Nearly 100 years ago in Wyoming, fossil hunters unearthed a duck-billed dinosaur that looked like a sun-dried mummy, its scaly skin preserved in astonishing detail. Now, two additional mummies of the same species reveal a complete fleshy outline, including a neck-to-trunk crest, a row of tail spikes, and wedge-shaped hooves — the first ever found in a reptile — while explaining how such features survived in a drought-prone, oxygenated environment, researchers report October 23 in Science.
Duck-billed dinosaurs like Edmontosaurus annectens roamed the floodplains of western North America at the very end of the dinosaur era, about 66 million years ago. These plant-eaters grew up to 12 meters long and weighed several tons, moving on two or four legs. While their bones are common fossils, intact skin and other soft parts are rare, raising questions about how they fossilize without rotting away.
These so-called dinosaur mummies have turned up mostly in wet, oxygen-poor settings that slow decay. But the Wyoming site, part of the Lance Formation, was a coastal area with seasonal droughts and floods, conditions that should speed up decomposition.
Revisiting the area around the original finds, researchers uncovered a “mummy zone” less than 10 kilometers across, yielding the two new Edmontosaurus specimens plus well-preserved remains of Triceratops and Tyrannosaurus. The new mummies, one a late juvenile about 2 years old, the other an early adult 5 to 8 years old, offer the clearest view yet of the dinosaur’s outer form.
A fleshy crest runs along the midline from neck to trunk, reaching up to 28 centimeters high in adults, with bands of small scales 1 to 4 millimeters across separated by fine grooves. Over the hips, it transitions into interlocking spikes that extend to the tail tip, each positioned above a vertebral spine and fringed with smooth edges. The spikes interdigitate like puzzle pieces, a setup seen today in some lizards.
The feet show another surprise: broad, flat-bottomed hooves sheath the toe bones of the hind limbs, extending well beyond the claws with striated surfaces and a shallow ventral groove, much like horse hooves. The front feet have a single central hoof, forming a padded structure for quadrupedal walking. “The disparity in fore and hindfoot posture seems possible only in a facultative quadruped with a posterior-centered body mass closer to the hindleg and with foreleg support limited to slow speeds,” the researchers write.
These features paint Edmontosaurus as a bipedal reptile with mammalian-like hooves, evolving them independently at large body size during the Cretaceous — far earlier than in mammals.
Tests including CT scans, microscopy, and chemical analyses show the skin, spikes, and hooves preserved as a thin clay layer less than 1 millimeter thick, with no original tissue or organics left. This template formed when a bacterial film on the decaying surface drew clay particles from the sediment, capturing the outer shape before everything else dissolved. The process unfolded in four stages: drought killed the animals, desiccating their carcasses without scavenging; floods buried them quickly in sand; breaches allowed infilling to maintain 3D form; and clay templating sealed the surface during final decay.
“Recently discovered duck-billed dinosaur mummies suggest that ‘mummification’ occurred after drought-induced mortality involving four stages: carcass desiccation and initial subaerial decay, sudden burial with minimal transport, rapid carcass infilling, and templating in clay and final decay,” the researchers explain. “The cyclic drought-flood monsoonal climate of the Lance Formation and its extraordinary thickness under the mummy zone provided the environmental and geologic conditions, respectively, for rapid burial of desiccated, unscavenged carcasses without sediment reworking.”
Citations
P. C. Sereno et al. Duck-billed dinosaur fleshy midline and hooves reveal terrestrial clay-template “mummification”. Science. Published online October 23, 2025. DOI: 10.1126/science.adw3536
