a definitive abelisaurid theropod dinosaur from the early late

Journal of Vertebrate Paleontology 22(1):58–69, March 2002 q 2002 by the Society of Vertebrate Paleontology

A DEFINITIVE ABELISAURID THEROPOD DINOSAUR FROM THE EARLY LATE CRETACEOUS OF PATAGONIA ´ N D. MARTIŃEZ2, and JOSHUA B. SMITH1 MATTHEW C. LAMANNA1, RUBE Department of Earth and Environmental Science, University of Pennsylvania, 240 South 33rd Street, Philadelphia, Pennsylvania 19104-6316, USA, [email protected]; 2 Laboratorio de Paleovertebrados, Universidad Nacional de la Patagonia ‘‘San Juan Bosco’’, C. C. 360 (9000), Comodoro Rivadavia, Argentina 1

ABSTRACT—A nearly complete, well-preserved maxilla of an abelisaurid theropod from the early Late Cretaceous (middle Cenomanian-Turonian) Lower Member of the Bajo Barreal Formation of Chubut, Argentina represents the first definitive member of the abelisaurid clade from pre-Senonian (Coniacian–Maastrichtian) deposits. The new maxilla shares derived characters with the maxillae of Carnotaurus and Majungatholus, and with AMNH 1955, a maxilla previously referred to Indosuchus, suggesting that it pertains to the abelisaurid subclade Carnotaurinae. Abelisaurus shares apomorphic characters with Carnotaurinae, but many of these characters are also found in the carcharodontosaurid allosauroid Giganotosaurus. As it is known only from cranial material lacking carnotaurine synapomorphies, Abelisaurus may represent a late-surviving carcharodontosaurid derivative. The presence of the Bajo Barreal predator in the early Late Cretaceous indicates that the origin of Abelisauridae had occurred by then. The occurrence of the new maxilla is nearly concurrent with the accepted interval of tectonic divergence between South America and Africa. Its discovery thus weakens support for the recent hypothesis that the abelisaurid clade could not have penetrated Africa. The known occurrence of Abelisauridae may reflect a former panGondwanan distribution, and is thus of limited utility in the support of Late Cretaceous paleogeographic hypotheses. RESUMEN.—Un maxilar casi completo y bien preservado de un tero´podo abelisaúrido del Cretaćico Tardıó temprano (Cenomaniano medio–Turoniano), proveniente del Miembro Inferior de la Formacioń Bajo Barreal de Chubut, Argentina representa el primer miembro definitivo del clado abelisaúrido de depo´sitos pre-Senonianos (Coniaciano–Maastrichtiano). El nuevo maxilar comparte caracteres derivados con los maxilares de Carnotaurus y Majungatholus, y con AMNH 1955, un maxilar referido previamente a Indosuchus, sugiriendo que e´ste pertenece al subclado abelisaúrido Carnotaurinae. Abelisaurus comparte caracteres apomo´rficos con Carnotaurinae, pero muchos de estos caracteres se encuentran tambień en el carcharodontosaúrido allosauroide Giganotosaurus. Como es conocido solo por material craneano, careciendo de sinapomorfıás carnotaurinas, Abelisaurus puede representar un sobreviviente tardıó de un carcharodontosaúrido derivado. La presencia del predador de Bajo Barreal en el Cretaćico Tardıó temprano indica que el origen de Abelisauridae ha ocurrido en ese momento. La aparicioń del nuevo maxilar es casi concurrente con el intervalo aceptado de diver´ frica. De este modo este descubrimiento debilita el sosteń de la reciente hipo´tesis gencia tectońica entre Sudame´rica y A ´ frica. La conocida presencia de Abelisauridae puede reflejar una de que el clado abelisaúrido no habrıá penetrado A antigua distribucioń pan-Gondwańica, siendo ası´ de limitada utilidad en el sosteń de las hipo´tesis paleogeogra´ficas para el Cretaćico Tardıó.

Sereno et al., 1994, 1996; Sampson et al., 1998). However, as fundamental questions remain concerning theropod evolution on southern continents, these hypotheses are problematic. One crucial limiting factor has been poor knowledge of the time intervals in which Gondwanan theropod clades appeared. The abelisaurids are arguably the most distinctive of the Southern Hemisphere predatory dinosaurs (e.g., Bonaparte, 1986, 1991b). Sereno (1998) and Padian et al. (1999) define Abelisauridae as all descendants of the most recent common ancestor of Abelisaurus and Carnotaurus. The existence of the clade in several Gondwanan landmasses is well-documented, with the identification of Abelisaurus comahuensis (Bonaparte and Novas, 1985), Carnotaurus sastrei (Bonaparte, 1985), and a new genus (Coria et al., 2000) from Argentina, Indosuchus raptorius and Indosaurus matleyi (Huene and Matley, 1933) from India, and Majungatholus atopus (Sues and Taquet, 1979; Sampson et al., 1998) from Madagascar. Putative abelisaurids have been reported from the Late Cretaceous of Europe (Buffetaut et al., 1988; Buffetaut, 1989; Astibia et al., 1990; LeLoeuff and Buffetaut, 1991) and the early Late Cretaceous of South America (Martıńez et al., 1986) and Africa (Russell, 1996), but these records have been strongly challenged (Coria

INTRODUCTION The past two decades have produced well-preserved specimens of several new or previously poorly known Cretaceous predatory dinosaurs (theropods) from areas that comprised the southern supercontinent of Gondwana. Exceptional finds have been made on many now disparate Gondwanan landmasses, including South America (Bonaparte, 1985, 1991a, 1996; Bonaparte and Novas, 1985; Martıńez et al., 1986; Bonaparte et al., 1990; Coria and Salgado, 1995; Novas, 1997a, 1998; Novas and Puerta, 1997; Coria and Currie, 1997; Martıńez and Novas, 1997; Coria and Salgado, 1998; Martıńez et al., 1999; Coria et al., 2000), Africa (Sereno et al., 1994, 1996, 1998; Russell, 1996; Taquet and Russell, 1998; De Klerk et al., 2000; Lamanna et al., 2000), India (Chatterjee and Rudra, 1996), and Madagascar (Sampson et al., 1998, 2000). These finds have greatly added to knowledge of the evolution of Southern Hemisphere theropods, and have led to the recognition of several previously unknown predatory clades. The spatial and temporal distribution of these clades has been used to support biogeographic hypotheses or to test various tectonic schemes pertaining to Cretaceous Gondwanan fragmentation (e.g., Bonaparte, 1986;

