INTRODUCTION

Mesoeucrocodylian crocodyliforms were remarkably diverse in the Cretaceous of Gondwana, showing a wide morphological diversity that contrasts with the characteristic anatomy of extant crocodylians. The most conspicuous Cretaceous crocodyliform group from Gondwana is Notosuchia, an assemblage of small terrestrial taxa remarkably diverse in dental morphology (including multicusped teeth; Clark et al., 1989; Carvalho, 1994; Gomani, 1997; Buckley et al., 2000) and indications of fore–aft jaw movements (Clark et al., 1989; Bonaparte, 1991; Pol, 2003). Gasparini (1971) first recognized this group to include these forms and a rather generalized form from the Santana Formation, Araripesuchus gomesii (Price, 1959).

Buffetaut (1981) described a crocodyliform from the Early Cretaceous of Niger and referred it to this genus, Araripesuchus wegeneri. More recently, Ortega et al. (2000) described a third species, Araripesuchus patagonicus, from the Cenomanian of Patagonia. Additionally, new remains preliminarily referred to this genus were recently reported from Late Cretaceous beds of Madagascar and Niger (Buckley et al., 1997; Sereno et al., 2004; Turner, 2004b). Despite the growing knowledge on the diversity, temporal, and geographic distribution of Araripesuchus (and other notosuchians), the phylogenetic relationships of these forms are still disputed in current phylogenetic analyses (Clark, 1994; Wu and Sues, 1996; Buckley and Brochu, 1999; Ortega et al., 2000; Sereno et al., 2003; Pol, 2003; Turner, 2004a).

A recently discovered locality from the Cenomanian–Turonian (Corbella et al., 2004) of Rio Negro Province (Argentina) yielded new remains of crocodyliforms that, despite being fragmentary, show a combination of derived similarities exclusively present in Araripesuchus. Here, we describe two of these specimens, erecting a new taxon based on one of them (MPCA-PV 235). The second, more fragmentary specimen (MPCA-PV 236) is described and its taxonomic status is discussed. The relationships of these materials are analyzed through a cladistic analysis within the context of Crocodyliformes.

The following acronyms are used throughout this work:

SYSTEMATIC PALEONTOLOGY

DESCRIPTION

The specimen MPCA-PV 235 consists of a skull in articulation with its lower jaws. The anterior tip of the snout is missing and the external surface of most skull bones is partially damaged. The external surface of the braincase has not been preserved, although fragments of the exoccipitals and supraoccipital are partially exposed.

The anteroposterior length of the skull is almost entirely preserved (130 mm), being the largest specimen of all the described Araripesuchus species. Although the external surfaces of the skull bones are poorly preserved, the original ornamentation pattern has been preserved in some elements. This pattern varies depending on the skull region, being composed either by small circular pits (e.g., anterior palpebral, ventral surface of mandibular symphysis) or by shallow grooves and ridges (e.g., posterior region of nasals, anterior region of frontal, dorsal surface of postorbital). The snout is oreinirostral (sensu Busbey, 1994), being slightly wider than high, and its anteroposterior length is approximately equal to the remainder of the skull. The dorsal surface of the rostrum is slightly concave in lateral view, although this could be accentuated by the poor preservation of the nasals. Most of the rostral region has a constant lateromedial width, except at its anterior tip that tapers anteriorly (at the premaxilla-maxilla contact) and its posterior end where the skull broadens gradually. The lateromedial width of the skull increases toward the posterior region of the orbit, where it reaches its maximum width. Posteriorly, along the temporal region, the skull has a rather constant width. The skull roof has a flat and wide dorsal surface, as in all crocodyliforms (Clark, 1994).

The orbits are rather large, occupying approximately one-fourth of the skull's anteroposterior length. The external margins of the supratemporal fossae are not preserved. The supratemporal fenestrae are subcircular, being slightly longer than their lateromedial width. The infratemporal fenestrae face laterally and are triangular, being slightly longer than high. The antorbital fenestrae are not preserved due to poor preservation of the lateral surfaces of the lacrimals and maxillae. The suborbital fenestrae are large and elongated, being approximately three times longer than wide. The choanal opening is long, narrow, and completely septated. The external mandibular fenestrae were not preserved.

Most of the premaxillae are missing, except for their posterodorsal process. This process is smooth and is exposed on the dorsal surface of the rostrum. The posterodorsal process is long, acute, and wedges between the maxilla and nasal (fig. 1). The external surface of the left maxilla of MPCA-PV 235 is damaged, but the anterior and posteroventral regions of the right element are well preserved. Most of the maxilla is vertically oriented except for its dorsal region, which is dorsally deflected, facing dorsolaterally. The anterior region of the maxilla bulges laterally at the level of the enlarged maxillary tooth and seems to be constricted at the premaxilla-maxilla contact (fig. 1). Posterior to its contact with the posterodorsal process of the premaxilla, the maxilla is bordered by the nasal along a straight suture that is longitudinally oriented. The buccal edge of the maxilla is concave posterior to the enlarged maxillary tooth, as in A. gomesii (AMNH 24450). Posteriorly, the maxilla is overlapped by the jugal and extends posterior to the anterior orbital margin. Unfortunately, its posterodorsal region and its contact with the lacrimal have not been preserved in MPCA-PV 235. The palatal processes of the maxillae meet on the midline, forming an extended secondary palate. Posterolaterally, they contact the anterolateral process of the ectopterygoids and posteromedially they receive the rounded anterior process of the palatines (fig. 2). The posterior edges of the palatal branches of the maxillae form the anterolateral margins of the suborbital fenestra.

The nasals are narrow and elongated, having their lateral edges subparallel to each other (fig. 1). Their lateromedial width increases slightly along the posterior region of the nasal-maxilla suture; however, the nasals narrow markedly at their posterior end (along their contact with the prefrontals). The posterior edge of each nasal is anteromedially oriented, receiving a long and acute anterior process of the frontal. This morphology contrasts with the transverse and interdigitated suture present in A. gomesii (AMNH 24450) and A. patagonicus (MUC-PV 269). The nasal-lacrimal contact cannot be determined since the latter element has not been preserved in MPCA-PV 235.

