Baurusuchid crocodyliforms as theropod mimics: clues from the skull and appendicular morphology of Stratiotosuchus maxhechti (Upper Cretaceous of Brazil)

Authors

  • DOUGLAS RIFF,

    Corresponding author
    1. Instituto de Biologia, Universidade Federal de Uberlândia, Campus Umuarama, Bloco 2D – sala 28, Rua Ceará, s/n, Uberlândia, Minas Gerais, 38400-902, Brazil
      E-mail: driff2@gmail.com
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  • ALEXANDER WILHELM ARMIN KELLNER

    1. Departamento de Geologia e Paleontologia, Museu Nacional, Universidade Federal do Rio de Janeiro, Quinta da Boa Vista, s/n, Rio de Janeiro-RJ, 20940-040, Brazil
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E-mail: driff2@gmail.com

Abstract

The Baurusuchidae crocodyliforms are usually interpreted as active terrestrial predators, but only some positive evidence of such habits has been described to date, mainly the relative position of external nares and orbits. Here we describe features that support this view in a complete specimen of the Baurusuchidae Stratiotosuchus maxhechti, and have executed a parsimony analysis to confirm their phylogenetic position. S. maxhechti exhibits theropodomorph features that have been previously recognized in skulls of the Baurusuchidae, as well as postcranial characteristics related to a parasagittal gait, showing that the similarities between the Baurusuchidae and theropods extend beyond the cranial morphology. These include a well-developed supracetabular crest, a relatively medially offset femoral head and a caudally orientated calcaneal tuber. The orientations of the surfaces for muscular attachments imply that the appendicular movements of S. maxhechti were mainly anteroposterior, with abduction significantly constrained. S. maxhechti presents features that mimic some present in theropods, including a ‘fossa brevis’ on the ilium and tubercles on the ischium and femur similar to the obturator process and accessory trochanter. The relative proportions of the femur, tibia, and longer metatarsal are more similar to those of Postosuchus than to other Crocodylomorpha. In the skull, besides the theropodomorph (ziphodont) dentition concentrated in the anterior half of the rostrum, the baurusuchids are remarkable by the fusion of the nasals, which can be related to a large resistance against feeding forces acting on a high-profile skull. The appendicular morphology of S. maxhechti strengthens the interpretation that the Baurusuchidae were active land-dwelling predators in the Upper Cretaceous of south-eastern Brazil, occuping ecological niches typical of small to medium-sized theropod dinosaurs.

© 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 163, S37–S56.

INTRODUCTION

Since the original description of the Cretaceous crocodyliform Baurusuchus pachecoi by Llewellyn Ivor Price (1945), palaeontologists have noted in this species some features reminiscent of theropod dinosaurs (Price, 1945; Buffetaut, 1982; Gasparini, 1984; Kellner & Campos, 1999; Riff & Kellner, 2001). Most conspicuous are the high and laterally compressed skull and the dentition. The latter consists of blade-like compressed teeth, reduced in number and retricted to the anterior half of both jaws, with posteriorly bent crowns bearing fore and aft denticulate carinae, a dental morphology recognized as true ziphodont (sensuPrasad & Broin, 2002). This taxon raised the question as to whether the numerous isolated teeth reported from the Cretaceous of Brazil, mainly from the Bauru Group, are indeed dinosaurian, as reported many years ago (Price, 1950). Other general features of Baurusuchus are the absence of the antorbital fenestrae, paired anteriorly located external nares, a huge choana, and orbits located lateroanteriorly.

For decades the only undisputable representative of the Baurusuchidae was the holotype of B. pachecoi, which consists of an almost complete skull with mandible from the Upper Cretaceous (Campanian−Maastrichtian) Adamantina Formation (Bauru Basin) of north-western São Paulo State (Paulo de Farias town), Brazil. Despite being only briefly described by Price (1945), the peculiarities noted above have drawn attention, leading some authors to associate Baurusuchus with the also oreinirostral and ziphodont crocodyliform Sebecus icaeorhinusSimpson, 1937 from the Eocene of the Chubut Province (Argentina), in the suborder Sebecosuchia (Colbert, 1946; Romer, 1956). Walker (1968), however, stressed the differences between the taxa and proposed that the infraorder Baurusuchia should be restricted to Baurusuchus.

Presently, the phylogenetic position of Baurusuchus (and related taxa) is still in dispute. Several phylogenetic analyses have rejected the monophyly of the Sebecosuchia, indicating that Sebecus is more closely related to the Neosuchia or to the Peirosauridae, with Baurusuchus occupying a more basal position within the Metasuchia, or even regarded as a derived Notosuchia (Clark, 1994; Wu, Russell & Cumbaa, 2001; Larsson & Sues, 2007; Andrade & Bertini, 2008). Other studies that included Baurusuchus and Sebecus, or additional putative sebecids (e.g. Bretesuchus), have found a monophyletic Sebecosuchia, related to, or inserted within the Notosuchia (Gasparini, Fernandez & Powell, 1993; Gomani, 1997; Ortega et al., 2000; Pol, 2003; Company et al., 2005; Turner & Calvo, 2005; Turner, 2006; Gasparini, Pol & Spalletti, 2006; Nascimento & Zaher, 2011, this volume).