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LAMANNA ET AL.—EARLY ABELISAURID and Rodriguez, 1993; Coria and Salgado, 1998; Sampson et al., 1998). Definitive abelisaurids are known only from the Senonian (Coniacian–Maastrichtian) of South America, India, and Madagascar (Ardolino and Delpino, 1987; Sampson et al., 1998; Forster, 1999). The biogeographic and temporal distribution of the clade has recently been cited as evidence in support of Late Cretaceous terrestrial vertebrate dispersal between South America and India-Madagascar (Sampson et al., 1998). Contrary to most current paleogeographic scenarios (e.g., Scotese and Golonka, 1993; Smith et al., 1995; Roeser et al., 1996; Scotese, 1997), but consistent with the work of Hay et al. (1999), this hypothesis assumes ‘‘land bridges’’ between these areas persisting well into the Late Cretaceous. Recent hypotheses of abelisaurid biogeography by Sampson et al. (1998) also presume that abelisaurid origins occurred subsequent to the tectonic separation of South America and Africa, preventing the clade from dispersing to the latter. Martıńez et al. (1993) briefly reported an abelisaurid maxilla from the pre-Senonian Lower Member of the Bajo Barreal Formation of central Patagonia. The early occurrence of the specimen strengthens questions concerning the aforementioned hypotheses and the utility of the abelisaurid clade as a proxy for Late Cretaceous paleogeographic reconstruction. Institutional Abbreviations AMNH, American Museum of Natural History, New York; FMNH, Field Museum of Natural History, Chicago; GSI, Geological Survey of India, Indian Museum, Calcutta; MACN, Museo Argentino de Ciencias Naturales, Buenos Aires; MC, Museo Provincial de Cipolletti, Rıó Negro, Argentina; MWC, Museum of Western Colorado, Grand Junction; NCSM, North Carolina State University Museum, Raleigh; PVL, Fundacıón-Instituto Miguel Lillo, Tucumań, Argentina; UMNH, Utah Museum of Natural History, Salt Lake City; UNPSJB, Universidad Nacional de Patagonia ‘‘San Juan Bosco,’’ Comodoro Rivadavia, Argentina; USNM, National Museum of Natural History, Washington D. C. Anatomical Abbreviations aof, antorbital fossa; aofe, antorbital fenestra; d, depression at base of rostromedial process; df, dental foramina; idp, interdental plates; ll, lamina lateralis of ascending ramus; lm, lamina medialis of ascending ramus; ls, lateral sculpture; mx, maxillary tooth position; pf, promaxillary fenestra; rp, rostromedial process; rr, rostral ramus; tm, torus maxillaris.

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FIGURE 1. Geographic location and geologic context of the new abelisaurid maxilla (UNPSJB-PV247), after Martıńez et al. (1986). The specimen was obtained from the Lower Member of the early Late Cretaceous (middle Cenomanian–Coniacian) Bajo Barreal Formation, west of Lago Musters, Estancia ‘‘Ocho Hermanos,’’ Chubut Province, Argentina.

SYSTEMATIC PALEONTOLOGY THEROPODA Marsh, 1881 NEOCERATOSAURIA Novas, 1991 ABELISAURIA Novas, 1992 ABELISAUROIDEA Bonaparte, 1991b ABELISAURIDAE Bonaparte and Novas, 1985 CARNOTAURINAE Sereno, 1998 gen. et sp. indet. Material UNPSJB-PV247, a nearly complete left maxilla, missing only the most dorsal portion of the ascending ramus and several tooth crowns. Locality Estancia ‘‘Ocho Hermanos,’’ Sierra de San Bernardo, Chubut Province, Patagonia, Argentina (Fig. 1). Horizon and Age Lower Member of the Bajo Barreal Formation (Upper Cretaceous: middle Cenomanian-Turonian). The Bajo Barreal is part of the Chubut Group, stratigraphically overlying the Castillo Formation and overlain by the Laguna Palacios Formation. At the ‘‘Ocho Hermanos’’ locality, the Bajo Barreal is 255 m thick, with the Lower Member (148 m) somewhat thicker here than the Upper Member (107 m). UNPSJBPV247 was recovered from strata high within the Lower Member, approaching the contact between the two members (Fig. 1). The Bajo Barreal Formation has traditionally been regarded as Senonian (Coniacian–Maastrichtian) in age (e.g., Huene, 1929; Bonaparte and Gasparini, 1979). However, the palynol-

ogy of a subsurface equivalent of the Bajo Barreal, the Caleta Olivia Member of the Canãdoń Seco Formation, suggests a late Albian-Cenomanian age (Archangelsky et al., 1994). Moreover, palynomorph assemblages indicate an Albian-early Cenomanian age for the Huincul Formation of Neuqueń Province (Vallati, 1998); the underlying Candeleros Formation is therefore older. The Bajo Barreal Formation shares two sauropod genera with the Candeleros, suggesting that both units were deposited nearly simultaneously (Sciutto and Martıńez, 1997). Finally, recently reported Ar-Ar ages for the Bajo Barreal range from 95.8 to 91.0 Ma, corresponding to the middle Cenomanian-late Turonian (Gradstein et al., 1995, 1997; Bridge et al., 2000). Collectively, the data indicate a middle Cenomanian–early Coniacian age (96.0–88.5 Ma) for the Bajo Barreal Formation (Bridge et al., 2000). The stratigraphic position of UNPSJBPV247 indicates a late Cenomanian–early Turonian age. The Bajo Barreal is lithologically dominated by mudstone and sandstone, with abundant volcaniclastic sediment. The strata are fluvial and lacustrine, with general sediment dispersal to the east. More detailed reconstructions of depositional environment differ in several regards (see Bridge et al., 2000, and references therein).