Only the medial regions of the prefrontals are well preserved in MPCA-PV 235. These elements are long and narrow, extending longitudinally on the dorsal surface of the skull (fig. 1). They form the anterior half of the dorsal margin of the orbits and extend anteriorly to them along the prefrontal-nasal contact. Their lateral region is badly preserved, although they seem to be depressed for the articulation with the large anterior palpebral. The right anterior palpebral was preserved dislocated, inside the orbit. The preserved portion of this element is subrounded and remarkably broader than in other species of Araripesuchus. The anterior palpebral of A. patagonicus (MUC-PV 269) and A. gomesii (AMNH 24450) has a moderately broad anterior edge and rapidly tapers posteriorly, forming a wing-shaped, acute, and elongated posterior process. The dorsal surface of the anterior palpebral of MPCA-PV 235 is ornamented on its anteromedial region with small and well-separated pits (fig. 1). The anterior edge is straight and the medial margin is slightly convex and probably overlapped the prefrontal. The extensive width of the anterior palpebral would have covered most of the orbit.

The frontals are completely fused into a single element, as in all mesoeucrocodylians (Clark, 1994). Most of the external surface is damaged, except for its anterior and posterior regions. In these areas, the frontal dorsal surface is poorly ornamented. The anterior region of the frontal extends beyond the anterior orbital margin, narrowing markedly between the prefrontals (fig. 1). The anterior end of the frontals extends between the posterior region of the nasals, forming an acute V-shaped process. Posteriorly, the frontal broadens laterally, forming the posteromedial margins of the orbit. The frontal posterior margin contacts the parietal and postorbital through a strongly interdigitated suture (fig. 1). The medial region of the frontal-parietal suture extends transversely on the skull roof, at the level of the anterior margin of the supratemporal openings. This suture extends laterally into the supratemporal fossa and reaches to the anterolateral corner of the supratemporal fenestra (fig. 1). Thus, the frontal enters widely into the supratemporal fossa, forming part of the anterolateral margin of the supratemporal fenestra. This condition contrast with that of A. gomesii (AMNH 24450) and A. patagonicus (MUC-PV 269), in which the frontal barely enters the supratemporal fossa and does not reach the supratemporal fenestra. The frontal-postorbital suture extends anterolaterally from the anterior margin of the supratemporal fenestra along the supratemporal fossa. This contact continues on the dorsal skull surface, reaching the posteromedial region of the orbital margin (fig. 1).

The parietals are completely fused into a single element, as in all crocodyliforms. Most of the parietal dorsal surface is damaged and its posterior region was not preserved. Its anterior margin is sutured to the frontal along a transversely oriented, interdigitated suture. This contact extends on the dorsal surface of the skull table and the smooth surface of the supratemporal fossa floor (fig. 1). The dorsal surface of the parietal is markedly restricted in MPCA-PV 235, being much narrower than the interorbital region of the frontal. Although the minimum width of the parietal dorsal surface cannot be determined due to the poor preservation of this element, the dorsal surface of this element at the frontal-parietal suture is remarkably narrow, being approximately one-third the maximum lateromedial width of the supratemporal fossa. In contrast, the parietal of A. gomesii (AMNH 24450) and A. patagonicus (MUC-PV 269) is notably wider, having this region approximately equal to the maximum lateromedial width of each supratemporal fossa. Lateral to its dorsal surface, the parietal forms a wide and smooth medial and posteromedial surface of the supratemporal fossa.

The squamosals are poorly preserved in MPCA-PV 235. Most of their dorsal surface has been eroded, and its posterior and occipital regions are missing. On the left element most of its lateral flange overhangs the quadrate and quadratojugal, forming a rather deep otic recess. Unfortunately, its contact with the postorbital, quadrate, and exoccipitals cannot be determined in MPCA-PV 235.

The postorbitals are also poorly preserved in MPCA-PV 235, although the left element has most of its dorsal surface complete. This surface is exposed horizontally, forming part of the flattened “skull roof” characteristic of Crocodyliformes. The medial edge of the dorsal surface of the postorbital contacts the frontal and extends posterolaterally, forming an oblique anterolateral margin of the skull roof, as in A. gomesii (AMNH 24450), A. patagonicus (MUC-PV 269), and most notosuchian crocodyliforms (e.g., Notosuchus terrestris MACN-RN 1037; Malawisuchus mwakayasyunguti MAL-45; Simosuchus clarki UA 8679; Libycosuchus brevirostris BSP 1912.VIII.574; Baurusuchus pachecoi DGM 299-R). The dorsal surface of the postorbital is poorly ornamented and slightly narrow toward its posterolateral region. The posterolateral end of the left postorbital of MPCA-PV 235 is damaged, and therefore its contact with the squamosal cannot be observed. The postorbital has a small, subtriangular, pointed process on its anterolateral corner (fig. 1). The dorsal surface of this process is flat and ventrally recessed from the postorbital dorsal surface. In other crocodyliforms a similar process is present and is overlapped by the posterior palpebral (e.g., A. gomesii AMNH 24450; A. patagonicus MUC-PV 269; Notosuchus terrestris MACN-RN 1037; Malawisuchus mwakayasyunguti MAL-45; Baurusuchus pachecoi DGM 299-R; Libycosuchus brevirostris BSP 1912.VIII.574; Theriosuchus (Clark, 1986); Zaraasuchus shepardi IGM 100/1321). The descending process of the postorbital is wide, smooth, and flat. Its posterior margin is sutured to the quadrate and quadratojugal. The postorbital descending process forms part of the dorsal and anterodorsal margins of the infratemporal fenestra and contacts the ascending process of the jugal through an oblique and posteroventrally directed suture. The postorbital descending process of MPCA-PV 235 seems to lack the large postorbital foramen present above the jugal-postorbital suture in A. gomesii (AMNH 24450).

