Shortened Tails of Early Birds Caused Evolutionary Explosion in Leg Diversity

The early bird gets the worm--especially when that worm happens to be an evolutionary edge. Scientists have discovered that a radical shortening of their bony tails about 100 million years ago gave birds a leg-up in evolution--literally. It allowed them to develop versatile legs which gave them a huge advantage.

In order to learn a little bit more about the ancient birds on our planet, researchers examined fossils of the earliest birds from the Cretaceous period. These creatures existed about 145 to 66 million years ago and included species such as Confuciusornis, Eoenantornis and Hongsharnornis. These birds existed at the same time of dinosaurs, using powered flight to soar through the skies.

What scientists didn't know, though, was how bird legs evolved. The researchers made detailed measurements of early bird fossils from all over the world, including China, North America and South America. After studying these fossils, the researchers found that the loss of their long bony tails led to an explosion of diversity in their hind limbs.

"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 Roger Benson, one of the researchers, in a news release. "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."

In fact, it was this evolution of highly versatile legs that led to the relatively rapid diversification of birds. Dinosaurs, in contrast, diversified slowly and didn't show the same range of leg forms that birds did.

"These 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 Benson. "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 findings are important for better understanding the evolutionary history of birds. In addition, it may show why birds were able to survive into the present day while dinosaurs ended up becoming extinct.

The findings are published in the journal Proceedings of the Royal Society B.