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

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: 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, Dr. 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,” Dr. 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, Dr. 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, 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.

The chelonian-crunching method can also be applied to other turtle-rich fossil sites where shallow burial histories have been suspected but tough 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, Dr. 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, Dr. Petermann and Dr. 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, Dr. Petermann said, in part because of their changing colors under certain levels of heat and pressure. However, none of them are immediately intuitive.