Giant deinonychosaurian theropod from the Late Cretaceous of Patagonia

NOVAS, FERNANDO EMILIO; CANALE, JUAN IGNACIO; ISASI, MARCELO

USA

Jornada; Annual Meeting of the Society of Vertebrate Paleontology; 2003

Exploration conducted in Bajo de Santa Rosa (a productive fossil locality in Rio Negro Province, Northern Patagonia) resulted in the discovery of a new theropod dinosaur. Fossils come from the Allen Formation (Campanian-Maastrichtian), which also yielded remains of fishes, turtles, hadrosaurs, and titanosaurs. The only theropod named from this unit is Quilmesaurus curriei, a probable abelisauroid.   We add here the finding of a large (6 m long), but gracile deinonychosaurian represented by several cranial and poscranial elements. The  frontal resembles that of Troodon and Sinornithosaurus in being triangular-shaped in dorsal view, and together with the postorbital define a wide and rounded orbital cavity. Jaws are elongate and low, 25 tooth alveoli on the dentary. As in other deinonychosaurians, the cervical vertebrae centrun exhibits cranial articular surface almost at the same plane as the ventral one, neural spine of dorsal vertebrae transversely enlarged at their distal extremity, manual ungual phalanges strongly curved and with prominent extensor tubercle, phalanx 2 of pedal digit 2 with constrcted  ´neck´ between both proximal and distal articular surfaces, and with a caudoventrally projected ´heel´. Bizarre traits of the new animal concern with teeth morphology, which are conical and circular in cross-section, and devoid of serrations and carinae. In regards with the postcranial skeleton, a notable disparity in transverse width of pedal phalanges 2.2 and1.4 suggests a marked assymmtry in foot construction.   The new discovery, along with the Patagonian maniraptorans Patagonikus, Alvarezsaurus, and Unenlagia, as well as the Malgalasy Rahonavis, support the interpretation that an important adaptive radiation of derived coelurosaurians took place in Gondwana at least from Turonian through Maastrichtian times.