Scientists reveal the complex early evolution of the bird's 'breastbone'
It has always been difficult to understand how birds evolved from dinosaurs because of the strange combination of features observed in taxa inferred to be situated near this great evolutionary transition. For example, the sternum, also called the 'breastbone', is a large bone to which the lower ends of the bird's ribs are attached. It is intrinsic to modern avian flight, providing the attachment surface for the two largest muscles in the body, the primary fight muscles the pectoralis and supracoracoideus. This bone is present in many dinosaurs inferred to be closely related to birds (e.g. Microraptor, Epidexipteryx) and most basal birds (e.g. Confuciusornis, enantiornithines) but strangely is absent in troodontid dinosaurs and the early birds Archaeopteryx and Sapeornis.