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 quantifies 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. …