New carnivorous dinosaur from the Late Cretaceous of NW Patagonia and the evolution of abelisaurid theropods
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- Canale, J.I., Scanferla, C.A., Agnolin, F.L. et al. Naturwissenschaften (2009) 96: 409. doi:10.1007/s00114-008-0487-4
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A nearly complete skeleton of the new abelisaurid Skorpiovenator bustingorryi is reported here. The holotype was found in Late Cenomanian–Early Turonian outcrops of NW Patagonia, Argentina. This new taxon is deeply nested within a new clade of South American abelisaurids, named Brachyrostra. Within brachyrostrans, the skull shortening and hyperossification of the skull roof appear to be correlated with a progressive enclosure of the orbit, a set of features possibly related to shock-absorbing capabilities. Moreover, the development of horn-like structures and differential cranial thickening appear to be convergently acquired within Abelisauridae. Based on the similarities between Skorpiovenator and carcharodontosaurid tooth morphology, we suggest that isolated teeth originally referred as post-Cenomanian Carcharodontosauridae most probably belong to abelisaurids.
The generic name derives from the Latin words skorpios (scorpion) and venator (hunter) because of the abundance of living scorpions moving around the excavation. The specific name honours the late Manuel Bustingorry, owner of the farm where the specimen was excavated.
MMCH-PV 48 (Museo Municipal “Ernesto Bachmann”, Villa El Chocón, Neuquén, Argentina): an articulated skeleton (Fig. 1b, c) represented by an almost complete skull and mandibles and most of the postcranial bones, lacking the right forearm and the distal half of the tail.
Locality and horizon
Bustingorry’s farm, 3 km NW of Villa El Chocón, Neuquén Province, NW Patagonia, Argentina (Fig. 1a). The specimen was found in fluvial sandstones with intercalated mudstones, belonging to the lower levels of the Huincul Formation (Late Cenomanian–Early Turonian; Corbella et al. 2004). This formation has yielded other theropod taxa, including the carcharodontosaurid Mapusaurus roseae (Coria and Currie 2006) and the abelisaurid I. aguadagrandensis (Coria and Salgado 2000). Skorpiovenator differs from the latter taxon in the morphology of the postorbital and quadrate bones (see below).
Skorpiovenator is distinguished from other abelisaurids on the basis of the following unique traits: (1) ascending process of maxilla homogeneously wide rostrocaudally, (2) maxillary horizontal ramus dorsoventrally deep with subparallel dorsal and ventral margins, (3) maxilla/jugal contact subvertical, (4) 19 maxillary teeth, (5) lacrimal rostrally projected and with well-developed suborbital process, (6) quadratojugal with pronounced caudal notch, (7) dentary with caudoventral process bifurcated to receive rostral end of angular, and (8) angular with rostral end dorsoventrally deep to fit between splenial and prearticular.
The contact zone between the dentary and postdentary bones is extensive; thus, the mandibular fenestra is smaller than in other abelisaurids (e.g. Carnotaurus, Majungasaurus; Bonaparte et al. 1990; Sampson et al. 1998). The caudoventral process of the dentary bifurcates to receive the angular, a feature not reported before from abelisaurids.
The number of maxillary teeth is 19, which is more than in other known abelisaurids (Sampson and Witmer 2007). The general shape of the crowns is similar to other abelisaurids (i.e. Majungasaurus), with strongly curved mesial margins and straight distal margins (Smith, 2007). Arcuate enamel wrinkles are present next to the marginal serrations (Fig. 2c), a condition resembling carcharodontosaurid allosauroids (Sereno et al. 1996). The maxillary and dentary teeth exhibit shallow blood grooves.
The postcranial axial skeleton of Skorpiovenator was found in articulation up to the 12th caudal vertebra. The dorsal and sacral vertebrae are complete, but remain unprepared. The cervical vertebrae have hypertrophied epipophyses, but without the cranial processes reported in Carnotaurus, Noasaurus and Aucasaurus (Bonaparte 1991; Coria et al. 2002). The ribs of the cranial cervicals are devoid of foramina and bear proximal aliform expansions and rod-like distal shafts. As in Majungasaurus and Carnotaurus (O’Connor 2007), the aliform process is distally separated from the shaft by a deep notch.