The only other taxon putatively referred to the Baurusuchidae during the 20th century was Cynodontosuchus rothiWoodward 1896, a tiny crocodyliform from the Coniacian−Santonian Bajo de la Carpa Formation of the Neuquén Province, Argentina (Gasparini et al., 1993; Leanza et al., 2004; Martinelli & Pais, 2008). Although Cynodontosuchus was formerly considered closely related to Notosuchus in the definition of the Notosuchidae by Nopcsa (1928), other authors included Cynodontosuchus in the Baurusuchidae based mainly on the high profile of the skull, absence of the antorbital fenestra, and the dental formula (Romer, 1956; Price, 1959; Gasparini, 1972; Buffetaut, 1980, 1982; Gasparini, Chiappe & Fernandez, 1991; Gasparini et al., 1993).

However, some important morphological differences between Cynodontosuchus and Baurusuchus are noticeable, mainly the presence in the former of a proportionally elongated premaxilla, absence of an enlarged caniniform premaxillary tooth, and absence of a large lateral occlusive notch above the premaxilla−maxilla suture. Despite the fact that the incompleteness of the holotype of Cynodontosuchus (a poorly preserved partial rostrum and dentary) precludes detailed comparisons and confidence about the relationships of this taxon, the very small size points to an ecological niche distinct from that of Baurusuchus (Bonaparte, 1996).

All studies focusing on the possible behaviour of the Baurusuchidae in the last century were based on the holotype of B. pachecoi and fragmentary specimens, some including appendicular elements tentatively associated to this taxon (Brandt-Neto et al., 1991; Brandt-Neto, Manzini & Bertini, 1992; Bertini, Manzini & Neto, 1999; Manzini, Brandt-Neto & Vizotto, 1996). More recently other taxa have been regarded as baurusuchids, including fragmentary cranial remains from the Late Cretaceous of Pakistan (Pabwehshi pakistanensisWilson, Malkani & Gingerich, 2001) and Argentina (Wargosuchus australisMartinelli & Pais, 2008). The phylogenetic position of the Pakistani form as a baurusuchid is not consensual, being supported by Turner (2006) but not by Larsson & Sues (2007), who included Pabwehshi as more closely related to Sebecus and to the Peirosauridae in the clade Sebecia, which excludes the Baurusuchidae.

The incomplete nature of the holotype of Pabwehshi (a partial rostrum and mandible) precludes a definitive taxonomic consensus, and the dental features usually taken as evidence of a close relationships with Baurusuchus are either not found in the Brazilian taxon (see Riff & Kellner, 2001) or also occur in the peirosaurid crocodyliforms (sensuLarsson & Sues, 2007), or in Kaprosuchus (Sereno & Larsson, 2009). Wargosuchus, despite the fragmentary condition of the holotype, shows features strongly reminiscent of the Baurusuchidae, such as the presence of a large and anteriorly placed palpebral, as well as prefrontals very close at midline, despite being isolated from each other by a slender anterior process of the frontal (prefrontals meet medially in Baurusuchus and S. maxhechti, isolating the frontal from any contact with the nasal). Therefore, a close relationship between Wargosuchus and the Brazilian Baurusuchidae is very probable, but new and more complete specimens are required.

Even without appropriate postcranial elements, the cranial features noted above have been used by almost all authors to infer terrestrial and predatory habits for Baurusuchus, as well as for Sebecus and other ziphodont crocodyliforms (e.g. Price, 1945, 1955; Buffetaut, 1982; Busbey, 1986; Buffetaut & Marshall, 1991; Gasparini et al., 1993; Riff & Kellner, 2001; Martinelli & Pais, 2008), although this was not an unanimous view (e.g. Colbert, 1946, who regarded their habits as not distinct from the extant crocodylians).

In recent years, many new baurusuchid specimens have been found in the fine sandstones of the Adamantina Formation, including several articulated skeletons (Campos et al., 2001; Arruda, Carvalho & Vasconcellos, 2004; Carvalho, Campos & Nobre, 2005; Pinheiro et al., 2008; Nascimento & Zaher, 2011, this volume). Amongst them is S. maxhechti, whose skull was briefly described by Campos et al. (2001). Here we present several new pieces of anatomical evidence that support the traditional view of baurusuchids as terrestrial top predators, and show that the theropodomorph features extend beyond the skull, indicating complete terrestrial habits and a fully erect posture for this peculiar group of crocodyliforms. We also performed a parsimony analysis to determine the phylogenetic position of S. maxhechti.

MATERIAL AND METHODS

The reconstruction of the musculature of S. maxhechti and other fossil crocodylomorphs was based on the available information on extant Crocodylia and on personal observations. The following taxa were used for the present study:

  • • S. maxhechti: DGM 1477-R (holotype); Museu de Ciências da Terra, Departamento Nacional de Produção Mineral, Rio de Janeiro, Brazil.
  • • Postcranial skeletons associated with the holotype of Baurusuchus salgadoensis: UFRJ DG 285-R and UFRJ DG 288-R. Instituto de Geociências, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil (figured in Arruda et al., 2004).
  • • Postcranial skeleton of Mariliasuchus amarali: UFRJ DG 105-R. Instituto de Geociências, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil.
  • • Partial skull and postcranial skeleton of Baurusuchus sp. URP RC-5. Universidade Estadual Paulista, São José do Rio Preto, Brazil (cited in Manzini & Brandt-Neto, 2000).
  • • Cleaned skeletons of Caiman yacare (seven) and Melanosuchus niger (two), housed at the Museu Nacional (Universidade Federal do Rio de Janeiro, Rio de Janeiro), and Universidade Federal de Uberlândia, Uberlândia (not catalogued).
  • • Cleaned skeleton of a large Melanosuchus niger (one 4.5-m adult), housed at Universidade Federal do Acre, Rio Branco, Brazil (UFAC-R207).

