Montealtosuchus arrudacamposi, Carvalho, Vasconcellos & Tavares, 2007

Carvalho, Ismar De Souza, Vasconcellos, Felipe Mesquita De & Tavares, Sandra Apare- Cida Simionato, 2007, Montealtosuchus arrudacamposi, a new peirosaurid crocodile (Mesoeucrocodylia) from the Late Cretaceous Adamantina Formation of Brazil, Zootaxa 1607, pp. 35-46 : 38-44

Montealtosuchus arrudacamposi gen. nov. et sp. nov.

( Plate 1 , 2)

Etymology: The species name, arrudacamposi , is in honor of Antonio Celso de Arruda Campos, eminent Brazilian paleontologist and professor.

Holotype: MPMA-16-0007-04 (Museu de Paleontologia de Monte de Alto). Skull, mandible and almost complete axial and appendicular skeleton, and articulated dorsal and ventral exoskeleton (osteoderms).

Locality: 14 km of Monte Alto, near the border between Monte Alto and Taiaçu counties, São Paulo State, Southeast of Brazil.

Stratigraphic context: Bauru Basin, Adamantina Formation, Upper Cretaceous (Turonian-Santonian).

Diagnosis (skull): Snout moderately narrow, tubular in cross-section. Skull triangular in dorsal view. Anterior nasal process at the tip of snout, hanging over the external naris. Continuously sloping surface connecting the rostrum and skull table. A small slit-shaped notch for the 4th mandibular tooth at the contact between maxilla and premaxilla. Five premaxillary conical teeth progressively increase in size posteriorly, all evenly spaced. Wedge-like maxillary process of the premaxilla present. Foramen incisivum with maxillary marginal contribution. Fourteen maxillary teeth in sinuous outline, with moderate heterodonty, displaying anterior sharp conical teeth and posterior low, laterally compressed globular teeth. Anterolateral external nares in vertical position. Nasals participating in a non-septate external nares. Orbits subsquare. Antorbital fenestrae present. Two triangular supraorbitals bordering dorsally the orbit. Postorbital bar ascending from the mesial border of the jugal. Supratemporal fenestrae elliptical, smaller than orbits, and set in a non-sculptured depression, bordered by the parietal, postorbital, squamosal and small contribution of the frontal. Supratemporal with minimal participation in skull table. Quadratojugal participates with the quadrate in skull-mandible articulation. Deep, square choanae, evenly composed by palatines and pterygoids. Palatine fenestrae elliptical, bordered by maxillae, palatines, ectopterygoids and pterygoids. Basioccipitals inclined toward the palatal surface. Exoccipitals wide and bend anteriorly and half of its height. Eustachian foramina closely disposed in “v” pattern, in the palatal view. Splenials participating in 1/3 of the symphysis. Symphysis extending to the 10th tooth. Mandible arched upward at anterior and posterior ends. Articular presents deep concave articular surface, with contribution from the surangular. Retroarticular process paddle-shaped and inclined dorsally and mesially. Mandibular fenestrae elliptical, aligned with the laterotemporal fenestrae. Eighteen dentary teeth show two morphotypes: anterior ones conical and sharp, with hypertrophied 4th tooth; posterior teeth globular, low, and laterally compressed. All premaxillary, except the first, maxillary and dentary teeth bear constricted basal crown and fine serrated carinae.

