The once fearsome and enormous dinosaurs have evolved into smaller creatures that fly. How this exactly happened still poses a number of questions but a group of scientists appear to have found a missing piece in the puzzle by revealing how the large ancient reptiles developed their bird wings.

In an evolutionary process that occurred over a span of millions of years, the wrists of the dinosaurs gradually changed into flexible wings. Findings of a new study conducted by researchers from the University of Chile in Santiago showed how this happened.

For the study "New Developmental Evidence Clarifies the Evolution of Wrist Bones in the Dinosaur-Bird Transition" which was published in the journal PLOS Biology on Sept. 30, Alexander Vargas, a biologist from the University of Chile, and colleagues examined the fossils of birdlike dinosaurs and the anatomy of modern day birds. They also used a new technique that allowed them to trace the prenatal development of the modern bird as well as see whether a bone was the product of the fusing of two separate structures or evolved from one component.

The researchers were then able to reconstruct how the bird's wings have evolved and found that the bone known as radiale among paleontologists and which ornithologists call as scapholunare are one and the same. The researchers likewise found that the bone known as the semilunate is actually the fusion of two different wrist bones found in dinosaurs.

The bone known as the pisiform which develops inside a wrist tendon, however, has disappeared and reappeared again. The pisiform disappeared when birdlike dinosaurs called therapods emerged on Earth about 230 million years ago but the bone was found to have re-emerged in modern birds.

"This discovery clarifies how dinosaur arms became bird wings," Vargas said.  "It shows that some bones fused, other bones disappeared, and one bone disappeared and then reappeared in evolution."

Vargas said that the pisiform bone is crucial for the animals that walk using their four legs and this may have disappeared when the birdlike dinosaurs started to walk on their two legs. The tiny bone, however, is also crucial in flight and this may be the reason why it has come back in modern birds.

The pisiform is absent in bird-like dinosaurs, which are known from several articulated specimens," the researchers wrote. "The combined data provide compelling evidence of a remarkable evolutionary reversal: A large, ossified pisiform re-evolved in the lineage leading to birds, after a period in which it was either absent, nonossified, or very small, consistently escaping fossil preservation."

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