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7/30/2019 Ngo 20130426 Bird
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Birds' " Crouching" Gait Born in Dinosaur
Ancestors
Getting closer to the ground helped dinosaurs take flight, says a new study.
The ancestors of modern birds developed a crouched stance to compensate for larger forelimbs,
or wings.
Illustration courtesy Luis Rey
Brian Handwerk
forNational Geographic News
Published April 24, 2013
Watch a bird's odd, bent-legged gait and you'll see anevolutionary adaptationborn millions of
years ago in itsdinosaurancestors 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
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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 Labat the University of London, and
co-author ofa study published this weekin 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 likeTyrannosaurus rexgave 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 J ohn 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 likeMicroraptorandArchaeopteryx, and in the featheredVelociraptor. (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.
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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 forwardwhich is what happens with larger forelimbs like wingsyou
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." Led street light
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."
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mark Riggle
16 hours ago
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
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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?