They’re Turtles Down in the Fossil Record

There has also been a lot of basic research done on how turtle shells perform under pressure, which helped inform the creation of the Turtle Compression Index. The team looked at about 70 complete shells: 21 from Corral Bluffs, 44 from Cretaceous rocks of the Hell Creek Formation in the Western United States and five from other earlier Cretaceous and Jurassic sites. Then, they measured where the shells fall, Dr. Petermann said, from “a typical turtle shell to a pancake.”

All the shells showed some stable behavior at different stress levels. First, the shells exploded above the waist. Now, on the back side. Dr. “The more I tolerate the sand in it, the better it gets,” Petermann said. “When they are flat, they will have a small wall around them. That’s the point of the shell.”

Another important aspect of the Turtle Compaction Index is to see how porous the sediment is – the open space between each grain, such as the difference between coarse sand and thick sand. The relationship between porosity and depth is well understood in geology, Dr. Petermann said: Petroleum geologists will drill a sample to determine the depth, then check how porous the sample is to predict the presence of oil and gas reservoirs. A group of turtles recently worked behind the scenes – they found out how porous the area is, how much pressure is needed to crack a turtle’s shell, and how deep it is settled.

Using the Turtle Compaction Index at Corral Bluff, Dr. Petermann said, they found that many of the turtles were buried underwater, and over time under silt beds about 1700-1800 feet deep. The thicker the original mud, the deeper the turtles were buried.

The chelonian-crunching method can also be used at other turtle fossil sites where a history of shallow burial is suspected but difficult to confirm. “If you have turtles, then you can start to figure out how well these things are buried,” said David Fastovsky, a paleontologist at the University of Rhode Island who was not involved in the study. He added that the paper is “very neat.”

The strength of the turtles may not be the only way to measure these types of shallow areas, Dr. Petermann said. The skulls of mammals from the Cenozoic era tend to break around the nasal cavity, while the skulls of crocodiles tend to break in the narrow area between the eyes. It will take work to determine how these patterns relate to specific depths.

If these types of solutions to geological problems seem out of left field, Dr. Petermann and Dr. Fastovsky both show, it is because they take a certain amount of lateral thinking to create. Proxies using fossil dust and the teeth of eel-like vertebrates called conodonts are common methods of measuring deep burials, Dr. Petermann said, in part because their colors change under certain conditions of temperature and pressure. However, none of them are immediately felt.

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