The fossil shell of Hoplochelys, an extinct genus of aquatic turtle, one of several types of turtle shells collected from sites in Montana and Colorado.Credit...Rick Wicker/Denver Museum of Nature and Science
The New York Times' Trilobite column has an interesting article about fossil turtle shells. A paper in the journal Geosphere, used compacted fossil turtle shells to determine how deeply a fossil site was originally buried before other geologic events settled in. This seemingly obscure statistic gives researchers clues to the environmental conditions that led to the burial. The measurement as dubbed the "Turtle Compaction Index.
Over millions of years, the sediments that bury a given site are compacted and shifted by geological processes before erosion reveals them. Accurately measuring the original burial depth is vital to understanding what conditions were like when fossils were laid down, Dr. Petermann said. Most methods for determining burial depth — analyzing the color of fossilized pollen, for instance — only work at sites subsequently entombed under a mile of stone. Shallower deposits — the sort likely to be buried only about 1,000 feet down — are harder to accurately measure, because they tend to lack clear indicators.
Dr. Petermann and his colleagues have studied different fossil sites in Corral Bluffs, Colo., which is composed of rocks about 63-million years old. These ancient sediments preserve glimpses of an aquatic ecosystem knitting itself back together after the asteroid impact that caused the Cretaceous-Paleogene extinction of non-avian dinosaurs. The minerals at these sites could be an important clue about the environmental conditions that formed them — but only if the burial depth is clear.
“We tried a bunch of ways of figuring it out,” Dr. Petermann said, “and then we realized we had all these complete turtle shells.”
Turtles — formally known as chelonians — evolved around 230 million years ago. They quickly became an ubiquitous part of freshwater ecosystems like rivers and ponds: the very sorts of inland environments that tend to collect fossils.