The jugal is a triradiate bone, with its anterior (suborbital) process being approximately twice as high as its posterior (infratemporal) process. Its anterior process extends beyond the anterior orbital margin, overlapping the maxilla (fig. 3). The lateral surface of this region is badly damaged although it seems to be slightly concave below the orbit. The left jugal of MPCA-PV 235 has a moderately developed longitudinal ridge running on its lateral surface along the posterior third of the orbit, close to its buccal margin. The ascending postorbital process of the jugal of MPCA-PV 235 is located posterior to the anteroposterior midpoint of the jugal (fig. 3). This process is narrow, cylindrical, and posterodorsally directed. The base of this process is continuous with the lateral surface of the jugal on its anterior margin, as in A. patagonicus MUC-PV 269, instead of being inset as in A. gomesii AMNH 24450. The posterior margin of this process is also superficially located, although this margin is located slightly more medially than the anterior margin since the major axis of the postorbital bar is slightly deflected posteromedially. The postorbital process of the jugal forms most of the anterior margin of the infratemporal fenestra and contacts the descending process of the postorbital. The left jugal preserves most of the infratemporal bar, which is slightly flattened and forms most of the ventral margin of the infratemporal fenestra. Most of its lateral surface is smooth, except for the region below the postorbital bar. The posterior end of the jugal and its contact with the quadratojugal has not been preserved in MPCA-PV 235.

Only the ascending process of the quadratojugal has been preserved in MPCA-PV 235. This element is exposed dorsolaterally as a narrow process extending into the otic recess. The base of the ascending process of the quadratojugal is slightly constricted while its dorsal end is broader than the rest of the process. The quadratojugal forms the posterior edge of the infratemporal fenestra. Its posterior margin contacts the anterodorsal process of the quadrate along a straight suture. Dorsally, it contacts the postorbital along a slightly interdigitated suture. The anterodorsal edge of the quadratojugal's ascending process is not indented as in A. gomesii (AMNH 24450).

The quadrates of MPCA-PV 235 preserve only their anterodorsal branch. This process of the quadrate is smooth and extends into the otic recess overhung by the lateral flange of the squamosal (fig. 3). The quadrate contacts the quadratojugal along a straight suture. The anterodorsal end of the quadrate is sutured to the posterior margin of the descending process of the postorbital. The dorsal edge is badly preserved and its contact with the squamosal has not been preserved in MPCA-PV 235, except at the anterodorsal margin of the right otic notch (fig. 3). The posterodorsal edge of the anterodorsal branch of the quadrate is markedly concave, forming the anterior and ventral edges of the large otic notch. Presumably, the squamosal closed the otic aperture posteriorly as in all mesoeucrocodylians. However, this is not preserved in MPCA-PV 235. The right quadrate of MPCA-PV 235 has a remarkably large ovoid siphoneal foramen located anteriorly to the otic notch (fig. 3). This opening is significantly larger than that of A. gomesii (AMNH 24450). The lateral surface of the quadrate is slightly depressed posteroventral to this opening, although this depression is not as developed and does not extend around the otic notch as in A. gomesii (AMNH 24450).

The ectopterygoids of MPCA-PV 235 are only exposed in ventral view. Their posteromedial process is a thin and subcylindrical bar that extends along the lateral edge of the pterygoid flange (fig. 2). This process is slightly deflected with respect to the anterior half of the ectopterygoids, which is oriented longitudinally, forming the entire lateral margin of the suborbital fenestra. Anteriorly, the ectopterygoids contact the posterior end of the palatal process of the maxilla. Its suture with the jugal and the presence of an ectopterygoid-postorbital contact cannot be determined in MPCA-PV 235.

The palatines of MPCA-PV 235 are exposed ventrally in articulation with the rest of the palate. The palatines are medially sutured to each other, extending posteriorly to form the secondary palate characteristic of mesoeucrocodylians. As in A. gomesii AMNH 24450 and A. patagonicus MUC-PV 269, the anterior end of the palatines extends anterior to the suborbital fenestra, forming a rounded anterior process that fits between the palatal processes of the maxillae (fig. 2). The palatines are strongly sutured to the maxillae instead of overlapping them as in basal crocodyliforms. The palatines form the anterior and medial margins of the suborbital fenestra. The posterior edge of the palatal surface of the palatines is concave and delimits the anterior margin of the choanal opening (fig. 2). At the medial part of this margin, the palatines are sutured to the wide anterior end of the choanal septum. The posterolateral regions of the palatines form the lateral borders of the choanal groove and contact the pterygoids.

The pterygoids of MPCA-PV 235 are exposed in ventral view. Anteriorly, the pterygoids are markedly depressed, forming a trough-shaped choanal groove. This elongated depression extends posteriorly up to the anteroposterior midpoint of the pterygoid flanges (figs. 2, 4A). In contrast, in A. gomesii (AMNH 24450) and A. patagonicus (MUC-PV 269), the posterior limit of the choanal opening is located anterior to the pterygoid flanges (fig. 4). The lateral walls of the choanal groove are thin and subparallel to each other, forming the medial margins of the suborbital fenestra and reaching the posterolateral projections of the palatines.

The choanal groove bears a sagittally oriented process formed by the pterygoids that reaches the posterior edge of the palatines. This choanal septum completely divides the choanal groove, from the roof of the nasopharyngeal passage to its palatal surface, resembling the condition of A. gomesii (AMNH 24450), A. patagonicus (MUC-PV 269), and Simosuchus clarki (UA 8679). The choanal septum is T-shaped in cross section, having its palatal surface flat and lateromedially wider than the rest of the septum (fig. 4). This condition is exclusively shared with A. gomesii (AMNH 24450), A. patagonicus (MUC-PV 269), and Simosuchus clarki (UA 8679). However, the flat palatal surface of the choanal septum of A. gomesii (AMNH 24450) and A. patagonicus (MUC-PV 269) tapers anteriorly, while in Simosuchus UA 8679 and MPCA-PV 235 it is broad anteriorly and tapers posteriorly (fig. 4).

