Chicago Tribune Article on Gigantoraptor

[Guy Leahy <xrciseguy@sbcglobal.net>]

http://www.chicagotribune.com/news/nationworld/chi-070613dino,1,3092896.story?coll=chi-news-hed

Big Bird joins cast of dinosaur fossils
Giant flies in face of evolution theory
By Robert Mitchum
Chicago Tribune staff reporter

June 13, 2007, 11:53 PM CDT

Weighing over a ton, with slender legs twice as tall
as a man, a toothless beak and ornamental feathers on
its arms, the newly discovered Gigantoraptor was the
Goliath of birdlike dinosaurs.

The species, discovered by fossil hunters in Inner
Mongolia, could not fly but had avian characteristics,
including a beak and lightweight, hollow bones.
Roughly 25 feet long and more than 16 feet tall, the
dinosaur would have towered over its turkey-sized
relatives.

The discovery of such a large specimen adds complexity
to the story of how birds evolved from dinosaurs, a
gradual shrinking process by which the huge "thunder
lizards" gave way to small, light and feathered
creatures. Finding a dinosaur 300 times heavier than
similar species underscores the twists and turns that
evolution can take.

Reported by a group of Chinese paleontologists in this
week's issue of Nature, the discovery also greatly
expands the view of diversity during the Late
Cretaceous period, when dinosaurs were at their most
varied and bird populations began to rise.

"This species is the largest of its kind, and a lot
bigger than anyone ever expected these animals to
get," said Peter Makovicky, assistant curator of
dinosaurs at the Field Museum.

"When animals get big they tend to look less birdlike;
for instance, we have no birds that reach a ton or
more in body size," said Makovicky, who was not
involved with the discovery. "But here, some of the
birdlike traits are actually retained or developed
even though the animal is very large."

Given the Latin name Gigantoraptor erlianensis, the
species roamed the prehistoric continent of Asiamerica
roughly 75 million to 95 million years ago, late in
the dinosaur era.

Xing Xu of the Institute of Vertebrate Paleontology
and Paleoanthropology in Beijing and colleagues found
the relatively complete fossil used to characterize
Gigantoraptor in the Erlian Basin in northeastern
China.

The region has attracted paleontologists' attention
since the 1920s, Xu said, but several interesting
discoveries have been made recently in the area,
including the four-winged Microraptor and several
other feathered dinosaurs.

By analyzing the state of growth in the fossil's calf
bone, researchers determined the dinosaur was
relatively young when it died, about 11 years old.
That suggests that a full-size Gigantoraptor may be
even larger than this specimen indicates.

Gigantoraptor shared gigantic size with dinosaurs of
the tyrannosaur family?it is about two-thirds the size
of a Tyrannosaurus rex?but it weighed much less, in
part due to differences in bone structure.

The T. rex was able to balance on two hind limbs in
part because it had air pockets in the bones in the
front of its body. Gigantoraptor, however, possessed
these light, hollow bones throughout its body?the same
modification that later allowed birds to fly.

The Gigantoraptor fossil also had longer, more slender
limbs than are usual for an animal of its size, as
well as a toothless "beak" rather than the
tooth-filled mouth characteristic of large carnivore
dinosaurs. The beak suggests to experts that the
species may have had an omnivorous diet of plants,
eggs and meat, similar to other dinosaurs with avian
features.

Although the fossil cannot tell scientists directly
whether Gigantosaurus had feathers, researchers agree
that it likely did. Recent discoveries revealed that
several birdlike dinosaurs in the same family, the
Dromaeosaurids, were feathered.

"One would assume Gigantoraptor is feathered since its
ancestry lies with a group of animals that we know
also had feathers," Makovicky said.

However, Xu said, these feathers were not for flight.

"Being 3,000 pounds heavy, no one would expect this
species to fly," Xu said.

Feathers likely evolved at first as a form of
insulation for animals, similar to fur in mammals. But
because it is easier for large animals to retain heat,
researchers suggest the Gigantoraptor's feathers
served a more colorful purpose: communication.

"Dinosaurs were clearly animals that liked to display
to each other; that's why they had crests, horns and
bumps," said Philip J. Currie, professor of dinosaur
paleobiology at the University of Alberta. "Feathers
almost certainly developed first as insulation in
small species, but other dinosaurs surely started
using them as display. A species might have crown
feathers at the top of its head or feathers at the end
of its tail."

Whether Gigantoraptor truly used feathers in this
peacock-like way and why such a large animal retained
its bird-like features remain mysteries.

"One identified pattern is that large-sized dinosaurs
are less birdlike than their small-sized relatives,"
Xu said. "The discovery of Gigantoraptor complicates
this pattern: It is much larger, but has more birdlike
features than its small relatives. This implies the
presence of multiple underlying mechanisms in the
evolution of avian characters."

rmitchum@tribune.com