A remarkable pair of dinosaur “mummies” featuring fossilized soft-tissue remains is helping shed light on how these ancient reptiles looked, defying paleontologists’ past reconstructions.
Millions of years ago, while the dinosaurs from this location (found in modern-day Wyoming) died and began to decay, microbes helped produce clay molds that left a lasting cast of the creatures’ skin, spikes, and hooves. Now, a researcher from the University of Chicago has followed the trail of a historical mystery stretching back over a century to locate new specimens of these mysterious dinosaur “mummies.”
Wyoming Discovery
While not truly mummified in the sense of having their actual tissues preserved, the remains offer the best glimpse yet of dinosaurs’ proper physical forms. Eastern Wyoming’s cache of remarkably preserved duck-billed dinosaurs, Edmontosaurus annectens, was first discovered in 1908 by fossil collector C. H. Sternberg.
“Paleontology started out as a wealthy man’s hobby,” lead author Paul Sereno of the University of Chicago told The Debrief. “Sometimes, with wealth, they would hire people to go out and find fossils. This is most famously done by Cope and Marsh at the end of the 19th century here in North America, hiring competing crews, sometimes sabotaging each other out west in the very beds I’m working in.”
“Sometimes they would come back, do the work, and become researchers. Sternberg wrote books: popular books,” Serreno added. “He was an incredible collector.”
Searching for Dinosaur Mummies
Sereno worked with colleagues to rediscover the original site by combing through historical documents for clues to its location. From photographs and Sternberg’s correspondence, combined with the remembrances of local ranchers in the area today, they isolated a 10-kilometer-wide patch of land they termed the “mummy zone.”
Although Sternberg quickly wrote a paper describing his find in Wyoming, a critical difference between modern journals and those of the early twentieth century led to the loss of information about the site’s location for a century. At the time, articles did not reference the images, which were only included at the end. The photos of Sternberg’s Wyoming dig appeared at the end of his subsequent paper about a discovery in Kansas, rather than in the piece about the Wyoming mummies.
“Nobody knew this,” Serreno said. “Everybody thought [there were] no photographs of the greatest dinosaur mummy ever found. Then, a researcher a decade ago, found three photographs of the quarry that were supposed to be included with the article Sternberg wrote the year before.”
“Sternberg never mentioned that the photographs got mixed up, so nobody knew.”
Digging in the Mummy Zone
Their ensuing dig uncovered a late juvenile and an adult E. annectens, marking the first discovery of a large-bodied dinosaur with a fully preserved flesh outline. Duck-billed dinosaurs belong to a larger classification of hadrosaurids, and this is the first example of that type to be recovered with a fully intact tail spike row. Additionally, it is the earliest known reptile with hoofed feet, whose form was discovered in the recent discovery that defies earlier attempts at recreations.
Paleontologists have continually revised their expectations for what living dinosaurs may have looked like in light of growing evidence. The midline crest and tail spikes in the new specimens also contradict expectations, featuring a much more complex hide along these structures than paleontologists had envisioned. Intriguingly, the findings suggest possible similarities in the functionality and morphology of some living reptiles.
Explaining the Dinosaur Mummies
Initially, how the skin texture and flesh were preserved was a mystery, although Sternberg referred to the finds as “skin impressions,” not actual mummies. Typically, soft tissue preservation occurs in oxygen-poor lagoons, yet these specimens were found in oxygen-rich riverbeds.
After conducting a detailed analysis of the specimens using optical, CT, electron microscopy, and X-ray spectroscopy, Sereno’s team determined that they were examining thin layers of clay bounded by sandstone. Most likely, the area rapidly sank during the seasonal nought-flood cycle, leading to the high level of preservation.
There is no evidence that any organics remain within the clay, which likely formed in layers aided by biofilms as the creatures decayed. Discovering this new preservation mechanism opens up a much wider range of settings in which paleontologists may find such preserved tissue.
“Everyone thought that this was a marine process that required anoxic environment with lots of clay mud, cold water, darkness,” Serreno explained. “Those conditions would give you the time for clay to create this mask on a soft material.”
“But what we’re finding is—and it probably happened very quickly—is that, wow, this happens in river sediments to something as large as a dinosaur.”
More to Discover
Beyond those specimens, the site also yielded a Triceratops skeleton with some flesh imprints, along with a fully articulated Tyrannosaurus rex skeleton. Serreno says that the scene will be more fully explained in forthcoming papers. As an example of the last days of the dinosaurs, the three species found at the site display remarkable differences, demonstrating the variety of dinosaur species.
“You’re going to have an animal that has small scales at the size of a T-Rex, you’re going to have a T-Rex that has no scales, smooth bodies, feathers, and you’re going to have an animal that’s got the largest scales anybody has ever recorded on a reptile, in the same environment, bumping into each other,” Serreno said.
All of this should excite people entering the field, Serreno says, as significant discoveries like this show there is much life left in looking into the past.
“That’s a message that young listeners need to hear, because we are in the heyday of dinosaur discovery,” Serreno concluded. “We’re naming more dinosaurs than ever. Part of it is just getting out there and getting into the field.
“I didn’t have mummies in my mind when I went there,” he adds. “I was trying to teach University of Chicago students what paleontology was about.”
The paper, “Duck-billed Dinosaur Fleshy Midline and Hooves Reveal Terrestrial Clay-template ‘Mummification’,” appeared in Science on October 23, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.