Weighty proposition: It’s turtles all the way down in the fossil record
NEW DELHI: You never know where a bit of unusual scientific research is going to lead. Consider a 2012 study about turtle shells. Researchers subjected the skeletal remains of pond sliders, diamondback terrapins, painted turtles and box turtles to incremental increases in mechanical forces and measured where and how the shells began to buckle.
This may sound a little sadistic, but no living turtles were hurt in the study. Other scientists understand the appeal of looking at the material properties of the interlocking plates and ribs that make up turtle shells. “It’s actually fun to just play around with them and see how they bend under a point or certain loading regimes,” said Holger Petermann, a paleontologist with the Denver Museum of Nature and Science. Earlier this year, Dr. Petermann and colleagues took the unusual project and applied it to an unrelated problem. The flatness of a turtle shell, he found, could help paleontologists figure out how deeply a fossil site was originally buried before eons of other geological activity. They came up with a simple but catchy name for their measurement method, now in an article published in the journal Geosphere: 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 — analysing the color of fossilised 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 realised 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. There’s also been a great deal of basic research done on how turtle shells perform under pressure, which helped to inform the invention of the Turtle Compression Index.
Using the Turtle Compaction Index at Corral Bluff, Dr. Petermann said, they found that many of the turtles had been buried in the ooze at the bottom of waterway, and over time under beds of silt around 1700-1800 feet deep. The denser the original sediment, the more deeply the turtles had been buried.
In the historical imaginations of many cultures — particularly those of India, China and the Americas — cosmic chelonians are said to carry the world on their backs. When it comes to analysing the buried worlds of the past, it really is turtles all the way down.