A new abelisauroid theropod from northwestern Patagonia

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A new abelisauroid theropod from northwestern Patagonia Rodolfo A. Coria, Philip J. Currie, and Ariana Paulina Carabajal

Abstract: The Argentinean record of abelisauroid theropods begins in the Early Cretaceous (Ligabueino) and spans most of the Late Cretaceous, from Cenomanian (Ilokelesia, Xenotarsosaurus, and Ekrixinatosaurus) to Campanian– Maastrichtian (Abelisaurus, Carnotaurus, Aucasaurus, and Noasaurus). A fragmentary specimen of a theropod dinosaur was collected in 2000 from the middle section of the Lisandro Formation (Turonian?) at Cerro Bayo Mesa, Neuquén Province, Argentina. The fossil-bearing level, which is part of the Lisandro Formation that also yielded the remains of the basal ornithopod Anabisetia saldiviai, corresponds to a reddish, massive mudstone linked with fluvial channel deposits. The theropod identified as MCF-PVPH-237 is an abelisauroid theropod that increases our knowledge about the evolution of South American Abelisauroidea and is the first record of this clade from the Lisandro Formation. Résumé : Le registre argentin des théropodes abélisauroïdes commence au Crétacé précoce (Ligabueino) et s’étend sur presque tout le Crétacé tardif, du Cénomanien (Ilokelesia, Xenotarsosaurus et Ekrixinatosaurus) au CampanienMaastrichtien (Abelisaurus, Carnotaurus, Aucasaurus et Noasaurus). Un spécimen fragmentaire d’un dinosaure théropode a été prélevé en 2000 de la section médiane de la Formation de Lisandro (Turonien?) à Cerro Bayo Mesa, dans la province de Neuquén (Argentine). Le niveau fossilifère, qui fait partie de la Formation de Lisandro dans laquelle des restes de l’ornithopode basal Anabisetia saldiviai ont également été retrouvés, correspond à une argilite massive rougeâtre associée à des dépôts de chenaux fluviaux. Le théropode, un abélisauroïde désigné MCF-PVPH-237, vient accroître les connaissances concernant l’évolution des Abélisauroïdés sud-américains et constitue le premier cas connu de ce clade dans la Formation de Lisandro. [Traduit par la Rédaction]

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Introduction Abelisauroid theropods are currently considered as typical representatives of the Gondwanan meat-eating dinosaur fauna. The group was first recognized in the late 20th Century by Bonaparte and Novas (1985) with the description of Abelisaurus comahuensis. Abelisauroid remains were subsequently collected or recognized from many localities in South America (Bonaparte 1996; Coria and Salgado 1998; Calvo et al. 2004), Madagascar (Sampson et al. 1998) and India (Huene and Matley 1933; Wilson et al. 2003), Africa (Sereno et al. 2004), and probably southern Europe (Buffetaut et al. 1988; Accarie et al. 1995). At present, the highest diversity of abelisauroids corresponds to species described from Argentinean Patagonia. Here, a fragmentary specimen of a theropod dinosaur with undoubted abelisauroid affinities is described. It was collected from the middle section of the Lisandro Formation (Turonian?) at Cerro Bayo Mesa (Fig. 1). This is the first theropod record Received 3 October 2005. Accepted 7 March 2006. Published on the NRC Research Press Web site at http://cjes.nrc.ca on 15 November 2006. Paper handled by Associate Editor H.-D. Sues. R.A. Coria,1 and A. Paulina Carabajal. CONICET — Museo Municipal “Carmen Funes”, Córdoba 55 (8318) Plaza Huincul, Neuquén, Argentina. P.J. Currie. University of Alberta, Department of Biological Sciences, Edmonton, AB T6G 2E9, Canada. 1

Corresponding author (e-mail: [email protected]).

Can. J. Earth Sci. 43: 1283–1289 (2006)

for this formation and adds to our knowledge about abelisauroid evolution before their Late Cretaceous diversification. Institutional abbreviations MCF-PVPH, Museo Carmen Funes, Vertebrate Paleontology, Plaza Huincul, Neuquén, Argentina.

