It’s Turtles All the Way Down in the Fossil Record

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.

This year, 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, 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.

Petermann and his colleagues have studied different fossil sites in Corral Bluffs, Colorado, 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 nonavian 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,” 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.

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. The team looked at roughly 70 complete shells, including 21 from Corral Bluffs, 44 from the Cretaceous rocks of the Hell Creek Formation in the Western United States and five from other earlier Cretaceous and Jurassic period sites. Then, they measured where the shells fell on the spectrum, Petermann said, of “normal perfect turtle shell to pancake.”

All of the shells showed certain consistent patterns at various levels of compression. First, the shells cracked above the hip. Then, along the side of the spine. “The more sand I bear onto it, the flatter it gets,” Petermann said. “When they get really flat, they’ll have a little wall running around them. That’s the edge of the shell.”

The other key to the turtle compaction index is to figure out how porous the sediment of a site is – how much open space exists between each grain, such as the difference between coarse sand and fine-grained, dense silt. The relationship between porosity and depth is well understood in geology, Petermann said: Petroleum geologists will drill a sample knowing the depth, then work out how porous the sample is to predict the existence of oil and gas reservoirs. The turtle team just worked in reverse – they figured out how porous the site was, figured out how much pressure was required to crack a turtle shell, and solved for depth.

Using the turtle compaction index at Corral Bluff, 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 1,700-1,800 feet deep. The denser the original sediment, the more deeply the turtles had been buried.

The chelonian-crunching method can also be applied to other turtle-rich fossil sites where shallow burial histories have been suspected but difficult to confirm. “If you have the turtles, then you really can begin to figure out how much burial these things have undergone,” said David Fastovsky, a paleontologist at the University of Rhode Island who was not involved in the study. He added that the paper is “really neat.”

Turtle power might not be the only method for measuring these sorts of shallow sites, Petermann said. Mammal skulls from the Cenozoic era tend to shatter around the opening of the snout, he said, while crocodile skulls often break at a weak spot between the eyes. It’ll take some work to figure out how these patterns relate to specific depths.

If these sorts of solutions to geological problems seem to come from left field, Petermann and Fastovsky both point out, that’s because they take a certain amount of lateral thinking to invent. Proxies using fossil pollens and the teeth of eel-like vertebrates called conodonts are the traditional methods of measuring deep burials, Petermann said, in part because of their changing colors under certain levels of heat and pressure. However, none of them are immediately intuitive.

“It took a lot of creativity to realize those colors are associated with different pressure and temperature,” Petermann said. “Finding a pattern, that requires a lot of looking at fossils and a lot of imagination.”

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 analyzing the buried worlds of the past, it really is turtles all the way down.