The choanal groove is closed posteriorly by the pterygoids, which seem to be completely fused at this point. The pterygoid flanges extend laterally and are slightly deflected ventrally. The ventral surfaces of these flanges are smooth and anteroposteriorly narrower in comparison with those of A. gomesii (AMNH 24450) and A. patagonicus (MUC-PV 269). The anterior margins of the pterygoid flanges are markedly concave while their posterior margins are oriented transversely and are only slightly concave (fig. 4). The posterior half of the pterygoid flanges of MPCA-PV 235 bulges ventrally, indicating pneumatic spaces within them (exposed in the broken areas of both pterygoids; fig. 4A). Anterior to these swollen areas, the ventral surface of the pterygoid flanges bears a transversely elongated depression running parallel to the posterior margin of the suborbital fenestra (fig. 4A). This suite of characters of the pterygoid flanges of MPCA-PV 235 is absent in A. gomesii (AMNH 24450) and A. patagonicus (MUC-PV 269). In these taxa, the pterygoid flanges are broad and laminar, lacking the pneumatic spaces described above (fig. 4).

The lateral edges of the pterygoid flanges of MPCA-PV 235 are sutured to the posteromedial process of the ectopterygoids. At the posteromedial region, the pterygoids are deeply notched as in the other species of Araripesuchus and other mesoeucrocodylians. The base of the quadrate processes of the pterygoids is markedly narrow as in most mesoeucrocodylians (fig. 4), although further details of this region and its contact with the quadrate and basisphenoid have not been preserved in MPCA-PV 235.

Only the dentaries, splenials, and a fragment of the surangular have been preserved from the lower jaw of MPCA-PV 235. The mandibular symphysis is moderately long and tapers anteriorly in its dorsoventral and lateromedial dimensions (figs. 3, 5). The ventral surface of the dentaries is slightly ornamented with small and well-spaced circular pits. On this surface, the dentaries form the anterior two-thirds of the mandibular symphysis, while the posterior third is formed by the splenials. The lateral margins of the dentaries diverge posteriorly at the anterior symphyseal region, and they are moderately bulged at the enlarged anterior dentary tooth (fig. 5). In lateral view, the dentaries gradually increase the dorsoventral height of the mandibular symphysis.

Posterior to the mandibular symphysis, the buccal margin of the dentaries protrudes dorsally, although it seems to lack the abrupt step present in Araripesuchus gomesii (DGM 423-R; AMHN 24450) and MPCA-PV 236 (see below). The lateral surfaces of the dentaries are flat, vertically oriented, and lack the ornamentation present on their ventral surface. This smooth region also seems to lack neurovascular foramina. In contrast, the lateral surface of A. gomesii, A. patagonicus, and neosuchians is laterally convex below the alveolar region (Ortega et al., 2000). The dentaries of MPCA-PV 235 bear a slightly marked longitudinal groove that runs parallel to the buccal edge on the posterior region of the toothrow (fig. 3). The left dentary has preserved its contact with the anterior branch of the surangular, being a vertically directed sinusoidal suture. Unfortunately, the posteroventral region of the dentaries and their contact with the angular has not been preserved in MPCA-PV 235.

The splenials are involved in the mandibular symphysis, contacting the dentaries through a U-shaped suture (fig. 5), in contrast to the acute V-shaped suture present in A. gomesii (AMNH 24450), A. patagonicus (MUC-PV 269), and most other crocodyliforms (except for Notosuchus terrestris MACN-RN 1037 and Comahuesuchus brachybuccalis MOZ P 6131). The splenials have a blunt-ended peg on the posterior edge of the symphysis (fig. 5), similar to the condition of A. patagonicus MUC-PV 269 and other basal mesoeucrocodylians (e.g., Uruguaysuchus aznarezi (Rusconi, 1933; fig. 19); Notosuchus terrestris MACN-RN 1037, 1040; Malawisuchus mwakayasyunguti MAL-49; Simosuchus clarki UA 8679). This posterior peg, however, is absent in A. gomesii (AMNH 24450). Lateral to this peg, the splenial is pierced by the anterior foramen for the mandibular branch of the trigeminal nerve.

Posterior to the mandibular symphysis, the splenials expand dorsally to cover the entire medial surface of the mandibular rami. This splenial lamina is medially convex and dorsoventrally high, extending from the alveolar margin to the ventral surface of the mandibular ramus. The splenial lamina lacks any signs of a posterior opening for the mandibular branch of the trigeminal nerve (such as those present in derived alligatoroids).

The dentition of MPCA-PV 235 is poorly preserved. None of the premaxillary teeth has been preserved and most of the maxillary tooth crowns are missing. The anterior maxillary teeth seem to be implanted in discrete alveoli, and the second preserved maxillary tooth is markedly enlarged with respect to the other maxillary teeth (fig. 3). All the preserved maxillary teeth are conical, with smooth enamel surfaces, and they seem to lack serrations or denticles on their mesial and distal margins. The dentary teeth are mostly hidden by the upper dentition. The anterior dentary teeth seem to be small except for a slightly enlarged tooth located at the level of the premaxilla-maxilla contact, which produces a moderate swelling of the dentaries (fig. 5). Probably, there were two or three teeth anterior to this element.

DESCRIPTION OF MPCA-PV 236

The specimen MPCA-PV 236 was found in the same locality and horizon as MPCA-PV 235 and consists of the anterior half of the lower jaws and fragmentary remains of the palate. This specimen differs from MPCA-PV 235 and the other described species of Araripesuchus in the morphology of the medial surface of splenials. In MPCA-PV 236 this surface is markedly concave, in contrast to the flat or slightly convex surface of all other species of Araripesuchus. Additionally, the medial edge of the posterior half of the alveolar groove (formed by the splenial) is dorsally located with respect to the lateral alveolar edge formed by the dentary. In contrast, in A. gomesii (AMNH 24450) the medial and lateral alveolar edges are located at the same level. Unfortunately, this information is not available for the other three species of Araripesuchus. Although these differences may suggest that MPCA-PV 236 belongs to a different taxon, its fragmentary nature prompted us to wait for more complete remains in order to make a formal taxonomic decision. Despite the fragmentary nature of this specimen, it shows several characters that suggest close phylogenetic affinities with the Araripesuchus clade (see below).

