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MARGINALIA

Freed to Fly Again

Through CT scans that can show sub-millimeter details, an imprisoned fossil reveals its secrets

Pat Shipman

An Existential Crisis

Most readers of this magazine will have heard of the computed tomography, or CT, scans used in healthcare settings. CT scanners use x rays to take pictures from many different angles, and a computer then combines those pictures to create a three-dimensional image. The micro-CT scanner applies the same principle on a smaller scale. It was developed largely for industrial purposes and is able to show features smaller than the width of a single hair—much more detail than a physician needs. Most "patients" studied with micro-CT are inanimate. Recently, these subjects have included fossils of animals dead for millions of years.

Paleontologist Nick Fraser of the Virginia Museum of Natural History in Martinsville, Virginia, has gotten to know all about micro-CT scans in recent years following the discovery of a unique fossil from the Solite Quarry in Virginia, a trove some 220 million years old. Fraser collaborates with Paul E. Olsen of the Lamont-Doherty Earth Observatory of Columbia University, who found the site in 1975.

The Solite Quarry is famous as a Konservatt Lagerstätte, a fossil site that shows exquisite preservation of an entire ecosystem, including the smallest and most fragile specimens. Excavations have yielded a wealth of fossilized insects, even showing the veining on the delicate wings of waterbugs or caddis flies. The rocks also contain fish, including the very primitive coelacanth, and plant remains from the damp waterside habitat, such as conifers, ferns, cycadophytes and gingko-like trees. You can find winged seeds, the kind that children often call "helicopters" or "pollynoses," that look almost as if they could have fallen from a maple tree today.

After years of revealing the litany of species in that ancient marsh, the Solite Quarry recently yielded a surprise: two specimens that preserve the features of a creature never seen or suspected to exist before. As Fraser tells the story, "It was touch and go for the specimen—certainly it was seconds away from biting the dust! I was on my own in the quarry splitting through the shale on a very hot, sultry day. Often, once the shale is split and exposed to air, the black surfaces become instantly covered with a layer of calcium carbonate, making it almost impossible to discern any fossils. This is exactly what happened with this specimen. I actually had it poised over my shoulder to throw, when the light just caught the ribs and threw them into relief. I took a second look, thought it was a coelacanth tail and put a field number on it. It was only at the end of the day when I was going through all the specimens with my field notes that I realized the 'tail' was right at the end of a small skull and long, slender neck!"

Fraser then remembered another enigmatic find he had made in 1994, which he almost threw away as unimportant. After twisting the slab of rock back and forth to get oblique lighting, he had decided there were faintly visible limbs on the slab and kept it.

"At that stage," Fraser recalls, grinning, "I thought I might have a gliding animal. Paul, on the other hand, thought I had been out in the sun too long and even questioned whether there was a fish there, let alone a glider!"

He and Olsen had never identified and studied the 1994 specimen because it was too difficult to prepare. They hoped this new specimen would answer their questions, but its preparation proved just as intractable. The surrounding stone, or matrix, was very hard and nearly the same color as the delicate fossilized bones. The danger of damaging the fossil was too great, leaving Fraser and Olsen with a serious problem. If the specimen could not be seen, then according to science it didn't exist.




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