Paleontologists use CT scans to identify unreachable fossils.
Recently, paleontologists used a CT scanner to identify a newly discovered 220-million-year-old reptile without cracking open the encasing rock. The technique, which preserves the fossils undisturbed in their original configuration, could help paleontologists identify other hard-to-access fossils.
Nick Fraser, curator of vertebrate paleontology at the Virginia Museum of Natural History, decided to try a new approach once traditional methods failed to remove the fossils from the stone surrounding them. This stone, the sedimentary shale of the Solite Quarry on the Virginia-North Carolina border, is known for beautifully preserving Triassic specimens, Fraser says. It also makes the fossils difficult to study. “I could just see the ribs,” Fraser says. But he was sure there was more to discover, inside the stone.
Over the millennia, pressure had blurred the boundaries between stone and bone. The fossils were the same color as their surroundings, and Fraser and his associates couldn’t chisel away the rock or dissolve it with chemicals for fear of accidentally damaging the fragile fossils.
“They were almost hair-like bones,” says Pete Kroehler, a Smithsonian researcher who also worked on the specimen. Kroehler says he and Fraser decided to x-ray the specimens to see if they could use the images as a guide for preparing the fossils.
Fraser took the rock to the medical CT scanner at the local hospital in Martinsville, VA. “It was a little unusual for them,” he admits.
Although the bones had proved impossible to separate any other way, in the x-rays, the difference in density made them stand out from the rock. Since CT scans did reveal images of the bones, Fraser sent the specimens to researchers at Penn State who examined them with a higher-resolution industrial CT scanner. According to Tim Ryan, a Penn State research associate, an industrial scanner can have resolutions as close as 5 to 150 microns, compared with resolutions of about a millimeter on the best medical machines. Ryan says that a medical scanner’s resolution can reveal the larger parts of the bone, but not the fine-scale features necessary to really study a creature. “We were trying to substitute the CT scanner for the preparation process, so we could go in and digitally prepare it,” he says.
Scanning the fossils was tricky, Ryan says, because the CT scanner isn’t designed to scan flat, oblong objects like the sheets of rock encasing the specimen. X-rays weaken, or attenuate, as they pass through rock, so they could not travel all the way through the long side of the stone. The Penn State researchers turned the problem to their advantage by orienting the rock so that the x-rays penetrated only as far as the fossil before weakening. Ultimately, this allowed them to get a better picture of the fossil.
The scans could not fully resolve some of the smallest bones, but the resulting image was good enough for Fraser to study and identify the fossils without breaking apart the rock. He determined that the creature was long-necked, used its wings to glide, and had curved feet that suggest it lived in trees. Because the fossils remain encased in their original position, there is no doubt about how bones are connected at joints.
Tim Rowe, director of the Vertebrate Paleontology Lab at the University of Texas at Austin, where one of the first industrial CT scanners was installed, says the number of instances of difficult specimens is growing, and he expects this type of approach to become increasingly common. “I think in the next 10 to 20 years … it will become a standard technique,” he says. Rowe believes that the approach, in addition to accessing hard-to-get-to fossils, can save a lot of time. “It probably took a day or two to scan that thing of Nick’s, while it might have taken a year or two to prepare it in a traditional way,” he says.
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June 11-12, 2019