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JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 22, NO. 1, 2002 TABLE 1.

Tetrapod fauna of the Bajo Barreal Formation.

Chelonia Chelidae indet. (2 species) (Broin and Fuente, 1993) Crocodyliformes indet. (Martıńez et al., 1986) Dinosauria Theropoda Theropoda incertae sedis gen. et sp. indet. (Martıńez et al., 1999) Neoceratosauria incertae sedis Xenotarsosaurus bonapartei (Martıńez et al., 1986) Abelisauridae Carnotaurinae gen. et sp. indet. (Martıńez et al., 1993; this paper) ?Coelurosauria gen. et sp. nov. A (Martıńez and Novas, 1997) Sauropoda Sauropoda indet. (Powell et al., 1989; Sciutto and Martıńez, 1994) Neosauropoda Diplodocoidea indet. (Sciutto and Martıńez, 1994; Martıńez, 1998c) Titanosauriformes gen. et sp. nov. B (Martıńez, 1998b, 1999) Titanosauria ?Andesaurus sp. (Sciutto and Martıńez, 1997) Titanosauridae Titanosauridae indet. (Powell et al., 1989; 5 Antarctosaurus wichmannianus, Laplatasaurus sp., Weishampel, 1990; Sciutto and Martıńez, 1994) Epachthosaurus sciuttoi (Powell, 1990) ?Argyrosaurus superbus (Lydekker, 1893; Bonaparte, 1996) Argyrosaurus sp. (Powell, 1986; Bonaparte, 1996) Titanosaurinae indet. (Powell et al., 1989; Sciutto and Martıńez, 1994) Ornithopoda Notohypsilophodon comodorensis (Martıńez, 1998a)

The Bajo Barreal has produced a diverse terrestrial vertebrate fauna (Table 1), consisting of chelonians, crocodyliforms, and dinosaurs (Martıńez et al., 2000). Of the latter, theropods are represented by a fragmentary long-clawed genus (Martıńez et al., 1999), the neoceratosaurian Xenotarsosaurus bonapartei (Martıńez et al., 1986; Coria and Rodriguez, 1993), the maxilla described here, and a small probable coelurosaur known from postcranial remains of several individuals (Martıńez and Novas, 1997). The sauropod fauna is diverse as well, and includes diplodocoids (Sciutto and Martıńez, 1994; Martıńez, 1998c), an undescribed titanosauriform represented by an excellent skull and cervical vertebrae (Martıńez, 1998b, 1999), the basal titanosaur Andesaurus sp. (Sciutto and Martıńez, 1997), and the titanosaurids Epachthosaurus scuittoi (Powell, 1990) and Argyrosaurus (Lydekker, 1893; Powell, 1986; Bonaparte, 1996). Ornithischian dinosaurs are represented by the ornithopod Notohypsilophodon comodorensis (Martıńez, 1997, 1998a). DESCRIPTION Lateral Much of the lateral surface of UNPSJB-PV247 (Fig. 2A) is ornamented with distinctive sculpturing, composed of subparallel, crescentic ridges and furrows recalling the maxilla of the abelisaurid Majungatholus (Sampson et al., 1998). The element is dorsoventrally deep relative to its length compared to other large theropod maxillae such as those of Allosaurus fragilis (Madsen, 1976) and Acrocanthosaurus atokensis (Currie and Carpenter, 2000) (Table 2). Again, this condition is similar to that in other abelisaurids, especially Majungatholus (Sampson et al., 1998) and AMNH 1955, a maxilla previously attributed to Indosuchus (Chatterjee, 1978). UNPSJB-PV247 does not possess lateral convexities corresponding to the roots of erupted

crowns and replacement teeth, which are clearly visible in some theropods (e.g., Buffetaut et al., 1988; Sereno et al., 1996). The rostral ramus of the new maxilla is strongly reduced, in contrast to many non-abelisauroid theropods. In Carnotaurus, the rostral ramus is practically nonexistent (Bonaparte et al., 1990), whereas in Noasaurus and Majungatholus it is plesiomorphically more rostrocaudally elongate (Bonaparte and Powell, 1979; Sampson et al., 1998). UNPSJB-PV247 exhibits an intermediate morphology (Fig. 3). The antorbital region of the new maxilla shows several characters suggestive of relationship to Abelisauroidea. The rostral margin of the ascending ramus of the maxilla is not inclined caudally as it is in many theropods (e.g., Allosaurus, Madsen, 1976; Carcharodontosaurus, Sereno et al., 1996; Coelophysis, Colbert, 1989; Ceratosaurus, Madsen and Welles, 2000); instead it is nearly vertical, to a degree surpassing that of some abelisaurids (Fig. 3). A single expansive recess invades the ascending ramus, rendering it entirely hollow. This vacuity communicates with the antorbital fossa via a prominent fenestra that is obscured in lateral view by the lamina lateralis of the ascending ramus. The caudally-facing opening is elliptical in shape and narrows dorsally. The identity of this structure is problematic; nevertheless, it appears homologous to the promaxillary fenestra of tetanuran theropods (Witmer, 1997). As in Dilophosaurus wetherilli and abelisauroids (Witmer, 1997), there is no evidence of a true maxillary fenestra in UNPSJBPV247. A remnant of an antorbital fossa is present on the lateral surface of the maxillary body as a shallow depression. It is clearly demarcated by an abrupt change in surface texture, from Majungatholus-like rugose sculpturing to a smooth, even texture. Caudolaterally, there is another marked change in texture associated with the jugal articulation. As in other abelisaurids, the jugal laterally overlaps the maxilla and its contact is elongate. Medial The element’s most notable feature in medial view is a subhorizontal row of prominent foramina, presumably for the passage of the dental arteries (Fig. 2B). Only ten are well-preserved, but there appear to have been an additional four. Their spacing varies with their position on the specimen (Table 3). The row of dental foramina is bordered dorsally by a conspicuous increase in thickness, a medial maxillary ‘‘shelf’’ [the torus maxillaris of Witmer (1997)], very similar to that present in AMNH 1955 (Chatterjee, 1978). The shelf becomes ridgelike caudally, beginning at mx9, and maximizing in dorsoventral depth between mx10–mx11. The ridge tapers to a width of approximately 1 mm at the caudal margin of the maxilla. The ridge demarcates a noticeable change in maxillary surface topography, from a series of heavily ridged and furrowed fused interdental plates (as in Majungatholus and AMNH 1955) to an even texture. This fusion of the interdental plates is comparable to the condition in Allosaurus (Madsen, 1976:pl. 6) but contrasts with that in Sinraptor dongi (Currie and Zhao, 1993:fig. 4), in which the interdental plates are clearly distinct from one another. In the new Patagonian maxilla, the ridges and furrows on the interdental plates are subvertical at the premaxillary articulation. Caudally, their dorsal portions become inclined rostrally. Small depressions are present immediately dorsal to the alveoli, and are more clearly defined rostrally. The slight ridges between them probably correspond to tooth roots and replacement teeth inside the element. A prominent, hollow rostromedial process arises ventral to the ascending ramus. A depression borders the rostromedial process ventrally, similar to the condition reported in AMNH 1955 (Chatterjee, 1978).