Description and comparison

Skull

General shape: The skull is triangular in dorsal view. The postorbital region is twice as wide as the rostrum. The skull table is slightly convex, and not much wide. The orbits are not visible from this region because of the interference of supraorbitals. In lateral view skull is moderately high and narrow, the postorbital part is twice as high as the rostrum. The lateral flanges of the cranial table are festooned and overhang the temporal region. The rostrum is tubular in cross-section (broad oreinirostral). A gentle and continuous slope connects the rostrum with the skull table. The posterior part of the skull is inclined ventrally and laterally, and the occipital region is vertical to the ventral limit of the basioccipital and then bends sharply anteriorly. The skull and mandible are uniformly sculptured with swallow grooves and pits, except in the temporal region. The total length of of the skull is 265 mm, its height at orbit is 85 mm, its width at 3rd maxillary tooth is 55 mm, its width at orbit is 87 mm, and its width at quadrate is 124 mm. The general shape of the skull of Montealtosuchus is distinctive from that of Peirosaurus , Lomasuchus and Uberabasuchus . It has a relatively shorter rostrum than Uberabasuchus and narrower than Lomasuchus and the Argentinian Peirosaurus . In fact the Argentinian Peirosaurus dorsal rostrum outline is greater than the other three Peirosauridae , including the Brazilian Peirosaurus holotype. The parallel maxillary wall present in Lomasuchus and Uberabasuchus differs from the triangular aspect of Montealtosuchus . The rostrum-skull table slope is slightly different between Uberabasuchus , Peirosaurus and Montealtosuchus , as the former two display a clear discontinuous surface and the later lacks this feature. This is also seen in Lomasuchus .

Cranial openings: The supratemporal fenestra is wide and subellipitical bearing two distinct margins, one external and other internal; the surface between them is smooth, except for the temporal canal foramen. It is extensively surrounded by the parietals and postorbitals with smaller contributions from the squamosals and the frontals. The frontals hardly contribute to the inner outline of the fenestrae. The orbits are rectangular, being completely lateral and vertical, roofed by two supraorbital bones. The laterotemporal fenestra is triangular and smaller than the orbit, three times longer than high; it is formed by the quadratojugal and jugal and to a smaller extent by the postorbital. The incisura otica (superior lateral fenestra) is encircled by the quadrate and squamosal. The temporal region also bears two other pneumatic foramina, lying anteroventrally to the incisura otica. The rostrum bears a semicircular antorbital fenestra, where a small and thin bone blade projects from the posterior and interior border of the fenestra. It is bordered by the lacrymal and maxilla in equal proportions. A small slit-shaped notch for the 4th mandibular tooth is present at the contact between maxilla and premaxilla. The foramen incisivum is elliptical reaching to the fourth premaxillary tooth, being bordered by maxilla and premaxilla. The palatine fenestra is elliptical, bordered by the maxilla anteriorly, by the palatines mesially, and by ectopterygoid and pterygoid posteriorly. The choana bears a small bony ridge in its dorsal surface (septum?), and is uniform in width and length; it is set in a smooth depression with featureless borders, composed equally by the palatines (anteriorly) and pterygoids (posteriorly). In frontal view the external nares are vertical and anterolateral in position and, although lacking septa, they exhibit a prenasal bony arch over its opening. The external nares are bordered by the premaxilla with a smaller contribution by the nasal and its arch. The medial eustachian and lateral eustachian foramina are visible in palatal view, due to the ventrally inclined position of the occipital surface. The lateral eustachian foramina are located posterodorsally in a “v” pattern, in relation to the medial eustachian foramen, being bordered by the basisphenoid, basioccipital and exoccipital. The foramina for the exit of cranial nerve XII and the foramen magnum are only visible from the occipital view.

The absence of an antorbital fenestra appears to be an autapomorphy for the Argentinian Peirosaurus among other Peirosauridae , although the preservation of this critical region is not perfect. The shape of orbits in the new species is rectangular, as in Lomasuchus , whereas in Uberabasuchus these are circular. The choanal shape of Montealtosuchus is similar to Lomasuchus , but this feature is not preserved in other Peirosauridae . The choanal outline is also very similar to that seen in the Araripesuchus gomesii Price, 1959 .

Another similarities between Peirosauridae and Araripesuchus are the composition of the supratemporal fenestrae, general shape of the antorbital fenestrae (seen only in Montealtosuchus ) and general outline of the skull in lateral and dorsal views.