The fragmentary skull remains of MPCA-PV 236 are poorly preserved and only exposed in palatal view. Although the palatal branches of the premaxillae are badly damaged, they seem to meet medially, forming the anterior region of the secondary palate. The premaxilla-maxilla palatal contact cannot be precisely determined; however, the palatal branches of the maxillae clearly contact each other medially, forming a posteriorly extended secondary palate. The ventral surface of the maxillary secondary palate is markedly concave at its anterior third, although this was probably accentuated by preservation causes. Posteriorly, the palatal surface becomes progressively flattened toward its contact with the anterior process of the palatines. Only the ventral half of the lateral surface of the maxilla was preserved in MPCA-PV 236. As in A. gomesii (AMNH 24450), this surface is ornamented with closely spaced pits extending down to the buccal margin of the maxilla, lacking the smooth maxillary surface present in more derived notosuchians (e.g., Notosuchus terrestris MACN-RN 1037; Malawisuchus mwakayasyunguti MAL-52; Sphagesaurus huenei RCL-100). The maxillary buccal margin is straight along most of its length, but it extends ventrally at the anterior portion of the maxilla, where a hypertrophied maxillary tooth is located. The palatines contact each other medially and the palatal processes of the maxillae anteriorly, forming the posterior part of the secondary palate. A well-developed anterior process of the palatines projects anteriorly and laterally between the palatal branches of the maxillae, forming a semicircular anterior end. In ventral view the palatines narrow posteriorly, where they probably form the floor of the nasopharyngeal duct. At this point, the lateral edges of the palatines form the medial and anteromedial margins of the suborbital fenestra.

Both rami of the lower jaw were completely preserved from their anterior end to the caudal end of the toothrow. The mandibular symphysis is moderately long, extending along two-thirds of the mandibular toothrow and is formed by both dentaries and splenials. At the symphyseal region, the dorsoventral and lateromedial dimensions of the dentaries taper anteriorly (figs. 6–8). The ventral surface of the dentaries is heavily ornamented with grooves and ridges. On ventral view, the dentaries only form the anterior two-thirds of the mandibular symphysis (fig. 7). Their lateral edges diverge slightly posteriorly in the symphyseal region, but they are strongly deflected laterally posterior to the symphysis. In the symphyseal region, the dorsal surface of the dentaries forms an anteroposteriorly elongate concavity between the toothrows (fig. 6). This shallow, trough-shaped, smooth surface narrows anteriorly and is not anteriorly delimited by an elevated anterior alveolar margin. This suite of characters is also present in A. gomesii (DGM 423-R; AMHN 24450). Posterior to the mandibular symphysis, the medial surface of the dentaries is completely hidden by the splenials.

In lateral view, the ventral edge of the dentaries extends posteroventrally, gradually increasing the dorsoventral height of the mandibular rami (fig. 8). The anterior region of the dorsal edge of the dentaries is straight and subparallel with the longitudinal axis of the mandible. However, at the level of the seventh mandibular tooth, the dorsal alveolar margin protrudes dorsally rather abruptly. This marked step causes an abrupt increase in the dorsoventral height of the dentaries, as seen in lateral view (fig. 8). Posterior to this step, the dorsal margin of the dentaries is straight and extends posterodorsally at a low angle. A similar lateral profile of the dorsal edge of the dentaries is also present in A. gomesii (DGM 423-R; AMHN 24450). Some other mesoeucrocodylians also have a drastic step in the dorsal margin of the dentaries (e.g., Sebecus icaeorhinus AMNH 3160), but this occurs at the level of the premaxilla-maxilla suture (approximately at the third mandibular alveolous), and the dorsal margin is distinctly concave posterior to this tooth. Comahuesuchus brachybuccalis (MOZ P 6131) also bears an abrupt step, although it is located at the posterior end of the toothrow where the enlarged dentary tooth is located. In most notosuchians, however, the dorsal edge of the dentaries is straight along the entire toothrow, having anterior dentary teeth located at the same level as the posterior teeth (e.g., Notosuchus terrestris MACN-RN 1037, 1040; Mariliasuchus amarali UFRJ 50-R; Malawisuchus mwakayasyunguti MAL-49). The ventral half of the lateral surface of the dentaries is ornamented with deep grooves and ridges while the dorsal half of the lateral surface of the dentaries is smooth and perforated by large neurovascular foramina (fig. 8). Posterior to the symphyseal region, the lateral surface of the dentaries is convex, a character noted to be present only in Araripesuchus and neosuchians (Ortega et al., 2000).

The splenials are largely involved in the mandibular symphysis and contact the dentaries differentially on the ventral and dorsal surfaces. In dorsal view, the splenial-dentary suture is transversely oriented along most of its length, except for its medialmost region (at the contact between the two splenials). In this area, the splenials have an anteriorly projecting process, and the splenial-dentary suture is directed anteromedially (fig. 6). In ventral view, instead, the splenial-dentary suture is V-shaped along its entire length, contributing to one-third of the anteroposterior symphyseal length (fig. 7). The splenials have a posteriorly oriented peg on the posterior edge of the symphysis, although it is notably smaller than that of MUC-PV 235. A similar morphology is present in A. patagonicus (MUC-PV 269) and most notosuchians (e.g., Uruguaysuchus aznarezi (Rusconi, 1933; fig. 19); Simosuchus clarki UA 8679; Notosuchus terrestris MACN-RN 1037, 1040; Comahuesuchus brachybuccalis MOZ P 6131), but not in A. gomesii (AMNH 24450).

At the posterior third of the mandibular symphysis, the splenials expand dorsally, forming a vertical lamina that covers the medial surface of the dentaries. The splenial lamina is lateromedially compressed and dorsoventrally high, extending from the medial alveolar margin down to the dorsal surface of the mandibular symphysis. The medial surface of the splenials is pierced by the anterior opening of the mandibular branch of the trigeminal nerve, located lateral to the mandibular symphysis and facing posteriorly (fig. 9). A similar condition is also present in A. gomesii (AMNH 24450) and Simosuchus clarki (UA 8679), but not in derived notosuchians (e.g., Notosuchus terrestris MACN-RN 1037; Comahuesuchus brachybuccalis MOZ P 6131). As in the specimen MPCA-PV 235, the medial surface of the splenial lamina lacks a posterior opening for the mandibular branch of the trigeminal nerve.

