Shortening of tails that took place over a 100 million years ago, gave earliest birds the chance to develop versatile legs that enabled the evolutionary diversification.

Early birds such as Confuciusornis, Eoenantiornis, and Hongshanornis lived alongside their dinosaur kin approximately 145 million years ago. They are believed to have evolved from  theropod dinosaurs. These birds had bony tails, which have now been replaced by feathers in modern birds.

According to a new study, conducted by researchers from Oxford University, the shortening of tails that took place over a 100 million years ago, gave the earliest birds a chance to develop versatile legs enabling evolutionary diversification of early birds.

During the time when these early birds existed alongside dinosaurs, they had already evolved powered flight, necessitating changes to their forelimbs. Hence in the new study, researchers looked at fossils of earliest birds from the Cretaceous Period to understand how this new lifestyle led to changes in their hind limbs.

"These early birds were not as sophisticated as the birds we know today -- if modern birds have evolved to be like stealth bombers then these were more like biplanes," said Dr Roger Benson of Oxford University's Department of Earth Sciences, who led the research in a press release. "Yet what surprised us was that despite some still having primitive traits, such as teeth, these early birds display an incredibly diverse array of versatile legs."

The team of researchers measured the main parts of the legs of early birds and compared these measurement to those of their dinosaur relative.

"Our work shows that, whilst they may have started off as just another type of dinosaur, birds quickly made a rather special evolutionary breakthrough that gave them abilities and advantages that their dinosaur cousins didn't have," said Dr Rogers. "Key to this special 'birdness' was losing the long bony dinosaur tail - as soon as this happened it freed up their legs to evolve to become highly versatile and adaptable tools that opened up new ecological niches."

According to the researchers, it was the development of these versatile legs rather than the pwered flight that enabled the evolutionary diversification of early birds to proceed faster than dinosaurs.