Inside a duck-billed dinosaur: Vertebral bone microstructure of Huallasaurus (Hadrosauridae), Upper Cretaceous of Patagonia.

Dinosaurs evolved a unique respiratory system with air sacs that contributed to their evolutionary success. Postcranial skeletal pneumaticity (PSP) has been used to infer the presence of air sac systems in some fossil archosaurs. While unambiguous evidence of PSP is well documented in pterosaurs and post-Carnian saurischians, it remains absent within Ornithischia, challenging phylogenetic predictions. We used computed tomography to examine the internal vertebral microanatomy of three Huallasaurus australis specimens, a saurolophine hadrosaur from the Late Cretaceous of Patagonia, Argentina. The internal structure reveals a relatively dense trabecular architecture lacking evidence of invasive pneumaticity across the centra, neural arches, and neural spines, contrasting with the condition in post-Carnian saurischians. The internal vertebral pattern of Huallasaurus resembles that of silesaurs more than other apneumatic archosauriforms. These observations are consistent with the hypothesis that invasive air sac diverticula did not evolve in Ornithischia and align with the previously proposed "pelvic bellows" ventilation model for the group. The internal vertebral architecture in this hadrosaur shows superficial similarities to the trabecular structure seen in some large mammals, although functional equivalence remains speculative. The absence of invasive air sacs in Huallasaurus, combined with dense trabecular matrix and thin cortical walls, may have supported large body sizes or accommodated intraosseous fat reserves, though this requires further testing. This stock of fatty tissues could have provided energy for these hadrosaurs during regional migration, as observed in modern migratory mammals.