Premaxilla: It is bordered by the maxilla and nasals. its contact with nasal is straight; its suture with maxilla shows a zig-zag pattern in dorsal view and in a wedge-like, delta-shaped, from palatal view. Although partially eroded, a smooth surface lateral to the external nares is recognizible, as in Uberabasuchus and Peirosaurus . a feature similar in all Peirosauridae .

Nasal: The bone is long with parallel mesial and lateral borders. It is slightly concave before the orbits, and gently slopes towards the frontals. It possesses two small projections between the prefrontals and frontals; it contacts the premaxilla, maxilla, prefrontal and frontal. With the exception of Uberabasuchus , in all Peirosauridae , including Montealtosuchus , the nasals do not contact the lacrimals.

Maxilla: Subvertical and moderately high. Its anterior border contacts the premaxilla, mesially contacts the nasal and its posterior border contacts the lacrimal, prefrontal and jugal.

Prefrontal: It is elliptical, longer than wider. The element is bounded by the nasal, maxilla, lacrimal, supraorbital and frontal. The internal portion of the prefrontal contributes to the anteromesial wall of the orbit.

Lacrimal: The bone is square and concave and forms the posterior part of antorbital fenestra and its internal laminae. It contacts the maxilla, jugal, supraorbital anterior and prefrontal, but not the nasals. It is very distinct from Uberabasuchus which is triangular and composed most of the anterior, dorsal and posterior borders of the antorbital fenestrae.

Supraorbital (anterior and posterior): It is also referred as palpebrals. The first one is larger than the second. Both are subtriangular in shape. The anterior one is longer than wide, and the posterior is as wide as long, both covering two-third of the dorsolateral border of the orbital region. They present a gentle convexity on the lateral margin of the skull that protects the orbits. In contrast, Uberabasuchus has a single supraorbital bone.

Frontal: It is long, triangular and completely fused. The bone borders anteriorly the nasals and both prefrontals, and posteriorly the supraorbital and postorbital. It is flat in the skull roof area but becomes slightly convex in its bordering limits.

Jugal: The bone contacts the maxilla at the length of posterior border of the antorbital fenestrae, where it forms the lower, slightly sinuous margin. In anterior view, the jugal occupies the lowermost portion beneath the orbits and is laterally inclined and presents a small ventrolateral expansion. Posteriorly to the postorbital bar it has a gentle downward slope. The projection to the postorbital starts from its upper mesial surface.

Postorbital: In dorsal view it bends gently to meet the supraorbital and follows the curvature of the orbits, where it shows a concave shape. At its opposite border, it follows the curvature of the supratemporal fenestrae. The postorbital bone is concave externally.

Postorbital bar: Positioned mesially, the postorbital bar is stout, column-like, and smooth without any ornamentation. Its upper part, the postorbital component, is more steeply inclined than the lower part; its jugal component originates mesially as seen in Uberabasuchus .

Parietal: It is flattened in the center, convex at its posterior border and sharply inclined in the borders of the supratemporal fenestrae. The parietal has straight contacts, with the frontal, postorbital and squamosal and supraoccipitals.

Squamosal: Most of this bone projects posteriorly from the rest of the skull roof. The bone is triangular, its squamosal surface is inclined lateroventrally along its lateral border, and posteroventrally in its posterior portion. This posteroventral part of the squamosal presents an extensive contact with the quadrate, running downward as it follows the quadrate morphology. Anteriorly it overhangs the temporal area along the postorbital. It displays a flange that bends ventrally and dorsally to enclose the temporal region. The sculpting is more evident than in any other skull bone.

Quadratojugal: The bone is elongated and strongly curved, projecting ventrally and posteriorly, from the posterior region of the skull. Although it extends extensively along the quadrate it has minimal lateral contribution to the jaw articulation.

Quadrate: It is moderately flexed ventrally to contact the quadratojugal. Here a small foramina is present, facing posteroventrally. Behind this foramen, the quadrate is strongly flexed ventrally, and expanded and differentiated into two condyles mesiolaterally. The innermost condyle is larger and more expanded mesially and posteriorly. The lateral condyle is composed by the quadrate with a small contribution from the quadratojugal.