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FIGURE 2. Abelisaurid maxilla, UNPSJB-PV247. A, lateral view; B, medial view; C, rostral view; D, ventral view. Abbreviations listed in text. Scale bar equals 10 cm.

TABLE 2. Measurements (mm) and length/depth ratios of the new maxilla, compared to the corresponding element in the abelisaurid Majungatholus (FMNH PR2100; Sampson et al., 1998) and the allosauroid Acrocanthosaurus (NCSM 14345; Currie and Carpenter, 2000). The similarity of C and D in both abelisaurids represents the nearly parallel dorsal and ventral margins of their maxillae. Note the much lower length/depth ratios in the abelisaurids, indicating maxillae deeper per unit length than that of A. atokensis. * 5 element incomplete. UNPSJB-PV247 A. Rostrocaudal length B. Maximum dorsoventral depth C. Dorsoventral depth at jugal articulation D. Dorsoventral depth, alveolar margin to base of lamina medialis E. Length/depth at jugal articulation (A/C) F. Length/depth at base of lamina medialis (A/D)

330 186* 94 102 3.51 3.24

Majungatholus atopus ;290 182 70 83 4.14 3.49

Acrocanthosaurus atokensis 820 312 76 131 10.79 6.26

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FIGURE 3. Theropod maxillae in lateral view. A, Allosaurus fragilis (after Madsen, 1976). B, Carcharodontosaurus saharicus (after Sereno et al., 1996). C, Coelophysis bauri (after Colbert, 1989). D, Ceratosaurus dentisulcatus (after Madsen and Welles, 2000). E, Noasaurus leali (after Bonaparte and Powell, 1980). F, Abelisaurus comahuensis (after Bonaparte and Novas, 1985). G, Carnotaurus sastrei (after Bonaparte et al., 1990). H, AMNH 1955 (after Chatterjee, 1978). I, Majungatholus atopus (after Sampson et al., 1998). J, UNPSJB-PV247. Note the caudal inclination in the maxillary ascending ramus of A. fragilis, C. saharicus, C. bauri, and C. dentisulcatus. This structure is nearly vertical in abelisauroids, most apparent in C. sastrei, AMNH 1955, and UNPSJB-PV247. Scale bar equals 10 cm in A, D, F–J; 50 cm in B; 2.5 cm in C, E.

Rostral

Caudal

Three prominent depressions are visible in rostral view (Fig. 2C). They may indicate an undulating premaxillary–maxillary suture. The maxilla and the premaxilla (judging from its articulation) are laterally convex.

A prominent groove for articulation with the jugal is evident. This suture gradually sweeps laterally approaching the ventral margin of UNPSJB-PV247, indicating an increase in skull width. Ventral

TABLE 3. Distances between medial dental foramina (in mm), UNPSJB-PV247, measured midpoint-midpoint. Number

Distance between

1–2 2–3 3–4 4–5 5–6 6–7 7–8 8–9 9–10 10–11 11–12 12–13 13–14

18.0 20.5 20.0 21.0 20.0 21.5 21.0 17.0 23.0 21.0 17.0 20.0 16.0

The 14–15 alveoli are distinguishable in ventral view (Fig. 2D). The lateral alveolar margin extends farther ventrally than the medial, most noticeably in the region of mx4–mx9. The row of alveoli displays a slight sigmoidal curvature. The rostral portion (encompassing mx1–mx3) of the tooth row arches gently laterally, then becomes inclined roughly caudally, parallel to the midline of the skull. In the area of mx12 the ventral margin flares more sharply, again in a lateral direction. Dentition Erupted or partially erupted tooth crowns are present in alveoli 1, 3, 4, 5, 7, 8, 10, 11, 12, 13, and 14. Of these, mx12 is the best preserved. It is labiolingually compressed and possesses denticulate mesial and distal carinae. It is unknown whether the carinae are denticulate to the crown tip, although the con-

