Theropoda Marsh, 1881

Tetanurae Gauthier, 1986

Neovenatoridae Benson et al., 2010

Megaraptora Benson et al., 2010

Megaraptora gen. et sp. indet.

Material: MPMA 08-003-94, vertebral centrum, Museu de Paleontologia de Monte Alto (MPMA).

Locality, horizon and age: Municipality of Ibirá, São Paulo State, Brazil; São José do Rio Preto Formation, Maastrichtian, Upper Cretaceous (Bertini and Menegazzo, 2009).

The theropod vertebral centrum (MPMA 08-003-94) was discovered at the Municipality of Ibirá, São Paulo State, Brazil (Fig. 1), from beds belonging to the São José do Rio Preto Formation (Maastrichtian; Bertini and Menegazzo, 2009).

The vertebra only preserves the centrum (Fig. 2). This is almost complete, except for the extreme right dorsal of the anterior articular surface, which is broken.It is long (11,8 cm), low (6,3 cm), and wide (8,5 cm), with a length to height ratio of nearly 1.9. This ratio is similar to that found in caudal vertebra 22 and more posterior vertebrae of Allosaurus (Madsen, 1976) and Tyrannosaurus (Brochu, 2003). Mid-caudals of Aerosteon (Sereno et al., 2008) and Orkoraptor (Novas et al., 2008) exhibit a ratio of 1.2 and 1.3, respectively, similar to the ratio found in caudal vertebrae 14 to 18 of Allosaurus and Tyrannosaurus. Based on these ratios, the centrum here described probably corresponds to a caudal vertebra from the distal half of the tail.

The centrum is amphicoelous, with subcircular articular surfaces, slightly broader than tall. Centrum margins close to the cranial and caudal articular facets are strongly decorated by longitudinal striations (Fig. 2B, D). The ventral surface of the centrum is almost flat, but bear two slight collateral ridges that separate the ventral surface from the lateral ones (Fig. 2B). The centrum is devoid of facets for articulation with the haemal arches.

On both lateral sides, the centrum exhibits on its upper half a large and elliptical depression, which occupies two quarters of the vertebral body length, containing two air chambers or pneumatic foramina and a shallow hollow in the most posterior part of the depression (Fig. 2A). These chambers are separated by thin septa (Fig. 2C). Furthermore, posterior to the pneumatic depression, the lateral surface of the vertebral body exhibits some kind of small pneumatopores.

Caudal pneumaticity is rare condition in theropod dinosaurs documented only in a few clades (i.e., Megaraptora, Oviraptorosauria, Therizinosauria, and Carcharodontosauridae; Benson et al., 2011). So far, the presence of remains of therizinosaurs in South America has not been documented and the oviraptorosaurs is at least doubtful (Agnolín and Martinelli, 2007). Also noteworthy is that the distal caudal vertebrae of oviraptorosaurs not bear pleurocoels, while they are present in the Brazilian centrum and Aerosteon. Stromer (1931), described an anterior caudal vertebra assigned to Carcharodontosaurus, which has pneumatic features. However, the general morphology and proportions (length to height ratio is 1,1 in Carcharodontosaurus) of the vertebrae of this African theropod specimen does not coincide with those observed in MPMA 08-003-94, as well as those of other members of Megaraptora (Orkoraptor, Aerosteon, Megaraptor).

Among theropod groups known from the Cretaceous of South America (i.e., abelisauroids, megaraptorans, maniraptorans, carcharodontosaurids, spinosaurids), only megaraptorans (e.g., Aerosteon, Megaraptor, Orkoraptor), exhibit pneumatic foramina on their caudal vertebrae. South American carcharodontosaurids (e.g. Tyrannotitan, Giganotosaurus) show a broad, shallow depression on the lateral surfaces of caudal vertebrae but not a pneumatic foramen as is present in megaraptorans and the Brazilian specimen. Pneumatic features present in MPMA 08-003-94 resemble the mid and posterior caudals of Aerosteon riocolaradensis (Sereno et al., 2008; Fig. 9B), in which the sides of the centrum bear pleurocoels (Fig. 3E) that enter the vertebral body, as well as the presence of small pneumatopores on the lateral surface of the centrum. A similar situation is also recorded in isolated caudal vertebrae of Megaraptor namunhuaiquii (J.D. Porfiri, pers. comm.).