Palatine: The bone has a shape of a blunt arrow in its contact with the maxilla, and encloses anteromesially the palatine foramen. It contacts the pterygoid at the mid-length of the choanae. Two lateral projections of the palatines embrace the anterior region of the choanae.

Pterygoid: The bone is large, laterally expanded and vertically flexed. It borders the palatine in the choanae. Its anterolateral part contacts the ectopterygoid, and after this contact, both bones are more ventrally flexed. The posteromesial region bears two small triangular expansions, diverging laterally from the mesial axis. This feature is seen in Araripesuchus gomesii .

Ectopterygoid: The bone is stout and coplex. It joins the pterygoid in a long process lateroposteriorly, meets the jugal dorsally and anteriorly, and makes a brief contact to the maxilla.

Supraoccipital: It has a small contribution to the skull roof as it is visible in the occiput. It is laterally expanded and slightly concave. It contacts the squamosal laterally and the occipital ventrally. This feature may be an autapomorphy for Montealtosuchus among Peirosauridae .

Exoccipital: The bone is vertical and slightly concave, and slopes at its middle height to the palatal surface along with the bordering bones. It contacts the supraoccipital, squamosal, quadrate, and basioccipital. The exoccipital is pierced by two foramina for the XII cranial nerves, which are positioned more lateroventrally. The topographic position of these two foramina are similar in Lomasuchus , although less laterally developed.

Basioccipital: The bone is subhorizontal and visible only in palatal view, but less flattened than in Lomasuchus . It is saddle-shaped and bears two concave lateral surfaces bordering a small crest. This crest extends from the medial eustachian foramen to two-third of the length of the basioccipital. The borders are swollen with strong lateral knobs. The contact between basioccipital, quadrate and basisphenoid is penetrated by the lateral eustachian foramina.

Basisphenoid: It is broad and anteriorly inclined. The bone is compressed between the basioccipital and palatines, and bears a stout lateral ridge at the contact of the quadrate and exoccipital.

Mandible

General shape: The mandible is concave in lateral view, arched upward at the anterior and posterior ends. It is tubular to conform the shape of the rostrum, but becomes higher and wider posterior to the last tooth. Median symphysis is moderately long, reaching as far as the 10th mandibular tooth (52 mm) whereas in the Argentinian Peirosaurus it reaches the 8th tooth position. Sinuous grooves ornament the ventral and lateral surfaces. Mandibular fenestra is large and long, subtriangular in shape, with a bony lamina originating in the dentary mesially at the anterior margin. In contrast, the Uberabasuchus mandible in lateral view is flat at its mid-length, not concave as seen in Montealtosuchus .

Dentary: This is the longest element in the mandible and is stout and highly ornamented. The anteriormost part of the dentary is flattened; the dorsal surface is slightly concave behind the symphysis. Behind this flattened region, the dentary becomes vertical, inclined mesially, and becomes thin and compressed. It extends as far back as the mandibular fenestra, where the anterior border becomes expanded into a mesial smooth bone lamina inside the mandibular fenestra. In Uberabasuchus this bony lamina is sculptured. The dentary articulates the angular and the surangular in intricate pattern. Its contact with the splenial is tight and sinuous. In Montealtosuchus the symphysial part of the dentary is narrower than the condition in Uberabasuchus . The symphysis in Montealtosuchus is comparatively longer than that of Uberabasuchus and the Argentinian Peirosaurus , reaching considerably backward in the dentary tooth row.

Splenial: It forms one-third of the symphysis (1.5 mm) and articulated with the dentary in a sinuous, tight contact. The splenial symphysis accommodates 4 teeth along its length. It borders mesially the dentition from the 11th to the end of the tooth row in the form of a thin bony lamina. Its posterior end extends up to the position of the pterygoid.

Surangular: The bone extends anteriorly to the posterior end of the antorbital fenestra. The surangular encloses the upper border of the mandibular fenestra. It has the same length as the angular and it extensively contacts this bone in a butt joint. It participates lateroventrally in the jaw articulation and postarticular process. This feature is absent in Uberabasuchus .

