New specimens of the problematic large theropod dinosaur Megaraptor from the Late Cretaceous of central Patagonia.

LAMANNA, MATTHEW C.; MARTÍNEZ, RUBÉN D.; LUNA, MARCELO; CASAL, GABRIEL A.; IBIRICU, LUCIO M.; IVANY, EDMUNDO

Denver; Colorado

Congreso; SVP; Reunión anual de Paleontología; 2004

Society of Vertebrate Paleontology (SVP)

Recent discoveries have revealed a surprising diversity of tetanuran theropod dinosaurs in Upper Cretaceous sediments of Patagonia, Argentina. Among the most enigmatic of these taxa is the large predator Megaraptor namunhuaiquii from the early Late Cretaceous (Turonian-Coniacian) Portezuelo Formation of Neuquén Province in northern Patagonia. The holotype of Megaraptor consist of an ulna, manual phalanx I-I, an incomplete metatarsal III, and a giant manual ungula I that was originally identified as pertaining to the pes. The only other described specimen pertaining to this genus is a newly discovered partial skeleton that preserves a cervical and two proximal caudal vertebrae, haemal arches, a scapula, coracoids, radius, ulna, a complete manus, pubis, and metatarsal IV. Because its anatomy is inadequaly known, the phylogenetic relationships of Megaraptor have remained unresolved. We describe two new partial skeletons of Megaraptor from the early Late Cretaceous (middle Cenomanian- Turonian) Lower Bajo Barreal Formation of southern Chubut Province in central Patagonia. The smaller skeleton, possibly pertaining to a subadult, includes a cranial dorsal vertebra,two dorsal ribs, three incomplete proximal-mid caudal vertebrae, manual phalanx ?II-2, incomplete manual unguals I-III, ? femur and ?fibula fragments, the distal end of metatarsal II, and two fragmentary noungual phalanges. The larger specimen consists of manual unguals I and III, a fragmentary femur and tibia, a nearly complete fibula, the distal end of metatarsal I, metatarsal II, several pedal phalanges and indeterminate fragments. The Bajo Barreal skeletons preserve axial and appendicular elements not previously known for Megaraptor, thus adding considerably to our understanding of its anatomy. A phylogenetic analysis resolves the position of Megaraptor within the carnosaurian clade Allosauroidea, as one of its youngest known representatives. Megaraptor is thus a relatively basal tetanuran and not a coelurosaur as originally proposed. The Bajo Barreal specimens represent the first occurrences of this genus outside of northern Patagonia.