In a study published in Science, paleontologists from the University of Chicago have detailed how the remains of the duck-billed dinosaur Edmontosaurus annectens were preserved as so-called “mummies” approximately 66 million years ago. The research outlines a process called clay templating, where a thin clay mask—measuring less than 1/100th of an inch—formed over the dinosaur’s body after burial, preserving intricate details such as scales and hooves.
Using various imaging techniques, including hospital and micro-CT scans, X-ray spectroscopy, and 3D surface imaging, the scientists reconstructed the animal’s external appearance. Their findings revealed features like a tall crest along the neck and trunk, a row of spikes down the tail, and hoof-like structures encasing its toes. These reconstructions provide what researchers describe as the most complete view yet of this large dinosaur’s soft anatomy.
“It’s the first time we’ve had a complete, fleshed-out view of a large dinosaur that we can really feel confident about,” said Paul Sereno, Professor of Organismal Biology and Anatomy at UChicago and senior author on the paper. “The badlands in Wyoming where the finds were made is a unique ‘mummy zone’ that has more surprises in store from fossils collected over years of visits by teams of University undergrads.”
Sereno explained that these dinosaur mummies differ from human-prepared mummies; no organic material remains. Instead, skin, spikes, and hooves are preserved as sub-millimeter clay films formed shortly after burial. “This is a mask, a template, a clay layer so thin you could blow it away,” Sereno said. “It was attracted to the outside of the carcass in a fluke event of preservation.”
After sun-dried carcasses were buried by flash floods in river sands found in east-central Wyoming—a region mapped out by Sereno’s team—the surface biofilm on each carcass pulled clay particles from wet sediment to form this thin template. The original tissue decayed over time while skeletons fossilized beneath.
Careful cleaning was required to expose these delicate boundaries, led by Fossil Lab manager Tyler Keillor. Postdoctoral scholar Evan Saitta and other researchers used additional imaging methods to analyze both new specimens—a late juvenile and an early adult—and earlier finds from the same area.
Digital artists collaborated with scientists to create models showing how Edmontosaurus might have looked and moved through its environment near the end of the Cretaceous period.
“I believe it’s worth taking the time to assemble a dream team in order to generate science that can be appreciated by the general public,” Sereno said. “We’ve never been able to look at the appearance of a large prehistoric reptile like this—and just in time for Halloween.”
Working with two well-preserved mummies allowed researchers to reconstruct Edmontosaurus’ full profile for the first time. “The two specimens complemented each other beautifully,” Sereno said. “For the first time, we could see the whole profile rather than scattered patches.”
Among their discoveries was evidence for continuous midline features: fleshy crests running along necks and trunks that transitioned into rows of spikes on tails—each spike corresponding with one vertebra. Most scales measured only 1–4 millimeters across despite belonging to animals exceeding 40 feet in length; wrinkles above ribcages suggested thin skin.
A significant finding involved hoof-like structures on hind feet—the earliest known hooves documented in any land vertebrate according to Sereno: “There are so many amazing ‘firsts’ preserved in these duck-billed mummies —the earliest hooves documented in a land vertebrate, the first confirmed hooved reptile, and the first hooved four-legged animal with different forelimb and hindlimb posture.”
Beyond anatomical insights, this research introduces new preparation techniques for studying soft tissues in dinosaurs—including workflows from fossil discovery through digital modeling—and offers criteria for identifying similar cases elsewhere.
“This may be the single best paper I’ve released,” Sereno said. “From field to lab to 3D reconstructions along with a suite of useful terms defined, it’s a tour de force, and it tells a coherent story about how these remarkable fossils come to be and what we can learn from them.”