Birds Have Retained “Unique” Ascending Process of Dinosaur Ankle, But Re-Evolved Its Developmental Mechanism

In the 19th century, Darwin’s most vocal scientific advocate was Thomas Henry Huxley, who is also remembered as a pioneer of the hypotheses that birds are living dinosaurs. Huxley noticed several similarities of the skeleton of living birds and extinct dinosaurs, among them, a pointed portion of the anklebone projecting upwards onto the shank bone (also known as the “drumstick”). This “ascending process” is well known to specialists as a unique trait of dinosaurs. However, until the late 20th century, many scientists were doubtful about the dinosaur-bird link. Some pointed out that the ascending process in most birds was a projection of the neighboring heel bone, rather than the anklebone. If so, it would not be comparable, and would not support the dinosaur-bird link. Some argued that, in bird embryos, the ascending process develops from the anklebone in dinosaur-like fashion, while others considered that its development in birds is unique and different from dinosaurs. Nowadays, the dinosaur-bird link is mainstream science, thanks to new methods of data analysis, and a dense series of intermediate fossils (including feathered dinosaurs). However, the disagreements about the composition and embryology of the avian ankle were never clarified fully. Now, a new study, published online on November 13, 2015 in an open-access article in Nature Communications, and authored by Luis Ossa, Ph.D., Jorge Mpodozis, Ph.D., and Alexander Vargas, Ph.D., all from the University of Chile, provides a careful re-examination of ankle development in six different major groups of birds, selected specifically to clarify conditions in their last common ancestor. The study also utilizes new techniques that allow three-dimensional analysis of fluorescent embryonic skeletons, using advanced spin-disc confocal microscopy and software.
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