Notably, a caudal vertebra with the same kind of pneumatic features was originally described by von Huene (1929) (Plate 41, 6b) as an isolated caudal of an indeterminate carnosaur. This material was collected along the south bank of the Senguer River (south of Chubut Province, Patagonia, Argentina) by Carlos Ameghino, by the end of the 19th century, from beds that may belong to Cenomanian-Turonian Lower Member of the Bajo Barreal Formation (Novas, 2009). The morphology of the caudal centrum described by von Huene is very similar to that from Brazil in the shape of the pneumatic opening (Fig. 3C), but differs substantially in the shape of the ventral surface, because the centrum from Senguer River has a well defined median keel (Fig. 3D), similar to that observed in the medial caudal vertebrae of Aerosteon (Fig. 3F) and Megaraptor. In contrast, the ventral surface is flat in the centrum from Brazil. Such difference reflects the respective position of the centrum along the tail, being more distal the material from Brazil. This is in agreement with the proportions of the centrum that match with those present in the caudal vertebrae down to 20 of basal tetanurans (e.g., Allosaurus) or large coelurosaurs (e.g., Tyrannosaurus), while in smaller coelurosaurs (e.g., Deinonychus) this ratio is present only in the anterior caudals. The absence of a conspicuous contact surface for the haemal arch is morphologically similar to that observed in the vertebrae posterior to caudal 22 of Allosaurus, while in Tyrannosaurus this ventral shape is present from caudal 29.In sum, the isolated caudal centrum here described from the Late Cretaceous of Brazil is considered as belonging to Megaraptora by having true pneumatic foramina in the mid-caudal vertebra, thus constituting the first record of this group in Brazil, extending the geographical distribution of this clade in South America.

For the first time, a megaraptoran remain is reported from the Brazilian Bauru Group. No autapomorphies were identified to diagnose a new genus and species within Megaraptora.

This new finding allows the extension of the record of theropod groups that inhabited Brazil and enlarges the biogeographical distribution of megaraptorans, which until now in South America were only known from Argentina. It also allows detailed knowledge of anatomical features present at least in the South American megaraptorans. These features are still not attributable to other members of Neovenatoridae, not belonging to less inclusive clade Megaraptora, because preserved caudal vertebrae of Neovenator (Brusatte et al., 2008) are apneumatic and Chilantaisaurus (Benson and Xu, 2008) do not preserve any caudal vertebrae. At the same time, the present study allows us to clarify the taxonomic allocation of certain old discoveries made at the end of the 19th century, such as a “carnosaurian” caudal centrum from Senguer River, Chubut Province, Argentina (von Huene, 1929).

Interestingly, the fossil record of South American megaraptorans is concentrated in different sedimentary units considered Cenomanian to Maastrichtian in age. This is the case for Megaraptor namunhuaiquii (Portezuelo Formation, Turonian-Coniacian; Calvo et al., 2004 and Novas, 1998), Aerosteon riocoloradensis (Plottier Formation; Novas et al., in press and Sereno et al., 2008), Orkoraptor burkei (Pari Aike Formation, Maastrichtian; Novas et al., 2008 and Novas et al., in press), the Senguer River specimen (presumably from the Lower Member of the Bajo Barreal Formation, Cenomanian-Turonian; Novas, 2009 and von Huene, 1929), and the specimen from Bauru Group (São José do Rio Preto Formation, Fernandes, 1998). This lends support to the interpretation that megaraptorans were relatively diverse in South America during the Late Cretaceous.

This finding from the Ibirá area demonstrates that members of the clade Megaraptora lived alongside with abelisaurids (Kellner and Campos, 2002 and Novas et al., 2008), and deinonychosaurian theropods (Bertini and Franco-Rosas, 2001, Bertini et al., 1997, Franco-Rosas, 2001 and Franco-Rosas, 2002).