Posterior to the mandibular symphysis, the splenial lamina is further expanded dorsoventrally, covering the entire medial surfaces of the mandibular rami, being slightly exposed on the ventral surface of the mandible (fig. 9). The presence of such an extensive splenial lamina is not uncommon among crocodyliforms. However, the splenial's medial surface in MPCA-PV 236 is markedly concave instead of being flat or slightly convex (fig. 9). The dorsal margin of the splenial lamina is slightly expanded lateromedially, forming a dorsally facing flat surface slightly ornamented with grooves and ridges. This broad surface forms the medial margin of the alveolar groove (figs. 6, 9).

There are at least four premaxillary teeth on the left side of MPCA-PV 236, but none of them preserves its crowns. The teeth are implanted in discrete alveoli separated by complete bony septa. The third of these elements is the largest. The anterior maxillary teeth are also poorly preserved, although the presence of a hypertrophied maxillary tooth can be observed close to the anterior edge of the left maxilla. Three posterior maxillary teeth were preserved on the right side. These elements lie in an alveolar groove, and the last one of them is bulbous and has a marked constriction between its root and crown. There are 12 dentary teeth preserved on the left dentary and they show some degree of heterodonty. The dentary likely bears 3 or 4 teeth in addition to the 12 teeth preserved in MPCA-PV 236. The anteriormost six teeth are slightly procumbent, small, and conical. Anterior procumbent teeth in the lower jaw are absent in A. gomesii (AMNH 24450) and A. patagonicus (MUC-PV 283) and were previously known to occur only in Anatosuchus minor (Sereno et al., 2003), Comahuesuchus brachybuccalis (Martinelli, 2003), Mariliasuchus amarali (Carvalho and Bertini, 1999), Bretesuchus bonapartei (PVL 4735), and Sebecus icaeorhinus (AMNH 3160). However, in these taxa this condition is much more developed than in MPCA-PV 236. The crowns of the last five mandibular teeth are bulbous and separated from their roots by a marked constriction, a widespread condition among mesoeucrocodylians. Unfortunately, most of the posterior crowns were not completely preserved and characters of their apical region cannot be determined.

The first preserved dentary tooth is implanted in separate alveoli, while posterior mandibular teeth are arranged on a continuous alveolar groove. Between subsequent teeth the groove is lateromedially constricted by expansions of the dentary, although these do not contact each other, forming complete alveolar septa (fig. 6). Anteriorly, the groove is lateromedially narrow but it markedly widens at the level of the five most posterior teeth. At this point, the buccal and lingual edges of the tooth groove are lateromedially wide with a flat dorsally facing surface (figs. 6, 9). Discrete alveoli were previously noted in Araripesuchus and neosuchians (Ortega et al., 2000). However, the type specimen of A. gomesii (DGM 432-R) and the specimen MPCA-PV 236 clearly show the presence of a continuous alveolar groove for the posterior mandibular teeth. Furthermore, the alveolar groove of A. gomesii (DGM 432-R) is remarkably similar to that of MPCA-PV 236, since the left and right toothrows are subparallel to each other along most of their length, but they markedly diverge laterally at the level of the last four teeth. In some extant crocodyliforms (e.g., alligatorids), the posterior dentary alveoli are incompletely septated in young individuals and become increasingly septated during ontogeny. However, in several notosuchian crocodyliforms the posterior mandibular teeth remain arranged on a continuous alveolar groove in presumably adult specimens (e.g., Notosuchus terrestris MLP 64-IV-16-13, Malawisuchus mwakayasyunguti MAL-49, Libycosuchus brevirostris BSP 1912.VIII.574, Simosuchus clarki UA 8679).

PHYLOGENETIC RELATIONSHIPS

Araripesuchus was traditionally considered to be related to Notosuchus and other forms from the Cretaceous of South America (Price, 1959; Gasparini, 1971). However, in recent cladistic analyses, Araripesuchus was depicted in different positions within the basal clades of Mesoeucrocodylia (Clark, 1994; Wu and Sues, 1996; Wu et al., 1997; Buckley and Brochu, 1999; Buckley et al., 2000; Ortega et al., 2000; Pol, 2003; Sereno et al., 2001, 2003; Pol and Norell, 2004a).

In order to analyze the phylogenetic affinities of these specimens, a cladistic analysis was conducted using a dataset of 230 characters scored for 50 taxa. This dataset is an extension of that used in previous studies (Pol et al., 2004; Pol and Norell, 2004b) with the addition of 1 character from Wu and Sues (1996), 3 characters from Buckley and Brochu (1999), 17 characters of Ortega et al. (2000), and 10 new characters. The sampled taxa included numerous basal mesoeucrocodylians, including the three previously described species of Araripesuchus and 20 other non-neosuchian crocodyliforms. This dataset was analyzed with equally weighted parsimony using TNT (vers. 1.0, Goloboff et al., 2003). A heuristic tree-search strategy was conducted performing 1000 replicates of random addition sequences plus TBR branch swapping (holding 10 trees per replicate), followed by a final round of TBR branch swapping. Zero-length branches were collapsed if they lacked support under any of the most parsimonious reconstructions (i.e., rule 1). This analysis resulted in six most parsimonious trees of 750 steps (CI = 0.373; RI = 0.679) found in all replicates.

All the most parsimonious hypotheses cluster the three previously known Araripesuchus species together with A. buitreraensis and the specimen MPCA-PV 236 (fig. 10), corroborating the monophyly of this genus and justifying the taxonomic assignment of the specimens described here.

