The Lance Formation in eastern Wyoming is rich in prehistoric fossils. And one area in particular — less than 10 kilometers (6 miles) in diameter — has yielded scientists at least half a dozen remarkably well-preserved specimens of dinosaurs with scaly skin, hooves and spines.
Paleontologist Dr. Paul Sereno and his colleagues have dubbed it the “Mummy Zone” in a new study that aims to explain why the area has yielded so many amazing finds and define exactly what a dinosaur “mummy” is.
In the early 1990s, fossil hunter Charles Sternberg discovered two specimens of Edmontosaurus annectens in the Lance Formation. The skeletons were so intact that Sternberg, along with paleontologist HF Osborne of the American Museum of Natural History in New York, could distinguish large areas of skin with noticeable scales and a fleshy ridge that appeared to run along the reptile’s neck.
The lead author of the study, Paul Sereno of the University of Chicago, marvels at the preserved hooves on the foot of an adult Edmontosaurus mummy. – Kieth Ladzinski
Sereno, the study’s lead author and a professor of organismal biology and anatomy at the University of Chicago, described the initial discovery as “the largest dinosaur mummy — possibly until the baby one — that we’ve found” in 2000.
Sereno and his team’s find, which is nearly a century apart, shared features with Sternberg’s: the skeletons were preserved in a three-dimensional pose and made it clear that skin and other features do not normally survive 66 million years on Earth. “Osborne said in 1912 that he knew it wasn’t real, dehydrated skin like the Egyptian mummies,” Sereno said. “But what was that?”
Whatever it was, “we didn’t really know how it was preserved,” he said. “It was a secret.”
The new study has solved this mystery and could help paleontologists find, identify and analyze future mummy finds for tiny clues about what the giant dinosaurs really looked like.
The death of a dinosaur cast from clay
Sereno and his colleagues used computed tomography, 3D imaging, electron microscopy and X-ray spectroscopy to study two Edmontosaurus mummies they discovered in the Lance Formation in 2000 and 2001. – a minor and a young person. “We looked and we looked and we looked and we took samples and we tested, but we didn’t find any ‘soft tissue remains,'” Sereno said.
Instead, the team found a thin layer of clay, less than a hundredth of an inch thick, formed on the animals’ skin. “It looks so real it’s unbelievable,” he said.
A thin layer of clay over the fossilized skeleton of a young Edmontosaurus preserved large areas of scaly, wrinkled skin and a fleshy ridge on its back. – Tyler Keillor / Fossil Lab
Sternberg and Osborn referred to skin “impression” in their examples, while Sereno’s work suggests an alternative term, “rendering,” which he says is more accurate.
The study identifies the conditions that would allow such a representation to be created. During the Late Cretaceous, when Edmontosaurus roamed the present-day American West, the climate fluctuated between drought and monsoon rains. The original mummy found by Sternberg and described by Osborne and other animals whose fossils were found nearby were determined to be caused by drought. Assuming the same is true of new specimens, the carcasses would have dried in the sun within a week or two.
A flash flood then buried the bodies in the sediment. Decaying carcasses would have been covered by a film of bacteria capable of electrostatically attracting clay in the surrounding sediments. A thin coating of clay persisted long after the underlying tissues had completely disintegrated, maintaining detailed morphology and forming a perfect clay mask.
“Clay minerals can attract and adhere to biological surfaces, providing a formation that can accurately recreate the outermost surfaces of the body, such as skin and other soft tissues,” said Anthony Martin, a professor of practice in the Department of Environmental Sciences at Emory University in Atlanta, who was not involved in the study. “So it makes sense that these clays would have formed such great portraits of dinosaur scales, spikes and hooves.”
Dr. Stephanie Drumheller-Horton, a vertebrate paleontologist at the University of Tennessee, Knoxville, who was also not involved in the study, is an expert in taphonomy, which she described as “the study of everything that happens to an organism from the time it dies until we find it.” She is particularly interested in how these fossils were formed.
“Dinosaur mummies have been known for over a hundred years, but there has definitely been more focus on describing their skin and less understanding of how they fossilized in the first place,” she said in an email. “If we can understand how and why these fossils form, we can better direct where to look to find more of them.”
Detailed portrait of a duckbill dinosaur
Together, the two recently unearthed mummies allowed Sereno and his team to create a detailed reconstruction of what Edmontosaurus probably looked like.
According to their analysis, the dinosaur, which could have grown up to 12 meters (40 feet) long, had a fleshy ridge along its neck and back, and a row of spines extended from its tail. The animal’s skin was thin enough to form delicate folds on the rib cage, and the skin was studded with small, pebble-like scales.
The clay mask revealed that the animal had hooves, which were previously only found in mammals. That makes it the oldest land animal with hooves and the first known example of a ungulate, Sereno said. “Sorry mammals, you didn’t come up with that,” he joked. “Did we suspect? Yes, we suspected it contained a hoof from the tracks, but seeing is believing.”
The study was published Thursday in the journal Science.
Amanda Schupak is a science and health journalist in New York.
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