An international team of scientists from USA, Argentina and Canada stumbled upon rare evidence of a bird-like breathing system in a predatory dinosaur from the Upper Cretaceous period. The amazing thing about this fossil is that it exhibits traces of air sacs in the skeletal bone, which suggest that this dinosaur, although carnivorous, breathed through a system that can only be encountered in birds.

Birds living today are known to rely on a pulmonary system made of air sacs that act as bellows, which push the air through a rigid lung. The problem paleontologists have encountered in establishing a complex evolutionary system for birds based on fossils is that the air sacs, which differentiates them from other vertebrates, only rarely penetrate the bone to live a mark of their presence.

Fortunately for the scientists involved in this study, the newly discovered fossil, which belongs to a Cretaceous dinosaur in Argentina, Aerosteon riocoloradensis, exhibits traces of pneumatization in the skeletal bone, which suggests a connection with the pulmonary system in birds today. In addition to that, the paleontologists also found several “stomach ribs” that are also connected with the air sac system.

The dinosaur shows postcranial bones, which according to the scientists involved, can be linked to intrathoracic air sacs used in lung ventilation. Furthermore, the bones in the rib cage are filled with air, and bear the same imprint of air sacs. Thanks to an excellent state of preservation, scientists believe the fossil will help constrain hypotheses for the evolution of the bird respiratory system.

As the authors of the study explained, the understanding of how the same mechanism that works for birds today also worked for dinosaurs is of extreme importance. Furthermore, Aerosteon could also be the key to revealing more about the evolution of the avian respiratory system, and how far back it can be traced.

The scientists concluded that while cervical air sacs in theropods developed no later than early Late Triassic, the origin and evolution of air sacs may have been driven by three elements: flow-through lung ventilation, locomotory balance, or bidirectional lung ventilation.

In addition to being a carnivorous dinosaur with avian respiratory system, Aerosteon is also of particular interest for the scientific community, as it represents a previously unrecorded lineage of large-bodied predators in South America’s Late Cretaceous.

While most large bodied Cretaceous theropods (bipedal dinosaurs) in South America, Africa, Madagascar, India, belonged to one of three distinctive groups (abelisaurids, spinosaurids, and carcharodontosaurids), Aerosteon is the representative of a distinctive lineage that survived in South America and could be linked to the Jurassic allosauroid radiation, the scientists explained.

The scientists involved in the study were Paul Sereno from the University of Chicago, USA; Ricardo Martinez from the Museo de Ciencias Naturales in San Juan, Argentina; Jeffrey Wilson from the University of Michigan, USA; David Varricchio from the Montana State University, USA; Oscar Alcober from the University of Chicago, USA; and Hans Larsson from the McGill University, Montreal, Canada. The findings appeared on September 30 in PLOS One.