During a 1908 dig in Wyoming, the fossil hunter Charles H. Sternberg made an unfathomable find: a dinosaur skeleton covered in scaly skin.
The duck-billed Edmontosaurus specimen ended up at the American Museum of Natural History. When it was unveiled in 1909, The New York Times proclaimed the find “was not only a skeleton, but a genuine mummy.”
A year later, in the same part of Wyoming, Mr. Sternberg and his sons discovered a second Edmontosaurus mummy, which they shipped to a museum in Germany.
Nearly a century later, a team of paleontologists returned to Wyoming’s “mummy zone” and unearthed two more Edmontosaurus mummies that preserve an array of rarely fossilized features, including the first example of dinosaur hooves. In a paper published on Thursday in the journal Science, the researchers describe the fossils and propose a mummification process that involved microbes and took place more than 66 million years ago.
“For the first time, I think that we’ve got Edmontosaurus’s look completely down,” said Paul Sereno, a paleontologist from the University of Chicago and lead author of the paper. “Based on our drawings, you can put it in a Hollywood movie and it’s going to be accurate head to toe.”
In 2001, Dr. Sereno took students to explore eastern Wyoming’s fossil beds, which date to the end of the Cretaceous period. They made a series of discoveries, including the Edmontosaurus specimens. One was a juvenile while the other was the back-half of an older dinosaur.
The two Edmontosaurus specimens, which the team nicknamed Ed Jr. and Ed Sr., eventually ended up in Dr. Sereno’s lab in Chicago. As the surrounding rock was removed, the team realized they were untombing mummies.
Ed Jr. had the outline of a fleshy crest running along its neck and back that was covered in small, pebble-like scales similar to those on modern lizards. Ed Sr. possessed wrinkled skin, wedge-shaped hooves rather than claws on its hind feet and a row of spikes running along its tail.
Dr. Sereno realized the new fossils were collected in the same region where Mr. Sternberg had discovered his mummies. Further research revealed that all four Edmontosaurus mummies were discovered in a six-mile-wide swath that the team calls the “mummy zone.”
During the late Cretaceous period, what is now eastern Wyoming was a coastal river valley prone to seasonal flooding. The team believes these floods buried the two Edmontosauruses along with some of their contemporaries — nearby rocks have also yielded Tyrannosaurus rex and Triceratops fossils that retain skin impressions.
To determine how the Edmontosauruses were mummified, the researchers made CT-scans of the fossils and examined the skin under electron microscopes.
The tests found no traces of organic compounds, revealing that the dinosaurs’ flesh, skin and hooves had all rotted away as the carcasses decayed and dried out. When the floodwaters buried the desiccated dinosaurs, microbes entombed the leathery hides in a biofilm. These microorganisms left behind a thin layer of clay that covered the carcasses like a mask at a spa. “When you rub clay on your face, it goes into every pore,” Dr. Sereno said. “When you touch these fossils and then look at your fingers, you still see traces of clay.”
This clay layer preserved the fine details of scales, spikes and hooves, creating a detailed mold of the dinosaurs that remained after their bones mineralized. This preservation method, Dr. Sereno said, is primarily seen in calm marine environments. He said it was surprising to find a dinosaur delicately preserved this way in a habitat like a flooding river valley.
Stephanie Drumheller-Horton, a paleontologist at the University of Tennessee, Knoxville, who was not involved with the paper, said the findings present an intriguing new way to mummify a dinosaur. In her own work, she has studied a partly eaten Edmontosaurus mummy from North Dakota and is excited to see there are multiple ways to form these “ridiculously beautiful fossils.”
Dr. Drumheller-Horton thinks such research can help paleontologists find more Mesozoic mummies.
“Understanding what conditions you need to get a mummy in the first place can help paleontologists target where we’re looking,” she said. “There’s so few mummies out there, and every single one is this amazing treasure trove of information.”
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