular process medially deflected. Although additional material is still needed in order to provide better grounds on which to evaluate the validity of B. salgadoensis, we here consider it as a valid species based on the characteristics stressed above. By contrast, B. albertoi differs markedly from both B. pachecoi and B. salgadoensis by the presence of five skull characters: posterodorsally expanded, lateromedially flattened, and vertically orientated retroarticular process; anteriorly convex posterolateral process of the squamosal; well-developed ventromedial crest of the quadrate, separating the descending body of the quadrate into medial and anterior faces; anterior portion of the jugal with a triangular and rugose ventrolateral projection; jugal with a narrow infratemporal bar that is dorsoventrally slender and anteroposteriorly short (i.e. as long as the inferior edge of the orbit) (see Fig. 1 and discussion below).

Figure 1.

Comparison in lateral view of the reconstructed skulls of Baurusuchus pachecoi, Baurusuchus salgadoensis, and Baurusuchus albertoi. Dotted lines indicate nonpreserved parts. Diagenetic deformation was removed to increase clarity. Scale bar = 10 cm.

In the present work, we provide a detailed description of the skull of B. albertoi, which was only briefly described by Nascimento & Zaher (2010). We also provide a phylogenetic analysis in which we evaluate the affinities of B. albertoi and test the monophyly of the genus Baurusuchus and family Baurusuchidae. The almost complete postcranial skeleton of B. albertoi was described in details by Nascimento & Zaher (2010).

GEOLOGICAL SETTINGS

With an area of approximately 370 000 km2, the Bauru Basin provides a diverse fossil record of vertebrates, including actinopterygian and sarcopterygian fishes, anurans, snakes, sauropods, theropods, and Crocodyliformes (see Bertini et al., 1993; Dias-Brito et al., 2001; Candeiro et al., 2006). The latter is by far the most common group of vertebrate, with 12 genera and 15 species of crocodyliforms described so far for the Bauru, representing an important continental Mesozoic record.

Soares et al. (1980) proposed the first stratigraphical definition for the Cretaceous sediments belonging to the Bauru basin. These authors considered a traditional vertical succession of units without lateral facies variations. Many stratigraphical scenarios were subsequently suggested (e.g. Fernandes & Coimbra, 1996a, b, 2000; Milani, 1997; Dias-Brito et al., 2001; Zaher et al., 2006) (Fig. 2A). Presently, there is no consensus regarding the nomenclatural status and litostratigraphical hierarchy of the Bauru Basin suprabasaltic units (Soares et al., 1980; Fernandes & Coimbra, 1996a, b, 2000; Milani, 1997; Paula e Silva, Kiang & Caetano-Chang, 2005). Failure to recognize accurately in the field an exact depositional sequence for the Bauru Basin represents an impediment to the general use of one of the proposals listed above. We follow Riccomini (1997), Paula e Silva et al. (2005), and Zaher et al.'s (2006) stratigraphical model that incorporates both Araçatuba (sensuSuguio, 1981) and Presidente Prudente formations (sensuFernandes & Coimbra, 2000) into the Bauru Basin (Fig. 2B).

Figure 2.

A, Bauru Basin: 1. Cauiá Group, Bauru Group [2. Adamantina Formation (Form.), 3. Marília Form., 4. Presidente Prudente Form.], 5 basin depocentres (Grohmann et al., 2005). B, compartmentalization suggested for the Bauru Basin (Zaher et al., (2006).

Riccomini (1997) traced the main discontinuities that affected deposition and deformation in the Bauru Group, pointing out evidence of tectonic pulses during sedimentation conditioned to reactivation of ancient faults. The fossil record also reveals evidence of tectonic pulses acting on taphonomic history, especially on final burying and fossil diagenesis. The Upper Cretaceous Bauru fossil record is related to a high variety of sedimentation systems, in time and space. Amongst the causes are the variation cycles in the basin basal level and the active tectonism (reflexes from Atlantic tafrogenesis). These actions delineate the basin edges (e.g. Alto Paranaíba Uplift) and the depocentre deformation (Pires-Domingues et al., 2007).

Grohmann et al. (2005) suggested that fossil assemblages from the Adamantina and Presidente Prudente Formations are restricted to the Paranapanema Jales-Mirante Graben, a tectonic structure that was kept active during deposition of the Bauru Basin (Fig. 2A). The crocodyliform diversity of General Salgado is conditionally preserved by flood event cycles (Pires-Domingues et al., 2007), with all palaeoflow vectors, including those from bioclasts (articulated, disarticulated, and fragmentary specimens), being consistently orientated in a north-west direction (Pires-Domingues et al., 2007). This suggests a block rotation with a north-west dip, which is in agreement with geophysical data provided by Paula e Silva et al. (2005).

