Megaraptoridae: a gracile model of large predators? insights from a multiproxy approach.

Megaraptora constitute a distinctive clade of Cretaceous theropods that evolved long and powerful arms, long and unsculptured skull and, particularly, highly pneumatized and slender skeletons in comparison to their relatives (e.g., tyrannosaurs). Furthermore, younger and late branching megaraptorans become progressively bigger and more pneumatic, which has been related with the extinction of carcharodontosaurids in the Cenomanian-Turonian limit and the niche shifting of megaraptorans as top predators of their ecosystems. Notwithstanding, the progressive increment of the pneumaticity and body mass could have more intimate relations that previously thought. Using recently published body estimations and segmented CT scan data, the relation between degrees of pneumatization vs. body mass of megaraptorids was evaluated in order to (1) confirm the increment of pneumaticity through phylogeny and (2) to test if the increment of pneumatization had any relation with the body mass of this clade. Moreover, CT data of sacral and caudal vertebrae and haemal archs of Aoniraptor and cervical, dorsal and caudal vertebrae and ribs of Maip reveal that pneumatic cavities occupy a huge proportion of bone volume, which probably reduced considerably the skeletal weight. Contrarily, similar data in non megaraptoran theropods like dorsal and sacral vertebrae Bicentenaria and cervical and dorsal vertebrae and ribs of Taurovenator, show notably lower volumes. Furthermore, body mass estimations based on femoral circumference (made in previous authors) and tibial circumference (made in this work) indicate that megaraptorids was 40-70% lighter than non-avian theropods of similar body length. This mass reduction combined previously estimated body length, suggests that megaraptorans deviates from the body mass expected for Theropoda. In this regard, it is proposed that the progressive increase of pneumatization is potentially coupled with proportionally low body mass indicating an evolutionary trend toward such "light" model of large-bodied predation. In this regard, these results highlight megaraptorids as a possible different group in contrast with the more "heavy" clades such as megalosauroids, carcharodontosaurids or tyrannosaurids.