Un nuevo Abelisaurodo del Cretácico Superior de Brasil

Elaine B. Machado¹*, Diogenes de A. Campos², Jorge O. Calvo³, and Alexander W. A. Kellner¹

¹ Laboratório de Sistemática e Tafonomia de Vertebrados Fósseis, Departamento de Geologia e Paleontología, Museu Nacional/Universidade Federal do Rio de Janeiro (UFRJ), Brasil. *machado.eb@gmail.com

² Museu de Ciencias da Terra, Departamento Nacional de Produqao Mineral, S.A.N. Quadra 01 Bloco B, 70041-903, Brasilia, Brasil.

³ Centro Paleontológico Lago Barreales (CePaLB), Universidad Nacional del Comahue, Ruta Provincial N° 51, km 65, Neuquén, Argentina.

Manuscript received: March 23, 2012
Corrected manuscript received: November 5, 2012
Manuscript accepted: November 7, 2012

Abstract

Although important dinosaur specimens have been recently described from Brazil, the theropod record in this county is still rather scarce, particularly from Cretaceous strata. Here we describe a complete right tibia (MCT 1783-R) from the Marilia Formation (Maastrichtian, Bauru Basin) near Peiropolis (Uberaba, Minas Gerais). This is the best theropod specimen recovered from this unit after five decades of collecting. MCT 1783-R shows a well developed cnemial crest with a lobular distal end, that projects significantly above the proximal articular surface, features that are characteristic of the Abelisauroidea. It further shows unique characters such as a marked asymmetry of lateral and medial condyles in posterior view, and a very deep triangular articular face for the astragalus, suggesting that it represents a new taxon.

Key words: tibia, theropod, Abelisauroidea, Cretaceous, Bauru Basin, Brazil.

Resumen

Palabras claves: tibia, terópodo, Abelisauroidea , Cretácico, Cuenca Bauru, Brasil.

INTRODUCTION

The Bauru Group crops out in several states of Brazil (Paraná, São Paulo, Minas Gerais, Goiás and Mato Grosso do Sul) and also in Paraguay (Fernandes and Coimbra, 1996). The stratigraphy of the Bauru Group has changed over the years (Soares et al., 1980; Souza, 1984, Fernandes

Except for teeth (Bertini et al., 1993; Kellner, 1996; Franco-Rosas, 2002; Candeiro et al., 2004), the record of theropod dinosaurs in the Bauru Basin is considerably scarce, as elsewhere in Brazil (Kellner and Campos, 1999; 2000; Bittencourt and Langer, 2011). The only named taxon is the abelisaurid Pycnonemosaurus nevesi Kellner and Campos 2002. This species is known from an incomplete skeleton collected in Cretaceous deposits of Mato Grosso State that has been tentatively correlated with the Bauru Group (Kellner and Campos, 2002). However, these deposits most likely are a distinct stratigraphic unit, the Quilombinho Formation (Weska, 2006). Some isolated non-avian theropod bones were also reported from the Late Cretaceous Adamantina Formation (Bertini, 1996), but most remains come from outcrops of the overlying Maastrichtian Marilia Formation (Dias-Brito et al., 2001).

The region known as Veadinho Hill ("Serra do Veadinho", see Campos and Kellner, 1999), in the area of Peiropolis (Uberaba, Minas Gerais State), has yielded several isolated specimens. These include one manual ungual (Novas et al., 2005) and one scapula (Machado et al., 2008) assigned to an unidentified maniraptoran theropod, as well as one incomplete femur and one pedal phalanx attributed to Abelisauridae (Novas et al., 2008). To date these specimens indicate the presence of, at least, two distinct theropod taxa in these strata. However, none of the specimens show any autapomorphy or unique combination of characters that would allow the proposal of a new taxon.

