Histology and pneumaticity ofAoniraptor libertatem(Dinosauria, Theropoda), an enigmatic mid-sized megaraptoran from Patagonia

Aoniraptor libertatemis a mid-sized megaraptoran that comes from the Late Cretaceous (Turonian) Huincul Formation at Rio Negro province, Patagonia, Argentina. In this study, we conducted a detailed analysis of pneumaticity of the sacrum and tail ofAoniraptor. This shows a complex structure within these vertebrae, being composed by small diverticulae surrounding large pneumatic canals and a central chamber that opens outside through pleurocoels or pneumatic canals. Further, we carried out a histologic analysis which confirms the pneumatic nature of these anatomical features. Both analyses found that chevrons inAoniraptorwere invaded by pneumaticity, a feature that appears to be unique to this taxon. In addition, a comparative analysis betweenAoniraptorand other theropods (e.g.Gualichoand other megaraptorans) was carried out. This resulted in the modification of previous schemes about the evolution of pneumaticity through Theropoda, the finding of some evolutionary pneumatic traits through Megaraptora, and the usefulness of pneumatic traits as a taxonomic tool.