For the first time, paleontologists have successfully reconstructed the profiles of two massive duck-billed dinosaurs, down to their stony skin and unexpected hooves. Based in part on remains found decades ago in the Wyoming lowlands Edmontosaurus annectens the specimens were preserved only because of the extremely rare, delicate process of “mummification”.
Meet the ducklings
About 39 feet long and about 6.2 tons in weight, E. annenctens was one of the largest and most widespread dinosaurs in what is now North America during the Late Cretaceous period. It was also literally one of the last of its kind. The fossil record, dating from 68 to 66 million years ago, shows that E. annenctens most likely existed before the Cretaceous-Paleogene extinction event that eventually killed off all non-avian dinosaurs.
Edmontosaurus it must have been quite impressive to look at the long, low skull and one of the most prominent duck-billed snouts of any hadrosaur. Although paleontologists had previously discovered various sections of the skeleton and more than 20 partial to complete skulls, they were not entirely sure of its general anatomy.
Scaly ridge skin on the back of a younger duckbill dinosaur, Edmontosaurus annectens, nicknamed “Ed Jr.” The duckbill baby, which was about two years old when it died, is estimated to be the only young dinosaur mummy ever discovered. Credit: Tyler Keillor/Fossil Lab
How a dinosaur becomes a “mummy”
To better understand the creature, a University of Chicago team led by anatomist Paul Sereno tracked the historical sites of previous dinosaur finds in east-central Wyoming. Specifically, they identified a small area known for its “mummies,” including two new ones E. annenctens The specimens discovered by Sereno’s group are a late juvenile and an early adult.
“Bloglands in Wyoming, where the finds were made, is a unique ‘mummy zone,'” Sereno said in a statement.
These are not mummies in the traditional sense of the word. No organic remains, but dinosaur anatomy such as skin, hooves and spines preserved in a sub-millimetre film of clay through a process called clay templating. Making one of these mummies requires what Sereno calls an “unexpected preservation event” that occurs shortly after the dinosaur’s death and burial.
“It’s a mask, a template, a layer of clay so thin you can blow it off,” he explained.
Sereno’s team used several imaging techniques, such as micro-CT scanning and X-ray spectroscopy, to closely analyze how this process might occur. They believe the most likely scenario begins when a flash flood engulfs a sun-dried dinosaur carcass. As water rushes through the body, the biofilm on the animal’s skin electrostatically pulls clay from the surrounding sediment, forming a delicate “template layer.” This layer then preserved the surface beneath it in three dimensions as the dinosaur’s organic matter continued to decompose until its skeleton eventually fossilized.
While the dinosaur template is often unprecedented in its detail, it’s also extremely delicate—experts at the UChicago Fossil Lab spent hours cleaning the clay border. Other members of the team continued to perform surface and CT imaging of the skeleton and study the fossilized footprints. The final results are described in a study published on October 23 Science offer never-before-seen glimpses of a pair of dinosaurs.
“The two specimens complemented each other very well. For the first time, we could see the whole profile instead of scattered patches,” Sereno said.
A scene from about 66 million years ago, depicting an anti-spiked dinosaur as it appeared in life, is based on mummies found in east-central Wyoming that record its scaly skin and hooves. It had a fleshy crest over its neck and trunk, a row of fleshy spines over its hips and tail, and hooves covering the toes of its hind feet. Credit: Dani Navarro
Single spines, polygonal scales and hooves
Paleontologists were able to identify a continuous middle anatomical detail E. annenctens which started as a ridge along the neck and trunk and then turned into a single row of spines across the hips and down the tail. Each spike grew directly above and attached one corresponding vertebra. The lower body and tail of the dinosaur had larger polygonal scales, although most were the size of small pebbles. Usually dinosaurs are as big as E. annenctens characterized by larger scales. Wrinkles left on the ribs also indicate thin skin.
But the most unexpected discovery is on the back feet of the larger mummy. Each had three toes covered with a wedge-shaped, flat-bottomed hoof similar to a horse’s. Along with additional studies of fossilized duck beak marks from the same era, the team reconstructed the general appearance. Now it seems that E. annenctens walked with its front feet touching the ground only with its hooves, while its hind feet also had a fleshy heel pad.
“There are so many wonderful ‘firsts’ preserved in these antibeak mummies,” Sereno said. “The earliest recorded hooves in a land vertebrate, the first confirmed hoofed reptile and the first hoofed quadruped with distinct forelimb and hindlimb posture.
The authors of the study hope that other scientists can now apply the methods described in their study to other dinosaur mummies. In the meantime, they plan to search Wyoming to find similar examples for their work.