Herein we describe another isolated element, a tibia that was collected at the Veadinho Hill locality by Lewellyn Ivor Price and his team in 1969. The specimen is housed at the Museu de Ciencias da Terra of the Departamento Nacional de Producao Mineral (MCT 1783-R; cast at Setor de Paleovertebrados of the Departamento de Geologia e Paleontologia - Museu Nacional/UFRJ - MN 7370-V). It can be referred to the clade Abelisauroidea, and is the most representative theropod dinosaur material thus far known from the Marilia Formation (Maastrichtian; Dias-Brito et al., 2001). It most likely represents a new species. However, due to the incompleteness of this specimen we refrain of giving the taxon a formal designation until more theropod material from this site becomes available.

MCT 1783-R, is herein referred to as the "Cambará theropod" (in allusion to the former name of the area of Peiropolis) and compared with all abelisauroid taxa for which a tibia is known, as follows: Aucasaurus garridoi Coria et al., 2002; Berberosaurus liassicus Allain et al., 2007; Ekrixinatosaurus novasi Calvo et al., 2004; Eoabelisaurus mefi Pol and Rauhut, 2012; Majungasaurus crenatissimus Lavocat, 1955; Masiakasaurus knopfleri Sampson et al., 2001; Pycnonemosaurus nevesi Kellner and Campos, 2002; Quilmesaurus curriei Coria, 2001; Rahiolisaurus gujaratensis Novas et al., 2010; Rajasaurus narmadensis Wilson et al., 2003; Skorpiovenator bustingorryi Canale et al., 2009; Xenotarsosaurus bonapartei Martinez et al., 1986 (Allain et al., 2007; Calvo et al., 2004; Canale et al., 2009; Coria, 2001; Coria et al. ,2002; Kellner and Campos, 2002; Martinez et al., 1986; Novas et al., 2010; Pol and Rauhut, 2012; Sampson et al., 2001; Wilson et al., 2003). Most comparisons are based on the original descriptions, complemented with additional published information for some of the taxa (Carrano et al., 2002; Carrano, 2007; Carrano and Sampson, 2008; Novas et al., 2004; Juarez-Valvieri et al., 2007).

The tibia MCT 1783-R belongs to the right side and is well preserved, showing no apparent distortion (Figure 1a). The cortical bone is fractured in some parts and the tibio-fibular crest is slightly abraded. The maximum length, including the dorsally projected cnemial crest, is 395 mm while the length of the shaft is 380 mm. The shaft is compressed anteroposteriorly, and has an oval transverse section (middle shaft dimensions: 44.7 mm x 31.5 mm; circumference: 123 mm).

The proximal articulation is 55.8 mm wide and 119.5 mm long, including the tip of the cnemial crest. Both proximal condyles project posteriorly and are separated by a well-marked intercondylar groove, followed by an irregular surface, the latter likely for muscle attachment (Figure 1b). The intercondylar groove is more developed than in Majungasaurus crenatissimus, Quilmesaurus curriei, Pycnonemosaurus nevesi, but less conspicuous than in Scorpiovenator bustingorryi and Ekrixinatosaurus novasi. In posterior view, the lateral condyle is larger and square, while the medial one is more triangular, having a straight medial margin. The accentuated asymmetry in shape of the condyles is more marked in this specimen than in other abelisauroids where this portion is preserved. The most noticeable difference between the Cambará theropod and Eoabelisaurus mefi is in the position of the lateral condyle, which is placed more posteriorly in the former, but more laterally in the latter. It differs from Majungasaurus crenatissimus, in that the posterior edge is not angled obliquely relative to the mediolateral axis.

