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.
The technique allowed the authors to determine that the embryonic semilunate in birds evolves as two separate cartilages which fuse into a single bone, consistent with what palaeontologists had been saying.
"These findings eliminate persistent doubts that existed over exactly how the bones of the wrist evolved, and iron out arguments about wrist development being incompatible with birds originating from dinosaurs," said Vargas.
The study also produced an interesting surprise for the research team when they discovered a wrist bone called the pisiform, which was present in early sauropod (four-legged, long-tailed, long-necked) dinosaurs, but had disappeared in later theropod (two-legged, two-armed) dinosaurs.
The authors found the pisiform had reappeared in early birds, probably as an adaptation for flight, where it allows transmission of force on the downstroke while restricting flexibility on the upstroke.
"We think the pisiform was lost when dinosaurs became bipedal," said Vargas. "Quadrupedal animals used this bone because they walk with their forelimbs, but bipedal dinosaurs no longer walked with their forelimbs and lost the bone. However they regained it when they began using their forelimbs for locomotion in flight."
This is a compelling scenario for a rare case of evolutionary reversal, said Professor Mike Archer of the University of New South Wales, who was not involved in the study.
"That's the most fascinating thing coming out of this work, because all of a sudden we're understanding how complex and yet how flexible and deliciously plastic embryological development can be," said Archer.
"There may be a gene lying around dormant and doing nothing, which suddenly gets kick-started and produces a structure that had been lost in the ancestor of the same animal."