National Geographic News
An illustration depicts dinosaurs and birds.

The ancestors of modern birds developed a crouched stance to compensate for larger forelimbs, or wings.

Illustration courtesy Luis Rey

Brian Handwerk

for National Geographic News

Published April 24, 2013

Watch a bird's odd, bent-legged gait and you'll see an evolutionary adaptation born millions of years ago in its dinosaur ancestors while they were still confined to the ground.

The crouched stance developed to compensate for the growth of larger forelimbs that eventually made flight possible, according to new research that digitally "fleshed out" fossils to show physical changes over the eons as bipedal dinosaurs evolved into birds. (Read about the evolution of feathers in National Geographic magazine.)

Birds and humans are the most common bipedal species in the modern world, but their legs are strikingly different. Humans are basically straight-legged, which allows their bones to support their resting body weight. But bird legs are bent into a zigzag, putting them in a crouched position that requires much more muscular effort to stand.

"It's more efficient to bear weight passively, in a straight line down your long bones [like] a pillar," said Vivian Allen of the Royal Veterinary College's Structure and Motion Lab at the University of London, and co-author of a study published this week in the journal Nature.

"In a crouch you have to use your muscles a lot more to resist gravity. Think about how much easier it is to hold something when you're standing up straight than it is when you are crouched down. So if you were designing an animal, this seems slightly odd from the perspective of mechanical principles." (Related: "Baby Dinosaurs Flexed Muscles Inside Their Eggs.")

Surprising Results

To uncover the origins of this odd stance, Allen's team used fossils to create 3-D skeletons of 17 archosaurs, a group that includes both dinosaurs and living land animals like birds. They then digitally fleshed out their bodies, using detailed muscular reconstructions and estimates taken from CT scans of living relatives, to reproduce the body shapes of the extinct animals.

The results allowed the team to trace biomechanical changes over time from 245-million-year-old, crocodile-like quadrupeds, to the earliest winged and feathered dinosaur Archaeopteryx 150 million years ago, to modern birds like the red junglefowl (Gallus gallus).

Big, heavy-tailed beasts like Tyrannosaurus rex gave way to animals with thinner or shorter tails. And although this most obvious physical change has long been noted, the new research suggests these modifications didn't drive the development of modern bird posture as was commonly believed.

"We'd never doubted the hypothesis that the tail was responsible for the major changes in dinosaur balance and posture," co-author and Royal Veterinary College evolutionary biomechanics expert John R. Hutchinson said in a statement.

"The tail is the most obvious change if you look at dinosaur bodies. But as we analyzed, and reanalyzed, and punishingly scrutinized our data, we gradually realized that everyone had forgotten to check what influence the forelimbs had on balance and posture, and that this influence was greater than that of the tail or other parts of the body."

The enlarged forelimbs that gave rise to a crouching posture, and eventually flight, can be seen in winged dinosaurs like Microraptor and Archaeopteryx, and in the feathered Velociraptor. (Pictures: "Dinosaur's Flashy Feathers Revealed.")

"The point we found is that if you get these big forearms, it does change the way your hind legs work as well," Allen explained. There's a pretty good statistical relationship between increasing limb mass and the position of a dinosaur's center of mass, he added, but none between the location of the center of mass and the mass of the tail.

To imagine how the adaptation occurred, Allen said, you must remember that a biped's foot position is strongly related to its center of mass.

"You might imagine a seesaw," he said. "If you move the hinge point of the seesaw you also have to move the support base of it. So you need to have your feet pretty well under the center of mass."

If you move that center of mass forward—which is what happens with larger forelimbs like wings—you also have to move your foot forward, continued Allen. "And that means that the limb has to bend at the joints and become more crouched. If you look at an ostrich or an emu, you'll see a femur that's almost horizontal and very short."

The Journey to Flight

Hans-Dieter Sues, curator of vertebrate paleontology at the Smithsonian's National Museum of Natural History in Washington, D.C., wasn't involved in the study but said he found the analysis quite compelling.

"Past studies had focused on the change in tail length, but this new paper is the first to look at the impact of the forelimbs on posture," he said.

What he liked about the study, Sues added, was the fact that the researchers tracked the changes in posture in the context of the evolutionary history of early birds and their dinosaur relatives.

The growing forelimbs appeared well before any dinosaurs took flight, so other needs drove their initial development.

"The forelimbs presumably changed in length in the stem group of birds for the purpose of seizing and/or manipulating prey," Sues said. He noted, however, that these changes were a significant step on the road to flight.

The long forelimbs definitely became wings in birds, he said, and "the tails became reduced as other flight adaptations in the skeleton developed."

Allen said a future study might reveal more about how some dinosaurs finally got off the ground by looking at how the center of lift evolved with early gliders and flyers. "That's more difficult and you really need to have an idea of the shape and size of the feathers," she said. "But enough specimens exist that someone could give it a go."

kayla spurlock
kayla spurlock

i wonder if billions of years in the future the scientist will say "the elephants that lived in 2013 are very different from today and in fact, they had feathers!" scientists don't know anything they just think they do. what mammal have feathers? we will never know what a dinosaur looks like so why can't we just live and know that dinosaurs are the coolest thing that we've ever seen and not make it into a huge debate.