In medial view, the proximal portion shows numerous muscle scars, especially near the cnemial crest. There is a shallow and elongate depression (= lateral fossa) located on the medial surface close to the posterior margin (Figure 1c). The cnemial crest is very well developed, curves laterally and is directed anterodorsally, a typical feature of abelisau-roids (Kellner and Campos, 2002; Carrano, 2007; Carrano and Sampson, 2008). In lateral view, the main axis of the cnemial crest forms an angle of about 115° relative to the shaft, rising well above the proximal articulation surface (Figure 1d). This condition is similar to Majungasaurus crenatissimus and Eoabelisaurus mefi and differs from Masiakasaurus knopfleri and Xenotarsosaurus bonapar-tei where it projects less relative the dorsal margin of the proximal articulation. In proximal view, this structure is strongly curved laterally, more than in other abelisauroids except for Masiakasaurus knopfleri and Ekrixinatosaurus novasi (Figure 2). In medial view, the dorsal margin of the cnemial crest is concave, in contrast with the straight ventral border. At the anterior end of the cnemial crest, there is a longitudinal groove on the lateral surface for the knee extensor tendon. A well-developed lateral fossa is located between the cnemial crest and the shaft.

Despite being abraded, the fibular crest is clearly broad, and begins near the cnemial crest, ending dorsal to the fibular facet. A shallow and triangular depression is present posterior to this crest. Several anteroposteriorly oriented blunt ridges can be delineated on the lateral surface, dorsal to the beginning of the fibular crest, and partially extending over the posterior surface of the shaft. These are interpreted as muscle scars.

The fibular facet is well developed (180 mm) and extends for about half the total length of the shaft, suggesting an almost total lack of mobility between the tibia and fibula. The extent of this facet is similar in Masiakasaurus knopfleri and Xenotarsosaurus bonapartei, but much larger than in Majungasaurus crenatissimus and Rahiolisaurus gujaratensis. Ekrixinatosaurus novasi also has an extended fibular facet that is broader than in MCT 1783-R.

The shaft is curved, with the medial margin concave and the lateral margin convex, differing from the relatively straight condition observed in Pycnonemosaurus nevesi, Quilmesaurus curriei, Ekrixinatosaurus novasi and Rahiolisaurus gujaratensis. As in Masiakasaurus knopfleri, the anterior surface bears a blunt ridge that according to Carrano et al. (2002) acts as a limit for musculature. Between this ridge and the fibular facet is a shallow elongate groove not reported in other abelisauroids.

The distal portion of the shaft is expanded mediolaterally (88.3 mm x 17 mm), and forming in anterior view, two asymmetric malleoli. The lateral (fibular) malleolus is broader and projects farther distally than the medial one. It differs from Majungasaurus crenatissimus, Pycnonemosaurus nevesi, Quilmesaurus curriei and Rajasaurus narmadensis in that the fibular malleolus projects less distally, and in having a more rounded end.

The anterior articular facet for the astragalus is triangular and deeper than in other abelisauroids. Based on the articulation facet and the muscle scars above it, the astragalar ascending process was apparently of moderate height. The distal end of the tibia is complete, and neither the astragalus nor the calcaneum are preserved. This suggests that those elements were not fused in this specimen (Figure 3).

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DISCUSSION

The Cambará theropod shows a well-developed cne-mial crest with a lobular distal end which projects considerably above the proximal articular surface. These features are characteristic of the Abelisauroidea (Rauhut, 2003; Carrano et al., 2002; Novas et al., 2004), and allow the assignment of the specimen to this clade.

Among abelisauroids there are about twenty-nine taxa, most based on incomplete material (Carrano and Sampson, 2008; Canale et al., 2009; Ezcurra et al., 2010; Novas et al., 2010). Only twelve of these have the tibia, at least, partially preserved, allowing for comparison with the Cambará theropod (Figure 4). The least complete is the tibia in the holotype of Berberosaurus lias-sicus. Based on published information, this african species comes from Early Jurassic deposits (Allain et al., 2007), and apparently has a cnemial crest smaller than in the Cambará theropod.

Among other african abelisauroids, the Cambará specimen differs from Majungasaurus crenatissimus in lacking the marked angle of the posterior end of the tibia relative to the main axis, which can be better observed in proximal view (Carrano, 2007). Furthermore, the brazilian species has a fibular malleolus that projects less distally, with a more rounded end, and a much larger fibular facet.