General remarks and the geological context ofStratiotosuchus Maxhechti

The holotype of S. maxhechti was the first complete skeleton of a baurusuchid ever found and is one of the most complete fossil crocodyliform from Brazil. Housed at the Earth Science Museum (DGM-1477-R), the specimen was collected in the 1980s from the town of Irapuru, north-western São Paulo State (Campos & Suarez, 1988) without proper taphonomic data. Notwithstanding, a strong flexion of the arm can be noted (Fig. 1).

Figure 1.

Block with partial sequence of dorsal vertebrae and right scapular girdle and member of S. maxhechti (holotype, DGM 1477-R) before full preparation. Abbreviations: Co, coracoid; Ph, phalanges; Ra, radius; Ul, ulna; Um, humerus; Un, ulnare. Scale bar = 10 cm.

The cranial and dental features of S. maxhechti allow an immediate association with the Baurusuchidae. The massive skull (Fig. 2) shows the same degree of oreinirostry as Baurusuchus and the teeth are equally true ziphodont, all finely serrated. With a reduced dental formula (three premaxillary and five maxillary teeth in S. maxhechti; four premaxillary and five maxillary teeth in Baurusuchus), the dentition of the Baurusuchidae is notable for the considerable size variation, with enormous caniniforms along very small ones, the first maxillar and the third mandibular teeth being the smallest (Fig. 3). In occlusion, upper and lower teeth alternate with the edges shearing past each other in a scissor-like fashion, but with all dentary teeth occupying a mesial position relative to the upper teeth. These dental features were the main ones that led several authors to postulate a terrestrial top predator habit for this group.

Figure 2.

Holotype skull of S. maxhechti (DGM 1477-R) in dorsal (A), lateral (B), ventral (C), occipital (D), and frontal (E) views. Abbreviations: bo, basioccipital; bs, basisphenoid; co, occipital condyle; ect, ectopterygoid; esq, squamosal; exo, exoccipital; f pr, perinarial fossa; fm, foramen magnum; fo, foramen; fr, frontal; fti, inferior temporal fenestra; j, jugal; l, lacrimal; ls, laterosphenoid; m, maxilla; me, external auditory meatus; n, nasal; ne, external nares; or, orbit; pa, parietal; paa, anterior palpebral; pap, posterior palpebral; pcq, cranium−quadrate passage; pm, premaxilla; po, postorbital; pp, paraoccipital process; pr nar, prenarial process of the premaxilla; prf, prefrontal; pt, pterygoid; q, quadrate; qj, quadratojugal; so, supraoccipital. Scale bar = 15 cm.

Figure 3.

Symphyseal portion of the left dentary of Baurusuchus pachecoi (DGM 299-R) in lateral view. Note the size variation of the teeth. The larger caniniform tooth is the fourth tooth. Scale bar = 2 cm.

Besides the dentition, another remarkable feature present in the skull of S. maxhechti is the fusion of the nasals (Fig. 4). A similar condition is observed in tyrannosaurid theropods, where it has been regarded as a result of high feeding forces acting on a high-profile skull (Snively, Henderson & Phillips, 2006).

Figure 4.

Horizontal computed tomography scan slices of the holotype skull of S. maxhechti (DGM 1477-R) taken at 13.2 mm (top) and 15.6 mm (bottom) below the dorsal surface. Abbreviations as in Figure 2. Note the absence of a suture between the nasals in both slices. The black area behind the prefrontals represents the dorsal concavity of the frontal and the hole over the nasal is a broken area. Regions in dark grey correspond to the sedimentary matrix that fills the skull.

Regarding the geological provenance, S. maxhechti was found in the Adamantina Formation, which is also the stratigraphical unit from which all other Brazilian baurusuchid come from. Its geographical distribution is restricted to western and north-western Sao Paulo State, with some undescribed specimens from the ‘Triângulo Mineiro’ region of the Minas Gerais State (Montefeltro et al., 2010).

As classically defined, the Adamantina Formation is a complex stratigraphical unit that comprises the largest continental deposit of the Bauru Group. Consisting of fine to very fine sandstones and siltstones, these layers are regarded to represent a meandering fluvial system intercalated with ephemeral lakes of Campanian−Maastrichtian age (Soares et al., 1980; Fernandes & Coimbra, 1996; Gobbo-Rodrigues, Petri & Bertini, 1999).

Covering approximately 117 000 km2, the original Adamantina Formation was recently divided into four distinct formations by Fernandes & Coimbra (2000): Vale do Rio do Peixe, Araçatuba, São José do Rio Preto, and Presidente Prudente, although geologists have not reached a consensus on these (e.g. Batezelli et al., 2003; Paula e Silva, Kiang & Caetano-Chang, 2009). Besides, many of the fossil vertebrates (including crocodyliforms) were found by chance and the lack of precise stratigraphical control undermines an accurate notion of the contemporaneity of the faunal elements of the former Adamantina Formation.

