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Ghost in the Fossil

Even for the lucky few creatures that are preserved in the fossil record, soft tissues such as skin and feathers typically disappear over time. But a newly developed technique has found a way to bring them back to life in some cases. Researchers have now used the approach to resurrect the teeth and recognize the carcass of a 50-million-year-old fossil of a lizard, long thought to be merely preserved remnants of skin shed from the reptile.

"This is incredibly uncharted territory," says Gregory Erickson, a vertebrate paleontologist at Florida State University in Tallahassee. "This technique reveals that there's literally more to fossils than meets the eye."

Discovered in the 1980s, the lizard fossil is one of only two known examples of reptile skin unearthed from the Green River Formation of the western United States, a finely layered mudstone best known for its exquisite fish fossils. Even though soft tissues are incredibly rare in the fossil record, being preserved only in unusual environmental circumstances, this lizard fossil survived the ages, says Phillip Manning, a vertebrate paleontologist at the University of Manchester in the United Kingdom. It's easy to see the remnants of individual scales in the skin, but the rock doesn't include any visible remains of bones or other hard tissue—a combination that led researchers to believe that the skin had been shed by a living creature and then preserved.

But recently, to learn more about the fossil, Manning and his colleagues turned to a relatively new x-ray analysis technique—dubbed synchrotron rapid scanning x-ray fluorescence—with surprising results. Instead of enabling scientists to see inside or through rock, he notes, the intense x-rays produced by this technique cause particular elements or compounds to fluoresce, revealing previously unrecognized chemical remnants that are invisible to the naked eye but persist in the rocks at very low concentrations.

When the researchers illuminated the fossil with x-rays that cause sulfur and copper to fluoresce, the skin remnants showed up in remarkable detail. But when they lit the fossil with x-rays that cause phosphorus to glow, the technique revealed many small spots in the lizard's head where that element was concentrated—regularly spaced spots that appear where the creature's jaws would have been. The arrangement prompted the researchers to interpret the traces of phosphorus as the chemical remnants of teeth. Because lizards don't shed their teeth when they molt their skin, the technique reveals the unusual fossil to be the partially preserved remnants of a full carcass, the researchers report online this month in Applied Physics A: Materials Science & Processing.

The fossil's state of preservation reveals a lot about the environmental conditions where the carcass ended up, presumably after being washed into the lake soon after it died. Lake-bottom waters at this particular spot likely had little or no oxygen, enabling preservation of the skin. But the waters apparently were also acidic, which totally dissolved the creature's bones and left only scant traces of its teeth. The chemical vestiges of the teeth were most likely preserved because tooth enamel typically has a low concentration of organic matter and large crystals of phosphate minerals, both of which render the teeth more resistant to decay.

The x-ray technique the team used "will open the curtain to a whole new way of studying extinct animals and the conditions in which they lived and died," Manning says. Another benefit of the approach, he notes, is that it's nondestructive.

Previous studies using the technique have revealed the chemical residues of pigments in feathers, providing insight into the color patterns that ancient birds might have sported. The technique also offers the opportunity to discern remnants of soft tissues that are only rarely preserved, such as the pigment-filled retinas of eyes, the ink sacs of ancient squid, and possibly other tissues such as muscles—at least as far as the naked eye is concerned.

Results of the new study "are fantastically interesting," says Mark Norell, a vertebrate paleontologist at the American Museum of Natural History in New York City. "There's a whole lot more preserved with fossils than we ever thought there was."

Erickson agrees. "This technique will prompt paleontologists to revisit a lot of classic fossils," he says. "Who knows what got missed during the first 150 years of paleontology?"