It is commonly thought that birds are living dinosaurs, based on skeleton similarities among the two species. Not all scientists are quick to jump on this bandwagon, though. In an attempt to explore the hypothesis, researchers from the University of Chile re-examined ankle development in six different major bird species that were chosen based on their last common ancestor, according to a news release, and noted several similarities between birds and dinosaurs.

One of these similarities involves a pointed portion of the ankle bone projecting upwards onto the shank bone – also known as the "drumstick" – that continues to be debated. Some scientists believe this "ascending process" in modern birds is a projection of the neighboring heel bone, rather than the ankle bone. The recent study, however, revealed a new insight. Researchers found that the ascending process does not develop from either the heel bone or the ankle bone; rather, it develops from a third element known as the intermedium. (Scroll to read more...)

"It puts the final nail in the anti-dinosaur coffin" Jacques Gauthier, a vertebrate paleontologist and professor at Yale University, said in a statement. "The dinosaurian ascending process is retained in all birds, though it has changed its association from ankle to heel bones in neognath birds."

Today, there are two living clades of birds: Paleognaths and neognaths. Throughout the ancient lineage of paleognath birds such as ostriches the intermedium grows closer to the anklebone, similar to those found in dinosaurs. However, neognaths – which includes most species of living birds – have an intermedium that comes closer to the heel bone. This gives the impression that it is a different structure, but in reality it is the same.

Additionally, the recent study revealed a surprising case of evolutionary reversal with ancient traits have reappearing in modern birds. The intermedium fuses to the anklebone shortly after it forms in the embryos of land egg-laying animals such as crocodiles, lizards and turtles. But this is not the case for birds, who develop more like their distant amphibian relatives. This is surprising because modern birds clearly belong within the category of landegg-laying animals, suggesting they have somehow resurrected a long-lost amphibian-like developmental pattern with three separate elements, one of which becomes the dinosaurian ascending process. 

The study was recently published in the journal Nature Communications. 

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