SYSTEMATIC PALAEONTOLOGY

Crocodyliformes Hay, 1930sensu Clark, 1986

MesoeucrocodyliaWhetstone & Whybrow, 1983

BaurusuchidaePrice, 1945

BaurusuchusPrice, 1945

Baurusuchus albertoi Nascimento & Zaher, 2010

Holotype: Museu de Zoologia da Universidade de São Paulo, Vertebrate Paleontology collection (MZSP-PV) 140, the right posterior portion of a skull with mandible, part of the hyoid apparatus (Cornu branchiale I), a complete postcranium lacking only the 12th, 17th, and terminal caudal vertebrae, and parts of left appendicular skeleton.

Horizon and locality: The specimen was collected in August 2004, in an outcrop of the Boa Esperança farm (20°34′01.4″S, 50°27′49.8″W), located in the district of Prudêncio e Morais, near the city of General Salgado, state of São Paulo. The referred region belongs to the Adamantina Formation, Bauru Group, north-western part of the Bauru Basin, possibly of Campanian to Maastrichtian age (Santucci & Bertini, 2001; Pires-Domingues, 2006).

Diagnosis: Jugal with a triangular and rugose ventrolateral projection in the anterior portion, more ventrally developed than in other Baurusuchidae; jugal infratemporal bar dorsoventrally slender, and anteroposteriorly as long as the inferior edge of the orbit; retroarticular process lateromedially flattened and vertically orientated, with a posterodorsal expansion; lateral projection of the vertical portion of the squamosal posteriorly concave; well-developed ventromedial crest of the quadrate, dividing the descendent body of the quadrate into a medial and an anterior face.

Description

We compared the skull material of B. albertoi with the holotypes of B. pachecoi (Museu de Ciências da Terra, Departamento Nacional de Produção Mineral do Rio de Janeiro, DGM 299-R), B. salgadoensis (Museu de Paleontologia de Monte Alto, MPMA 62–0001-02), S. maxhechti (DGM 1477-R), and specimens of Mariliasuchus amarali Carvalho & Bertini, 1999 (Museu de Zoologia da Universidade de São Paulo, MZSP-PV 50 and 51). We follow Iordansky (1973) and Romer (1956) for the osteological nomenclature, Cleuren & De Vree (2000), Schumacher (1973), and Iordansky (1964) for muscle nomenclature.

The specimen lacks the whole rostral and left portions of the skull. Preserved bones of the skull include the posterior half of the right mandible formed by the angular, the surangular, the articular, and a fragment of a dentary; the right posterior part of the braincase formed by the squamosal, postorbital, anterior and posterior palpebral, a fragment of the anterior palpebral, quadratojugal, jugal, quadrate, otoccipital, and a piece of the basioccipital; the right posterior portion of the palatal complex formed by the pterygoid and ectopterygoid; and both elements of the Cornu branchiale I. Only a fragmented pterygoid was found from the left side of the skull. Even without the anterior portion of the skull, it is possible to recognize many Baurusuchidae features in B. albertoi. The infratemporal fenestra is formed by the postorbital, jugal, and quadratojugal, possessing a triangular shape, with almost the same anteroposterior length of the orbit. The external mandibular fenestra is formed by the angular, surangular, and dentary, and has a parallelogram shape (see character 261). The adductor fossa is not preserved, but there is no fracture posterior to the external mandibular fenestra, suggesting that the adductor fossa was placed anteriorly to the fenestra.

Postorbital: The postorbital contacts the posterior palpebral anterolaterally, and the squamosal, quadratojugal, and possibly the quadrate posteriorly. The anteromedial portion is missing. It possesses a descendent anterior ramus that contacts the jugal anteroventrally, forming the dorsal half of the postorbital bar that separates the orbit from the infratemporal fenestra (Fig. 3). Medially, it forms the anterolateral portion of the supratemporal fenestra (Fig. 4). The contact with the posterior palpebral is straight and oblique, the lateral edge being more posterior than the medial one in dorsal view. The lateral margin of the postorbital and squamosal overhangs the quadrate and quadratojugal, forming a deep external otic recess (Iordansky, 1973). The medial postorbital wall is extremely thin and, as a result of the material's fragility and the diagenetic factors, it is difficult to visualize the postorbital−quadratojugal suture, which seems to form a straight contact without interdigitations. As in the other Baurusuchus (but not in Stratiotosuchus), the suture with the squamosal is shaped as an inverted ‘C’ when observed in lateral view (character 258, see phylogeny) (Fig. 3). The contact between the postorbital and the squamosal is much more anterior in B. albertoi and B. salgadoensis than in B. pachecoi (Fig. 1). The contact with the jugal cannot be perfectly distinguished, and the descending postorbital ramus is smooth and cylindrical. The postorbital is ornamented just dorsally, showing a foramen near the lateral contact with the squamosal. This foramen is not present in other baurusuchids.

