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Microscopic Bone Evidence Supports Dinosaur-Bird Evolution Link

Story by
Nancy Joseph

Did birds evolve from dinosaurs? That popular notion came under assault recently with the discovery of fossil evidence of a feathered reptile that pre-dates birds. But recent findings by John Rensberger and a Japanese colleague once again suggest a dinosaur-bird connection. 

Rensberger, UW professor of geological sciences and curator of vertebrate paleontology at the Burke Museum, and Mahito Watabe of the Hayashibara Museum of Natural Sciences in Okayama, Japan, have found similarities in bone structure suggesting that birds did, in fact, evolve from a group of dinosaurs called coelurosaurs. 

Their findings are based on the study of bone canaliculi—submicron-sized channels that connect bone cells and blood vessels within the bone. In coelurosaurs, 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. (In contrast, the canaliculi in ornithischians—another group of dinosaurs—take very direct and parallel routes, a structure similar to that in modern mammals.) 

Rensberger and Watabe’s research 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.

This research doesn’t necessarily prove that birds had to derive from dinosaurs but, at least from the data we’ve seen, that appears to be a logical conclusion.

In modern vertebrates, this type of irregular 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. “Right now, the thing that is closest to what we see in the bones of birds is in the bones of coelurosaurs,” says Rensberger. 

Rensberger and Watabe made their comparisons using fossilized dinosaur bones collected from the Gobi Desert in China and from the Hell Creek geological formation in Montana. They 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 through comparisons between species, Rensberger says. 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. 

“This research doesn’t necessarily prove that birds had to derive from dinosaurs,” Rensberger adds, “but, at least from the data we’ve seen, that appears to be a logical conclusion.”