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Revealing Stone-Embedded Fossils sans Hammer and Chisel

BioPhotonics
Aug 2007
Michael J. Lander

Some 200 million years ago, two gliding reptiles sank to the floor of a lake in present-day Virginia. Scientists have characterized the skeletal structures of the creatures — long since entombed in hard gray mudstone — and found them to be unique. But they did not make the identification after count-less hours of toiling with tiny tools and brushes. Instead, the researchers used CT scanning techniques.

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From CT images, the scientists made detailed skeletal sketches. This illustration shows the reptile’s long neck, a feature that clearly separates it from other gliding diapsids of the same period. Reprinted with permission of the Journal of Vertebrate Paleontology.

Nicholas C. Fraser from the Virginia Museum of Natural History in Martinsville and colleagues from there, from Columbia University’s Lamont Doherty Earth Observatory in Palisades, N.Y., and from Pennsylvania State University in University Park started with fossils that had defied removal from the thin sheets of surrounding stone via mechanical and chemical methods.

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A computer-generated illustration of Triassic gliding reptiles generated from fossilized remains provides an idea of what the animals might have looked like on the wing as well as in their presumed arboreal habitat. Reprinted with permission of the Journal of Vertebrate Paleontology.

To penetrate the rocks and reveal the preserved structures’ forms, they turned to x-rays. With an X-Tek CT system, they took 90 perpendicular scans of one sample and 37 of the other. Reconstruction of the slices into composite images revealed mostly intact skeletons of an animal with a pointed rostrum, a long neck, and elongated thoracic and lumbar ribs.

From a more detailed analysis of the skulls’ structure, the researchers clearly identified the remains as those of a diapsid reptile species, whose numerous small teeth meant that they probably munched on insects like those fossilized in nearby stones. The feet, with their hooked posture and right-angle orientation to the lower leg, suggested that the reptiles spent much of their time in trees.

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The researchers obtained images of the fossilized creature by uniting multiple scans taken perpendicular to the rocks’ surfaces, as seen in the red-outlined box. The darker sections of the shaded region beside the scan indicate regions of low x-ray penetration. Courtesy of Tim Ryan, Pennsylvania State University.

The ribs impressed the scientists most, however. The bones’ excessive length led the team to suspect that a sheet of skin had once stretched between them. Unlike kuehneosaurs, another period glider, the reptile appears to have enjoyed control over lift and drag by moving its anterior rib. Although sharing a pointed head and relatively long neck with the ancient diapsid Sharovipteryx, the latter’s gliding membrane was stretched between its hind limbs. The newly discovered animals also seem to have possessed a fairly rigid neck that might have made for unsteady gliding. The researchers have dubbed the reptile Mecistotrachelos apeoros, from the Greek words for “longest neck” and “soaring,” respectively. A full summary of their work can be found in the June issue of the Journal ofVertebrate Paleontology.

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A close-up of the CT image highlights the long ribs extending from the animal’s vertebrae. The large dark splotch running across it represents an area of poor x-ray penetration. Courtesy of Tim Ryan, Pennsylvania State University.

The scientists’ elucidation of the reptiles’ morphology thus indicates that at least three Triassic reptile groups had evolved unique gliding mechanisms. And although CT sees fairly regular use in modern archaeology, the investigators’ effective use of the technique for a complete reconstruction based on a thin, delicate sample may set a trend. CT not only lessens damage to preserved structures but also maintains their relative orientation — a detail that can offer clues to a specimen’s lifestyle and place in evolutionary history.


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