Similarities between the Cambará theropod and Masiakasaurus knopfleri are noticeable, and include the curvature of the shaft and the extension of the lateral fossa. Among the main differences is the less developed and projected cnemial crest in Masiakasaurus knopfleri, which also has the tarsal elements fused to the tibia (Carrano et al. , 2002). Furthermore, the Cambará specimen is almost two times larger.

There are two abelisauroids from India with the tibia preserved. Rahiolisaurus gujaratensis differs from the Cambará theropod in having a more elongate cnemial crest, an almost straight shaft in anterior view, and a smaller fibular facet. Rajasaurus narmadensis only preserves the distal portion of the tibia, which differs from the brazilian species in having the lateral malleolus more projected distally.

The Cambará specimen differs from Aucasaurus garridoi in having the distal end of the cnemial crest blunt and rounded, and in lacking the lateral process present in the latter. The argentinean species further has the tip of the distal margin of the cnemial crest ventrally projected, giving it a 'hatchet' shape. In Aucasaurus garridoi the astragalus and calcaneum are fused to the tibia (Coria et al., 2002), which is not the case in the brazilian specimen. Lastly, in Eoabelisaurus mefi the condyles differ not only in shape, but in position compared to the brazilian specimen. The Jurassic specimen is not only from a larger and more robust animal, but it also differs by having a fused distal end of the tibia.

Compared to Ekrixinatosaurus novasi, the Cambará specimen possesses a less developed intercondylar groove and a comparatively narrower fibular facet. The fibular facet in Ekrixinatosaurus novasi is also straight in anterior view, and the tarsal elements are fused to the tibia (Calvo et al., 2004). Quilmesaurus curriei, a species based on the distal articulation of a femur and a complete tibia (Coria, 2001), has a straight shaft and a slightly expanded distal end of the cnemial crest, features not present in the Cambará theropod. The argentinean species also has a more projected fibular malleolus, a less developed intercondylar groove, and belongs to a much larger animal.

Size is also a difference between the new brazilian specimen and Xenotarsosaurus bonapartei and Scorpiovenator bustingorryi; both are much larger. Xenotarsosaurus further has a cnemial crest more triangular and less projected, and a straighter shaft (Martinez et al., 1986). Scorpiovenator bustingorryi displays a wider intercondylar groove between the proximal condyles (Canale et al., 2009). Both also have the tarsal elements fused with the tibia.

The tibia in the only named abelisaurid from Brazil, Pycnonemosaurus nevesi, is more robust, has a straight shaft and is about double the size of the Cambará specimen. Another difference is the hatchet-shaped condition of the cnemial crest, diagnostic of Pycnonemosaurus nevesi. Furthermore, the lateral malleolus is more lobular in MCT 1783-R, while in Pycnonemosaurus nevesi this structure is more elongate, projecting laterally and downward (Kellner and Campos, 2002). Based on those differences, and on the fact that the new material comes from a different stratigraphic unit, it is unlikely that the Cambará theropod represents a juvenile or immature individual of Pycnonemosaurus nevesi.

Besides those differences, there are two potentially unique features in the Cambará theropod: 1) the condyles at the posterior margin of the proximal articulation are strongly asymmetric, with the lateral condyle square and the medial condyle triangular, and 2) the articular facet for the astragalus on the anterior surface is triangular and very deep. There is also a shallow elongate groove on the anterior surface, lateral to the fibular facet, which is not present in Pycnonemosaurus nevesi and has not been reported in any other abelisauroid. This may also be a unique feature of the Cambará theropod.

