For the first time, paleontologists have successfully reconstructed the profiles of two massive, duck-billed dinosaurs, right down to their pebbled skin and unexpected hooves. Based in part on remains recovered decades ago in the badlands of Wyoming, the pair of Edmontosaurus annectens specimens were preserved only thanks to an extremely rare, delicate “mummification” process.
Meet the duckbills
At around 39 feet long and weighing about 6.2 tons, E. annectens was one of the largest and most common dinosaurs in present day North America during the Late Cretaceous period. It was also literally one of the last of its kind. Fossil records dating back to 68 to 66 million years ago indicate E. annectens likely existed until the Cretaceous–Paleogene extinction event that eventually killed off all non-avian dinosaurs.
Edmontosaurs were probably pretty striking to look at with a long, low skull and one of the most prominent duck-bill snouts of any hadrosaur. Although paleontologists previously discovered various skeletal sections and over 20 partial-to-complete skulls, they weren’t entirely sure of its overall anatomy.
How a dinosaur becomes a ‘mummy’
To gain a better understanding of the creature, a team at the University of Chicago led by anatomist Paul Sereno tracked down the historical locations of previous dinosaur finds in east central Wyoming. Specifically, they identified a small area known for its “mummies,” including two new E. annectens specimens discovered by Sereno’s group—a late juvenile and an early adult.
“The badlands in Wyoming where the finds were made is a unique ‘mummy zone,’” Sereno said in a statement.
These aren’t mummies in the traditional sense of the word. There are no organic remains, but dinosaur anatomy like skin, hooves, and spikes preserved in a sub-millimeter clay film during a process called clay templating. Making one of these mummies requires what Sereno calls a “fluke event of preservation” that occurs soon after a dinosaur’s death and burial.
“This is a mask, a template, a clay layer so thin you could blow it away,” he explained.
Sereno’s team utilized multiple imaging techniques such as micro-CT scans and X-ray spectroscopy to closely analyze how this process might occur. They believe the most likely scenario starts when a flash flood covers a sun-dried dinosaur carcass. As the waters rush over the body, a biofilm on the animal’s skin electrostatically draws clay from the surrounding sediment to form a delicate “template layer.” This layer then preserved the surface underneath it in three dimensions as the dinosaur’s organic matter continued decaying before its skeleton eventually fossilized.
While the dinosaur template is often unprecedented in its detail, it’s also extremely delicate—experts at UChicago’s Fossil Lab spent hours cleaning up the clay boundary. Other team members continued conducting surface and CT imaging of the skeleton and studied fossilized footprints. The end results described in a study published on October 23 in Science offer never-before-seen looks at the dinosaur pair.
“The two specimens complemented each other beautifully. For the first time, we could see the whole profile rather than scattered patches,” said Sereno.
Single spikes, polygon scales, and hooves
Paleontologists were able to identify a continuous midline anatomical detail on E. annectens that started as a crest along its neck and trunk, before transforming into a single row of spikes over its hips and down its tail. Each spike grew directly over and fitted a single, corresponding vertebra. The dinosaur’s lower body and tail skin displayed larger polygonal scales, although most of them were the size of small pebbles. Usually, dinosaurs as large as E. annectens featured larger scales. Wrinkles preserved over its ribcage also indicate thin skin.
However, the most unexpected discovery is located on the larger mummy’s hind feet. Each included three toes covered in a wedge-shaped, flat-bottomed hoof similar to a horse. Combined with additional examinations of fossilized duck-bill footprints from the same era, the team reconstructed the overall appearance. It now appears that E. annectens walked with its forefeet touching the ground with only hooves, while its hindfeet also featured a fleshy heelpad for support.
“There are so many amazing ‘firsts’ preserved in these duck-billed mummies,” said Sereno. “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.”
The study’s authors hope other researchers can now apply the methods described in their study on other dinosaur mummies. In the meantime, they plan on conducting searches around Wyoming to find similar examples for their own work.
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