Embryos resolve the enigmatic origin of the theropod wishbone.

Developmental evidence supports a composite origin of the avian wishbone from both clavicles and the interclavicle, with a key pattern observed in Nothoprocta perdicaria.

• Why it matters: Understanding the homology of the furcula is crucial for clarifying theropod dinosaur evolution and avian origins, as its precise developmental source has been debated for decades.
• What they did: The study examined ossification and mesenchymal condensations in a broad sample of birds and reptiles, focusing on the relative positions and fusion patterns of clavicular and interclavicle-like structures.
• The result: Findings suggest the furcula derives from both clavicles and the interclavicle, with early fusion leading to seamless integration, which may explain the scarcity of fossil evidence for this composite origin.