While exploring an Arctic mountaintop in 2008, paleontologists unearthed a small skeleton that resembled a coiled sea serpent imprinted into a slab of 240 million-year-old rock. The remarkably complete skeleton, nicknamed Oda, was deposited in the collection of the University of Oslo’s Natural History Museum.

It was clear that Oda was an ichthyosaur, but no one could say if it was a known species of the marine reptiles, which were like a mash-up of a crocodile and a dolphin. While most of its skeleton remained, eons under a muddy seafloor had squeezed Oda into a two-dimensional jumble of bones.

To identify the reptile, paleontologists stuck the perplexing patient under an X-ray machine to piece together the petrified puzzle. In a paper published Wednesday in the journal PLoS One, the researchers described the anatomical details they had gleaned from the ghostly glow of Oda’s X-rayed bones.

“The contrast of these bones are bright as day,” said Neil Kelley, a paleontologist at Vanderbilt University who studies marine reptiles and was not involved in the new study. “I’m very jealous — that’s exactly the result that you want when you put something in an X-ray.”

The findings, he added, show the potential of the technique to add new dimensions to mysteries in the fossil record that have been flattened by the passage of time.

The enigmatic skeleton was discovered on a windswept plateau on Edgeoya island in Svalbard, an Arctic archipelago north of Norway that’s home to reindeer and polar bears. But during the middle Triassic Period, the area was a deep sea shelf off the northern coast of the supercontinent Pangea and a haven for marine reptiles.

Victoria Sjoholt Engelschion, a doctoral researcher at the Natural History Museum in Oslo, came across bits of bluish ichthyosaur bones when she was making computed tomography scans of clumps of fossilized clams from the area. A colleague recommended scanning Oda for identifying clues.

For more than a century, paleontologists had to break open fossils to analyze internal anatomy, often destroying their prized specimens. In recent decades, scientists have turned toward nondestructive techniques like CT scanning to create three-dimensional renderings of fossils. Because Oda’s bones were stamped into the rock, Ms. Engelschion and her colleagues opted to go for a more traditional approach by shooting X-rays through the fossil to render two-dimensional images.

Fitting Oda, which is preserved with its spine curled, its tail bent and its flipper and rib bones strewn about, into an X-ray machine proved daunting.

“We do not have any machine that can make radiographs of large specimens, but luckily our colleagues at the Cultural History Museum did, as archaeologists use this technique much more often,” Ms. Engelschion said.

In the initial scans, Oda’s fossilized bones leaped off the X-rays. This contrast was a result in part of the fact that the material inside the animal’s bones had been entirely replaced by barite, a sulfate mineral that is used today as a radiographic contrast agent for medical exams.

“The ichthyosaur’s bones were no longer bones, which caused them to light up,” Ms. Engelschion said.

Because the barite gave the ichthyosaur’s bones a bright glow, the team was able to observe anatomical features that had been overlooked or obscured. They discovered that the animal’s alligator-like skull was considerably longer than previously thought. They also pinpointed previously invisible limb bones and vertebrae.

“This study illustrates the importance of using some of the more ‘tried and tested’ techniques that may still reveal new data,” said Dean Lomax, a paleontologist at the University of Manchester in England who specializes in ichthyosaurs and was not involved in the new study.

The crucial clue was in the creature’s teeth. The X-rays revealed that Oda’s larger teeth had grooves in them that were reminiscent of teeth found in the jaws of Phalarodon atavus, a small and sleek ichthyosaur that has been found in mainland Europe and China. According to Ms. Engelschion, finding this ichthyosaur in Svalbard sheds light on how widespread and successful the species was during its heyday.

Dr. Kelley added that finding Oda’s rightful place in the fossil record helped add context to the rise of ichthyosaurs, which would dominate marine ecosystems for 150 million years. He said he thought that re-examining other marine reptile fossils under X-rays might reveal hidden clues to how these reptiles evolved.

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