Figure 3.

Right lateral view of the skull of Baurusuchus albertoi. The descending lateral process of the squamosal was removed. Abbreviations: ap, anterior palpebral; ect, ectopterygoid; f, foramen; itf, infratemporal fenestra; j, jugal; oc, otic cavity; or, orbit; po, postorbital; pp, posterior palpebral; pt, pterygoid; q, quadrate; qd, quadrate depression; qj, quadratojugal; sq, squamosal. Scale bar = 1 cm.

Figure 4.

Dorsal view of the skull of Baurusuchus albertoi. Anterior palpebral, ectopterygoid, and part of the pterygoid were removed. Abbreviations: itf, infratemporal fenestra; j, jugal; or, orbit; po, postorbital; pop, paroccipital process; pp, posterior palpebral; qj, quadratojugal; sq, squamosal; stf, supratemporal fenestra. Scale bar = 1 cm.

Jugal: The jugal has a triradiate shape, following the typical crocodylomorph pattern (Riff, 2003), with an anterior ramus that contacts the ectopterygoid (with a broken anterior part that probably contacted the maxilla and the lacrimal), an ascending process that contacts the postorbital, and a posterior ramus that contacts the quadratojugal (Fig. 3). It forms anterodorsally the ventral and posteroventral borders of the orbit, and, posterodorsally, the ventral and anteroventral borders of the infratemporal fenestra. In B. albertoi and B. pachecoi, but not in the other Baurusuchidae, both apertures are almost the same length. The anterior ramus is flat mediolaterally and developed dorsoventrally, being more than twice higher than the posterior ramus. This portion is vertical, differing from the laterally deflected anteroventral projection of Stratiotosuchus. In all species of Baurusuchus the dorsal edge of the jugal that defines the lower margin of the orbit is smooth and concave, differing from Stratiotosuchus in which it is rugose and convex (character 260 – see phylogeny). The ascending process of the jugal is slightly medially deflected, and has a cylindrical shape. The suture with the quadratojugal is strongly interdigitated and forms a V-shaped pattern with its vertex pointing posteriorly. This same suture is straight or has distinct form in other Baurusuchidae. The posterior portion of the jugal is rod-shaped, as in other baurusuchids, but it is more slender and longer than in the other species of Baurusuchus and in Stratiotosuchus. The lateral surface of the jugal is highly ornamented, except for the ascending ramus. Additionally, the anterolateral surface shows a triangular-shaped depression that broadens anteriorly (Fig. 3). Such a depression is present in all baurusuchids and in Sphagesaurus (Pol, 2003), but shows conspicuous differences in each species. Contrary to the other species of Baurusuchus, the depression found in B. albertoi is continuous, ending in the ventral limit of the jugal.

Quadratojugal: The quadratojugal contacts the jugal anteroventrally, the postorbital dorsally, and the quadrate posteriorly, not reaching the mandibular condyles. It forms just a small part of the posterior border of the infratemporal fenestra. In lateral view, the suture with the quadrate is wide and extends along its entire posterior border (Fig. 3). The quadratojugal is narrow lateromedially, having laterally a convex shape. It forms the posterior border and the posteroventral edge of the infratemporal fenestra. As in the other species of Baurusuchus and in Stratiotosuchus, the lateral surface of the quadratojugal of B. albertoi is striated. Such striation was interpreted by Riff (2003) as evidence of muscular insertion in that area. Riff (2003) suggested that the M. adductor mandibulae externus superficialis, which inserts in the dorsal portion of the quadratojugal in modern crocodiles (Cleuren & De Vree, 2000), posteroventrally expanded its area of origin in the Baurusuchidae. The posterior border of the quadratojugal is solidly sutured to the anterior border of the quadrate.

