LIFE & EVOLUTION
Robot re-creates a tetrapod’s moves
Early four-limbed animal had a relatively modern style of walking
B Y CAROLYN GRAMLING
Orobates pabsti lived between 280 mil-
lion and 290 million years ago, but it was
pretty advanced at doing the locomotion.
Using computer simulations, fossil
footprints, a re-created skeleton and
a walking robot, scientists found that
this ancient four-footed creature had a
surprisingly efficient gait. That result
suggests that evolving a more advanced
way of walking may not have been as
closely linked to the later diversifica-
tion of tetrapods as once thought, the
researchers report in the Jan. 17 Nature.
O. pabsti was an early amniote, a group
that arose about 350 million years ago
and includes reptiles and mammals.
Unlike amphibians, amniotes can live
entirely on land. Protective membranes
surrounding embryos allow amniotes to
bypass a tadpole-type life stage: Reptile
and bird eggs can be laid on land; mam-
mal embryos stay within the mother.
The amniotic membrane “is regarded
as a key evolutionary innovation, to be
able to colonize different habitats,” says
John Nyakatura, an evolutionary biolo-
gist at Humboldt University of Berlin who
led the new study. Understanding how
early amniotes walked on land could help
scientists better understand the origins
of amniotes themselves, and how they
A robot called OroBOT helped researchers analyze different gaits that Orobates pabsti, a four-legged animal that lived about 290 million years ago, might have used.
diversified across the continents, he says.
Researchers first described O. pabsti
in 2004, following the discovery of
beautifully preserved fossils at a site in
Germany known as the Bromacker locality. A few years later, researchers linked
the creature to a series of footprints,
called a trackway, found at the same place.
There’s more to visualizing walking
than knowing where an animal put its
feet. Scientists use various approaches to
study the locomotion of extinct animals,
including studying trackways, examining anatomy or even building robots,
says study coauthor and bioroboticist
Kamilo Melo of École Polytechnique
Fédérale de Lausanne in Switzerland.
What’s different about the new work,
Melo says, is that it combines several
tactics to get the best possible approximation of the gait.
The researchers first re-created the
skeleton to constrain the possible ranges
of motion of the legs. “You create a marionette and see what amount of angle
each joint can move,” Melo says. The
scientists also created a simulation that
included factors such as gravity, friction
and balance to examine how the animal
might have walked.
The team also looked to modern four-
footed species, including salamanders,
skinks, caimans and iguanas, to study
possible ranges of motion for tetrapods.
Skinks and salamanders, for example,
hold their bodies lower with their limbs
more sprawled out to the side, while
caimans tend to have more erect limbs.
Finally, the scientists created a tetra-
pod robot, dubbed the OroBOT, to act
out potential gaits and match the prints
they created to known fossil tracks.
The researchers ultimately considered
512 possible types of movement, scoring
them on energy consumption, balance
and precision to see how well the gaits
reproduced the fossil tracks without
slipping or sliding.
The data suggest that O. pabsti had
a relatively advanced style of walking,
one that many researchers previously
thought would have been possible only
in later tetrapods. O. pabsti probably
held its belly off the ground and had a
stable, efficient gait without a lot of side-to-side, salamander-like undulations.
That style of walking would have helped
the animal conserve energy.
Stuart Sumida, a vertebrate paleontologist at California State University, San
Bernardino, praises the multipronged
design of the study. The biomechani-cal analysis, he notes, has confirmed
something that previously was strongly
suspected only by the fossil’s finders: that
O. pabsti was indeed a fully terrestrial animal with a relatively modern gait.
Sumida and others have demonstrated
that amniotes from the Bromacker locality had a range of walking styles. Some had
erect limbs like O. pabsti, some sprawled,
and at least one animal walked on two
legs. “What these studies are showing is
that when amniotes first showed up, they
were doing lots of things more quickly
than we ever realized,” he says.
The new findings are just a start,
Nyakatura says. His group hopes its
approach will be a jumping-off point to
better understand O. pabsti and to examine other puzzles, such as the evolution
of active flight, bipedal locomotion in
human ancestors and the transition
from terrestrial to aquatic in marine
mammals. “We have a whole bag of interesting things to study,” he says. s