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dition in mx8 suggests this. The carinae are aligned with the mesiodistal axis of the tooth, and are not offset as in the distal maxillary crowns of some other theropods (e.g., Allosaurus, JBS., pers. obs. 1998; Acrocanthosaurus, many tyrannosaurids, Holtz, pers. comm., 1999). Mx12 is nearly fully erupted, and occupies much of the length of its alveolus. It measures 14 mm in crown base length [following Smith and Dodson, in press; 5FABL of several works (e.g., Currie et al., 1990)], approximately 11 mm in crown base width, and has an average mesial denticle density of 12/5 mm (2.4/mm). The crown is thus low compared to the size of the maxilla and strongly dissimilar to the tall crowns of many other large theropods (e.g., Ceratosaurus, Marsh, 1884; Madsen and Welles, 2000; allosauroids, Madsen, 1976; Currie and Zhao, 1993; Sereno et al., 1996; Currie and Carpenter, 2000; tyrannosaurids, Osborn, 1912; Lambe, 1914, 1917). Mx12 recalls crowns of Majungatholus and AMNH 1955. Crowns of Carnotaurus are elongate and slender, dissimilar from those of other abelisaurids (Bonaparte, 1991b). Comparison with Xenotarsosaurus bonapartei UNPSJB-PV247 was recovered from the same horizon of the Bajo Barreal, approximately 150 m from the type locality of the medium-sized theropod Xenotarsosaurus bonapartei (Martıńez et al., 1986). The stratigraphic and geographic proximity of the new maxilla to Xenotarsosaurus raises the possibility that the two entities pertain to the same taxon. However, no direct anatomical comparisons can be made between them. The holotype of X. bonapartei includes a femur, fibula, tibia, astragalocalcaneum, and two incomplete cranial dorsal vertebrae. The elements of Xenotarsosaurus exhibit neoceratosaurian characters, but as nearly all recognized abelisaurid synapomorphies pertain to cranial or cervical morphology, the taxon has been excluded from Abelisauridae in recent works (Coria and Rodrı´guez, 1993; Coria and Salgado, 1998; Sampson et al., 1998). The dimensions of the involved elements may provide evidence against the referral of UNPSJB-PV247 to Xenotarsosaurus. UNPSJB-PV247 measures 330 mm in rostrocaudal length, some 13.8% longer than the maxilla of the Malagasy abelisaurid Majungatholus (Table 2). The total body length of Majungatholus has been estimated at 7 to 9 m (Sampson et al., 1998). If it was of similar proportions, the Bajo Barreal theropod represented by the maxilla reached 8 to 10 m in length. The femoral length of Xenotarsosaurus is 598 mm, indicating an animal roughly 5 m in total length (based on comparison with Carnotaurus sastrei, Bonaparte et al., 1990). In addition, the dorsal neural arches of X. bonapartei are fused to their respective centra, possibly indicating that this animal had neared adult body size at the time of its death (Brochu, 1996). It is possible that the new maxilla, which may indicate an animal 8 m or more in length, does not pertain to Xenotarsosaurus. Two or more neoceratosaurian genera may be present in the Bajo Barreal fauna, as is the case in the Maastrichtian Lameta assemblage of India (Huene and Matley, 1933; Chatterjee, 1978; Sampson et al., 1998; Novas and Bandyopadhyay, 1999). PHYLOGENETIC ANALYSIS A preliminary phylogenetic analysis of neoceratosaurian taxa was conducted to assess the evolutionary relationships and biogeographic significance of the Bajo Barreal theropod. The seven ingroup taxa included Abelisaurus (MC 11098), AMNH 1955, Carnotaurus (MACN-CH 894), Ceratosaurus (USNM 4735; MWC 1; UMNH 5278), Majungatholus (FMNH PR2100), Noasaurus leali (PVL 4061), and UNPSJB-PV247. Nine binary and two multistate maxillary anatomical characters were coded for each taxon (Appendix 1). Multistate characters were considered unordered, and polarity was determined using the coelophysoid

FIGURE 4. Preliminary hypothesis of phylogenetic relationships for seven neoceratosaurian taxa based on a parsimony analysis of 11 maxillary characters and using Coelophysis bauri as an outgroup. Node designations follow these sources: Neoceratosauria, Novas, 1991; Abelisauroidea, Bonaparte, 1991b; Abelisauridae, Bonaparte and Novas, 1985; and Carnotaurinae, Sereno, 1998. The biogeographic and temporal distribution of all taxa is given (concept after Sampson et al., 1998; De Klerk et al., 2000). The earliest known abelisaurid is the Bajo Barreal form, UNPSJB-PV247, from the Late Cenomanian–Early Turonian (;94 Ma).

theropod Coelophysis bauri (Colbert, 1989) as an outgroup. Character states in each terminal taxon were determined using information from the literature, personal communication, or direct observation (Appendix 2). The analysis was conducted using PAUP* [(Phylogenetic Analysis Using Parsimony (*and Other Methods)], Version 4.0b4a (Swofford, 2000) with the ‘collapse’ option in effect. The small size of the matrix permitted use of the exhaustive search method. Due to missing anatomical information for several ingroup taxa, namely Abelisaurus, AMNH 1955, Noasaurus, and UNPSJB-PV247, preliminary attempts to incorporate non-maxillary cranial, axial, and appendicular morphology resulted in many most parsimonious trees, the consensus of which was invariably a phylogenetically uninformative polytomy. RESULTS The phylogenetic analysis produced a single most parsimonious tree of 16 steps (Fig. 4), demonstrating the abelisaurid affinities of UNPSJB-PV247. The tree has high consistency, retention, and rescaled consistency indices (0.81, 0.80, 0.65), due largely to the small size of the character matrix. The tree supports a monophyletic Abelisauridae, and within it, a Carnotaurinae composed of Carnotaurus, AMNH 1955, Majungatholus, and the Bajo Barreal form. Bremer support indices (Bremer, 1988) for each node determined using TreeRot, Version 2 (Sorenson, 1999) are as follows: Abelisauroidea 1.0; Abelisauridae 2.0; Carnotaurinae 1.0; and unnamed clade (AMNH 1955-Majungatholus-UNPSJB-PV247) 1.0.