The African A. wegeneri is located as the basalmost member of Araripesuchus, being the sister taxon of the clade composed by all South American Araripesuchus (fig. 10), as in a recently published analysis (Turner, 2004a). The monophyly of Araripesuchus (including A. wegeneri) is supported by the presence of two unambiguous synapomorphies regarding the morphology of the maxilla (common to all most parsimonious trees). The lateral edge of the snout of all these taxa is laterally bulged at the enlarged maxillary tooth and is markedly concave posteriorly to this point, having a sinusoidal contour (character 173–1; paralleled in some derived neosuchians). Additionally, the alveolar edge of the maxilla extends ventrally at the level of the enlarged maxillary tooth in all Araripesuchus taxa (character 178–1), except for A. patagonicus, where this condition is reversed. Both characters are also present in non-longirostrine neosuchians, and therefore they only provide support for the monophyly of this group given the phylogenetic position of the Araripesuchus clade as the most basal group of Notosuchia (fig. 10). Other putative synapomorphies of this clade are optimized as ambiguous due to the missing information in its basal members: (1) ascending postorbital process located on the posterior half of the jugal (character 143–2), paralleled in Simosuchus clarki and some basal crocodyliforms; (2) absence of ornamentation on the posteroventral region of quadratojugal (character 144–0), paralleled in several basal crocodylomorphs and baurusuchids; (3) concave, trough-shaped dorsal surface of mandibular symphysis (character 184–1). The first two characters are missing in A. wegeneri, A. buitreraensis, and MPCA-PV 236, while the third one is missing in A. wegeneri, A. buitreraensis, and A. patagonicus. Further study or remains of these taxa will determine if these characters are diagnostic of the entire clade of Araripesuchus or of a restricted subgroup.

All South American members of Araripesuchus in this analysis form a monophyletic group supported by three unambiguous synapomorphies. The posterior maxillary and dentary teeth are not compressed lateromedially at their base (character 140–0) in the South American species of Araripesuchus but are lateromedially compressed in A. wegeneri, most basal notosuchians (e.g., Uruguaysuchus aznarezi, Simosuchus clarki), and in several basal crocodyliforms (e.g., Hsisosuchus chungkingensis, the Fruita form). The lateral surface of the dentaries lacks a well-developed longitudinal ridge (character 188–0); this ridge divides the lateral surface of the dentaries in a dorsal region exposed laterodorsally and a vertical ventral region in most basal crocodyliforms and notosuchians (except for sebecosuchians). These two characters are paralleled in neosuchians and therefore provide support for this South American clade only when Araripesuchus is depicted as related to notosuchians. The third synapomorphic character is the presence of a straight dorsal edge of the dentary with an abrupt dorsal outgrowth (character 156–1) described above for the specimen MPCA-PV 236. As noted above, this morphology is also present in A. gomesii and is moderately developed in A. buitreraensis, but is currently unknown in A. patagonicus. Two additional putative synapomorphies from the morphology of the choana are ambiguously optimized and might also diagnose this clade: presence of a T-shaped choanal septum (character 186–1) that completely divides the internal nares opening (character 70–2). These two characters are missing in A. wegeneri and are also present in Simosuchus clarki. Further material of the African taxon and successive outgroups would help to determine if this character is synapomorphic of the South American Araripesuchus or if it diagnoses a larger group of taxa.

In the strict consensus of this analysis the South American Araripesuchus subclade lacks internal resolution; however, these taxa are collapsed due to the alternative positions of the specimen MPCA-PV 236 within that clade (fig. 10). Disregarding the alternative positions of MPCA-PV 236, all the most parsimonious trees agree in displaying A. buitreraensis as the sister taxon of the clade formed by A. gomesii and A. patagonicus (fig. 10). These two taxa share five characters to the exclusion of A. buitreraensis. The lateral margins of the nasals are markedly concave along their suture with the premaxillae (character 127–0) instead of being straight as in A. buitreraensis and A. wegeneri. The caudal edges of nasals are transversely oriented (character 162–1) instead of being caudally separated by an acute anterior process of the frontal as in A. buitreraensis. The postorbital process of the jugal is posteriorly inset from the lateral surface of the jugal (character 164–1). The pterygoid flanges are thin and laminar (character 193–0), lacking the dorsoventrally thick flanges with pneumatic spaces present in A. buitreraensis. Finally, the T-shaped choanal septum of A. gomesii and A. patagonicus tapers anteriorly (character 220–1), in contrast to anteriorly broad septum of A. buitreraensis and Simosuchus clarki.

Araripesuchus is consistently depicted as the most basal group of a large clade that is congruent with the traditional taxonomic content of Notosuchia (Gasparini, 1971) with the addition of sebecosuchians (depicted here nested within notosuchians; fig. 10). This phylogenetic position contrasts with the results of some previous cladistic analyses (e.g., Clark, 1994; Buckley and Brochu, 1999; Buckley et al., 2000; Ortega et al., 2000; Turner, 2004a), where Araripesuchus was depicted as more closely related to neosuchians than to notosuchians. However, the results presented here agree with the hypotheses proposed by other authors (Wu and Sues, 1996; Sereno et al., 2001, 2003; Pol 1999b; Pol and Norell, 2004a) in which Araripesuchus was depicted as closer to Notosuchus than to neosuchians. Sereno et al. (2001) recognized a clade composed by a subset of the taxa included here and defined the taxon Notosuchia as all crocodyliforms more closely related to Notosuchus than to Crocodylus.

In our dataset, the Araripesuchus clade is supported in this position by 10 synapomorphies present in all most parsimonious trees. The alveolar edge of the premaxilla-maxilla contact lacks a large notch or fenestra (character 9–0). The lateral surface of the maxilla is pierced by several large neurovascular foramina aligned above the alveolar margin (character 138–1), being paralleled in some basal crocodyliforms (e.g., Protosuchus richardsoni, Sichuanosuchus shuhanensis). The distal body of the quadrate (ventral to the triple contact between this element, the exoccipital, and the squamosal) is directed ventrally (character 149–1) rather than posteroventrally. The splenials bear the previously described peg located posteriorly on the ventral surface of the mandibular symphysis (character 181–1; reversed in A. gomesii). The dentary lacks a posteroventral process extending beneath the mandibular fenestra (character 70–1). The retroarticular process of these forms is formed by a broad rounded surface projected posteroventrally and facing dorsomedially (character 71–2). In some notosuchians this surface is slightly concave (e.g., Notosuchus terrestris) while in others it is flat or barely concave (e.g., A. gomesii). The area of insertion of the m. pterygoideus extends onto the lateral surface of the angular (character 76–1), a character paralleled in crocodylians. The cervical vertebrae have anteroposteriorly narrow, rodlike neural spines (character 90–1) and bear moderately developed hypapophyseal keels on their ventral surface (character 91–1). The sacrum is composed by three elements (character 115–1), the last two of which are fused in some taxa (Pol, 2005). Additionally, several other synapomorphies may diagnose Notosuchia, but they are ambiguously optimized due to lack of information on notosuchian outgroups. Some of the most conspicuous characters are the presence of a small foramen located on the lateral surface of the snout, at the premaxilla-maxilla suture (character 135–1), the presence of a vertical ridge on the posterior surface of the distal body of the quadrate (character 150–1), the absence of a posterior buttress in the glenoid facet of the articular (character 182–1), and the absence of an anterolateral process in the dorsal osteoderms (character 96–0; paralleled in derived neosuchians).

