Dinosaur Arms to Bird Wings: It's All in the Wrist

One of the last niggling doubts about the link between dinosaurs and birds may be settled by a new study that shows how bird wrists evolved from those of their dinosaur predecessors.
The study, reported in the Journal PLOS Biology, shows how nine dinosaurian wrist bones were reduced over millions of years of evolution to just four wrist bones in modern day birds.
"This discovery clarifies how dinosaur arms became bird wings," said one of the study's authors, Dr Alexander Vargas of the University of Chile in Santiago.
"It shows that some bones fused, other bones disappeared, and one bone disappeared and then reappeared in evolution."
Skeletal similarities between theropod dinosaurs and birds provide some of the strongest evidence showing how birds developed from dinosaurs. But the evolution of straight dinosaur wrists into hyperflexible wrists allowing birds to fold their wings when not flying, has remained a point of contention between palaeontologists and some developmental biologists.
Among the structures in question is a half-moon shaped wrist bone called the semilunate which is found in dinosaurs and looks very similar to a wrist bone also found in birds.
The semilunate originated as two separate dinosaur bones which eventually fused into a single bone. However some developmental biologists claim it evolved as a single bone in birds, and so isn't the same bone as that found in dinosaurs.
To help settle the debate, Vargas and colleagues examined the wrist bones of dinosaur fossils in the collections from several museums, and compared them to new developmental data from seven different species of modern birds.
"We developed a new technique called whole-mount immunostaining, which allows us to observe skeleton development better than ever before, including the expression of proteins inside embryonic cartilage," said Vargas.
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