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JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 22, NO. 1, 2002 DISCUSSION

Relationships of Ingroup Taxa Neoceratosauria: Ceratosaurus Ceratosaurus is a large theropod from the Upper Jurassic of western North America whose phylogenetic relationships have become controversial in recent years. The systematic work of Rowe and Gauthier (1990) allied the genus to coelophysoid theropods in a monophyletic assemblage termed Ceratosauria. Within this clade, Ceratosaurus is usually regarded as the sister-taxon to Abelisauria; together they constitute Neoceratosauria (Novas, 1991; Holtz, 1994, 1998; Coria and Salgado, 1998). Several preliminary analyses have raised evidence against the monophyly of Ceratosauria (Currie, 1995; Britt, 1995; Rauhut, 1998; Carrano and Sampson, 1999; Forster, 1999). In the current study, Ceratosaurus united with Abelisauroidea in a monophyletic Neoceratosauria. However, theropod genera traditionally regarded as tetanuran were not included in the present analysis, precluding scrutiny of the monophyly of Ceratosauria. Ceratosaurus is provisionally accepted as the sister-taxon to Abelisauria, pending several works in progress (Coria and Currie, 1997; Rauhut, 1998; Carrano and Sampson, 1999). Abelisauroidea: Noasaurus leali Noasaurus, from the ?Maastrichtian Lecho Formation of Argentina is generally regarded as an abelisaur, but outside of Abelisauridae and placed in Noasauridae (Bonaparte and Powell, 1980; Bonaparte, 1991b; Novas, 1997b, c; Coria and Salgado, 1998). The maxilla of this animal shares several derived characters with those of abelisaurids, such as a reduced rostral ramus and a subvertical ascending ramus. Nonetheless, it retains a very well-developed antorbital fossa on the maxillary body, and lacks several postcranial apomorphies of Carnotaurus (Bonaparte, 1991b). Noasaurus is regarded as a member of the sister-taxon to Abelisauridae. Abelisauridae The monophyly of Abelisauridae is supported by the analysis. Maxillary synapomorphies linking abelisaurids include external maxillary sculpture (apparently reversed in AMNH 1955) and a strong reduction in size of the antorbital fossa on the main body of the maxilla. Abelisaurus comahuensis Abelisaurus is a large Late Cretaceous Argentine theropod with hypothesized close affinity to Carnotaurus and its relatives (Bonaparte et al., 1990; Bonaparte, 1991b). However, Abelisaurus lacks or is indeterminate for synapomorphies of Carnotaurinae, and thus cannot be referred to that clade (Sereno, 1999). Furthermore, several maxillary characters that it shares with carnotaurines are also found in the allosauroid carcharodontosaurids (Rauhut, 1995; Sereno et al., 1996) Giganotosaurus carolinii and Carcharodontosaurus saharicus (Novas, 1997b; Sampson et al., 1998). A number of authors have commented upon remarkable similarities in cranial morphology between the abelisaurid and carcharodontosaurid clades (Coria and Salgado, 1995; Novas, 1997b; Holtz, 1998; Sampson et al., 1998; Forster, 1999). Similar features include reduced antorbital fossae, rugose nasals and maxillae, a lacrimal and postorbital union forming eave-like brows, and invasion of the orbital fenestrae by the lacrimal and postorbital (Coria and Salgado, 1995; Novas, 1997b). The resemblances have been explained either as the result of convergence (Coria and Salgado, 1995; Holtz, 1998; Sampson et al., 1998) or as evidence for close phylogenetic affinity between the two groups (Novas, 1997b, c). Due to the reported dissimilarity in postcranial morphology between Carnotaurus and Giganotosaurus (Coria and Salgado, 1995), it seems appropriate to invoke homoplasy as an explanation for these putative cranial synapomorphies. However, as Abelisaurus is at present known only from cranial remains and cannot be verified for any carnotaurine syapomorphies, it is possible that it pertains to Carcharodontosauridae. Its resemblance to Giganotosaurus is par-

ticularly striking. In addition to the possible synapomorphies detailed above, Abelisaurus and Gigantosaurus share strongly caudoventrally inclined quadrates resulting in a mandibular articulation far caudal to the occiput (Bonaparte and Novas, 1985; Coria and Salgado, 1995; Currie and Carpenter, 2000). As a similar situation is present in Ceratosaurus, however, this character may be plesiomorphic (Marsh, 1884; Gilmore, 1920). Postcranial material of Abelisaurus is needed to more adequately establish its affinities and biogeographic implications. Carnotaurinae The morphological characters assembled in this study support a subclade within Abelisauridae composed of Majungatholus, Carnotaurus, Indosuchus, and the Bajo Barreal abelisaurid termed Carnotaurinae (Sereno, 1998). The Bajo Barreal form extends the temporal range of carnotaurines from an exclusively Senonian to a late Cenomanian–Maastrichtian occurrence (Fig. 4). Maxillary synapomorphies linking carnotaurines include a promaxillary fenestra obscured in lateral view by the lamina lateralis of the ascending ramus, a maxillary body with subparallel dorsal and ventral margins, and possibly rostrocaudally shortened maxillae and an elongate maxilla-jugal contact. The former also occurs in several tetanuran theropods, particularly Allosaurus, Proceratosaurus, and some tyrannosaurids (Witmer, 1997; Holtz, 1998). Carnotaurus sastrei The osteology of Carnotaurus, recovered from northern Chubut Province and well known for its conspicuous frontal ‘‘horns,’’ has been of tremendous importance to abelisaurid systematics (Bonaparte et al., 1990). The holotype of C. sastrei, formerly believed to be from the AlbianCenomanian Gorro Frigio Formation, now appears to have come from the lower portion of the Campanian–Maastrichtian La Colonia Formation (Ardolino and Delpino, 1987; Sampson et al., 1998; Albino, 2000; Pascual et al., 2000). Indian Abelisaurids Over several decades, the Maastrichtian Lameta Formation of India has yielded significant theropod specimens, including Indosaurus matleyi and Indosuchus raptorius as well as many isolated elements (Huene and Matley, 1933). Type specimens for these taxa were selected from remains described in the original publication on each (GSI K27/ 565 and K27/685, respectively; Chatterjee, 1978). Both species are based upon incomplete braincase material. Indosaurus has traditionally been regarded as an Allosaurus-like taxon, a ‘megalosaur’ (Huene and Matley, 1933; Walker, 1964; Chatterjee, 1978). Recent works have strived for more precise identification, and modern workers generally treat this form as an abelisaurid (Bonaparte and Novas, 1985; Molnar, 1990; Sampson et al., 1998; Novas and Bandyopadhyay, 1999). The abelisaurid affinities of this species are suggested by its apparent possession of cranial hornlike structures seen in some members of this clade. While Indosaurus has been largely overlooked in the literature, Indosuchus has received more attention. Like the former, Indosuchus was originally regarded as an allosaurid (Huene and Matley, 1933); however, Walker (1964), referred this genus to Tyrannosauridae, noting a tyrannosaurid-like median crest between the supratemporal fenestrae in the specimen that was to be the lectotype. This assignment was followed by Chatterjee (1978), who, largely based on the alleged tyrannosaurid status of Indosuchus, referred to it several isolated cranial elements, including a maxilla included in the present analysis, AMNH 1955. In more recent years, comparisons of the lectotype with Abelisaurus and Carnotaurus have led to the current view of Indosuchus as an abelisaurid (Bonaparte et al., 1990; Molnar, 1990; Chatterjee and Rudra, 1996; Novas and Bandyopadhyay, 1999). Additional material from the Lameta Formation has been referred to this taxon, including cranial remains and a disarticulated postcranial skeleton (Chatterjee, 1978; Chatterjee and

