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Microscopic Bone Evidence of Dinosaur-Bird Evolution - Press Release



University of Washington
10-Aug-00

                       Microscopic Bone Evidence of Dinosaur-Bird
Evolution

Library: SCI
Keywords: BIRD EVOLVE DINOSAUR BONES LONGISQUAMA METABOLISM
Description: The popular notion that birds evolved from dinosaurs has
come under assault. A researcher at the University of
Washington and a Japanese colleague have found similarities in bone
structure suggesting that birds did, in fact, evolve from a group
of dinosaurs (Nature, 8-10-00).
8/9/2000



FROM: Vince Stricherz
(206) 543-2580
vinces@u.washington.edu

MICROSCOPIC BONE EVIDENCE SUPPORTS DINOSAUR-BIRD EVOLUTION LINK

The popular notion that birds evolved from dinosaurs has come under
assault recently with the discovery of fossil evidence of a feathered
reptile that pre-dates birds. Now a researcher at the Burke Museum of
Natural History and Culture at the University of Washington and a
Japanese colleague have found similarities in bone structure suggesting
that birds did, in fact, evolve from a group
of dinosaurs.

The research will be published in the Aug. 10 issue of the journal
Nature.

The study shows that in a group of dinosaurs called coelurosaurs, the
organization of bone canaliculi - submicron-sized channels that connect
bone cells and blood vessels within the bone - form in a randomly
branching network. The canaliculi take circuitous, meandering routes as
they make connections between the bone cells and nutrient sources. That
same pattern today is found only among birds. However, in a group of
dinosaurs called ornithischians, which includes horned creatures such as
Triceratops, the
canalicular organization follows a much more regular pattern with very
direct and parallel routes, a structure similar to that in modern
mammals.

The work also sheds light on another controversy - whether dinosaurs had
high metabolic rates like modern birds. The researchers found evidence
that bundles of collagen fibers - which bind bone minerals together in
much the same way that rebar binds concrete - have an irregular
structure in both birds and coelurosaurs. The layers are thicker in some
places and much thinner in others, and often they disappear completely
before reforming. In modern vertebrates, this type of structure only
occurs in bone that forms very
rapidly, as it does in birds. In mammals, such bone formation happens
only at young ages or in healing bone breaks, times when bone growth
rates are highest. Otherwise, among vertebrates other than birds,
collagen bundles show a much more uniform pattern, with little thickness
variation from one part of a layer to another because the layers are
growing more slowly.

"Right now, the thing that is closest to what we see in the bones of
birds is in the bones of coelurosaurs," said John Rensberger, a UW
geological sciences professor and curator of vertebrate paleontology at
the Burke museum.

Rensberger and Mahito Watabe of the Hayashibara Museum of Natural
Sciences in Okayama, Japan, made their comparisons using fossilized
dinosaur bones collected from the Gobi Desert in China and from the Hell
Creek geological formation in Montana.

The researchers sampled about 550 cross sections of bone, ground to a
few microns thick and viewed through a microscope. Most of the cross
sections were prepared by 35 students in a series of undergraduate
research courses, who took 10 or more samples from each major bone
collected from each species. The students, selected from among the top
students in a course on dinosaurs
that Rensberger teaches, spent some 3,000 hours on the work over three
years.

The sheer volume of samples allowed the researchers to understand the
variability among species, making it easier to draw conclusions from
comparisons between species, Rensberger said. That was particularly
important in making the observations of the differences in canaliculi
and fiber bundles, since the information describing those structures in
most vertebrates, and especially dinosaurs, has been limited.

Observations of some of the differences in modern species were recorded
in a scientific paper published in German in 1906 and another published
in Italian in 1947. But the references were very general, Rensberger
said, because the scientists didn't have the breadth of data about
variability in those bone structures.

"There aren't any textbooks that show this," he said.

The debate over bird evolution grew more heated in June when a team of
Russian and U.S. researchers suggested a fossil of a small flying
reptile with feathers, called Longisquama, came from 225 million years
ago. That's about the time dinosaurs first appeared but 75 million years
before the first birds. Longisquama was an archosaur, part of a group of
reptiles from which dinosaurs, birds and crocodiles (birds' closest
living relatives) are descended.

"It doesn't necessarily prove that birds had to derive from dinosaurs,"
Rensberger said of the new research. "But, at least from the data we've
seen, that appears to be a logical conclusion."

###

For more information, contact Rensberger at (206) 543-7036, (206)
616-1581 or rensb@u.washington.edu

High-resolution electronic images of cross-section slides are available.

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