Considering this stratigraphical arrangement, all published reports associated with Baurusuchus, as well most of the crocodyliforms from São Paulo State, came from outcrops that appear to correspond to the Vale do Rio do Peixe Formation. S. maxhechti came from a locality (Irapuru town) where the Presidente Prudente Formation is predominant. Furthermore, lithological characteristics of the matrix in which the specimen was found (lighter in colour and less cemented siltstone than the Vale do Rio do Peixe unit, with abundant millimetric to centimetric pellets of clay) agree with this tentative stratigraphical positioning.

During the preparation of the holotype (DGM-1477-R) of S. maxhechti, some elements were found to be duplicated: a pair of femora (incomplete), a fragment of a left ischium, and the proximal half of a right metatarsus (with four articulated metatarsi). These bones are identical in size and shape to the corresponding complete elements of DGM-1477-R, and indicate the presence of a second individual probably representing S. maxhechti (and numbered as MCT 1714-R). How they become preserved in the same place cannot be determined for the time being because of the lack of taphonomic information.

Phylogenetic analysis

In order to establish the phylogenetic position of S. maxhechti, we performed a parsimony analysis (see Character List S1 in the Supporting Information). Our results (Fig. 5) show that S. maxhechti is closely related to Baurusuchus, and support the validity of the node-based definition of Baurusuchidae proposed by Carvalho, Ribeiro & Avilla (2004): Baurusuchidae are the most recent common ancestor of Baurusuchus and S. maxhechti and all of their descendants. Unfortunately, a test including Pabwehshi and Wargosuchus resulted in a large polytomy in the Mesoeucrocodylia clade. As a major revision of the Mesoeucrocodylia phylogeny was beyond the scope of this paper, we excluded both fragmentary taxa. Our results support previous ones, cited above, suggesting the paraphyly of the classical Sebecosuchia.

Figure 5.

Strict consensus of six equally optimal trees generated by PAUP (Swofford, 1998). Length = 1028 steps, consistency index, excluding autapomorphies = 0.360; retention index = 0.666. Numbers above internal branches indicate jackknife (if alone) or bootstrap/jackknife support. Numbers below internal branches indicate Bremer support.

The clade Baurusuchus + S. maxhechti is supported by the following unambiguous synapomorphies: internarinal bar complete, with a larger contribution of the nasal [character (char.) 4, state (st.) 1; jugal bar rod-shaped beneath infratemporal fenestra (char. 18, st. 1); frontal twice as broad as nasals between the orbits (char. 20, st. 1); posterolateral process of squamosal elongated, robust, and almost vertically orientated, forming a plate (char. 36, st. 3); anterior dentary teeth opposite premaxilla−maxilla contact more than twice the length of other dentary teeth (char. 80, st. 1); mandibular symphysis in lateral view deep and anteriorly convex (char. 103, st. 2); absence of an unsculptured region along alveolar margin on lateral surface of maxilla (char. 107, st. 0); posterior process jugal not exceeding posteriorly the infratemporal fenestrae (char. 136, st. 1); participation of ectopterygoid in the palatine bar (char. 233, st. 1); dorsal surface of frontal, posterior to orbits, concave transversally (char. 260, st. 1); and prefrontals contacting mutually medially, isolating frontal from contact with nasal (char. 262, st. 1). Amongst the eight ambiguous synapomorphies, we emphasize the anterior part of the jugal more than twice as broad as the posterior part (char. 17, st. 2); dentary teeth posterior to tooth opposite premaxilla−maxilla contact more enlarged than the opposite smaller maxillary teeth (char. 81, st. 1); maxilla with five teeth (char. 108, st. 3); dentary with lateral concavity for the reception of the enlarged maxillary tooth (char. 158, st. 1); and absence of foramina in the perinarial depression (char. 237, st. 0).

This result supports the deep insertion of the Baurusuchidae amongst the Notosuchia, and a close relationship with the Sphagesauridae, as already pointed by Pol (2003), despite not supporting the position of Bretesuchus in this clade. The three unambiguous synapomorphies that support the clade Baurusuchidae + Sphagesauridae are: lateral surface of anterior process of jugal or with broad shelf below the orbit with triangular depression underneath it (char. 121, st. 1); antorbital region of jugal more expanded than infraorbital region (char. 130, st. 1); and skull roof with trapezoidal shape in dorsal view (char. 181, st. 1).

As the matrix used had an overrepresentation of skull features, the small number of coded postcranial features did not lead to major reconsiderations or have implications for the discussion conducted below.

The appendicular morphology ofS tratiotosuchus maxhechtiand the stance and gait of the Baurusuchidae

The appendicular elements of S. maxhechti have all osteological features generally associated with a fully erect posture in a quadruped animal (Parrish, 1986). Here we stress the main features that indicate a permanent parasagittal posture in the Baurusuchidae in general, and in S. maxhechti in particular. Furthermore we show some homoplastic similarities between the appendicular anatomy of the Baurusuchidae and the Dinosauria (mainly Theropoda). A full anatomical description of the holotype of S. maxhechti, including skull and the postcranial elements, is forthcoming.