After five decades of excavation in the Peiropolis area (Campos and Kellner, 1999), only a handful of theropod specimens have been recovered. Two were assigned to maniraptoran theropods, one ungual (Novas et al., 2005), and one scapula (Machado et al., 2008). Three others were regarded as abelisauroids, and due to their large size (from individuals over 5 m in estimated body length) tentatively attributed to the Abelisauridae (Novas et al., 2008). Of those, two came from the same deposit as the tibia described here: a pedal phalanx and the distal end of a femur. According to Novas et al. (2008), the distal articulation surface of the femur has a transverse section of 102 mm, which is almost double the transverse dimension of the proximal articulation of the tibia described here (MCT 1783-R; 55.8 mm). This clearly shows that the Cambará theropod tibia is from a much smaller individual.

Although difficult to determine accurately, MCT 1783-R is a little larger than half the size ofMasiakasaurus knopfleri (see Carrano et al., 2002, table 1) and about half the size of Pycnonemosaurus nevesi, suggesting that the Cambará theropod represents an individual with a body length of around 3.5 meters.

Although relationships among abelisauroids remain to be resolved, several authors recognize at least two distinct clades: the Abelisauridae and the Noasauridae, (Novas et al., 2004; Carrano et al., 2002; Carrano and Sampson, 2008). Among the main differences between them is size, with noasaurids and abelisaurids represented by comparatively small (less than 2.5 meters) and large (over 5 meters) species, respectively. Unfortunately, there is no synapomorphic feature present in the tibia that would allow a confident assignment of the Cambará specimen to one or the other abelisauroid clade. At the present level of knowledge, this species falls between the typical sizes of Noasauridae and Abelisauridae. It could potentially either be the largest known noasaurid or the smallest known abelisaurid. In any case, the occurrence of the Cambará theropod clearly indicates the presence of at least three distinct theropods at the Veadinho Hill site: one maniraptoran (Machado et al, 2008; Novas et al., 2008) and two abelisauroid taxa of distinctly different sizes.

An isolated abelisauroid tibia (MCT 1783-R) from the Veadinho Hill locality in Peiropolis, Minas Gerais, was recovered from the Maastrichtian Marilia Formation. After more than four decades of excavation there (Campos and Kellner, 1999), this is the first theropod specimen recovered that shows diagnostic features and a unique combination of characters, which along with its stratigraphic position and geographical location, suggest that it is a new species (Kellner, 2010). Nonetheless, due to the incompleteness of this material we refrain from erecting a new species until more theropod specimens from this unit may become available.

Based on the morphological features of the cne-mial crest, the Cambará specimen represents a member of Abelisauroidea. This group has been subdivided into two clades with different typical body lengths, the smaller Noasauridae (less than 2.5 m) and larger Abelisauridae (more than 5 m). The estimated body length for the Cambará theropod falls between those typical sizes, and it could represent either a large noasaurid or a small abelisaurid. The new specimen demonstrates the presence of two abelisauroid species in the Peiropolis region of Brazil, and indicates that theropods were more diverse here during Late Cretaceous time than previously recognized.

ACKNOWLEDGMENTS

We thank Juan Ignacio Canale (CONICET, Consejo Nacional de Investigaciones Científicas y Técnicas) and Juan Porfiri (Centro Paleontológico Lago Barreales (CePaLB), Universidad Nacional del Comahue) for send-ing unpublished pictures of Scorpiovenator bustingorryi and Ekrixinatosaurus novasi, respectively. Fernando Novas (CONICET, Museo Argentino de Ciencias Naturales) is thanked for sending us relevant bibliographic information. We are grateful to the reviewers Ariana Paulina-Carabajal (Museo Carmen Funes), Matthew Carrano (Smithsonian Institution) and the editor Thomas M. Lehman (Texas Tech University) for their helpful comments. This study was supported by the Fundacao Carlos Chagas Filho de Amparo á Pesquisa do Estado do Rio de Janeiro (FAPERJ no. E-26/102.779/2008 and E-26/102.737/2012) and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq no. 307276/ 2009-0). EBM acknowledges the CNPq for supporting her Ph.D. studies.

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