Buitreraptor gonzalezorum Makovicky, Apesteguía, and Agnolín, 2005

Gianechini, Federico A., Makovicky, Peter J. & Apesteguía, Sebastián, 2011, The teeth of the unenlagiine theropod Buitreraptor from the Cretaceous of Patagonia, Argentina, and the unusual dentition of the Gondwanan dromaeosaurids, Acta Palaeontologica Polonica 56 (2), pp. 279-290 : 281-283

Buitreraptor gonzalezorum Makovicky, Apesteguía, and Agnolín, 2005

Figs. 3–5 .

Holotype: MPCA 245 , consisting of an almost complete adult skeleton, including a partial, articulated skull, with both incomplete maxillae with teeth in situ, left jugal, both postorbitals, both quadrates, right squamosal, both incomplete nasals, both frontals, both parietals, the occipital condyle, and mandibular bones, including both dentaries with in situ teeth, both splenials, left angular, and left surangular. The holotype also includes several isolated teeth. The postcranium includes cervical, dorsal, sacral, and caudal vertebrae, cervical and dorsal ribs, chevrons, both scapulocoracoids, furcula, both ilia, right ischium, both humeri, right radius and ulna, metacarpals and manual phalanges, both femora, right tibia and fibula, left incomplete tibia and fibula, metatarsals, several pedal phalanges, and undetermined fragments of bone.

Type locality: “La Buitrera”, situated about 80 km southwest from Cipolletti, close to the south shore of the Ezequiel Ramos−Mexía Lake, northwestern Río Negro Province, Argentina.

Type horizon: Candeleros Formation, Cenomanian–Turonian, Upper Cretaceous.

Referred material.—MPCA 238, consisting of an incomplete sacrum, right ilium and pubis, right hindlimb (femur, tibia, astragalus, metatarsals, and phalanges), all preserved in articulation.

Description.—The holotype skull of Buitreraptor gonzalezorum bears six alveoli in the preserved part of the right maxilla and five more are located on its posterior section ( Fig. 3A, D ). The preserved part of the right dentary bears at least twenty alveoli, with three teeth in their sockets in the anterior region, and three more in the mid−region ( Fig. 3A–C ). The left dentary has traces of around fifteen alveoli and only preserves two teeth in situ at the rostral tip ( Fig. 3E–G ). The exact number of alveoli is difficult to establish due to breakage, but bedoi:10.4202/app.2009.0127

cause of the length of the dentaries, the extension of the dental row and the size of the alveoli it is estimated that about twenty five alveoli were present in the jaw. Although the articulation of the lower jaw precludes the observation of the lingual side of the maxillary teeth, this surface is available in five isolated teeth found associated with the skeleton during preparation.

Buitreraptor teeth are small when compared to skull height, ranging from 0.6 to 4.6 millimeters (mean CH: 2.19 mm; ratio between the height of the crown and the height of the lower jaw: 0.29) ( Table 1). They are distally curved and labiolingually compressed (CBR medium: 0.29), with a very acute end ( Fig. 4). All teeth, those in situ and those isolated, present the same morphology, differing only in their size. The latter becomes evident in the right dentary, where the anterior teeth are slightly larger than those from the mid−dentary ( Fig. 3A–C , Table 1). The preserved teeth are in different stages of eruption, with some completely erupted and others barely so. It is evident that continuous tooth replacement occurred in this species.

All crowns are straight in mesial and distal views, without any lateral deflection ( Fig. 4B, D). In side view, the crowns are often moderately recurved distally along their full length. However, in some teeth, especially the largest, the base of the crown is quite straight while the apex shows a strong curvature toward the tip ( Fig. 4A, C).

