How birds got their wings: Fossil data show scaling of limbs altered as birds originated from dinosaurs

Sep 18, 2013
Archaeopteryx, believed to be a transitional species between theropod dinosaurs and birds, had longer forelimbs and shorter hind limbs than its ancestors. Credit: Michael Skrepnick

Birds originated from a group of small, meat-eating theropod dinosaurs called maniraptorans sometime around 150 million years ago. Recent findings from around the world show that many maniraptorans were very bird-like, with feathers, hollow bones, small body sizes and high metabolic rates.

But the question remains, at what point did forelimbs evolve into wings – making it possible to fly?

McGill University professor Hans Larsson and a former graduate student, Alexander Dececchi, set out to answer that question by examining fossil data, greatly expanded in recent years, from the period marking the origin of birds.

In a study published in the September issue of Evolution, Larsson and Dececchi find that throughout most of the history of , limb lengths showed a relatively stable scaling relationship to body size. This is despite a 5000-fold difference in mass between Tyrannosaurus rex and the smallest feathered from China. This limb scaling changed, however, at the origin of birds, when both the forelimbs and hind limbs underwent a dramatic decoupling from body size. This change may have been critical in allowing early birds to evolve flight, and then to exploit the , the authors conclude.

As forelimbs lengthened, they became long enough to serve as an airfoil, allowing for the evolution of powered flight. When coupled with the shrinking of the hind limbs, this helped refine flight control and efficiency in . Shorter legs would have aided in reducing drag during flight—the reason modern birds tuck their legs as they fly—and also in perching and moving about on small branches in trees. This combination of better wings with more compact legs would have been critical for the survival of birds in a time when another group of flying reptiles, the , dominated the skies and competed for food.

"Our findings suggest that birds underwent an in their developmental mechanisms, such that their forelimbs and hind limbs became subject to different length controls," says Larsson, Canada Research Chair in Macroevolution at McGill's Redpath Museum. Deviations from the rules of how an animal's limbs scale with changes in body size—another example is the relatively long legs and short arms of humans—usually indicate some major shift in function or behaviour. "This decoupling may be fundamental to the success of birds, the most diverse class of land vertebrates on Earth today."

"The origin of birds and powered flight is a classic major evolutionary transition," says Dececchi, now a postdoctoral researcher at the University of South Dakota. "Our findings suggest that the limb lengths of birds had to be dissociated from general body size before they could radiate so successfully. It may be that this fact is what allowed them to become more than just another lineage of maniraptorans and led them to expand to the wide range of limb shapes and sizes present in today's birds."

"This work, coupled with our previous findings that the ancestors of birds were not tree dwellers, does much to illuminate the ecology of bird antecedents." says Dr. Dececchi. "Knowing where birds came from, and how they got to where they are now, is crucial for understanding how the modern world came to look the way it is."

Explore further: Shortening tails gave early birds a leg up

More information: onlinelibrary.wiley.com/doi/10.1111/evo.12150/abstract

Related Stories

Shortening tails gave early birds a leg up

Aug 13, 2013

A radical shortening of their bony tails over 100 million years ago enabled the earliest birds to develop versatile legs that gave them an evolutionary edge, a new study shows.

Recommended for you

Drones unearth more details about Chaco culture

Apr 22, 2014

Recently published research describes how archaeologists outfitted a customized drone with a heat-sensing camera to unearth what they believe are ceremonial pits and other features at the site of an ancient village in New ...

User comments : 0

More news stories

Male-biased tweeting

Today women take an active part in public life. Without a doubt, they also converse with other women. In fact, they even talk to each other about other things besides men. As banal as it sounds, this is far ...

Developing nations ride a motorcycle boom

Asia's rapidly developing economies should prepare for a full-throttle increase in motorcycle numbers as average incomes increase, a new study from The Australian National University has found.

High-calorie and low-nutrient foods in kids' TV

Fruits and vegetables are often displayed in the popular Swedish children's TV show Bolibompa, but there are also plenty of high-sugar foods. A new study from the University of Gothenburg explores how food is portrayed in ...