New research led by scientists at the American Museum of Natural History reveals that the unique way that birds breathe likely first appeared in their dinosaur ancestors. The findings, published recently in the journal PLOS ONE, build upon mountains of recent work exploring the origin of “bird-like” traits such as feathers, wishbones, and flight.
Birds have a highly efficient respiratory system powered by four sets of air sacs that pump air throughout their body. This adaptation allows the air to flow in one direction (as opposed to mammals, like us, who breathe in and out through the same tube) and is vital to birds’ ability to fly high over long distances.
During bird development, finger-like projections from these air sacs invade bone to create networks of internal chambers, a condition called pneumaticity. However, how and why this unique feature evolved in dinosaur ancestors is largely unknown. To directly see these pneumatic chambers in fossils, the researchers used high-resolution computed tomography (CT) scanning, for the first time, on the backbone of a dinosaur closely related to birds, 70-million-year-old Archaeornithomimus. The specimen used in the study was collected on the Museum’s Central Asiatic Expedition to Mongolia in 1923.
“Our work shows that at least part of the avian respiratory system was already in place in Archaeornithomimus, and therefore likely evolved in the common ancestor of birds and Archaeornithomimus over 150 million years ago,” said Aki Watanabe, the paper’s lead author and a student in the comparative biology doctoral program at the Museum’s Richard Gilder Graduate School. What they found was a remarkable network of pneumatic chambers in the vertebrae of the neck and chest, a detail never seen previously.
In modern birds, two types of air sacs invade the backbone: the cervical sacs, which occupy the neck; and the abdominal sacs, which invade the vertebrae associated with the back part of the chest, the hip, and the tail. The CT images of Archaeornithomimus show pneumaticity in the vertebrae of the neck, chest, and the base of the tail, but not in the hip region. This gap, also seen in modern birds, indicates that both cervical and abdominal air sacs were already present before the origin of birds.
Pneumaticity has previously been identified in pterosaurs, gigantic sauropod dinosaurs, and bird-line theropod dinosaurs, but these were largely based on external observations like holes that lead to internal chambers and broken regions of the fossils where some internal chambers are exposed. With CT scanning, this study provides full characterization of pneumaticity in dinosaurs.
“This project paves the way for additional studies documenting the early evolution of the avian respiratory system in unprecedented detail,” Watanabe said.