In a new paper in Science, researchers from the University of Chicago describe how, about 66 million years ago, the bodies of the duck-billed dinosaur Edmontosaurus annectens were transformed into remarkably detailed dinosaur “mummies” that preserved tiny features of skin, scales, and hooves. The team names the process “clay templating.” In this process, the outer soft tissues of the animal were preserved as a delicate clay coating that settled over the skeleton after burial, forming a mask less than 1/100th of an inch thick.
With a combination of advanced imaging methods, the scientists were able to recreate what the dinosaur would have looked like in life. Their reconstruction reveals a tall crest running along the neck and torso, a row of spikes down the tail, and hooves that enclosed the toes. When these results are combined with fossilized footprints, the overall appearance of this duck-billed dinosaur, long speculated about but never documented with this level of precision, becomes much clearer.
“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, PhD, 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.”
Rediscovering Wyoming’s dinosaur “mummy zone”
Using old field photographs and careful detective work, Sereno and his colleagues retraced the locations in east-central Wyoming where several classic dinosaur mummies had first been uncovered in the early 1900s. They mapped a compact “mummy zone” within these rock layers, which represent stacked river sands.
In this area, the team excavated two new Edmontosaurus mummies, a younger individual and a somewhat older one, both preserving large, continuous patches of the external skin surface. These fossils provided crucial pieces needed to build a full, fleshy profile of the animal.
Sereno emphasizes that these dinosaur mummies are very different from human-created mummies in Egyptian tombs; none of the original organic material is still present. In both the newly described specimens and earlier examples labeled as mummies (including those found at the same site in the 20th century), the preserved skin, spikes, and hooves exist not as tissue but as an extremely thin clay coating that formed on the outside of the carcass 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.”
Ultra-thin clay films capture dinosaur skin in 3D
To investigate how these extraordinary fossils formed, the team used a suite of imaging and analytical approaches. These included hospital and micro-CT scans, thin sections, X-ray spectroscopy, clay mineral analyses, and a close study of the rock layers where the fossils were found. All lines of evidence pointed toward a specific sequence of events that produced this rare style of preservation.
The researchers propose that after the dinosaurs died, their bodies dried in the sun before being rapidly buried in sudden flash floods. A microbial film on the outer surface of the carcass then attracted clay particles from the surrounding wet sediment through electrostatic forces. This process created a wafer-thin clay template that faithfully captured the animal’s outer shape in three dimensions. Over time, the soft tissues decayed away, leaving the clay film and the skeleton, which later fossilized on longer timescales.
Cleaning, scanning and digitally rebuilding the duckbill
Revealing a boundary as fragile as this paper-thin clay layer required painstaking work. Fossil Lab manager Tyler Keillor, a co-author on the study, led hours of meticulous preparation to uncover the crucial surfaces without destroying them.
Another group, led by postdoctoral scholar Evan Saitta, relied on 3D surface imaging, CT scans, and comparisons with fossil footprints from the same time period. They traced the preserved soft anatomy, examined the sediments inside and outside the mummy, and matched the dinosaur’s hooves back into a footprint. Digital artists then worked alongside the scientists to create lifelike reconstructions that showed how the duckbill would have looked and moved as it walked across soft mud near the very end of the dinosaur age.
“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.”
Crest, spikes, scales and thin skin
Working from the two newly described mummies, the researchers were able to assemble a complete, fleshy outline 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 discovered a continuous feature running along the center of the back that started as a fleshy crest over the neck and torso. Over the hips, this structure shifted into a single row of spikes along the tail, with each spike aligned above a vertebra and fitting neatly with its neighbors.
The team also documented the dinosaur’s scale patterns. The largest polygonal scales appeared along the lower body and tail, while most of the animal was covered in very small, pebble-like scales only 1-4 millimeters across, surprisingly tiny for a dinosaur that could reach more than 40 feet in length. Fine wrinkles preserved over the ribcage indicate that the skin of this duckbill was relatively thin.
Hooves and heel pads on a “hoofed” dinosaur
The most unexpected discovery came from the larger mummy’s hind feet: this dinosaur had hooves. The tips of each of the three hind toes were enclosed in a wedge-shaped hoof with a flat underside, similar to that of a horse.
To confirm what these feet looked like in life, the researchers combined CT scans of the mummified feet with 3D images of the best-preserved duckbill footprint from the same time period, carefully aligning the bones and soft-tissue impressions. With this information, they produced a detailed reconstruction of the hind foot. Unlike the forefoot, which makes contact with the ground only through its hooves, the hind feet also included 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.
A new toolkit for dinosaur soft-tissue research
Beyond revealing striking new anatomy, the research provides a practical framework for future studies of dinosaur soft tissue. The authors outline new preparation techniques, a clear set of terms for describing soft structures and scale types, a step-by-step imaging pathway from fossil specimen to fleshed-out model, and a recipe for how a dinosaur mummy can form under natural conditions.
Their work offers more than a series of isolated findings. It proposes a general model for dinosaur mummification based on clay templating, which can now be tested on other fossils that might have formed in similar ways.
The team also highlights the next steps: focused searches for additional specimens with this kind of preservation in the same Wyoming rock layers and in other regions; biomechanical studies that can now rely on accurate external body outlines; and complementary analyses designed to understand when and where clay templating is most likely to occur.
“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.”