66 million-year-old dinosaur ‘mummy’ skin was actually a perfect clay mask

By Amanda Schupak, CNN

(CNN) — In the badlands of eastern Wyoming, the Lance Formation is a trove of prehistoric fossils. And one area in particular — a region less than 10 kilometers (6 miles) across — has provided scientists with at least half a dozen remarkably well-preserved dinosaur specimens complete with details of scaly skin, hooves and spikes.

The paleontologist Dr. Paul Sereno and his colleagues dub it “the mummy zone” in a new study that aims to explain why this particular area has given rise to so many amazing finds and define exactly what a dinosaur “mummy” is.

In the early 1900s, a fossil hunter named Charles Sternberg found two specimens of a large duck-billed dinosaur, Edmontosaurus annectens, in the Lance Formation. The skeletons were so pristine that Sternberg, along with H.F. Osborn, a paleontologist at New York’s American Museum of Natural History, could make out what appeared to be large swaths of skin with discernible scales and a fleshy crest that seemed to run along the reptile’s neck.

Sereno, lead study author and a professor of organismal biology and anatomy at the University of Chicago, described the initial discovery as “the greatest dinosaur mummy — until maybe the juvenile that we found” in the year 2000.

Separated by nearly a century, Sereno and his team’s find shared common traits with Sternberg’s: The skeletons were preserved in three-dimensional poses and showed clear evidence of skin and other attributes that don’t usually survive 66 million years in the ground. “Osborn said in 1912 he knew that it wasn’t actual, dehydrated skin, like in Egyptian mummies,” Sereno said. “But what was it?”

Whatever it was, “we actually didn’t know how it was preserved,” he said. “It was a mystery.”

The new research puts that mystery to rest and can help paleontologists find, recognize and analyze future mummy finds for tiny clues into how giant dinosaurs really looked.

A dinosaur death cast in clay

Sereno and his collaborators used CT scanning, 3D imaging, electron microscopy and X-ray spectroscopy to analyze two Edmontosaurus mummies they discovered in the Lance Formation in 2000 and 2001 — a juvenile and a young adult. “We looked and we looked and we looked, we sampled and we tested, and we didn’t find any” remnants of soft tissue, Sereno said.

What the team found instead was a thin layer of clay, less than one-hundredth of an inch thick, which had formed on top of the animals’ skin. “It’s so real-looking, it’s unbelievable,” he said.

Whereas Sternberg and Osborn referred to the “impression” of skin in their specimens, Sereno’s paper proposes an alternate term — “rendering” — which he argues is more precise.

The study lays out the conditions that would produce such a rendering. In the Late Cretaceous Period, when Edmontosaurus roamed what is now the American West, the climate cycled between drought and monsoon rains. Drought has been determined to have been the cause of death of the original mummy found by Sternberg and described by Osborn, and of other animals whose fossils were found nearby. Assuming the same is true of the new specimens, the carcasses would have dried in the sun in a week or two.

Then, a flash flood buried the bodies in sediment. The decaying carcasses would have been covered by a film of bacteria, which can electrostatically attract clay found in the surrounding sediment. The wafer-thin coating of clay remained long after the underlying tissues decayed completely, retaining their detailed morphology and forming a perfect clay mask.

“Clay minerals have a way of attracting to and sticking onto biological surfaces, ensuring a molding that can faithfully reproduce the outermost surfaces of a body, such as skin and other soft tissues,” said Dr. Anthony Martin, professor of practice in the department of environmental sciences at Emory University in Atlanta, who was not involved in the research. “So it makes sense that these clays would have formed such fine portraits of dinosaurs’ scales, spikes and hooves.”

Dr. Stephanie Drumheller-Horton, a vertebrate paleontologist at the University of Tennessee, Knoxville, who also was not involved in the study, is an expert in taphonomy, which she described as “the study of everything that happens to an organism from when it dies until when we find it.” She is particularly interested in how these fossils formed.

“Dinosaur mummies have been known for over one hundred years, but there has definitely been more emphasis on describing their skin and less on understanding how they fossilized in the first place,” she said via email. “If we can understand how and why these fossils form, we can better target where to look to potentially find more of them.”

A detailed portrait of a duck-billed dinosaur

Together, the two more recently unearthed mummies allowed Sereno and his team to create a detailed update of what Edmontosaurus probably looked like.

According to their analyses, the dinosaur, which could grow to over 12 meters (40 feet) long, had a fleshy crest along the neck and back and a row of spikes running down the tail. The creature’s skin was thin enough to produce delicate wrinkles over the rib cage and was dotted with small, pebble-like scales.

The clay mask revealed that the animal had hooves, a trait previously preserved only in mammals. That makes it the oldest land animal proven to have hooves and the first known example of a hooved reptile, Sereno said. “Sorry, mammals, you didn’t invent it,” he joked. “Did we suspect it? Yeah, we suspected it had a hoof from the footprints, but seeing it is believing.”

The study was published Thursday in the journal Science.

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