Babu Ranganathan
Babu Ranganathan


All this talk of dinosaurs having feathers is nonsense and is not even accepted by most evolutionists. Some evolutionists have a certain interpretation of a fossil and this interpretation becomes extrapolate beyond into imagination and then you have artists giving expression to that imagination, but the media is all for such "news." Please read my popular Internet articles below.

Recently it was thought they had discovered fossils of dinosaurs with feathers until they found out that the so-called feathers were really scales, which only had the appearance of feathers. Scientists theorize the scales took upon a feather-like appearance during some brief stage of decomposition before being fossilized. Some structures may actually be fossilized skin filaments that some evolutionists interpret as proto-feathers.


Visit my newest and very popular Internet site: THE SCIENCE SUPPORTING CREATION

Babu G. Ranganathan*
(B.A. theology/biology)

I am author of the popular Internet article: TRADITIONAL DOCTRINE OF HELL EVOLVED FROM GREEK ROOTS

* I have had the privilege of being recognized in the 24th edition of Marquis "Who's Who in The East" for my writings on religion and science, and I have given very successful lectures (with question and answer time afterwards) before evolutionist science faculty and students at various colleges and universities.

Seth Forbis
Seth Forbis

what is with scientist these days?!?! they make these claims, like they always do, like they always should, but instead of claiming that they have theories or a hypothesis they basically claim their suppositions are facts every time...maybe it's just the journalists reporting them but science doesn't allow for infallable theories, and any scientist worth his salt would know that without years or research and hundreds of corroborating studies you can't claim anything as fact, yet with every science related story being published there is some claim that a barely tested idea must be the unequivocal answer to all of our questions...things that could never ever be definitively proven with the technology available today...

look at the higgs boson, they found the tiniest scrap of evidence that there may be a partical that might exist, yet every story on the internet says scientist have found it and that it is now part of our known world...

in reality scientists are using social media and online news outlets to hype the studies and results they are getting, so that they can generate bigger buzz and get more funding for their work...bastardizing the scientific method for grant money!

mark Riggle
mark Riggle

Birds do have a different bipedal gait than MOST humans. Children with cerebral palsy walk with one of 3 gaits; none of them is what is considered the typical human adult gait.  The most common gait they adopt is a toe-walk. That gait is exactly the bird gait; in both the CP child and the bird, the lower leg swings forward (and also somewhat inward for CP) until the toes jam into the ground.  This has two effects: the first is that the friction with the ground slows the toe-area's forward speed; the second is the geometry of the foot and knee then greatly increase the downward pressure which also greatly increases the friction with the ground.  That friction rapidly stops the foot's forward motion (like a hinged doorstop will do when it drops with the door moving). It is a stop so rapid it creates a shockwave in the tibia and then in the femur.  Unfortunately, in CP children, that shockwave will eventually damage the knee, and hip, and also twists the femur.

So the prior post asks the extremely relevant question: why (in a neural system way) did the therapods (the bipedal archosaur group) become bipedal and walk they way they do?  Does that answer tell us why CP children walk that way?  Interestingly, the other two gaits that are adopted by CP children also have strong shockwave generating impact strikes (and those also cause extreme joint and bone damage).  

And now add on to that that the normal adult walking gait also produces a strong impact shockwave from the heel strike.  That shockwave travels up the skeleton and then in some people may cause bone and joint problems (but much less than for CP for which it always occurs).  Even the human gaits found during development, from 1st toddler  steps until the mature adult gait is reached, all have this shockwave impact.  

To me, a shockwave generating impact seems a pretty common geature for the bipeds.  What does that mean?

John Jackson
John Jackson

Interesting that the forelimbs have a greater effect than the tail, but good that a reasonable approach has been made.  Does anyone still remember the Burgers and Chiappe paper that showed a standing dinobird with the c. of mass nowhere near above the feet?!

Actually, understanding the balance isn't as interesting as understanding why birds, and dinosaurs, and certain other archosaurs, were bipedal in the first place.  We do have a good explanation:


Some other points on the paper:

Saying: "the earliest winged and feathered dinosaur Archaeopteryx" cannot be right in any scheme.  How about Anchiornis as a winged and feathered dinobird?  Not top mention a significant few others.

"Big, heavy-tailed beasts likeTyrannosaurus rex gave way to animals with thinner or shorter tails".  That would be... mice?  It's not just an error that T rex itself had no descendants, but types like that didn't lead to birds either.

John Jackson
John Jackson

@mark Riggle  Hi MR- I don't claim to be an expert on gait details, but I think a shockwave is inevitable to some extent because a fair bit of forward thrust comes from the leg being accelerated backwards while in mid air, so that when the foot hits the ground it's moving backwards.  Otherwise it would tend to give the animal a slight backwards momentum on each footfall.  In order to gain any useful forward impetus on footfall and make use of friction, it is necessary to press hard on the ground, especially when accelerating rapidly.  For that reason, and because the time in contact with the ground is so short, slamming hard into the ground is inevitable really.

As for heel shock in humans - I believe people accustomed to running without shoes tend to run on their toes more (or at least the ball of the foot).

I'd say the CP humans you mention don't necessarily have a gait whose characteristics need to be explained in terms of evolutionary benefit.

As for dinobirds and indeed all bipedal archosaurs (inc croc ancestors and pterosaurs), they were bipedal because they had another use for their arms somewhat inconsistent with ground contact (and it's worth noting that obligate bipeds can't be cold).


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