Ancient “terror
bird” used powerful
beak to jab like an
agile boxer
International team
uses computer
modeling to
reconstruct kills
of prehistoric
predator

ATHENS, Ohio (Aug.
18, 2010)—The
ancient “terror
bird”
Andalgalornis
couldn’t fly, but it
used its unusually
large, rigid
skull—coupled with a
hawk-like hooked
beak—for a fighting
strategy reminiscent
of boxer Muhammad
Ali. The agile
creature repeatedly
attacked and
retreated, landing
well-targeted,
hatchet-like jabs to
take down its prey,
according to a new
study published this
week in the online,
open-access journal
PLoS ONE by
an international
team of scientists.
The study is the
first detailed look
at the predatory
style of a member of
an extinct group of
large, flightless
birds known
scientifically as
phorusrhacids
but popularly
labeled “terror
birds” because of
their fearsome skull
and often imposing
size. Terror birds
evolved about 60
million years ago in
isolation in South
America, an island
continent until the
last few million
years, radiating
into about 18 known
species ranging in
size up to the
7-foot-tall (2.1
meters) Kelenken.
Because terror birds
have no close
analogs among
modern-day birds,
their life habits
have been shrouded
in mystery. Now, a
multinational team
of scientists has
performed the most
sophisticated study
to date of the form,
function and
predatory behavior
of a terror bird,
using CT scanning
and advanced
engineering methods.
“No one has ever
attempted such a
comprehensive
biomechanical
analysis of a terror
bird,” said study
lead author Federico
Degrange of the
Museo de La
Plata/CONICET in
Argentina, who is
conducting his
doctoral research on
the evolution of
terror birds. “We
need to figure out
the ecological role
that these amazing
birds played if we
really want to
understand how the
unusual ecosystems
of South America
evolved over the
past 60 million
years.”
The terror bird
under study is
called
Andalgalornis
and lived in
northwestern
Argentina about six
million years ago.
It was a mid-sized
terror bird,
standing about 4.5
feet tall (1.4
meters) and weighing
in at a fleet-footed
90 pounds (40 kg).
Like all terror
birds, its skull was
relatively enormous
(14.5 inches or 37
centimeters) with a
deep narrow bill
armed with a
powerful, hawk-like
hook.
Article co-author
Lawrence Witmer of
the Ohio University
College of
Osteopathic Medicine
ran a complete skull
of Andalgalornis
through a CT
scanner, giving the
team a glimpse into
the inner
architecture of the
skull. The scans
revealed to Witmer,
Degrange and article
co-author Claudia
Tambussi, also from
the Museo de La
Plata/CONICET and
Degrange’s PhD
advisor, that
Andalgalornis
was unlike other
birds because it had
evolved a highly
rigid skull.
“Birds generally
have skulls with
lots of mobility
between the bones,
which allows them to
have light but
strong skulls. But
we found that
Andalgalornis
had turned these
mobile joints into
rigid beams. This
guy had a strong
skull, particularly
in the fore-aft
direction, despite
having a curiously
hollow beak,” said
Witmer, Chang Ying-Chien
Professor of
Paleontology and a
professor of
anatomy.
The evolution of
this large and rigid
bony weapon was
presumably linked to
the loss of flight
in terror birds, as
well as to their
sometimes gigantic
sizes.
From the CT scans,
Stephen Wroe,
director of the
Computational
Biomechanics
Research Group at
the University of
New South Wales,
Australia, assembled
sophisticated 3D
engineering models
of the terror bird
and two living
species for
comparison (an
eagle, as well as
the terror bird’s
closest living
relative, the
seriema). Using
computers and
software supplied by
Wroe, Degrange and
Karen Moreno of the
Université Paul
Sabatier in
Toulouse, France,
applied an approach
known as Finite
Element Analysis to
these models to
simulate and compare
the biomechanics of
biting straight down
(as in a killing
bite), pulling back
with its neck (as in
dismembering prey)
and shaking the
skull from side to
side (as in
thrashing smaller
animals or when
dealing with larger
struggling prey).
Color images created
by the program show
cool-blue areas
where stresses are
low and white-hot
areas where stresses
get dangerously
high.
The engineering
simulations
supported the
CT-based anatomical
results. “Relative
to the other birds
considered in the
study, the terror
bird was
well-adapted to
drive the beak in
and pull back with
that wickedly
recurved tip of the
beak,” remarked Wroe,
“but when shaking
its head from side
to side, its skull
lights up like a
Christmas tree. It
really does not
handle that kind of
stress well at all.”
A key part of the
engineering analysis
was determining how
hard of a bite
Andalgalornis
could deliver. To
examine bite force
in birds in general,
Degrange and
Tambussi worked with
zookeepers at the La
Plata Zoo to get a
seriema and an eagle
to chomp down on
their bite meter.
“Combining all this
information, we
discovered that the
bite force of
Andalgalornis
was a little lower
than we expected and
weaker than the bite
of many carnivorous
mammals of about the
same size.
Andalgalornis
may have compensated
for this weaker bite
by using its
powerful neck
muscles to drive its
strong skull into
prey like an axe,”
Degrange said.
Taken together, the
team’s results give
new insight into the
lifestyle of a
unique avian
predator. This
terror bird was no
slugger and couldn’t
wade into the fray
like a feathered Joe
Frazier. Its skull,
though strong
vertically, was too
weak from side to
side, and the hollow
beak was in danger
of catastrophic
fracture if
Andalgalornis
grappled too
vigorously with
large struggling
prey.
Instead, the study
shows that the
terror bird was
required to engage
in an elegant style
more like Muhammad
Ali, using a
repeated
attack-and-retreat
strategy, using
well-targeted,
hatchet-like jabs.
Once killed, the
prey would have been
ripped into
bite-sized morsels
by the powerful neck
pulling the head
straight back or, if
possible, swallowed
whole.
Feeding on a
diversity of
strange, now-extinct
mammals and
competing with the
likes of saber-tooth
marsupials, terror
birds became top
predators in their
environment. At
least one gigantic
terror bird,
Titanis,
eventually invaded
North America about
two to three million
years ago, but the
animals disappeared
from Earth shortly
after.
The research was
funded by grants to
Witmer from the U.S.
National Science
Foundation; to Wroe
from the Australian
Research Council and
the Australia and
Pacific Science
Foundation; and to
Tambussi from the
Fondo Nacional para
la Investigación
Científica y
Tecnológica
(Argentina).
Images and
supporting
materials:
Article:
http://www.plos.org/press/pone-05-08-degrange.pdf.
High-resolution
images and movies:
http://www.oucom.ohiou.edu/dbms-witmer/terror_bird/Terror_bird_media_graphics_with_captions_PLoS_ONE.pdf.
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