Araripesuchus is depicted as the most basal group of Notosuchia due to the absence of the following derived characters that diagnose the different notosuchian nodes: dorsal parasagittal osteoderms rounded or ovate (character 95–1; node 1 [a similarly composed group (including sebecosuchians and most notosuchians with the exception of Araripesuchus) was referred to as Ziphosuchia by Ortega et al., 2000]); quadrate-articular joint located ventral to the level of the toothrow (character 105–2; node 1); absence of an enlarged maxillary tooth (character 79–0; node 2); presence of a distinctly smooth surface above the alveolar margin on the lateral surface of the maxilla (character 107–1; node 2); extensive perinarial fossa with a rounded concave surface facing anteriorly (character 221–1; node 2); external surface of skull roof ornamented with grooves and ridges (character 1–1; node 3); dorsal surface of frontal with a sagittal ridge (character 22–1; node 3); dorsal edge of surangular markedly bowed (character 74–1; node 3); seven (or less) maxillary teeth (character 108–1; node 4); posterior end of choanal groove (at internal nares) wider than nasopharyngeal passage at the palatine bar (character 42–1; node 5); presence of posterolateral rodlike palatine bar (character 227–1; node 5); ectopterygoid extending medially on ventral surface of pterygoid flanges, approximately covering the ventral half of them (character 230–1; node 5).

As is evident above from the number of convergences and parallelisms, the character distribution among basal mesoeucrocodylians is rather conflictive, and homoplasy levels are remarkably high. In addition to this, the presence of fragmentary or poorly known forms in the analysis results in minimal Bremer support values for most of these nodes. The phylogenetic position of the Araripesuchus clade is also minimally supported, showing that new data can overturn this hypothesis. In particular, several authors have proposed Araripesuchus to be more closely related to neosuchians than to notosuchians (Clark, 1994; Ortega et al., 2000; Buckley and Brochu, 1999; Buckley et al., 2000; Turner, 2004a). However, in the dataset presented here, trees supporting neosuchian affinities for Araripesuchus are markedly suboptimal. The most parsimonious tree that depicts Araripesuchus in that position requires six extra steps.

DISCUSSION

As mentioned above, the phylogenetic position of Araripesuchus has been repeatedly debated in the recent literature. Most of previous studies have exclusively considered the information provided by the type species of this group, A. gomesii (but see Ortega et al., 2000 and Turner, 2004a). This taxon, known from the Romualdo Member of the Santana Formation (Middle–Late Albian), has a conflictive combination of characters that alternatively suggests neosuchian and notosuchian affinities (see diagnoses above). The remains found in the Candeleros Formation (Cenomanian–Turonian) of Patagonia (including the specimens described here and A. patagonicus) provide novel and interesting phylogenetic information. These remains (in particular A. buitreraensis and A. patagonicus) add an additional set of derived similarities with notosuchians that are absent in A. gomesii (e.g., posterior peg in splenial symphysis), demonstrating their relevance for understanding the phylogenetic relationships of this group.

Araripesuchus has been the focus of several studies due to its distribution in Cretaceous beds of both Brazil and Continental Africa at the time of the South Atlantic opening (Buffetaut, 1981; Kellner, 1994). Recently, the generic assignment of A. wegeneri was questioned (Ortega et al., 2000; Prasad and Lapparent de Broin, 2002). Under the phylogenetic hypotheses proposed by these authors, the biogeographic informativeness of the Araripesuchus clade would be notably diminished. In contrast, the phylogenetic analysis presented here depicts this fragmentary African taxon as the most basal member of the Araripesuchus clade, indicating its potential relevance for biogeographical studies as originally proposed (Buffetaut, 1981; see also Turner, 2004a). It must be noted, however, that the position of A. wegeneri is extremely poorly supported, and biogeographical scenarios made upon these results should be taken cautiously. The poorly supported position of the Early Cretaceous A. wegeneri is partially caused by the fragmentary nature of its type specimen but also by the presence of some conflict in its character distribution. Further studies and additional material on this taxon are extremely relevant for understanding the early evolution of Araripesuchus and its biogeographical implications. Furthermore, the study of recently reported materials preliminarily referred to this clade from the Late Cretaceous of Madagascar and Niger (Buckley et al., 1997; Sereno et al., 2004; Turner, 2004b), as well as from Coniacian–Santonian beds of Patagonia (S.A., in prep), will provide critical information to test the alternative biogeographical hypotheses for the Cretaceous of Gondwana (Krause et al., 1997; Turner, 2004a; Sereno et al., 2004).

CONCLUSIONS

The two crocodyliform specimens described here from the Early Late Cretaceous of Patagonia are closely related to previously described species of Araripesuchus (Price, 1959; Ortega et al., 2000). These specimens add relevant phylogenetic information for understanding the evolutionary history of this clade as well as its diversity during the Cretaceous in southern South America.

The set of most parsimonious trees found in the phylogenetic analysis presented here is congruent with some aspects of the two previous phylogenetic analyses of this genus (Ortega et al., 2000; Turner, 2004a), depicting Araripesuchus as a basal mesoeucrocodylian clade within which the South American taxa are more closely related to each other than to A. wegeneri (from the Early Cretaceous of Africa). However, the new information provided by the specimens described here and the new character data considered in this phylogenetic analysis suggest that the Araripesuchus clade is the most basal member of Notosuchia, the largest and most diverse clade of Cretaceous Gondwanan crocodyliforms.