The coracoid of S. maxhechti shows a posteroventrally glenoid process and the humerus has a wide and convex articular head, allowing large anteroposterior movements of the anterior member and keeping the limb in parasagittal plane (Fig. 6). These features are similar to those reported in the basal crocodylomorph Junggarsuchus, from the Jurassic of China described by Clark et al. (2004), who stressed this morphology as an indication of restricted parasagittal movement. The deltopectoral crest of S. maxhechti occupies 45% of the humeral length, and its extension throughout the cranial surface of the humerus in an oblique orientation provides a stronger protraction vector to all musculature there inserted. In the extant Crocodylia the muscles inserted in the laterally positioned deltopectoral crest (deltoideus clavicularis muscle, and especially the supracoracoideus complex) are the main protractors, with the deltoideus clavicularis muscle acting also as a stabilizer of the shoulder joint, mainly during the high walk (Meers, 2003). We suggests that the elongation and the obliquity of the deltopectoral crest in S. maxhechti (noted also in the specimens UFRJ DG 285-R and UFRJ DG 288-R, regarded as B. salgadoensis) increase the protractor moment arm by imposing a more anterior than lateral orientation of the associated muscles.

Figure 6.

Left humerus (at left) and right coracoid (at right) of S. maxhechti (DGM 1477-R). Humerus in cranial, lateral, caudal, and medial views. Coracoid in proximal (or dorsal), medial, lateral and caudal views. Abbreviations: dc, area for insertion of the muscle deltoideus clavicularis; sc, area for insertion of the muscle supracoracoideus. Scale bar = 10 cm.

Furthermore, on the lateral surface of the deltopectoral crest, adjacent to the insertion point of the supracoracoideus complex (at the apex of this crest), S. maxhechti and Baurusuchus show a marked rugose scar of lightly oblique and parallels ridges, corresponding to the insertion point of the deltoideus clavicularis muscle of the extant forms (Meers, 2003), which is more developed in the baurusuchids. The development of a muscle that in extant form is required mainly when the high walk is performed agrees with the notion of a permanent parasagittal posture of the Baurusuchidae.

The manus of S. maxhechti has metacarpals that are compressed together and not spread out. This pattern is reflected by proximal expansions on the metacarpals that cover the immediate lateral element, only absent in the thinner and longer metacarpal IV (Figs 7, 8). The phalangeal formula is incomplete, being 2-3-4?-?-?.

Figure 7.

Left manus (at left) in articulated position and left metacarpals I to V disarticulated (at right) of S. maxhechti (DGM 1477-R) in dorsal view (ungual phalanges, 1-I and 3-III, in medial view). Abbreviation: dc3, distal carpal 3. Scale bar = 10 cm.

Figure 8.

Right manus of S. maxhechti (DGM 1477-R), with articulated metacarpals and some proximal phalanges in dorsolateral view, proximal phalanges in dorsal view; and unguals in lateral view (A). Note the pathologically altered metacarpal V (see Cabral et al., 2011, this volume). Articulated metacarpals (except metacarpal V), some proximal phalanges and distal carpal in proximal (B), ventral (C), lateral (D), and medial (E) views. Abbreviations: Roman numbers represent metacarpals; 1-I, first (proximal) phalanx of the digit I; 1-II, first (proximal) phalanx of the digit II; 2-II, second (middle) phalanx of the digit II; dc 4 + 5, distal carpal 4 + 5. Scale bar = 10 cm.

The proximal phalanges in the manus and pes have two proximal projections. The ventral one is longer (except the proximal phalanx of the pedal digit IV, where it is subequal) and represents the insertion point of part of the flexor musculature of the digits (Fig. 9). The dorsal projection, narrower than the ventral one, fits within the intercondylar notch of the precedent phalanx, acting as a stabilizer to avoid lateral displacements. A similar condition also occurs in Herperosuchus agilis (Colbert, 1952) and in Caiman and Melanosuchus, but in the extant taxa only the median pedal phalanges have such projections at the proximal articular surface (but only the dorsal one is noticeable).

Figure 9.

Phalanx 1 (proximal) of the left pedal digit II of S. maxhechti (DGM 1477-R) in medial view. Abbreviations: dpp, dorsal proximal process; vpp, ventral proximal process. Scale bar = 5 cm.

All phalanges, even the most distal ones, have deep dorsal pits at the distal ends, acting as insertion points of the extensor musculature of the digits. The proximal and middle phalanges also show developed flexor tubercles. These features also occur in the pedal phalanges (Fig. 10) and are only weakly developed in the extant taxa. Although a more in-depth analysis regarding the mechanical implications of such distal extension of the extensor and flexor digital musculature is still required, some authors have correlated this feature to a parasagittal gait (Carrano & Hutchinson, 2002; Hutchinson, 2002).

Figure 10.

Left pes of S. maxhechti (DGM 1477-R) in dorsal view, unguals in medial view. Scale bar = 10 cm.

In the pelvic girdle, a hypertrophied and pendant supracetabular crest in the ilium represents an osteological constraint to an abduction of the femur of more than 70°. The deep and anteroposteriorly wide ventral surface of this crest accommodates a semispherical and medially offset femoral head, allowing large retraction−protraction movements of the femur (Fig. 11).

Figure 11.

Middle fragment of the left ilium (A) and caudal fragment of the right ilium (B) of S. maxhechti (DGM 1477-R). Left ilium in lateral (top), dorsal (middle), and ventral (bottom) views, and right ilium in lateral (top), medial (middle), and ventral (bottom) views. Abbreviations: ‘fb’, ‘brevis fossa’; ip, ischial process; pal, postacetabular lateral lamina; pap, postacetabular process; sac, supracetabular crest. Scale bar = 5 cm.

