First reptilian hooves found in mummified dinosaurs from Wyoming

Wyoming Public Radio | By Hanna Merzbach

Published November 3, 2025 at 11:16 AM MST

A man touches a large fossil with a distinct big head, against a black background. — Mummy of the juvenile duck-billed dinosaur Edmontosaurus annectens with Professor Paul Sereno of the University of Chicago. The dinosaur mummy nicknamed “Ed Jr.,” was covered by floodwaters some 66 million years ago, preserving its fossilized skeleton and, in a thin clay layer, large areas of scaly, wrinkled skin and a tall fleshy crest over its back.

Paleontologists have revealed the first reptiles with hooves in the form of mummified, duck-billed dinosaurs found in Eastern Wyoming.

Two Edmontosaurus annectens, called “Ed Jr.” and “Ed Sr.” for short, were initially discovered near the South Dakota border more than a century ago, but researchers just released their findings in the journal, Science.

“Wyoming has given us the clearest best picture of what a large dinosaur looks like ever,” said University of Chicago paleontologist Paul Sereno, who leads the research team. “It's given us the first reptilian hooves.”

Hooves have long been isolated to mammals. Paleontologists have suspected some dinosaurs might have had horse-like hooves, which are good for running away quickly on dry ground, but this is the clearest evidence, Sereno said.

“You just needed to find an upright reptile the size of a big mammal,” he added.

A painting of a large dinosaur with its tail up in a colorful sunsetting sky. One dinosaur has fallen dead in the background. — Scene painting some 66 million years ago showing the duck-billed dinosaur as it appeared in life based on mummies discovered in east-central Wyoming which document its scaly skin and hooves. It had a fleshy crest over neck and trunk, a fleshy spike row over hips and tail and hooves capping the toes of the hind feet.

It’s unusual to have this clear of a picture. Sereno said this discovery is possible because of the mummified forms.

The fossils were preserved head to toe under a layer of clay as thin as the kind you’d put on your face to clean your pores. It captured the original shape and texture of the bodies.

“That is preserving the shape, the wrinkles, the scales, the hooves, the spikes, all the detail,” Sereno said.

Mummy dinosaurs are rare but more common in this part of east-central Wyoming, between the towns of Lusk and Newcastle, because of the geological formations. Researchers think that the animals died next to a prehistoric river, then water rapidly rushed in and clay formed before they could decompose.

This story was produced by the Mountain West News Bureau, a collaboration between Wyoming Public Media, Nevada Public Radio, Boise State Public Radio in Idaho, KUNR in Nevada, KUNC in Northern Colorado, KANW in New Mexico, Colorado Public Radio, KJZZ in Arizona and NPR, with additional support from affiliate newsrooms across the region. Funding for the Mountain West News Bureau is provided in part by the Corporation for Public Broadcasting and Eric and Wendy Schmidt.