Pterosaurs have long suffered an identity crisis. Pop
culture heedlessly — and wrongly — lumps
these extinct flying lizards in with dinosaurs. Even
paleontologists assumed that because the
creatures flew, they were birdlike in many ways, such as
using only two legs to take flight.
Now comes what is believed to be first-time evidence
that launching some 500 pounds of
reptilian heft into flight required pterosaurs to use four
limbs: Two were ultra-strong wings that,
when folded and balanced on a knuckle, served as front
"legs" that helped the creature to walk — and
Publishing in Zitteliana, Michael B. Habib, of
the Center for
Functional Anatomy and Evolution at
the Johns Hopkins University School of Medicine, reports
his comparison of bone strength in the
limbs of pterosaurs to that of birds and concludes that
pterosaurs had much stronger "arms" than
legs. The reverse is true of birds.
"We've all seen birds take off, so that's what's most
familiar," Habib said. "But with pterosaurs,
extinct 65 million years and with a fossil history that
goes back 250 million years, what's familiar isn't
A supersized glitch is inherent in the traditional
bipedal launch model, Habib noted: "If a
creature takes off like a bird, it should only be able to
get as big as the biggest bird."
Birds use legs to launch, wings to flap. They don't
get launch power from wings or flight power
from legs. In fact, when a bird is aloft, its legs become
payload, or cargo. The muscle on the two back
limbs that provides the power to launch must be carried and
therefore limits size. Released of that
handicap by employing all four legs to launch, giant
pterosaurs could fly despite the fact that they
were roughly the same size and shape as modern-day
"The difference between pterosaurs and birds with
regard to critical mechanical properties is
very, very large, especially when you're talking about the
big pterosaurs, Habib said. "As the size gets
bigger, the difference gets bigger too."
For example, the wings of these fantastic hairy
reptiles, most notably those of Quetzalcoatlus
northropi, which spanned to an impressive 35 feet when the
creatures were aloft, propelled the
creatures into the air during takeoffs that Habib described
as leap-frogging long jumps. "Pterosaurs
had long, huge front limbs, so no partner was required," he
said. "Then, with wings snapping out, off
Using computer scans to obtain cross-sectional images
and geometric data for 155 bird
specimens representing 20 species, Habib calculated the
strengths of bones in bird limbs and
compared these to three species of pterosaurs, the bone
strengths of which he calculated using
measurements from previously published sources. Structural
strength, taking into account length and
diameter, among other things, is a measure of how much
force a bone can take before it fractures.
Habib also spent time crunching the numbers using the
old, bipedal launch model and simply
couldn't find a mathematical solution that would enable the
largest of the pterosaurs — using hind legs
alone — to launch at all.
"But using all four legs, it takes less than a second
to get off of flat ground, no wind, no cliffs,"
he said. "This was a good thing to be able to do if you
lived in the late Cretaceous period and there
were hungry tyrannosaurs wandering around."
It stands to reason that a large-bodied animal needing
to produce lots of power at takeoff
would use four legs instead of two, Habib said. "We put V8
engines in our biggest, heaviest cars, not V-
4s, like the one in my Camry," he said.
Assumption and convention — rather than reason
or data — held sway for centuries, ever since the
classical bipedal model of pterosaur takeoff was first
championed, he noted.
The research was funded by the Jurassic Foundation.
Habib is the sole author of the paper.