Quadrate: The quadrate contacts the quadratojugal anteroventrally, the squamosal dorsally, the paroccipital process posteromedially, the basioccipital ventromedially, and the pterygoid anteromedially (Figs 3, 5, 6). Contrary to Stratiotosuchus, in which the quadrate is isolated from the basisphenoid by the pterygoid (Riff, 2003), in B. salgadoensis and B. pachecoi, the quadrate contacts the basisphenoid posteroventrally, as in most crocodyliforms. The basisphenoid was not preserved in B. albertoi. The quadrate−squamosal contact is very narrow in posterior view. The wide and rounded concavity of the laterodorsal surface of the quadrate extends dorsally until the inferior edge of the otic aperture, where it ends abruptly (Figs 1, 3). The distal body of the quadrate of B. albertoi is slightly more ventrally deflected than in Stratiotosuchus. Its ventromedial surface has a pronounced crest that runs from the anterior edge of the medial mandibular condyle to the basioccipital (Fig. 6), similarly to the condition present in Sphagesaurus montealtensis (Andrade & Bertini, 2008). This crest is shallow in B. pachecoi, vestigial in Stratiotosuchus, and almost absent in B. salgadoensis, whose quadrate has an almost cylindrical shape in transversal section. The anterior surface of the distal body of the quadrate is concave and possesses gentle ridges for the origin of the adductor bundles (crests A and B sensuIordansky, 1973). As in the other species of Baurusuchus, the two mandibular condyles are well defined, the lateral one being larger and rounded whereas the medial one is smaller, lateromedially compressed, and projects more ventrally than the lateral condyle (Fig. 5). The condyles are separated by a well-developed groove. The margins of the lateral condyle have a rugose surface. The posterior cranioquadrate aperture is situated at the level of the contact between the quadrate and paroccipital process (see paroccipital process description) (Figs 5, 6). The foramen aëreum is very narrow and opens in the posterior surface of the quadrate, dorsally to the medial mandibular condyle. As a result of the vertical quadrate in Baurusuchidae, the foramen aëreum is ventrally deflected. Only one pneumatic foramen is present in all three species of Baurusuchus and in Stratiotosuchus (Riff, 2003), with three smaller apertures inside.

Figure 5.

Posterior view of the skull of Baurusuchus albertoi. Abbreviations: bo, basioccipital; cqp, cranioquadrate passage; fäe, foramen äereum; pop, paroccipital process; pt, pterygoid; q, quadrate; qj, quadratojugal; sq, squamosal; sql, squamosal lateral process. Scale bar = 1 cm.

Figure 6.

Ventral view of the skull of Baurusuchus albertoi. Jugal, ectopterygoid, and the pterygoid wing were removed. Abbreviations: bo, basioccipital; cqp, cranioquadrate passage; itf, infratemporal fenestra; or, orbit; po, postorbital; pop, paroccipital process; pt, pterygoid; q, quadrate; qj, quadratojugal; qvc, quadrate ventromedial crest; sq, squamosal; stf, supratemporal fenestra. Scale bar = 1 cm.

Squamosal: The right squamosal is almost completely preserved, contacting the postorbital anteriorly, the paroccipital process posteromedially, and the quadrate posteroventrally (Figs 3–5). The only missing part is the posteromedial portion that would probably have been in contact with the parietal. Anteromedially, the squamosal forms the posterolateral margin of the supratemporal fenestra. Posteroventrally, it forms the posterior portion of the otic recess, and the dorsal and posterior borders of the otic aperture. The squamosal is bow-shaped in lateral view, with the posterior half being orientated in an almost vertical position, a typical derived feature of Baurusuchidae (Riff, 2003; Nascimento, 2008) (Figs 1, 3). The lateral edge of the vertical portion of the squamosal in B. albertoi is medially convex and laterally concave, differing from other Baurusuchidae in which it is laterally convex and medially concave. Ventrally, the lateral edge of the squamosal extends slightly laterally, forming a free ending extremity. This ventral free projection is poorly developed in Stratiotosuchus and B. salgadoensis, but wide and posteriorly directed in B. albertoi (Fig. 7). The suture between the squamosal and quadrate extends ventrally from the anterior border of the otic aperture to the posterior limit of the skull. The dorsal surface of the squamosal is highly ornamented.

Figure 7.

Detail of the lateral process of the squamosal in baurusuchids. A, Stratiotosuchus maxhechti. B, Baurusuchus salgadoensis. C, D, Baurusuchus albertoi, in lateral and anterior views, respectively. Abbreviations: po, postorbital; q, quadrate; qj, quadratojugal; sq, squamosal. Not to scale.