LAMANNA ET AL.—EARLY ABELISAURID Rudra, 1996). Still, none of the elements attributed to Indosuchus have been associated with braincase material necessary to confirm their referral. Furthermore, it is likely that some specimens referred to Indosuchus pertain to other genera, as at least one other probable abelisaurid (Indosaurus) and a small probable abelisaur (Laevisuchus) have been recognized from the Lameta (Sampson et al., 1998; Novas and Bandyopadhyay, 1999). We reserve the name Indosuchus for the lectotype (GSI K27/ 685), and view all referred material (including AMNH 1955) as Abelisauridae, gen. et sp. indet. pending further discoveries. Majungatholus atopus Outstanding new specimens from the Maastrichtian Maevarano Formation of Madagascar have revealed the abelisaurid affinities of Majungatholus (Sampson et al., 1996; 1998; O’Connor and Sampson, 1998), originally described as a Gondwanan pachycephalosaur (Sues and Taquet, 1979; Sues, 1980). Sampson et al. (1998) postulated prolonged biotic and physical connections between selected landmasses of the former Gondwanan supercontinent, based in part upon abelisaurid biogeography. In light of the recognition of the Bajo Barreal predator as an abelisaurid closely related to Majungatholus, Carnotaurus, and AMNH 1955, these hypotheses are reevaluated below. Biogeographic Implications According to current paleogeographic reconstructions, the Cretaceous witnessed the disassembly of the southern supercontinent of Gondwana. However, many issues concerning the timing and sequence of this process remain unresolved. The majority of proposed Cretaceous paleogeographic scenarios (e.g., Scotese and Golonka, 1993; Smith et al., 1995; Roeser et al., 1996; Scotese, 1997) detail a rapid dissociation of Gondwanan landmasses, with India-Madagascar separating completely from the Antarctica–Australia complex by the upper Barremian (Early Cretaceous, ca. 125 Ma; Gradstein et al., 1995, 1997). According to these hypotheses, South America severed physical ties with Africa during the Aptian or Albian (prior to 100 Ma), but portions of these landmasses remained in fairly close proximity throughout much of the Cretaceous (Scotese and Golonka, 1993). India and Madagascar were among the last southern landmasses to dissociate, separating at ca. 88.0 Ma (Storey et al., 1995). The isolation of South America was finally completed in the Tertiary, when it broke from Antarctica. According to the recent hypothesis of Hay et al. (1999), a different scenario is likely: India-Madagascar and South America remained united via Sri Lanka, the Kerguelen Plateau, and Antarctica well into the Senonian, subsequent to the divergence of South America and Africa. Africa became an ‘‘island continent’’ for the entirety of the Late Cretaceous. Biogeographic study has potential to help resolve tectonic hypotheses. Continental dispersal of terrestrial vertebrates appears strongly controlled by the presence or absence of seas. Landlocked faunas developing on Gondwanan continents geographically isolated from one another should show increasing degrees of endemism over time. If the former, more conventional geophysical model reflects the actual sequence of events in Gondwanan breakup, then the Cenomanian–Turonian terrestrial vertebrate fauna of South America should resemble that of Africa more closely than those of India and Madagascar. Aptian–Albian faunal exchange between Africa and South America would have likely resulted in early Late Cretaceous clades native to both, but absent from an isolated India–Madagascar. Conversely, if the Hay et al. proposal proves correct, then the vertebrates of South America should be more phylogenetically allied to those of India and Madagascar than Africa. Late Cretaceous Africa should have hosted an isolated, endemic vertebrate fauna.

