Despite being known to science for more than a century, the duck-billed dinosaur Edmontosaurus continues to surprise. In 2013, paleontologists announced that some of these herbivorous dinosaurs had fleshy combs on the tops of their heads, much like a rooster’s. Now new research has uncovered another unexpected feature that experts had missed. Edmontosaurus had the reptilian equivalent of a hoof—its middle fingers connected and covered in a large nail.
The discovery comes from an analysis of an Edmontosaurus “mummy,” nicknamed Dakota, published today in PLOS ONE. Other exceptional Edmontosaurus fossils have been found, but Dakota is one of the most complete. In addition to the discovery of the hoof, it offers clues about one way dinosaur bones and tissues can be preserved through time.
Dinosaur fossils with preserved tissues such as skin are sometimes called “mummies,” and up until now “the assumption was that dinosaur mummies had to be rapidly buried,” says University of Tennessee, Knoxville paleontologist Stephanie Drumheller-Horton. Uncovered carcasses would attract scavengers that would pick away at the body, the thinking went, and exposure to the sun, wind, and rain could also contribute to the body’s breakdown. The faster a dinosaur was covered in sediment and shielded from destructive forces, researchers believed, the better the chance that some of its soft tissues would be preserved.
Yet these explanations were never fully satisfying, Drumheller-Horton says, because insects, fungi, and even microorganisms can still break down a carcass that’s buried. Something else must explain how these mummified fossils are formed, and Dakota reveals an unexpected pathway for skin and other features, such as the keratin of nails, to make it into the fossil record.
“Dinosaur skin is unexpectedly common in the fossil record,” Drumheller-Horton says, and the way Dakota became preserved may explain why.
Dried dino skin
Dakota wasn’t buried quickly. In fact, Drumheller-Horton and her colleagues propose, more than 66 million years ago the dinosaur’s carcass sat exposed on the ground and dried out over the course of weeks or months. Scavengers did come to pick at the body, evidenced by patches of Dakota’s skin that show bite marks where carnivores nibbled on the carcass before the dinosaur was eventually buried.
“We do know that at least two carnivores, and maybe more, consumed parts of the animal’s arm and tail after its death,” Drumheller-Horton says. Those opportunistic meat-eaters might even have helped Dakota’s skin become preserved.
The bite marks suggest that scavengers opened up the dinosaur’s body wall, which could have allowed the fluids, gases, and microbes that built up during decomposition to drain away. As internal viscera were eaten or rotted away, the hadrosaur’s scaly, durable skin was then better able to dry out in the sun, forming a kind of wrapper around the bones before the animal was buried. Drumheller-Horton and colleagues note that this is why Dakota looks “deflated,” with the skin shrunk to the bones instead of positioned as it was in life.
Paleontologists have puzzled over the preservation of dinosaur mummies for decades, and the new paper makes “a strong case” that Dakota was exposed to the elements before burial, says New York Institute of Technology paleontologist Karen Poole, who was not involved in the study. Even if it’s difficult to tell when scavengers fed on Dakota’s remains, she notes, it’s clear that the fossil represents a dinosaur that was somewhat rotted and desiccated rather than being a pristine corpse.
Dakota probably wasn’t the only hadrosaur to go through this process. Paleontologists often find skin or skin impressions with hadrosaurs but not remnants of other soft tissues, such as internal organs. The pattern might hint that hadrosaur skin was so tough that it wasn’t very enticing to carnivores and that it could persist for much longer than the softer parts in the dinosaur’s body, giving it a better chance of making it into the fossil record.
An early ‘hoof’
Specimens such as Dakota can rewrite what we think dinosaurs looked like, something that is often based upon bones alone. Among living animals, many important structures—such as the trunk of an elephant, or the wattles of a turkey—do not contain any bones, meaning that we wouldn’t know about them if we had only fossils of the animals. Dinosaurs also undoubtedly had ornaments and body parts that left no trace on the skeleton, and it takes the discovery of well-preserved fossil animals like Dakota to reveal them.
The front feet of Dakota came as a particular surprise to paleontologists. From skeletons, paleontologists knew that Edmontosaurus had four fingers on each hand. Other hadrosaur mummies and trackways indicated that these fingers were wrapped together in a kind of “mitt,” acting almost like a single pillar rather than multiple, spreading digits.
Dakota further clarifies this foot structure. “The term ‘hoof’ got applied to this specimen on social media when the images of the hand first leaked out, and it has stuck ever since,” says study co-author Clint Boyd. The structure isn’t the same as a horse’s hoof, in that the tough outer covering doesn’t extend to the bottom of the foot, but Dakota’s anatomy still has a hoof-like appearance, with a large nail over the middle fingers, unlike most other reptilian paws.
The hand suddenly starts to look like one hoof with a smaller dew claw on either side,” Poole says.
Dakota also likely has other secrets waiting to be discovered. “There’s often an assumption that all dinosaur skin is preserved as either a mold or a cast,” Drumheller-Horton says, but in Dakota “we can see that the skin itself is present in three dimensions.” It’s not just the shape of the skin, but the skin itself that’s been preserved for more than 66 million years.
“Discoveries like Dakota are a great chance for us to take a step back,” Poole says, “and think about what assumptions we’ve been making about the anatomy of these creatures.”