Embryos resolve the enigmatic origin of the theropod wishbone.

The origin of the avian wishbone (furcula)-present since theropod dinosaurs-remains unsolved. Its homology is still debated, with competing hypotheses proposing derivation from the clavicles, the interclavicle, or both. To obtain comparative developmental evidence, we studied ossification and pre-ossification mesenchymal condensations in a broad sample of birds and non-avian reptiles. Relative positions of paired condensations and their ossification centers support homology between the lateral arms of the furcula and reptilian clavicles. However, in the palaeognathous bird Nothoprocta perdicaria, we observed a separate interclavicle-like medial condensation. It quickly fuses with the clavicular condensations but then develops its own ossification center. The phylogenetic distribution of this developmental pattern across reptiles and birds supports a composite origin of the furcula from clavicles and the interclavicle. Early fusion of condensations leads to seamless integration, which may explain why fossil evidence of this composite origin has remained elusive.