Despite the fragmentary nature of the ilium, and the preservation of only the first sacral vertebra and fragments of the second one, the elongation of the iliac postacetabular process in S. maxhechti is consistent with the presence of three sacral vertebrae, as occurs in postcranial skeletons of undescribed specimens attributed to Baurusuchus (UFRJ DG 285-R and UFRJ DG 288-R). The supracetabular crest in these specimens, despite also being hypertrophied, is less developed than in S. maxhechti.

On the proximal portion of the ventral shaft of the ischium of Baurusuchus and S. maxhechti, between the pubic peduncle and the keeled border where the muscle adductor femoris (pars 1) was inserted, there is a laterally pointed tubercle (Fig. 12, ‘pit’), homologous to the point of origin of the puboischiotibialis muscle (PIT) in the living Crocodylia. This muscle is active throughout the stance period during the high-walk in Alligator, acting as an adductor and knee flexor (Gatesy, 1997; Reilly & Blob, 2003). In extant Crocodylia only a tiny scar denotes the muscle origin, but in S. maxhechti the prominence of this tubercle argues that this muscle was well developed, a condition consistent with a permanent parasagittal posture.

Figure 12.

Left ischium of S. maxhechti (DGM 1477-R) in medial (left) cranial (middle), and lateral (right) views. Abbreviation: pit, point of origin of the muscle puboischiotibialis. Scale bar = 10 cm.

The femora are very robust, straighter than in the extant Crocodylia, and present a thick cortical bone (Figs 13, 14). The proximal portion has only 36° of torsion relative to the plane of the condyles, whereas in the extant Caiman yacare this number is around 52° (Fig. 15). The condition observed in S. maxhechti suggests that the orientation of the proximal muscle insertions is more similar to Rauisuchidae, Poposauridae, and basal Dinosauria than to the extant (and most of the extinct) crocodyliforms (Carrano, 2000; Hutchinson, 2001a).

Figure 13.

Left and right femora of S. maxhechti (DGM 1477-R) in cranial view. Scale bar = 10 cm.

Figure 14.

Left and right femora of S. maxhechti (DGM 1477-R) in caudal view. Scale bar = 10 cm.

Figure 15.

Right femur of S. maxhechti (DGM 1477-R), at right, and Caiman yacare (Museu Nacional, without number of collection), at left, in proximal view, showing the angle of relative torsion between the femoral head and the plane of the condyles. Not to scale.

The lateral, cranial, medial, and caudal views of the proximal region of the femur in extant Crocodylia are approximately equivalent to the cranial, medial, caudal, and lateral views of S. maxhechti, respectively. Consequently, muscles that insert in the lateral surface of the proximal part of the femur in Crocodylia have in S. maxhechti a more protracting vector, improving its hip flexor moment arm.

A parasagittal posture in S. maxhechti also leads to a strong protractor moment for muscles that in extant forms are main abductors, such as the iliofemoralis muscle (Romer, 1923; Gatesy, 1997). The popliteal fossa, positioned between the condyles, is deep and strongly ornamented with longitudinal crests, suggesting that the tendons and muscle stabilizing the knee (flexor cruris) are well developed in S. maxhechti, as well as the muscle gastrocnemius, a major knee flexor and ankle extensor (Reilly et al., 2005).

The astragalus is remarkable for its wide and deep tibial articular surface, mainly for the ventrally expanded medial maleolous of the tibia, and by the reduction of the ‘anterior hollow’ (Sereno, 1991), which is restricted to a dorsal fossa at the cranial surface (Fig. 16). The caudally orientated calcaneal tuber (tuber calcanei) of the calcaneous in S. maxhechti is typical of orthograde quadruped forms (Fig. 17), in which the insertion of the muscle gastrocnemius runs in a caudally faced groove in the tuber and does not create a torsion moment to the pes (Parrish, 1987). The caudally projected calcaneal tuber makes the lateral wall of the calcaneum flat, whereas it is concave amongst the extant forms.

Figure 16.

Left (articulated) and right (disarticulated) proximal tarsals (astragalus and calcaneum) of S. maxhechti (DGM 1477-R) in cranial or distal view (top) and caudal view (bottom). Abbreviations: bt, buttress; cgct, caudal groove of the calcaneal tuber; dc, dorsal cranial fossa; dp, dorsal process of the astragalus; dt4as, articular surface for distal tarsal 4; fas, articular surface for the fibula; fcv, cranioventral fossae; fo, foramen; lmas, articular surface for the lateral malleolus of the tibia; mmas, articular surface for medial malleolus of the tibia; peg, astragalar peg or ventral process; pg, posterior groove; samtI/II, articular surface for metatarsals I and II; sc, socket of the calcaneum to reception of the astragalar peg; st, transverse groove; tc, calcaneal tuber; vt, ventral tubercle of the astragalus. Scale bar = 10 cm.

Figure 17.

Right proximal tarsals (astragalus and calcaneum) in preserved position and left disarticulated proximal tarsals of S. maxhechti (DGM 1477-R) in proximal (or dorsal) view (top) and ventral view (bottom). Abbreviations as in Figure 16. Scale bar = 10 cm.

