How dinosaurs' arms transformed into bird wings revealed

In a new study scientists have explained how arms of dinosaurs went from straight to bent and hyperflexible, turning into wings.

Using an interdisciplinary approach, the lab run by Alexander Vargas at the University of Chile has re-examined fossils stored at several museum collections, while at the same time collecting new developmental data from seven different species of modern birds. Joao Botelho, a Brazilian student in Vargas' lab, developed a revolutionary new technique that allows him to study specific proteins in 3D embryonic skeletons. By combining these data from both fossils and embryos, the research team has made a major step forward in clarifying how the bird wrist evolved.

The new data obtained by the Vargas lab has revealed the first developmental evidence that the bird semilunate was formed by the fusion of the two dinosaur bones. They went on to show that another bone, the pisiform, was lost in bird-like dinosaurs, but then re-acquired in the early evolution of birds, where it allowed transmission of force on the downstroke while restricting flexibility on the upstroke. Combined, the fossil and developmental data provide a compelling scenario for a rare case of evolutionary reversal.

The study by the Vargas lab also settled the identity of the other two bones of the bird wrist, which were commonly misidentified in both fields. This emphasized the downsides of not integrating all data sources, and revealed a situation perhaps akin to that of astronomy and experimental physics in the pursuit of cosmology: Together, palaeontology and development could come much closer to telling the whole story of evolution, this integrative approach resolved previous disparities that have challenged the support for the dinosaur-bird link and revealed previously undetected processes, including loss of bones, fusion of bones, and re-evolution of a transiently lost bone.

The study published in PLOS Biology.