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In a in Science, experts from the University of Chicago describe steps that took place some 66 million years ago to transform the carcasses of a duck-billed dinosaur, Edmontosaurus annectens, into dinosaur "mummies" preserving fine details of scales and hooves.

Called clay templating, the external fleshy surface of the dinosaur was preserved over the skeleton after its burial by a thin clay mask no more than 1/100th of an inch thick. Using an array of imaging techniques, the scientists reconstructed the fleshy appearance of the animal in life, from a tall crest over the neck and trunk to a spike row over its tail and hooves sheathing its toes. Combined with fossilized footprints, the appearance of a duck-billed dinosaur—long guessed at but never demonstrated in this detail—is now at hand.

"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 senior author Paul Sereno, Ph.D., Professor of Organismal Biology and Anatomy at UChicago. "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."

From field puzzle to full profile

Using historical photos and field sleuthing, Sereno and his team relocated the sites in east-central Wyoming where several famous dinosaur mummies were discovered in the early 20th century, mapping out a compact "mummy zone." In those stacked river sands, they excavated two new Edmontosaurus mummies—a late juvenile and an early adult—with large continuous areas of preserved external skin surface.

Sereno is quick to explain that his dinosaur mummies are not like the human-prepared mummies in Egyptian tombs; no organic material remains. Across both newly described specimens and previous ones labeled mummies (including those found at the same site in the 20th century), the skin, spikes, and hooves were preserved not as tissue, but as a sub-millimeter clay film that formed on the carcass surface soon 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."

The team used several imaging tools, including hospital and micro-CT scans, thin sections, X-ray spectroscopy, clay analyses, and examination of the discovery site, all of which pointed to how this unique preservation occurred. After each sun-dried dinosaur carcass was covered up suddenly in a flash flood, a biofilm on the carcass's surface electrostatically pulled clay out of the wet sediment to congeal into a wafer-thin template layer, capturing the true surface in three dimensions, after which the organic material decayed away and the skeleton below fossilized over longer timescales.

Exposing such a soft, paper-thin clay boundary required hours of careful cleaning led by Fossil Lab manager Tyler Keillor, a co-author of the paper. Other researchers, led by postdoctoral scholar Evan Saitta, used 3D surface imaging, CT scans and contemporary footprints to follow the soft anatomy, characterize the sediment inside and outside the mummy, and fit the hooves back into a footprint. Digital artists then joined the science team to reconstruct the fleshy appearance and movement of the duckbill, walking on soft mud near the very end of the dinosaur era.

"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."

Crests, spikes, and scales

Working with the two new mummies, the researchers reconstructed a complete, fleshy profile of Edmontosaurus annectens.

"The two specimens complemented each other beautifully," Sereno said. "For the first time, we could see the whole profile rather than scattered patches."

They identified a continuous midline feature that began as a fleshy crest along the neck and trunk and transitioned over the hips into a single row of spikes running down the tail—each spike positioned over a single vertebra and fitted to each other.

The lower body and tail had the largest polygonal scales, although most were tiny pebble-like scales just 1–4 millimeters across, surprisingly small for a dinosaur growing to over 40 feet in length. Wrinkles preserved over the ribcage suggest the skin of this duckbill was thin.

A hoofed dinosaur

The hind feet of the larger mummy held the biggest surprise: hooves. The tip of each of the three hind toes were encased in a wedge-shaped hoof with a flat bottom like that of a horse. The team used CT scans of the mummy's feet and 3D images of the best-preserved duckbill footprint from the same time period, fitting the former into the latter. Using information from both sources, they accurately reconstructed the appearance of the hind foot. Unlike the forefoot that touches the ground only with its hooves, the hind feet have a fleshy heel pad behind the hooves.

"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," Sereno said.

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Shaping the future of the field

Beyond the anatomical revelations, the study offers a toolkit for future research on dinosaur soft anatomy: new preparation methods, a clear lexicon for soft structures and scales, an imaging workflow from fossil to a flesh model, and a recipe for generating a dinosaur mummy. More than standalone discoveries, the team's mummy research offers a new model for dinosaur mummification involving clay templating to test on future finds.

The authors also point to what comes next: targeted searches for similarly preserved specimens in the same Wyoming strata and elsewhere; biomechanical models that now have reliable external boundaries; and companion analyses that probe when and where clay templating takes hold.

"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 came to be and what we can learn from them."