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Despite many recent finds, the Cretaceous record of terrestrial vertebrates, including dinosaurs, in Gondwanan landmasses remains incomplete. In all Southern Hemisphere continents, even comparatively well-documented South America, there is currently no information on dinosaurian assemblages for entire Cretaceous temporal ages. Particularly frustrating is the lack of specimens from time intervals pertinent to the resolution of the paleogeographic controversy described above. Examples include the lack of Senonian dinosaur remains from Africa and the paucity of pre-Campanian Cretaceous terrestrial vertebrates from India or Madagascar. Nevertheless, researchers have recently cited the geographic occurrence of abelisaurids as evidence in support of a Late Cretaceous dispersal route between India-Madagascar, Antarctica– Australia, and South America (Sampson et al., 1998). These authors consider the origins of Abelisauridae to have postdated rifting between South America and Africa, but to have predated the separation of India and Madagascar in the Coniacian. According to their hypothesis, abelisaurids never existed in Africa. Strictly speaking, the Bajo Barreal maxilla and the neoceratosaurian phylogeny recovered in this analysis are in accordance with this hypothesis, and with the tectonic model of Hay et al. (1999), as abelisaurids remain limited to Upper Cretaceous deposits of South America, India, and Madagascar. Still, the Cenomanian–Turonian occurrence of the Bajo Barreal form raises some questions concerning the timing of abelisaurid origins, the biogeography of the clade, and its utility in paleogeographic reconstruction. Prior to the recognition of UNPSJB-PV247 as an abelisaurid, the group was believed to be temporally limited to Campanian-Maastrichtian deposits (Sampson et al., 1998). The recovery of the new maxilla in Cenomanian–Turonian strata represents a substantial range extension for the clade. Estimates of the time interval during which abelisaurids originated depend on the use of delayed or accelerated transformations. If delayed divergence estimates are used, Abelisauridae originated immediately prior to the Cenomanian-Turonian. However, if accelerated estimates are employed, the clade may have originated many millions of years prior to the Late Cretaceous. It is likely that the rise of Abelisauridae predated the separation of Africa and South America, and thus, that abelisaurids could have occupied Africa during at least some Cretaceous intervals. Moreover, the known occurrence of Abelisauridae may reflect a former pan-Gondwanan distribution, and is thus of limited utility in the support of Late Cretaceous paleogeographic hypotheses until phylogenetic affinities within the clade are resolved. ACKNOWLEDGMENTS We thank M. Luna and G. Casal for aid in recovery, preparation, and casting of UNPSJB-PV247. We are indebted to J. F. Bonaparte, J. S. Bridge, E. Buffetaut, M. T. Carrano, S. Chatterjee, R. A. Coria, C. A. Forster, W. W. Hay, T. R. Holtz, Jr., E. A. Musacchio, D. A. Russell, S. D. Sampson, P. VallatiSandoval, and L. M. Witmer for valuable discussion and access to unpublished information. M. Luna provided a Spanish translation of the English abstract. Early drafts of the manuscript benefited greatly from reviews by P. Dodson, J. D. Harris, and C. E. Vaughn. M. T. Carrano, C. A. Forster, T. R. Holtz, Jr, and F. E. Novas provided valuable reviews of the final manuscript. This research was supported by the University of Pennsylvania (Paleobiology Stipends to Lamanna and Smith), the Universidad Nacional de la Patagonia ‘‘San Juan Bosco,’’ the National Geographic Society Committee for Research and Exploration (Grant #6646-99), the Jurassic Foundation, and the Delaware Valley Paleontological Society.

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APPENDIX 1 Characters used in phylogenetic analysis. All characters unordered. Modified from Holtz, 1998, and references therein: 1. Rostral ramus of maxilla: 0, present, rostrocaudal length of maxilla rostral to ascending ramus over ½ that of antorbital fenestra; 1, less than ½ length of antorbital fenestra. 2. Promaxillary fenestra: 0, completely absent; 1, ovoid depression present in its place; 2, present, visible in lateral view; 3, present, obscured in lateral view by lamina lateralis of maxillary ascending ramus. 3. Interdental plates: 0, absent; 1, present and separate in the caudal portion of the maxilla; 2, present and completely fused. Modified from Novas, 1992: 4. Lamina medialis of maxillary ascending ramus: 0, strongly caudodorsally inclined, resulting in an angular rostral margin of the antorbital fenestra; 1, subvertical, resulting in rounded rostral margin of the antorbital fenestra. Modified from Martıńez, et al., 1993: 5. Tooth crown height to crown base length ratio of tallest fully erupted crown: 0, 2.0 or more; 1, 1.5 or less. Modified from Coria and Salgado, 1995: 6. Main body of maxilla: 0, rapidly decreasing in depth caudally; 1, maintaining depth throughout much of its length, with subparallel dorsal and ventral margins. This was identified as an autapomorphy of the Argentine carcharodontosaurid Giganotosaurus carolinii by these authors. We found it to occur in several abelisaurids. Modified from Sampson et al., 1996: 7. Premaxillary articular surface: 0, angled strongly caudodorsally; 1, subvertical. Sampson et al., 1998: 8. Extensive sculpturing of maxillary lateral surface: 0, absent; 1, present. Modified from Sereno, 1999: 9. Short pre-orbital skull length. Maxillary length caudal to rostral margin of antorbital fenestra/depth caudoventral to base of lamina medialis: 0, 2.5 or more; 1, 2.0 or less. Sereno, 1999: 10. Maxilla-jugal contact, length: 0, short; 1; long and broad. This work: 11. Antorbital fossa on lateral surface of maxillary body: 0, present and well-developed; 1, strongly reduced or absent. Description: In Coelophysis, the antorbital fossa is well-developed as a depression occupying much of the lateral maxillary surface. Its maximum rostroventral extent is clearly delineated by a pronounced lateral ridge (Colbert, 1989:figs. 38, 48). Ceratosaurus and Noasaurus display a similar condition. In the abelisaurids Abelisaurus, Carnotaurus, Majungatholus, AMNH 1955, and UNPSJB-PV247 this ridge/depression complex has been markedly reduced or is totally absent.

APPENDIX 2

0

1 1

1 0

1 1 1

INGROUP Abelisaurus AMNH 1955

Carnotaurus Ceratosaurus

Majungatholus Noasaurus UNPSJB-PV247

1

OUTGROUP Coelophysis

OTU

3 ? 3

3 1

2 3

0

2

2 2 2

2 a

? 2

0

3

1 1 1

1 0

1 1

0

4

1 0 1

0 0

? 1

0

5

1 0 1

1 0

? 1

0

6

1 0 1

1 1

1 1

0

7

Character state

1 0 1

1 0

1 0

0

8

1 0 1

1 1

0 1

0

9

1 ? ?

1 0

? ?

0

10

1 0 1

1 0

1 1

0

11

Bonaparte and Novas, 1985; Sereno, 1999 Chatterjee, 1978; Martıńez et al., 1993; Witmer, 1997; Chatterjee, pers. comm., 2000; Witmer, pers. comm., 2000 Bonaparte et al., 1990; Sereno, 1999; Forster, 1999; pers. obs., 2000 Marsh, 1884; Gilmore, 1920; Witmer, 1997; Sereno, 1999; Madsen and Welles, 2000; Witmer, pers. comm., 2000 Sampson et al., 1998; Sereno, 1999 Bonaparte and Powell, 1980; pers. obs., 2000 Martıńez et al., 1993; this study

Colbert, 1989; Witmer, 1997; Sereno, 1999

Source

Phylogenetic data matrix. Character codings: 0, postulated plesiomorphic character state; 1, 2, 3, postulated derived character state; a, derived character states 1 and 2 present.

LAMANNA ET AL.—EARLY ABELISAURID 69