Otoccipital: Only the lateral portion of the paroccipital process of the right otoccipitals is preserved in B. albertoi. The paroccipital process contacts the quadrate ventrally and the squamosal dorsolaterally. The right paroccipital process is dorsoventrally wide and slightly narrower laterally (Fig. 5). The process forms two distinct surfaces delimited by a poorly developed transversely orientated ridge: one more dorsal and vertically exposed in a posterior view, and the other ventral and exposed in a posteroventral view. The ridge is very similar to that of B. salgadoensis, being less pronounced than that of Stratiotosuchus and B. pachecoi. The posterior aperture of the cranioquadrate passage opens between the paroccipital process and the quadrate, slightly medially to the contact with the squamosal.

Basioccipital: Baurusuchus albertoi only preserves a fragment of the basioccipital that represents the posterolateral corner of the bone (Figs 5, 6). The surface of the basioccipital fragment is strongly rugose and represents the area of attachment of the tendons of the Mm. basioccipitovertebralis and occipitotransversalis profundus (Iordansky, 1973). The basioccipital is exposed posteroventrally.

Ectopterygoid: The ectopterygoid of B. albertoi lacks the anterior part, contacting the jugal dorsolaterally and the pterygoid posteroventrally (Fig. 3). The contact with the jugal is wide and elliptical, expanded anterodorsally, and restricted to the suborbital region. Ventrally to that region, the ectopterygoid is less expanded, but still has an elliptical shape. The dorsal part of the bone is concave medially, forming the lateral border of suborbital (or palatine) fenestra. Ventrally, this surface ends in a medially deflected projection, which would have made contact with the palatine. The ventral projection of the ectopterygoid forms a lateromedially flat and posteroventrally orientated pointed structure that has a rugose surface in contact with the pterygoid flange. It is not possible to confirm if the ectopterygoid would have ventrally overlapped the pterygoid flange because the latter is disarticulated.

Pterygoids: There are many preserved pieces of the pterygoids of B. albertoi: a tiny piece that contacts the anteroventral portion of the quadrate (Fig. 5); the right pterygoid flange that contacts the ectopterygoid anteriorly (Fig. 3); a fragment of the left pterygoid flange; and the middle portion of the pterygoid, ventral to the braincase and dorsal to the pterygoid flanges (Fig. 6). The right pterygoid flange is similar to that present in B. salgadoensis, being ‘U’-shaped and flattened lateromedially. Anteriorly, in the area of contact with the ectopterygoid, the flange outline is slightly convex, with a highly rugose surface. The rugose surface continues laterally to this contact, forming the torose margin (torus transiliens), a friction area with the angular that permits slightly lateral movements of the mandible (Schumacher, 1973). The middle portion of the pterygoid, preserved disarticulated, has very thin walls and an elliptical shape dorsally that narrows lateromedially to form the pterygoid wings. The lateral surface of the middle part of the bone has a large concave depression, larger ventrally.

Palpebrals: Two palpebrals are present in B. albertoi. The anterior palpebral is only partially preserved whereas the posterior palpebral is complete. The anterior palpebral is highly developed in Baurusuchidae (as in Stratiotosuchus and B. salgadoensis), with a large lateral projection and exceeding the prefrontals anteriorly. The preserved portion of anterior palpebral of B. albertoi is ‘D’-shaped. The dorsal surface is ornamented and slightly convex. The posterior palpebral contacts the anterior palpebral anteriorly and the postorbital posteroventrally. It shows an almost triangular shape in dorsal view (Fig. 4), differing from the trapezoidal form present in Stratiotosuchus and B. salgadoensis. About a third of the total length of the bone is laterally projected, dorsally covering the orbit. The dorsal surface of the posterior palpebral is slightly convex laterally and ornamented with furrows, whereas the ventral surface is smooth and slightly concave.