Being a feature classically recognized as related to the parasagittal posture (Parrish, 1986, 1987; Sereno, 1991), the caudal orientation of the calcaneal tuber is a primitive and widespread feature within Crocodylomorpha, also occurring in forms such as Terrestrisuchus (Crush, 1984), Protosuchus (Colbert & Mook, 1951), Mahajangasuchus (Buckley & Brochu, 1999), and Baurusuchus (UFRJ DG 285-R and UFRJ DG 288-R), as well as in other Suchia, such as Aetosauria and Rauisuchia (Parrish, 1987; Sereno, 1991). Although typical of the Neosuchia, a laterocaudally orientated calcaneal tuber occurs early in the history of the Crocodyliformes, such as in Edentosuchus (Pol et al., 2004), Uruguaysuchus (Rusconi, 1933), and Araripesuchus tsangatsangana (Turner, 2006).

Regarding the proportion of the appendicular elements, S. maxhechti's hindlimbs are more similar to the medium to large suchian Postosuchus than to the sampled Crocodylomorpha (Table 1). Metatarsals II and III, which are the longer elements of the foot, make up 36% of the femoral length and 50% of the tibial length, whereas in both Caiman and Melanosuchus these values are 51 and 65%, respectively.

Table 1.  Comparative measurements of skull and appendicular lengths amongst S. maxhechti, some other crocodylomorphs, and Postosuchus (in mm)
TaxonSkull (total length)FemurTibiaLonger metatarsalHumerusRadiusLonger metacarpal
S. maxhechti (DGM 1477-R)470337245122.825022078.5
(MT II; MT III has 121,9)(MC IV)
Postosuchus kirkpatricki (‘intermediate specimen’, data from Chatterjee, 1985)45038328612622520540
(MT III)(both MC II and III)
Hesperosuchus agilis (data from Colbert, 1952)145140130809487
(MT III)
Protosuchus richardsoni (holotype, data from Colbert & Mook, 1951)1131008337665212
(MT III)(MC III)
Mariliasuchus amarali (UFRJ-DG 105-R)1307561,544
(estimated; rostral length: 41)
Melanosuchus niger (UFAC-R207)54027019013020012052
(MT III)(MC II)
Crocodylus acutus (data from Mook, 1921)793325227141168149
(MT III)

Muscular attachments and homoplastic theropodomorph features

Some osteological features related to muscular origin or insertion traditionally viewed as typically dinosaurian also occur in S. maxhechti and Baurusuchus. In the ventral portion of the ilium, caudal to the ischial process, there is a wide concavity limited medially by the wall of the postacetabular process and laterally and dorsally by a postacetabular lateral lamina (Fig. 11, ‘bs’). This concavity, absent in other Mesoeucrocodylia, is topologically homologous to the origin of the caudofemoralis brevis muscle of the living Crocodylia, a powerful femoral retractor (Romer, 1923; Gatesy, 1997). This postacetabular concavity in the baurusuchid ilium is very similar to the dinosaurian brevis fossa, where the same muscle originated and was related to ventral dislocation of the femoral retractors from the medial wall of the ilium (Gatesy, 1990; Hutchinson, 2001b).

Another interesting anatomical observation is the developed tubercle for the PIT muscle in the ischium of S. maxhechti, which is topologically very similar to the obturator tubercle of the maniraptoriform theropods. In birds this tubercle acts as an attachment to the ligamentun ischiopubicum (Hutchinson, 2001b). The absence of a similar sstubercle in other archosaurs (such as Rauisuchia, Aetosauria, and also other crocodylomorphs), and the different soft tissue attached there in S. maxhechti and Maniraptoriformes, denote its homoplastic origin.

The craniolateral face of the proximal region of the femur shows a strongly marked set of crests and rugosities (Fig. 19), indicating the insertion point of the muscle puboischiofemoralis internus pars dorsalis (PIFI2), the main femoral protractor of the extant Crocodylia (Gatesy, 1997; Reilly & Blob, 2003). The more cranial orientation of this femoral surface in S. maxhechti compared to other Crocodylia, as a result of the more medially orientated femoral head, increases the protractor vector of this muscle and reduces the adductor and torsional moment, as occurs in Crocodylia. Interestingly, the protrusion for insertion of the PIFI2 (Fig. 18: ‘pifi2’) is topologically similar to the accessory trochanter of the tetanuran dinosaurs, in which the same muscle PIFI2 was inserted (Makovicky & Sues, 1998; Hutchinson, 2001a). Its occurrence in S. maxhechti is considered a convergence related to the assumption of a fully erect gait.

Figure 19.

Right tibia (top) in cranial, lateral, medial, and caudal views, and right and left tibiae (bottom), in proximal view, of S. maxhechti (DGM 1477-R). The holes seen in the proximal surface of the right tibia are taphonomical boring marks (see Cabral et al., 2011, this volume). Abbreviations: cat, cranial crest (possible accessory point for insertion of the extensor tendon); cd, caudal; cr, cranial; cte, crest of the extensor tendon; ff, fossa flexoria; fs, fibular surface; l, lateral; li, linea intermuscularis; lm, lateral malleolus; m, medial; mm, medial malleolus; tmt, medial tubercle (possibly related to the origin of some ankle flexor). Scale bar = 10 cm.

Figure 18.

