Fossil Fingerprints: Rare earths tie bones to burial ground

In the summer of 2002, paleontologists traveled from Temple University in Philadelphia to Badlands National Park in South Dakota. There, they uncovered hundreds of fossils, among them 30-million-year-old specimens derived from ancient mammals and tortoises. Using measurements of cerium, europium, and other rare earth elements in the bones, the team uncovered even more: chemical signatures of the soil in which the fossils formed.

In the June Geology, the researchers demonstrate the power of the new data by distinguishing fossils that formed in ancient flood plains from those that formed in stagnant lakes. The scientists can now examine a fossil from the Badlands park and identify the setting in which it was initially buried.

“I was knocked off my feet,” says team member Christine A. Metzger, now a graduate student at the University of Oregon in Eugene. “I was surprised that [ancient soils] had such an effect on the geochemical signature in the bones.”

Rare earth elements—largely the lanthanide series at the bottom of the periodic table—show up in only minute concentrations in living creatures. When an animal dies, far larger quantities of rare earth elements in the soil leach into the crystallizing bones, replacing calcium atoms of similar size. These changes, which take up to 30,000 years, reflect a particular soil’s geochemistry, permanently capturing a snapshot of soil conditions in the fossilized bones.

Rare-earth analysis is a “very useful tool to enable us to figure out the ecology of ancient animals and how they were living on the landscape,” says ancient-soil specialist Gregory J. Retallack of the University of Oregon, who was not a member of the group. Researchers now can begin applying the technique, which relies on mass spectrometry to weigh the elements in a specimen, to museum fossils from the Badlands, thereby revealing more about their histories.

What’s more, the technique may eventually be used to verify whether a fossil was unearthed illegally. Scientists could perhaps compare the rare earth signature of a mystery fossil to a national database of signatures of fossils from protected lands.

Right now, however, it’s not clear whether geochemical profiles from other fossil-rich sites will be distinct enough from each other to enable geologists to draw firm conclusions, cautions Metzger.

Poaching of fossils is a major problem in the Badlands. Not only is it illegal, but—even more important to scientists—a poached fossil has lost crucial information about the conditions under which it formed, says Dennis O. Terry, a lead researcher of the team. “For all intents and purposes, it’s a paperweight.”

While on its 2002 dig, the Temple research team encountered a family illegally gathering fossils. Park officials turned over the bones to the researchers, so they could apply rare earth analysis to determine the geologic settings in which the fossils formed. “By scampering all over the Badlands, the family created for us a perfectly random sample over time and location,” says Terry.

He’s confident that the geochemical signatures will be a boon to paleontologists. “If this works out the way we hope, it might be the next big revolution in the study of fossil bones,” he says.