Surangular: This is an elongated element located in the dorsal half of the posterior portion of the mandibulae, forming anteroventrally the dorsal and posterodorsal edges of the external mandibular fenestra (Figs 1, 8). The surangular contacts the dentary anteriorly, the angular ventrolaterally, and the articular posteromedially. The suture with the dentary is mostly ‘S’-shaped in lateral view, similarly to that present in B. salgadoensis. The contact is poorly preserved in B. pachecoi. In medial view, the same suture extends anteriorly (Fig. 9). As in Mariliasuchus amarali (Zaher et al., 2006), the small and rugose surface that is dorsal to the anterodorsal extremity of the mandibular fenestra (Fig. 9) could represent a vestigial coronoid fused to the surangular. As in B. salgadoensis, the anterior ramus of the surangular in B. albertoi is longer than that of B. pachecoi (Fig. 1), giving the shape of a parallelogram to the mandibular fenestra of the former two taxa (character 261 – see phylogeny). The lateral suture of the surangular with the angular is located in the dorsal half of the mandible, contrasting with the other species of Baurusuchus in which this suture appears in a more ventral position of the lateral surface. Ventrally, the surangular possesses a thin and laminar prolongation that fits between the angular (laterally) and the articular (medially). The shape of this ventral lamina is straight and funnelled anteriorly (Fig. 9). The posterior extremity of the surangular is deflected dorsally at almost 30°, forming a small, rugose, and funnelled projection. This small projection is present in other species of Baurusuchus, being more robust in B. salgadoensis and clearly not dorsally deflected in B. pachecoi. The surangular ornamentation is only present in the lateral surface, near the angular contact.

Figure 8.

Lateral view of the mandible of Baurusuchus albertoi. Abbreviations: a, angular; art, articular; d, dentary; emf, external mandibular fenestra; pid, M. pterygoideous posterior insertion depression; rap, retroarticular process; sa, surangular; tm, torose margin; vpa, ventral protuberance of the articular. Scale bar = 1 cm.

Figure 9.

Medial view of the mandible of Baurusuchus albertoi. Abbreviations: a, angular; aca, anterior crest of the articular; art, articular; d, dentary; emf, external mandibular fenestra; gf, glenoid fossa; rap, retroarticular process; sa, surangular; tm, torose margin; vls, ventral lamina of the surangular; vpa, ventral protuberance of the articular. Scale bar = 1 cm.

Angular: The angular forms the ventral half of the posterior portion of the mandible, and shows a typically ‘U’-shaped form (Iordansky, 1973) with a straight ventral outline. The anterodorsal region forms the inferior edge and almost the entire posterior edge of the external mandibular fenestra. The suture with the dentary occurs in the anteroventral limit of the mandibular fenestra, and is much more anteriorly positioned medially than laterally. The mandibular portion of the torose margin (Schumacher, 1973), present in other Baurusuchidae, is located in the anteromedial portion of the angular and corresponds to a small protuberance that extends dorsally (Figs 8, 9). Laterally to this margin, two large foramina are present inside the ‘U’-shaped form, contrasting with Stratiotosuchus, which shows three foramina (Riff, 2003). Posteriorly to the mandibular fenestra, the lateral wall of the angular is higher than that of the surangular, in contrast with the condition present in other species of Baurusuchus, but similar to the one present in Stratiotosuchus (Pinheiro et al., 2008). The lateral wall of the angular contacts medially the ventral lamina of the surangular. The posterior portion of the angular is lateromedially flat and has a rounded outline that contacts medially the retroarticular process (Fig. 8). The lateral surface is ornamented with a series of furrows and small crests, especially in the anterior-most ramus. The lateral surface of the surangular retains a well-developed depression behind the mandibular fenestra that corresponds to the area of attachment of a greatly developed M. pterygoideous posterior typical of Baurusuchidae (Nascimento, 2008).

Articular: The articular contacts the angular anteroventrally and the surangular laterally, being placed posterior and medial to the external mandibular fenestra. It is a roughly triangular bone, with an acute anterior tip, a medially developed glenoid fossa, and a well-developed posterodorsally directed retroarticular process (Fig. 9). The anteroventral portion of the articular fits totally inside the ‘U’ of the angular. This portion possesses a crest that divides the bone into two faces: a dorsal horizontal one and a vertical medial one (Fig. 9). This form is very similar to that present in B. pachecoi, but is different from that of B. salgadoensis, where this part is rounded and does not possess a developed crest. As in other species of Baurusuchus, the glenoid fossa possesses two concavities divided by a middle prominence: a horizontal lateral one, and an almost vertical medial one that fits the fusiform medial condyle of the quadrate. Posteriorly and continuous to the glenoid fossa there is a rugose surface that could represent an attachment area of the M. depressor mandibulae. The retroarticular process of B. albertoi is verticalized, lateromedially flattened, and dorsally rounded (Figs 8, 9). This condition contrasts with the one present in the other species of Baurusuchus, where the retroarticular process is almost horizontal and not flattened (Fig. 1). Posteriorly, the transition of the glenoid fossa to the retroarticular process is very abrupt, forming an angle of almost 90° (character 262) (Fig. 9). This transition is gentle in the other species of Baurusuchus. The retroarticular process has a ventral expansion that forms a small, rounded, and rugose projection, anteromedially deflected (Figs 8, 9). The same projection is present in the other species of Baurusuchus, being more robust, medially deflected, and less prominent than in B. albertoi. This small projection could be an auxiliary attachment area for the retroarticular aponeurosis of the M. pterygoideus posterior (Iordansky, 1964, 1973).

