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As dinosaurs evolved into birds, their straight wrists gradually transformed into flexible wings.

The process, over the course of millions of years, altered their bone structure – but exactly how this happened has been a mystery to scientists.

Now researchers in Chile have discovered that the change involved the disappearance and reappearance of a bone known as the 'pisiform'.

Vanishing act: The pisiformbone (labeled 'ue') forms and vanishes during the development of a chicken embryo. The pisiform allowed bird wings to remain rigid on the upstroke. The study found it disappeared in bird-like dinosaurs, but modern birds later evolved to once again use this tiny bone

The pisiform allowed bird wings to remain rigid on the upstroke. The study found it disappeared in bird-like dinosaurs, but modern birds later evolved to once again use this tiny bone.

'This idea that a bone can disappear and reappear in evolution has been resisted a lot in evolutionary biology' Alexander Vargas, a researchers University of Chile told Stephanie Pappas at LiveScience.

While similarities in the wrists of dinosaurs and birds have long been known, the exact link between the two has been unclear.

Early dinosaur ancestors had around nine wrist bones, but birds have only kept four during the course of evolution.

Matching up: Pictured in A is the wrist bones of early dinosaurs Heterodontosaurus and Coelophysis. The coloured areas represent bones that are potentially still present in the avian wrist

Scientists don't know how these bones disappeared. Professor Vargas said part of the problem is that palaeontologists look at fossils, while biologists focus on embryos.

Scientists at Chile University brought the research together by looking at fossils as well as studying bird anatomy.

The team also traced certain at proteins in 3D embryonic skeletons that were linked to the creation of collagen which makes up connective tissue.

The method allowed them to uncover whether a bone was fused together from two separate structure, or evolved as a single piece.

As well as the disappearance and appearance of the pisiform bone, they found that a half-moon-shaped bone called the semilunate in birds, is the fusion of two wrist bones from dinosaurs.

The study, reported in the Journal PLOS Biology, follows research last week found there was 'missing link' between modern-day birds and their ancestors - as was previously thought.

Instead, an 'evolutionary explosion' 150 million years ago ultimately led to the thousands of avian species that are in existence today.

Research led by the University of Edinburgh proposes new evidence for how birds evolved from dinosaurs (illustration shown). Previously it had been thought there was a 'missing link' between the two.  But now it's thought an 'evolutionary explosion' may have led to the thousands of avian species alive today