Systematic palaeontology Theropoda Marsh, 1881 Ceratosauria Marsh, 1884 Abelisauroidea Bonaparte, 1991 SPECIMEN: MCF-PVPH-237, partial skeleton of a small-sized abelisauroid theropod that includes the last presacral vertebrae, a partial sacrum, an incomplete right ilium, fragments of the left ilium, pubes lacking distal ends, the proximal region of the right ischia, and other fragmentary remains. LOCALITY AND HORIZON:

Cerro Bayo Mesa, 30 km south of Plaza Huincul, Neuquén Province, Argentina. Lisandro Formation (Turonian?), Río Limay Subgroup, Neuquén Group, Neuquén Basin (Ramos 1981; Garrido 2000) (Fig. 1). The fossil-bearing horizon corresponds to a massive, reddish, silty mudstone that is interpreted as muddy channel deposits. The facies are linked to the deepest area of a point-bar system. The sequence of sediments has been identified as floodplain and fluvial channel deposits (Garrido 2000). Slightly below to these levels, the remains of the ornithopod Anabisetia saldiviai (Coria and Calvo 2002) were found (Fig. 1B).

doi:10.1139/E06-025

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Fig. 1. (A). Locality map. (B) Stratigraphical section of the site indicating the provenance of MCF-PVPH-237 and the specimens of Anabisetia saldiviai.

Description The specimen MCF-PVPH-237 is probably an adult because of the fusion of the posterior dorsal neural arch to its centrum, and of the sacral vertebrae to the ilium. The estimated ilium length suggests the remains correspond to a theropod dinosaur approximately 4 m long (Table 1). The total body length was estimated by comparing the ilia of the new animal, Carnotaurus, with Aucasaurus to calculate the femur length. The estimated femur length was then compared with total body length of theropods in general. The dorsal vertebra was found close to the anterior end of the sacrum (Fig. 2, Table 1). It is almost complete, lacking only the neural spine and the distal end of the right transverse process. The parapophysis is positioned high, almost at the level of the diapophysis (Fig. 2A). The anterior centroparapophyseal lamina is thin and curves gently ventrally at its distal end (Fig. 2A). These features indicate that the specimen is a posterior dorsal vertebra, probably the 12th dorsal, which is the last one of the presacral series in Carnotaurus (Bonaparte et al. 1990). The bone is proportionally tall (see Table1), and the dorsoventral axis of the complete vertebra would have been almost three times the length of the anteroposterior one. The centrum is anteroposteriorly short when compared with its dorsoventral height (Fig. 2B). This is different from other abelisaurids, where the centrum is longer than it is high (i.e., Ilokelesia, MCF-PVPH-35, Fig. 3A) or as long as it is high (i.e., Carnotaurus, MACN-CH-894, Fig. 3C). In cranial view, the prezygapophyses extent into a deep hypantrum, as in most theropods (Fig. 2A). The prezygapophyseal articular

surfaces are oriented craniodorsally, and their cranial sides have distinct pendant cranioventral processes (Coria and Salgado 1998; Paulina Carabajal et al. 2003). This feature may be synapomorphic for Abelisauroidea (sensu Coria and Salgado 1998) because the same character has been described for Ilokelesia (Fig. 3B) and Carnotaurus (Fig. 3D) (Bonaparte et al. 1990; Coria and Salgado 1998). The prezygodiapophyseal lamina is well developed and defines a gentle curve from the prezygapophysis to the distal end of the diapophysis, as in Carnotaurus and Ilokelesia (Fig. 2A). The anterior centroparapophyseal and prezygodiapophyseal laminae both extend laterally, as in all abelisaurid theropods whose dorsal vertebrae are known, including Carnotaurus and Ilokelesia (Figs. 2A, 2C). In lateral view, the posterior centrodiapophyseal and anterior centroparapophyseal laminae enclose a deep fossa (infradiapophyseal fossa) that communicates with pneumatic cavities, as in most theropods (Fig. 2B). The parapophysis has a semicircular articular facet for the rib. As in the posterior dorsal vertebrae of the holotype Ilokelesia aguadagrandensis (MCF-PVPH-65, Fig. 3A), there are no pleurocoels on the lateral sides of the centrum. In contrast, other abelisauroids, like Xenotarsosaurus bonapartei (Martínez et al., 1986), Ekrixinatosaurus novasi (Calvo et al., 2004), and Carnotaurus sastrei (Bonaparte et al., 1990), have shallow pneumatic depressions that are pierced by small openings (Fig. 3C). These openings are variable in Carnotaurus, in which the 10th dorsal centrum has two openings on one side and none on the other. It should be added that in Xenotarsosaurus, the posterior dorsal centra have not been recovered, and may © 2006 NRC Canada

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1285 Table 1. Measurements of MCF-PVPH 237 (in mm). (A) Dorsal vertebra Centrum length Centrum height (up to pneumatic depression) Centrum width (anterior articular facet) Centrum width (mid-length) Vertebra heigth