Dentary: The anterior portion of the dentary is not preserved in B. albertoi. The dentary is lateromedially narrow and laterally straight in B. albertoi, with its posteroventral border contacting the angular, and its posterodorsal border contacting the surangular medially. The dorsal boundary of the dentary is straight and anteriorly bent ventrally. Its ventral border is straight. The posterior border of the dentary forms the anterior boundary of the external mandibular fenestra. The lateral surface of the dentary is ornamented, as in all other baurusuchids. There is a shallow linear groove that runs horizontally along the lateral surface of the dentary, from the anteroventral extremity of the external mandibular fenestra to the broken anterior part (Fig. 8).

Hyoids: The only part of the hyoid apparatus that is totally ossified in crocodiles is the Cornu branchiale I (Schumacher, 1973). Both Cornua branchiale I of B. albertoi were found, semi-articulated to the skull. The two elements are 9 and 10 cm in length, are bow-shaped (Fig. 10), and resemble the ones of Crocodylus porosus (Schumacher, 1973). Comparing with the position of these bones in extant crocodiles, it is evident that the concave portion of the bones was medially directed. The two bones are elliptical in their extension, being more flattened at their extremities. The ventral surface is totally flat. Both the extremities are damaged, probably because of their cartilaginous nature. Both elements show scars in their surface that probably represent areas of insertions of the Mm. brachiomandibularis visceralis and branchiomandibularis spinalis (Schumacher, 1973; Cleuren & De Vree, 2000) (Fig. 10).

Figure 10.

Dorsal view of the Cornu branchiale I, from the hyoid apparatus of Baurusuchus albertoi. The white lines point to the muscular insertion scars. Scale bar = 1 cm.

DISCUSSION

In order to test the phylogenetic affinities of B. albertoi and the monophyly of the Baurusuchidae, we used an extended version of the data matrix of Gasparini et al. (2005). Four taxa were included in the analysis: S. maxhechti, B. salgadoensis, B. albertoi, and P. pakistanensis. We also added five more characters (Appendix). A total of 63 taxa was scored for 262 characters. Parsimony analysis using PAUP 4.0 (beta 10) (Swofford, 2000) with a heuristic search strategy (100 replicates of Wagner trees followed by tree-bisection-reconnection [TBR] branch swapping) resulted in only one most parsimonious tree with 933 steps (consistency index = 0.356; retention index = 0.714) (Fig. 11).

Figure 11.

Most parsimonious tree that resulted from the parsimony analysis. Unambiguous synapomorphies for the labelled nodes and Bremer indexes are: Node 1 (Bremer of 1): 95 (0), 105 (2). Node 2 (Bremer of 1): 79 (0), 107 (1), 226 (1). Node 3 (Bremer of 1): 1(1), 22 (1), 74(1), 260(2). Node 4 (Bremer of 1): 108(1), 136(1), 141(1). Node 5 (Bremer of 1): 140(0), 188(2). Node 6 (Bremer of 1): 232(1), 235(1). Node 7 (Bremer of 1): 104(2), 111(1), 127(0), 185(1), 237(1). Node 8 (Bremer of 1): 7(0), 121(1), 130(1), 134(1), 148(1). Node 9 (Bremer of 1): 106(3), 124(1). Node 10 (Bremer of 6): 9(2), 79(1), 80(1), 103(2), 107(0), 120(0), 128(0), 155(0), 158(1), 159(2), 193(0), 236(1). Node 11 (Bremer of 4): 44(1), 56(1), 78(0), 118(1), 228(1). Node 12 (Bremer of 3): 17(2), 18(1), 19(1), 20(1), 32(1), 36(3), 68(0), 81(1), 90(0), 108(3), 113(1), 206(0), 209(1), 257(1). Node 13 (Bremer of 1): 258(1). Node 14 (Bremer of 1): 9(1), 106(2), 124(1). Node 15 (Bremer of 1): 136(0).

