Microscopic bone evidence supports dinosaur-bird evolution link

August 08, 2000

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."
-end-
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.

University of Washington

Related Dinosaurs Articles from Brightsurf:

Ireland's only dinosaurs discovered in antrim
The only dinosaur bones ever found on the island of Ireland have been formally confirmed for the first time by a team of experts from the University of Portsmouth and Queen's University Belfast, led by Dr Mike Simms, a curator and palaeontologist at National Museums NI.

Baby dinosaurs were 'little adults'
Paleontologists at the University of Bonn (Germany) have described for the first time an almost complete skeleton of a juvenile Plateosaurus and discovered that it looked very similar to its parents even at a young age.

Bat-winged dinosaurs that could glide
Despite having bat-like wings, two small dinosaurs, Yi and Ambopteryx, struggled to fly, only managing to glide clumsily between the trees where they lived, according to a new study led by an international team of researchers, including McGill University Professor Hans Larsson.

Some dinosaurs could fly before they were birds
New research using the most comprehensive study of feathered dinosaurs and early birds has revised the evolutionary relationships of dinosaurs at the origin of birds.

Tracking Australia's gigantic carnivorous dinosaurs
North America had the T. rex, South America had the Giganotosaurus and Africa the Spinosaurus - now evidence shows Australia had gigantic predatory dinosaurs.

Ancient crocodiles walked on two legs like dinosaurs
An international research team has been stunned to discover that some species of ancient crocodiles walked on their two hind legs like dinosaurs and measured over three metres in length.

Finding a genus home for Alaska's dinosaurs
A re-analysis of dinosaur skulls from northern Alaska suggests they belong to a genus Edmontosaurus, and not to the genus recently proposed by scientists in 2015.

Can we really tell male and female dinosaurs apart?
Scientists worldwide have long debated our ability to identify male and female dinosaurs.

In death of dinosaurs, it was all about the asteroid -- not volcanoes
Volcanic activity did not play a direct role in the mass extinction event that killed the dinosaurs, according to an international, Yale-led team of researchers.

Discriminating diets of meat-eating dinosaurs
A big problem with dinosaurs is that there seem to be too many meat-eaters.

Read More: Dinosaurs News and Dinosaurs Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.