Microscopic Bone Evidence Supports Dinosaur-Bird Evolution Link

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."