Ancient Birds' Hind Leg Evolution Set Stage for Species Diversity

The evolution of birds has a history as old as the dinosaurs, and new research on avian evolution has documented through the fossil record just how birds adapted further evolutionary advantages after acquiring powered flight.

During the Cretaceous Period about 150 million years ago, ancient birds such as Confuciusornis, Eoenantiornis and Hongshanornis were already adept at powered flight, having evolved the necessary changes to their forelimbs.

In an attempt to better understand these early birds, an international team of researchers sought to examine the evolution of the birds' hind limbs after the creatures had already evolved the required apparatuses for flight.

After collecting detailed measurements from early bird fossils from around the world, the collective analysis revealed a progressive loss of the birds' boney tails after the creatures gained the ability to fly. This evolutionary landmark paved the way for a great proliferation of diversity in the hind limbs of ancient birds, which, in turn, became the foundation for the amazing variety we see in modern birds today.

"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 research leader Roger Benson of Oxford University's Department of Earth Sciences."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 birds' upper legs, shins and feet were compared to those of the birds dinosaur relatives, and in doing so the researchers were able to determine whether bird leg evolution was in any way exceptional to the evolution of their dinosaur contemporaries.

"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," Rogers said. "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."

The researchers contend that it was the rapid and diverse evolution of hind legs -- rather than wings -- that marked the early evolutionary diversification of early birds compared to dinosaurs.

The research, conducted in tandem by the University of Oxford and University of the Witwatersrand, South Africa, is published in the journal Proceedings of the Royal Society B.