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It’s Turtles All of the Way Down within the Fossil Record


There’s also been an amazing deal of basic research done on how turtle shells perform under pressure, which helped to tell the invention of the Turtle Compression Index. The team checked out roughly 70 complete shells: 21 from Corral Bluffs, 44 from the Cretaceous rocks of the Hell Creek Formation within 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 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. “Once they get really flat, they’ll have slightly wall running around them. That’s the sting of the shell.”

The opposite key to the Turtle Compaction Index is to determine how porous the sediment of a site is — how much open space exists between each grain, reminiscent of the difference between coarse sand and fine-grained, dense silt. The connection 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 discovered how porous the positioning was, discovered how much pressure was required to crack a turtle shell, and solved for depth.

Using the Turtle Compaction Index at Corral Bluffs, Dr. Petermann said, they found that lots of the turtles had been buried within the ooze at the underside of waterway, and over time under beds of silt around 1700-1800 feet deep. The denser the unique sediment, the more deeply the turtles had been buried.

The chelonian-crunching method may also be applied to other turtle-rich fossil sites where shallow burial histories have been suspected but tough to substantiate. “If you’ve the turtles, you then really can begin to determine how much burial these items have undergone,” said David Fastovsky, a paleontologist on the University of Rhode Island who was not involved within the study. He added that the paper is “really neat.”

Turtle power won’t be the one method for measuring these forms of shallow sites, Dr. Petermann said. Mammal skulls from the Cenozoic era are likely to shatter across 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 determine how these patterns relate to specific depths.

If these forms of solutions to geological problems seem to come back from left field, Dr. Petermann and Dr. Fastovsky each indicate, that’s because they take a certain quantity of lateral pondering to invent. Proxies using fossil pollens and the teeth of eel-like vertebrates called conodonts are the normal methods of measuring deep burials, Dr. Petermann said, partly due to their changing colours under certain levels of warmth and pressure. Nevertheless, none of them are immediately intuitive.

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