By Will Dunham
WASHINGTON (Reuters) – Two fossilized “mummies” unearthed by scientists in the Wyoming lowlands reveal the external anatomy of the duck-billed dinosaur Edmontosaurus in exquisite detail, including the surprising presence of hooves on its feet, a first for any dinosaur.
Two individuals of Edmontosaurus, dating to the very end of the Age of Dinosaurs, 66 million years ago, were about 40 feet (12.2 meters) long as an adult, and about half that length as a two-year-old juvenile. The contours of the outer fleshy surface of the two dinosaurs were preserved above the skeleton by a thin layer of clay, about one-hundredth of an inch (0.025 cm) thick, formed when they died.
Because fossils rarely preserve the shape of an animal’s soft tissues, it is generally difficult to recreate the appearance of dinosaurs and other extinct creatures. However, these two had large continuous areas of preserved outer skin surface, providing the most complete and detailed image of a large dinosaur to date.
“This is the first time we’ve seen a complete profile of a dinosaur,” said University of Chicago paleontologist Paul Sereno, who led the study, published in the journal Science. “We’re pretty sure what it looked like.”
Dinosaurs are not mummies in the same sense as the bodies carefully preserved in ancient Egypt for the afterlife. However, similar fossils were found more than a century earlier in the same area, although not as carefully excavated as these, which were dubbed mummies, and the term stuck.
“They don’t resemble Egyptian-style human mummies. And at least our mummies have no DNA, no tissue structure, nothing. It’s a clay mask,” Sereno said.
These Edmontosaurus individuals lived in the Cretaceous period just before the asteroid hit, abruptly ending the age of the dinosaurs. Edmontosaurus, which munched on plants with a broad and flat snout that vaguely resembled a duck, roamed western North America alongside the apex predator Tyrannosaurus, the horned dinosaur Triceratops, and the armored dinosaur Ankylosaurus.
“It’s the most common dinosaur” in its ecosystem,” Sereno said of Edmontosaurus. “It was a huge herd. It’s a cow of its time.”
At about 42 feet (12.8 meters) long, it is about as long as a tyrannosaurus. Other fossils with tooth marks indicate that this was T. rex’s favorite meal.
“There’s no doubt it’s on the menu,” Sereno said.
“It’s not an animal that’s easy to take down. That’s why you needed something like a tyrannosaurus.”
CONVERGENT EVOLUTION
Mammals such as horses, cows, goats, and sheep have developed hooves, structures that protect the toes, support the animal’s weight, provide traction, and absorb the shock of walking and running. But Edmontosaurus did it millions of years earlier. It is the first dinosaur, the first reptile, and the first land vertebrate to develop hooves.
“It’s designed for hard ground, efficient walking — maybe even running — on the surface,” Sereno said.
Its hooves are an example of a phenomenon called convergent evolution, in which different organisms independently develop similar features — such as the wings of birds, bats and extinct flying reptiles called pterosaurs — to adapt to similar environments or ecological niches.
Edmontosaurus appears to have moved slowly on all fours and on two legs when running, Sereno said.
“The only animal we can even point to as a parallel or analog is the kangaroo,” Sereno said.
As a result, the hooves on his front limbs were different from those on his hind limbs.
Fossils revealed that Edmontosaurus had a continuous midline on its body, with a fleshy ridge along its neck and trunk that turned into a single row of spines across its hips, extending into its tail. Its skin was covered in tiny, pebble-like scales, most of which were no larger than an average lizard.
The two individuals of Edmontosaurus appear to have died, possibly due to drought, and their desiccated carcasses were later covered by a flash flood that left them covered in a film of clay called a clay template.
Using historic photographs and field research, researchers have rediscovered where some dinosaur mummies were excavated in east-central Wyoming in the early 20th century. During fieldwork in this “mummy zone,” Sereno said, they also found Tyrannosaurus and Triceratops fossils, which must be detailed in separate studies. Sereno said the Tyrannosaurus fossil suggests it may have been feathered.
“The T. rex doesn’t even have scales,” Sereno said.
(Reporting by Will Dunham Editing by Rosalba O’Brien)