Angular: Its length is at least two-third of the mandibular ramus. It borders ventrally the mandibular fenestrae.

Articular: The bone is square in dorsal aspect and concave. Its anteriormost lateral border joins the surangular and the angular; the posteriormost component participates in the formation of the jaw articulation. The glenoid is concave, barrel-shaped, bordered anteriorly and posteriorly by a thin ridge and its mesial surface is inclined ventrally. The posterior portion of the articular (retroarticular process) is anteroposteriorly expanded, which rises vertically after a concave surface. Its mesial border gently slopes ventrally. In Uberabasuchus the articular is less developed laterally and the retroarticular process is not as long as in Montealtosuchus . Moreover, the surangular and the quadratojugal do not take role in the formation of the jaw joint in Uberabasuchus , unlike the condition in Montealtosuchus . In fact, this feature is unknown in other Peirosauridae .

Dentition

General shape: The dental formula is 5 premaxillary, 14 maxillary and 18 mandibular teeth. The dentary and maxillary dental formula matches with that of Lomasuchus , with 14 maxillary teeth, but differs from that of Uberabasuchus , which bears only 12 maxillary and 12 mandibular teeth. The anteriormost teeth of the mandible, maxilla and those of premaxilla are sharp and conical, but the posterior ones became more bulbous and lateromesially compressed. All teeth bear serrated carinae, but in some teeth, the feature is eroded and damaged. The posterior ones bear more pronounced carinae and serration. The serration and carinae are composed only of enamel, not dentine. The crown of each teeth is separated from the base by a well defined constriction, which is deeper in the posterior ones. The teeth are set on undulating mandibular and maxillary alveolar margins. Even considering the discordance of dental formula, the general pattern of tooth size, form and characteristics are similar. Montealtosuchus possesses comparatively more low bulbous teeth.

Premaxilla: These teeth become progressively bigger from the anterior to posterior region; the first three are small, and the last two are larger and more evenly spaced. The first tooth is the smallest in the series, followed by the slightly bigger second tooth; the third and fifth are bigger than the first and second ones. The fourth tooth is the largest in the series. The second to fifth premaxillary teeth have finely serrated keels. The size pattern of these teeth is similar to that of Peirosaurus , Lomasuchus and Uberabasuchus .

Maxilla: These teeth show some degree of heterodonty. The three most anterior teeth are conical but slightly compressed, and are progressively bigger, with the lingual side slightly flattened and the labial side convex. All teeth possess anterior and posterior keels, finely serrated. The largest tooth in this series is the third, followed by the second one. The remaining eleven teeth are smaller, bulbous, laterally compressed, and finely serrated. The tips of the maxillary dentition have a sinuous outline, displaying two waves of teeth in lateral view. The second and third teeth are dislocated more laterally, while the rest lies in a straight line in dorsal perspective. Three teeth pass the anterior margin of the palatine fenestra, unlike the condition in Lomasuchus , where only two teeth surpass this margin.

Dentary: The first three teeth are of the same size, displaying a slightly flat lingual surface. The fourth is the largest, conical in shape, slightly inclined posteriorly and mesially. The fourth dentary tooth projects upward and fits into a notch between the premaxilla and maxilla. In lateral view, the mandibular dentition shows a sinuous margin, marked by two sets of waves. The summit of first one is represented by the large 4th tooth, whereas the second summit is formed by the 12th tooth, also large. In either side of the waves the smaller teeth occur. In dorsal view, the posterior mandibular teeth are aligned in a straight line by the 11th; and became more lingual in position compared with the maxillary ones.

Figures

PLATE 1. Holotype of Montealtosuchus arrudacamposi gen. nov. et sp. nov. (MPMA-16-0007-04) in 1 dorsal view of skull and mandible; 2 lateral view of the skull; 3 lateral view of mandible; 4 palatal view; 5 dorsal view of mandible; 6 occipital view.