63 28 78 40 180

(B) Sacral vertebrae Position in sequence (as preserved)

Centrum length

1 2 3

91 110 62

(C) Ilium (right) Dorsal border length (as preserved) Heigth above acetabulum Dorsal ridge width Acetabulum (maximum transversal width)

526 168 15 80

(D) Pubis (right) Length (as preserved) Anteroposterior width shaft Below obturator foramen Mid-length of shaft

Centrum heigth (intervertebral contact) 75 75 —

330 43 50

Fig. 2. MCF-PVPH-237. Dorsal vertebra in (A) anterior, (B) lateral and (C) posterior views. c, centrum; dp, diapophysis; hi, hipantrum; hy, hyposphene; nc, neural canal; pp, parapophysis; poz, postzigapophysis; prz, prezygapophysis. Scale bar = 10 cm.

lack pleurocoels as in MCF-PVPH-237. The centrum of MCFPVPH-237 is constricted at mid-length and expands broadly at the intervertebral articular facets (Fig. 2B). In posterior view, only the left postzygapophysis is preserved, and it faces ventrally (Fig. 2C). The hyposphene is relatively deep and extensive compared with those of Carnotaurus and Ilokelesia. In lateral view, the postzygapophysis projects pos-

teriorly beyond the level of the posterior articular surface of the centrum. The first three sacral vertebrae are partially preserved (Fig. 4A). The centra are strongly compressed laterally, as in Carnotaurus and Aucasaurus (Bonaparte et al. 1990; MCFPVPH-236, Coria et al. 2002). The preserved parts of the neural arches are solidly fused to the ilium. Despite the fact © 2006 NRC Canada

1286 Fig. 3. Comparative drawings of posterior dorsal vertebrae of abelisauroid theropods in lateral and anterior views. (A, B) Ilokelesia, (C, D) Carnotaurus and (E, F) MCF-PVPH-237. (Figs. A, B, C, and D modified from Coria and Salgado (1998). Not to scale.

that these elements have been very damaged by compression, shallow pleurocoelic depressions can be observed on the centra sides. The first sacral has partially preserved the transverse process, which is not in contact with the internal side of the iliac blade. By comparison, the second sacral (technically the first real sacral vertebra) shows clearly the fusion between the sacral rib and the transverse process with the ilium. The postzygapophyseal–prezygapophyseal articulation


between sacrals 1 and 2 is clearly distinguishable, suggesting that they were not fused. Sacral 3 is badly damaged precluding any further description of the vertebra or of the attachments of the neural arches to the ilium. The ilia had been almost completely washed away when the specimen was found. The right ilium is the least damaged and has preserved enough information to allow us to estimate its size. Only a small fragment of the preacetabular blade of the left ilium is preserved. The proximal end retains the morphology found in both Carnotaurus and Aucasaurus, having a convex dorsal edge and a rather square profile. The proximal half of the acetabulum is preserved. The supraacetabular crest is very sharp and the acetabulum itself seems to be anteroposteriorly long and transversely narrow. On the lateral side of the preserved right ilia blade, a conspicuous ridge, can be seen parallel to the dorsal edge and placed approximately 15 mm away from it (Table 1). This ridge would have run all along the dorsal border of the ilium. The same feature is present in the fragmented left iliac blade. The ilium of MCF-PVPH-237 (Fig. 4B) seems to have been rather elongate and low, like those of Carnotaurus, Majungatholus, and Aucasaurus (Bonaparte et al. 1990; Sampson et al. 1998; Coria et al. 2002). The thickened anterior, dorsal, and posterior margins of the iliac blade define a shallowly depressed lateral surface. Both pubes are almost complete and lack only the distal boots. The proximal end is firmly fused to the ilium, with no evident sutural contact. The same condition is observed with the pubis–ischium contact. The obturator foramen is completely surrounded by bone, as in Carnotaurus (Fig. 5A) (Bonaparte et al., 1990), Masiakasaurus (Carrano et al., 2002), and Aucasaurus (MCF-PVPH-236). The pubes are coossified along most of the pubic symphysis. Each of the pubes projects cranioventrally, apparently at a sharper angle (50°) with respect to the longitudinal axis of the iliac blade than in Carnotaurus (65°) (Bonaparte et al. 1990). In lateral view, the distal third of each pubis curves ventrally (Fig. 5B), in contrast with the straight pubes of Carnotaurus (Fig. 5A). Together they both form a narrow, blade-like bar that is distinctively compressed craniocaudally. In cross section (Fig. 4C), the fused pubes are crescentic, with the concavity facing backwards. The lateral edges of the pubes are thick, whereas the adjacent medial regions, corresponding to the apron and the pubis symphysis, are clearly laminar (Fig. 4C). Only a proximal part of both fused ischia is preserved, but their contacts with the ilium are missing. A small fragment of the pubic process is preserved as a thin lamina of bone (Fig. 4). The small portions of the ischiatic shafts are solidly fused to each other, suggesting that they were rod-like, as in in other abelisauroids.

