Sternal Elements of Early Dinosaurs Fill a Critical Gap in the Evolution of the Sternum in Avemetatarsalia (Reptilia: Archosauria)

The sternum is a functionally important but poorly understood component of the pectoral girdle in both living and extinct dinosaurs. When ossified, the dinosaur sternum consists of paired plates situated just posterior to the clavicles and scapulocoracoid. In Avialae, the sternal plates are fused at the midline to form a single element, aiding in powered flight. Our understanding of the early evolutionary history of the theropod sternum is complicated by a lack of fossil material, especially outside Maniraptoriformes, both because of taphonomic biases and incomplete ossification. Here, we report the oldest-known dinosaur sternal plates from the Late Triassic taxon Tawa hallae recovered from the Hayden Quarry at Ghost Ranch, New Mexico, as well as an isolated sternal plate from just south of Petrified Forest National Park, Arizona. These specimens expand the fossil record and allow for reconstruction of early theropod sternal musculature. They bear morphological features that are surprisingly similar to sternal traits in avialans, including a sternocoracoidal process, costal processes, and possible homologs to the ‘coracoid facet’ and pila coracoidea. The presence of these features in T. hallae pushes their appearance back to the Late Triassic, well before the development of the modern avian sternum. The variable distribution of sternal character states across avemetatarsalians was previously recognized in later Cretaceous avialans and can now be shown to also characterize the early evolution of the theropod sternum.