Proximal portion of the right femur of S. maxhechti (DGM 1477-R) in cranial view. Abbreviation: pifi2 and adjacent area, insertion surface of the muscle puboischiofemoralis internus, pars dorsalis. Scale bar = 10 cm.

The tibia of the Baurusuchidae is naturally compressed lateromedially with the proximal portion twisted laterally, anticlockwise (Fig. 19). Consequently, the proximal articular surface has the anteroposterior axis longer than the mediolateral one, as in dinosaurs, whereas extant Crocodylia have a subrounded proximal articular surface in the tibia (compare with Hutchinson, 2002: fig. 2). The tibia of S. maxhechti has a robust but low longitudinal crest projecting anterolaterally close to the medial border of the proximal cranial surface, with a shallow longitudinal groove running parallel and laterally. This crest and groove are located in the same region where the extensor tendon (or triceps) is inserted in the extant Crocodylia (Fig. 19: ‘cte’), in which strong striations occur in the periosteum, but no crest is formed. This crest in S. maxhechti is very similar to the low cnemial crest occurring in dinosauriform non-Neotheropoda, where the same extensor tendon was inserted. However, in these taxa the cnemial crest was positioned close to the lateral border of the cranial proximal surface of the tibia (Hutchinson, 2002) whereas in Baurusuchidae it is close to the medial border.

CONCLUSION

The dominance of the Crurotarsi in terrestrial ecosystems has been viewed as a typical Triassic scenario that diminished gradually with the rise of dinosaurs. In the Upper Cretaceous of south-eastern Brazil, the Baurusuchidae crocodyliforms appear to have bounced back and might have at least partially occupied spatial and ecological niches dominated mainly by theropods in the remaining parts of the Cretaceous world.

All the features described above enabled the Baurusuchidae to act as terrestrial predators with at least some cursorial ability (sensuCarrano, 1999). Besides, the relative frequency of these crocodyliforms in Upper Cretaceous deposits of the Bauru Basin postulate a predominant role in the predatory niche. The scarcity of small to medium-sized theropods in the Bauru Group, in both the Marilia and Adamantina Formations, is notorious. Even after several decades of collection efforts, just a handful of teeth, considered to be of Maniraptora, as well an isolated ungual and a scapula, have been found (Bertini, Marshall & Gayet, 1993; Novas, Ribeiro & Carvalho, 2005; Machado, Campos & Kellner, 2008). Only two large abelisaurid theropods have been recorded so far, including Pycnonemosaurus nevesi (Kellner & Campos, 2002; Novas et al., 2008). In contrast, there is a relative abundance of titanosaurs, with at least eight taxa (Aeolosaurus, Gondwanatitan, Maxacalisaurus, Baurutitan, Adamantisaurus, Trigonosaurus, Uberabatitan, and an indeterminate Saltasaurinae) and several individuals, represented by isolated bones to incomplete skeletons, in many localities of the Bauru Group (Kellner & Azevedo, 1999; Kellner & Campos, 2000; Santucci & Bertini, 2001, 2006; Kellner, Campos & Trotta, 2005; Kellner et al., 2006; Salgado & Carvalho, 2008).

Besides other mesoeucrocodylian taxa, baurusuchids are the most common faunal element, with several taxa showing dozens of specimens (Arruda et al., 2004; Candeiro & Martinelli, 2006; Candeiro et al., 2006). This taphocenosis appears not to be a preservational artefact, but a real condition of this Brazilian Cretaceous fauna. It seems that the ecological niche occupied in other regions of the world by small and medium-sized theropods was occupied in the Bauru Group mainly by baurusuchid crocodyliforms during part of the Cretaceous. At the same time, the Upper Cretaceous Neuquen Group, in Argentina, has furnished a myriad of theropods from small to large sizes, sauropods, and crocodyliforms, but amongst the latter, top predaceous forms are very rare (Leanza et al., 2004). The two putative baurusuchids (Wargosuchus and Cynodontosuchus) are notably distinct from the Bauru forms, including the distinctive diminutive size of the Argentinean species.

This possibility of competition between theropods and baurusuchids was first suggested by Gasparini et al. (1993), and then gained more support (e.g. Candeiro et al., 2006; Martinelli & Pais, 2008). With a cranial length of 470 mm, a total body length of around 4 m, and all the morphological attributes described above, S. maxhechti was able to occupy a top terrestrial predator niche in the Upper Cretaceous ecological guilds (Fig. 20). This hypothesis offers a new perspective for the study of these extraordinary crocodylomorphs and the ecosystems in which they lived.

Figure 20.

Artistic reconstruction of an adult S. maxhechti attacking a juvenile titanosaur. Art by Maurílio Oliveira (Museu Nacional, Rio de Janeiro).

ACKNOWLEDGEMENTS

We thank the editors of this special volume, Diego Pol and Hans Larsson, for the organization of the pleasant and productive ‘Symposium on Crocodyliformes Evolution’ of the III Latin America Congress of Vertebrate Paleontology (Neuquen, Argentina), and for the invitation to participate in this volume. We also thank two anonymous referees for several valuable suggestions that greatly improved the manuscript. This project was partially funded by the Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG; grant APQ-00581-09 to D. R.), the Fundação Carlos Chagas de Amparo à Pesquisa do Rio de Janeiro (FAPERJ, grant E-26/152.885/2006 to A. W. A. K.), and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, grants 486313/2006-9 and 501267/2008-5 to A. W. A. K.).

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