Discussion MCF-PVPH-237 shares with Abelisauroidea the presence of cranial processes on the prezygapophyses and thickened margins of the iliac blade. Examination of this and other abelisauroid specimens discovered in recent years demonstrates that the abelisauroid pelvis possesses many derived features. The presence of coossified pelvic bones is a remarkable feature that links MCF-PVPH-237 with abelisauroids © 2006 NRC Canada

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Fig. 4. MCF-PVPH-237. Pelvis in (A) medial and (B) lateral views. (C) cross-sections of reconstructed pubes at two levels of pubic shaft. a, acetabulum; il, ilium; is, ischium; of, obturator foramen; op, obturator process; p, pubis; s1, s2 and s3, sacral vertebrae 1, 2, and 3; sy, pubic symphysis; 1 and 2, cross-sections of pubis shaft. Scale bar = 10 cm.

Fig. 5. Comparative drawings of pelvic girdle in lateral view of (A) Carnotaurus and (B) MCF-PVPH-237. (Fig. 5a from Bonaparte et al. 1990). Not to scale.

like Carnotaurus, Aucasaurus, and probably Genusaurus. However, this character was proposed as a synapomorphy for all Carnotaurini (Carnotaurus, Aucasaurus, and all descendants of their common ancestor, Coria et al. 2002). Therefore, the distribution of this feature among other abelisaurids has yet to be analyzed. The presence of a relatively low and long iliac blade seems to be a feature shared by MCFPVPH-237, Aucasaurus, Carnotaurus, Genusaurus, and Majungatholus. Consequently, although present in all abelisaurids with known ilia, this character could be more widely distributed among Abelisauroidea so as to include Ilokelesia, Noasauridae, Abelisauridae, and all descendants of their common ancestor (see Coria and Salgado 1998 for diagnostic features of these clades). Compared with the abelisaurids for which ilia are known (Aucasaurus, Carnotaurus, Genusaurus, and Majungatholus),

the thickened margins of the iliac blade may represent a unique abelisaurid character. Only Majungatholus seems to lack this character (Sampson et al. 1998, fig. 2f). The morphology described for the pubis of MCF-PVPH-237 seems unusual among abelisauroids. Other abelisauroids, like most theropods, have massive and rod-like pubic shafts in cross section. In the new specimen from Cerro Bayo, the slenderness of the pubis is noteworthy. The pubic shafts are thin laminae that define a trough-like structure, open posteriorly, with a crescentic cross section. (Fig. 4A). The South American abelisauroid forms were collected from different Cretaceous units, from Hauterivian to Campanian–Maastrichtian (Fig. 6). Current knowledge about abelisauroid evolution indicates that the oldest representative of this lineage would be Ligabueino andesi (Bonaparte 1996) from the La Amarga Formation, although the abelisauroid © 2006 NRC Canada

1288 Fig. 6. Stratigraphically calibrated cladogram of abelisauroid theropods from South America. Node 1: Abelisauroidea; Node 2: Noasauridae; Node 3: Abelisauridae; Node 4: Carnotaurini.

affinities of this taxon are controversial. The next known step of abelisauroid evolution in South America is from the Patagonian Cenomanian Candeleros, Huincul, and Bajo Barreal formations, with the records of Ekrixinatosaurus novasi (Calvo et al., 2004), Ilokelesia aguadagrandensis (Coria and Salgado, 1998), and Xenotarsosaurus bonapartei (Martínez et al., 1986), respectively. From the mid-Cretaceous Turonian Portezuelo Formation only a scattered, fragmentary specimen with abelisauroidian affinities has been recovered (MCF-PVPH380). Finally, a handful of Late Cretaceous formations have provided the latest South American manifestations of the Abelisauroidea: Carnotaurus sastrei from the La Colonia Formation (Bonaparte 1985), Abelisaurus comahuensis and Aucasaurus garridoi from the Anacleto Formation (Bonaparte and Novas 1985; Coria et al. 2002), and Noasaurus leali from the El Lecho Formation (Bonaparte and Powell 1980). MCF-PVPH-237 is a small abelisauroid compared with Aucasaurus, Carnotaurus, Ilokelesia, and Majungatholus. Its size compares better with that of Genusaurus (Accarie et al., 1995), which has an estimated ilium length of 30 cm. In contrast, noasaurids (Masiakasaurus, Noasaurus) represent significantly smaller animals. The discovery of MCF-PVPH-237 is not surprising because abelisauroids have been reported from units below and above the Lisandro Formation (Fig. 6). Definitive South American,