The second preserved tooth of the left dentary (MPCA 245 D8) is situated in an alveolus with a broken posterior rim. This reveals that the distal base of the crown is continuous with the root, without any constriction ( Fig. 3G ), and contrasting with troodontids and other groups of non avian coelurosaurs (e.g., ornithomimosaurs, Pérez−Moreno et al. 1994; oviraptorosaurs, Balanoff et al. 2009; therizinosaurs, Kirkland and Wolfe 2001; Clark et al. 1994). The curvature of the distal border of the crown changes at this point and the distal border of the root continues fully straight. Both mesial and distal borders of the crown are slightly rounded and devoid of carina and denticles in all preserved teeth ( Figs. 4B, D, 5A ).

The labial and lingual sides of the crowns are very similar to each other, and it is difficult to observe any difference in their convexity that could allow us to distinguish between these sides, especially in the isolated teeth. An important feature in all the observed teeth is an elongated longitudinal depression on both labial and lingual surfaces, which confers a figure−eight cross−section to the crown ( Figs. 3B–D, F , 4A, C, E). In some teeth, a groove arises from this depression and extends toward the apex without reaching it ( Fig. 4A). This median groove is flanked by shallow ridges in some teeth ( Fig. 3B, F ).

Another distinctive feature is the presence of a system of parallel longitudinal ridges and grooves, at both sides of the central depression ( Figs. 3B, F , 4A, C, 5B ). In some teeth these grooves and ridges are located closer to the distal border, distally with respect to the central depression ( Fig. 5B ). These ridges and grooves do not follow a defined pattern and their number is not constant, being sometimes asymmetrical in both sides of the same crown, and differing in their depth. They extend from the base of the crown almost to the apex ( Fig. 5B ) or can be also restricted to the apical half of the crown ( Fig. 4A, C). In several teeth there are two grooves on each side of the crown, framed by a ridge of the same extension on both sides ( Fig. 4A, C). To summarize, the complete topographical system of B. gonzalezorum teeth is composed by a median ridge, two framing grooves, a ridge on the mesial side, and one on the distal side. In other cases, three or more grooves and ridges are restricted to only the distal region, close to the edge ( Fig. 5B ). Some teeth do not have these features, except for the central depression on each side ( Fig. 5A ). The recognition of a pattern between the number and distribution of grooves and ridges and the position of the teeth in the tooth row is hampered due to a lack of more complete dental series ( Fig. 3A, E ).

Stratigraphic and geographic range.—Candeleros Formation (Cenomanian–Turonian), basal unit of the Neuquén Group, Río Limay Subgroup , Río Negro province, Argentina .

Figures

Fig. 3. In situ teeth of the unenlagiine theropod Buitreraptor gonzalezorum Makovicky, Apesteguía, and Agnolín, 2005 (MPCA 245) from the Upper Cretaceous of La Buitrera, northwestern Río Negro, Argentina. A. Right side of skull, showing the zones of the mandible and maxilla with preserved teeth. B. Second and third tooth preserved in the right mandible (the first tooth is very poorly preserved). In the second tooth the central groove is visible on the lateral side of the crown flanked by shallow ridges. C. Fourth, fifth and sixth teeth preserved in the right mandible. D. First to fourth tooth preserved in the right maxilla. Here the central grooves also are visible on the lateral side of the crowns, like B. E. Left side of the skull, showing the zone with preserved teeth. F. First and second teeth preserved on the left mandible. Also visible are the grooves and the ridges of the lateral sides of the crown. G. Posterior view of F. The arrow shows the broken zone of the mandible, where is visible the root of the second tooth.

Fig. 5. SEM micrographs of isolated teeth associated with the holotype of the unenlagiine theropod Buitreraptor gonzalezorum Makovicky, Apesteguía, and Agnolín, 2005 from the Upper Cretaceous of La Buitrera, northwestern Río Negro, Argentina. A. Mesio−lateral view of one isolated tooth (MPCA 245 A2). Note the total absence of carinae and denticles on the mesial edge, and the central depression of the lateral side. B. Lateral side of one isolated tooth (MPCA 245 A5). Note the grooves and the ridges located near of the distal edge of the crown, to the left of the image (grooves are marked with arrows).