The caudal vertebrae exhibit typical abelisaurid traits, such as large and distally expanded fan-shaped transverse process, with a slender anterior projection that contacts the transverse process of the preceding proximal caudal (Coria et al. 2002). The proximal caudals bear horizontally projected transverse processes, in contrast to the condition present in Carnotaurus and Aucasaurus, for example, where the transverse processes are laterodorsally directed.
The ilium has a continuously convex dorsal margin, in contrast to Aucasaurus and Carnotaurus in which this bone exhibits a straight dorsal margin. The femur is a stout bone, as in most abelisaurids, bearing a conspicuous large mediodistal crest on the distal end. Metatarsals II–IV are subparallel to each other, in contrast to Majungasaurus where metatarsal IV is distally divergent (Carrano 2007).
Skorpiovenator is one of the most complete and informative abelisaurids yet known. The almost complete and articulated skeleton of this new taxon offers valuable osteological details that help to clarify character distribution among abelisaurid theropods.
Abelisaurids were considered a key taxon for mutually exclusive palaeobiogeographical interpretations, namely the “pan-Gondwana model”, which suggest connections between major Gondwanan landmasses during the Early Cretaceous before all were finally severed at the beginning of the Late Cretaceous (Sereno et al. 2004), and the “Africa-first model”, which support close connections between South America and Indo-Madagascar (but not Africa) well into the Late Cretaceous (Sampson et al. 1998; see Krause et al. 2007, for a comprehensive review of these hypotheses). The recovery of an abelisaurid clade (Brachyrostra) that exclusively occurred in South America, suggests an alternative third model for abelisaurid diversification, i.e. that at least from the Cenomanian through the Maastrichtian, derived South American abelisaurids may have remained isolated from their African and Indo-Madagascar relatives. Such an alternative model is in concordance with palaeogeographical reconstructions of the sequence of isolation of the Gondwanan landmasses (Krause et al. 2007), but does not necessarily apply to the diversification of the remaining Cretaceous terrestrial tetrapods from Gondwana.
The discovery of Skorpiovenator also helps to clarify the chronostratigraphical distribution of other Gondwanan theropods, such as carcharodontosaurids. These were large-sized predatory dinosaurs that some authors (i.e. Veralli and Calvo 2004; Apesteguía et al. 2004; Martinelli and Forasiepi 2004; Candeiro et al. 2006) interpreted as surviving into the latest Cretaceous times. Such assumption was based on small-sized, carcharodontosaurid-like teeth collected in post-Cenomanian beds. However, these purportedly carcharodontosaurid teeth exhibit several features that are also present in Skorpiovenator among abelisaurids, such as straight dental crowns and well-demarkated enamel wrinkles (Fig. 2c, d, e). Thus, we consider the record of post-Cenomanian carcharodontosaurids as dubious. Given the possibility that these teeth actually represent abelisaurids, they cannot be used to contradict an early Late Cretaceous (Cenomanian) extinction of carcharodontosaurids in South America, as hypothesised by Novas et al. (2005).
We thank C. Albornoz, M. Berrondo, J. Castro, M. Castro, N. Gonzalez, A. Moretti, J. Ochoa, M. Ripoll, M. Soto, and R. Zapata for the excavation and preparation of the specimen; J. Ochoa for his detailed drawings; A. Haluza for their useful comments in early drafts of the manuscript; J. Calvo, R. Coria, A. Kramarz, C. Muñoz and J. Powell for access to specimens under their care; R. Candeiro and A. Martinelli for providing useful pictures of theropod teeth; and Matthew Lamanna and two anonymous reviewers for the useful comments they made that improved the quality of this paper. The Municipalidad of Villa El Chocón is thanked for their constant support. Financial assistance was received from Agencia Nacional de Promoción Científica y Técnica (PICTs 13.803 and 58) and Conicet (PIP 5153).