African, Malagasy, Indian, and European abelisauroid species include the Hauterivian Ligabueino andesi, the Albian Genusaurus sisteronis, the Cenomanian Ekrixinatosaurus novasi, Ilokelesia aguadagrandensis, Rugops primus and Xenotarsosaurus bonapartei, the Campanian Abelisaurus comahuensis and Aucasaurus garridoi, the Campanian–Maastrichtian Carnotaurus sastrei and Noasaurus leali, and the Maastrichtian Majungatholus atopus, Masiakasaurus knopfleri, and Rajasaurus narmadensis (Bonaparte and Powell 1980; Bonaparte 1985; Bonaparte and Novas 1985; Martínez et al. 1986; Accarie et al. 1995; Bonaparte 1996; Coria and Salgado 1998; Sampson et al. 1998; Sampson et al. 2001; Coria et al. 2002; Wilson et al. 2003; Calvo et al. 2004; Sereno et al. 2004). Additional, putative abelisauroid material is known from India, southern France, Brazil, and other nations (Huene and Matley 1933; Chatterjee 1978; Buffetaut et al. 1988; Kellner and Campos 2002). Although the presence of abelisauroid theropods in these countries is not disputed, the assignment of particular specimens to the Abelisauroidea needs to be better supported. Pycnonemosaurus nevesi Kellner and Campos (2002) from Brazil, for example, is a species whose identification is based on disarticulated elements of uncertain association. Only a single element, an isolated fragment of a caudal transverse process, suggests abelisauroid affinities. Other, more complete vertebral elements are either plesiomorphic for all theropods or lack diagnostic abelisauroid synapomorphies. Also, although the tibia assigned to Pycnonemosaurus, bears an abelisauroid-like cnemial crest, it has a shape more reminiscent of a basal tetanuran as it is proportionately stout and massive, has a contact area that suggests the ascending process of the astragalus was triangular, and displays a lack of fusion between the tibia and astragalus. In contrast, the abelisauroids Aucasaurus and Ekrixinatosaurus have slender tibia, tongue-shaped ascending processes of the astragalus, and fused tibiotarsi. For the same reasons, we consider that the assignment of the Late Cretaceous theropod Quilmesaurus curriei Coria to Abelisauroidea (sensu Kellner and Campos 2002) does not recognize the unusual features of this form. Indeed, there are several characters that support not only the taxonomic validity of Quilmesaurus but also differentiate it from the abelisauroids (Coria 2001; Coria and Salgado 2005; contra Juárez Valieri et al. 2004). MCF-PVPH-237 fills in a significant gap in abelisauroid evolutionary history. It precedes the explosive diversification of abelisauroids after the suspected extinction of carcharodontosaurs proposed by Coria and Salgado (2005), who point out the Turonian as a time when several faunal changes took place worldwide. In South America, the dinosaur record currently suggests that these changes affected at least some saurischian dinosaurs. Diplodocoid sauropods and carcharodontosaurid theropods, abundant in pre-Turonian Patagonia, seem to vanish after this age. As a consequence, titanosaurian sauropods and abelisaurid theropods demostrate increasing diversity throughout the Late Cretaceous. MCF-PVPH-237 documents a preview of post-Turonian abelisauroid success.

Acknowledgments The specimen was found by A.C. Garrido and was collected by A.L. Garrido, C. Giménez, and M. Vickaryous. Figures 2 © 2006 NRC Canada

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and 3 were drawn by Alfredo Geréz. The authors thank the Royal Tyrrell Museum of Palaeontology (Drumheller, Alberta, Canada) and the Museo Carmen Funes (Plaza Huincul, Neuquén, Argentina) for their institutional support in the field and laboratory. The manuscript was greatly improved through the review process, and the authors are grateful to Drs. M. Carrano and M. Lamanna for their advice.

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