According to the present analysis, the family Baurusuchidae forms a monophyletic clade deeply nested within Notosuchia, as the sister group to a clade formed by Bretesuchus and Iberosuchus. Monophyly of the Baurusuchidae is supported by the following 14 unambiguous synapomorphies: anterior portion of the jugal more than two times broader than the posterior part [character 17 (2)]; rod-shaped infratemporal bar of the jugal [character 18 (1)]; dorsal process of the quadratojugal extensively contacting the postorbital [character 19 (1)]; frontal width between orbits twice as broad as nasals [character 20 (1)]; parietal without a broad occipital portion [character 32 (1)]; posterolateral process of squamosal totally deflected ventrally [character 36 (3)]; relatively large supratemporal fenestrae, covering most of the surface of the skull roof [character 68 (0)]; enlarged dentary teeth posterior to tooth opposite premaxilla−maxilla contact [character 81 (1)]; cervical neural spines anteroposteriorly large [character 90 (0)]; maxilla with five teeth [character 108 (3)]; spool-shaped vertebral centra [character 113 (1)]; cranial table as wide as ventral portion [character 206 (0)]; paired ridges located medially on ventral surface of basisphenoid [character 209 (1)]; nasals partially or completely fused [character 257 (1)]. Within Baurusuchidae, Stratiotosuchus and Pabwehshi form a clade supported by three unambiguous synapomorphies: a notched premaxilla−maxilla contact [character 9 (1)]; three premaxillary teeth [character 106 (2)]; and dorsal border of external nares formed by the premaxillae and nasals [character 124 (1)]. This clade represents the sister group of the genus Baurusuchus, the monophyly of which is supported by the unambiguous synapomorphy of a ‘C’-shaped lateral suture of the postorbital and squamosal [character 258 (1)].

Baurusuchus albertoi appears as the sister group of a clade formed by B. pachecoi and B. salgadoensis. The latter clade is supported by just one unambiguous synapomorphy, the posterior process of the jugal exceeding the infratemporal fenestra posteriorly [136 (0)]. This difference may be related to individual variation, and the validity of this clade must be tested with additional material.

The clade formed by the family Baurusuchidae, Iberosuchus and Bretesuchus (clade 10) is well supported by the following unambiguous synapomorphies: 9 (2), 79 (1), 80 (1), 103 (2), 107 (0), 120 (0), 128 (0), 155 (0), 158 (1), 159 (2), 193 (0), 236 (1). Some of these features are classically related to the Sebecosuchia clade, such as large dentary teeth (character 80), the height of the mandibular symphysis (character 103), and ziphodont teeth (character 120).

Although B. albertoi, B. pachecoi, and B. salgadoensis are only known from one specimen each, they differ from each other in numerous aspects that are not ontogenetically related, as all specimens are clearly adult individuals. Indeed, B. pachecoi differs from the other two species by the following combination of characters: posterior end of surangular straight; anterior end of surangular and posterior end of dentary expanded dorsoventrally, and forming a dorsal convexity; ventral outline of the mandible straight in its middle portion; dorsal portion of the retroarticular process poorly developed and posteriorly orientated. Baurusuchus salgadoensis differs from the two other species of the genus by the following combination of characters: anterior portion of the articular lacking a marked crest that divides the dorsal surface from the medial one; ventral outline of the maxillae anteriorly convex and posteriorly concave; retroarticular process with a medially deflected ventral projection. Finally, B. albertoi differs from the other two species of the genus by the following combination of traits: anterior portion of the jugal with a triangular and rugose ventrolateral projection; infratemporal bar of the jugal slender and as long as the inferior edge of the orbit; retroarticular process flattened lateromedially and orientated vertically, with a posterodorsal expansion; lateral projection of the vertical portion of the squamosal posteriorly concave; well-developed ventromedial crest of the quadrate.

ACKNOWLEDGEMENTS

The authors thank Diógenes de Almeida Campos, Rita Cassab, and Alex Souto (Museu de Ciências da Terra Llewellyn Ivor Price of DNPM do Rio de Janeiro); Sérgio Alex Azevedo, Alexander Kellner, and Deise Henriques (Museu Nacional do Rio de Janeiro); Douglas Riff (Universidade Federal de Uberlândia); Ismar Carvalho and Felipe Vasconcellos (Universidade Federal do Rio de Janeiro); Antônio Celso Arruda and Sandra Tavares (Museu de Paleontologia de Monte Alto) for permission to access specimens under their care. Diego Pol provided important comments that improved the quality of the work. Mark Van Tomme (Université Libre de Bruxelles) helped with the acquisition of important references. We are also grateful to João Tadeu Arruda, for support in the field, and especially to Aparecido Barbosa for allowing us to excavate in his property. The present contribution benefited from grants of FAPESP (05/52411–8 and 02/13602–4) under the program BIOTA-FAPESP.

Ancillary