The bulk of what we know from dinosaurs comes from bones. Skeletal parts can reveal a dinosaur’s size, how fast they grew, and even if they were gravid with eggs when they perished, but all the ribs, jaws, and other pieces can only go so far in revealing what our favorite dinosaurs looked like in life. Fortunately, paleontologists have found some dinosaur bones are preserved with soft tissues like feathers, claw sheaths, and, with luck, entire dinosaur “mummies.”
These aren’t traditional mummies purposefully preserved through chemical embalming. Instead, they’re fossils that preserve skin impressions as a thin layer within the rock. As paleontologists have continued to sift through the fossil record, such specimens are finally offering some insights into the external appearance of familiar species. And through untangling the backstories of these fossils, experts are beginning to realize that such cases of exceptional preservation might not be as rare as once believed.
One familiar face getting fleshed out is Edmontosaurus annectens. The duckbilled dinosaur walked the floodplains of ancient Montana and Saskatchewan 68 to 66 million years ago, and today it’s one of the best-known and most common fossils found in the region. Several Edmontosaurus mummies have revealed soft tissue structures that paleontologists never expected to see, including a pair of mummies uncovered in Wyoming described October 23 in Science.
Some of the preserved features on the new E. annectens specimens, a juvenile and an adult, confirm what paleontologists predicted. The toes on the back feet of the dinosaurs, for example, had a tough coating on them, like a hoof or a toenail. A row of overlapping spiky scales also ran down their backs that connected to their spines, a feature that paleontologists had not seen before in its entirety. “This is sort of dragon-like,” says Paul Sereno, a paleontologist at the University of Chicago and a coauthor on the new paper.
The findings add to previous research that has revealed Edmontosaurus had a fleshy comb on their heads, front toes wrapped together in a mitt of flesh tipped with a hoof-like covering, and rough, shovel-like beaks jutting down from their snouts.
How do dinosaurs turn into mummies?
The newly-described fossils follow a long history of Edmontosaurus mummies found in western North America, including the first dinosaur mummy discovered in 1908. Scientists began calling these early specimen mummies because of the extensive skin impressions left in the rock that surrounded their skeletons. Since that time, paleontologists have uncovered scaly skin and other soft tissues among such fossils as well.
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But researchers often assumed that a dinosaur body would have to be rapidly buried in sediment very close to death in order to preserve such fine details. As experts have taken a deeper look, however, it seems that dinosaur mummies may not have required a quick burial, after all. In fact, laying exposed on the surface for weeks to months after death might have helped some of these unique fossils form.
In 2022, Clint Boyd, Stephanie Drumheller-Horton, and their colleagues proposed that the exceptional Edmontosaurus mummy “Dakota” had lain exposed on the surface long enough for scavengers to take a few bites and that this long exposure helped preserve it. Scavenging animals and the decay process left holes in the dinosaur’s body, releasing gases, fluids, and microbes that allowed the tougher, scaly skin and front “hoof” to dry out and persist alongside the bone until sediment eventually covered the carcass.
The new Wyoming mummies suggest yet another pathway to exceptional preservation. The spiky scales and back hoof-like features of the fossils described by Sereno and colleagues are made up of a thin clay layer sandwiched between harder rinds of sandstone. The thin layer of clay adhered to the dinosaur’s body as it decayed, taking the shape of the soft tissue and creating a natural cast of the hadrosaurs’ bodies.
“It’s a 100th of an inch thick clay layer,” says Sereno, who is also a National Geographic Explorer. It’s so thin that it would be easy to miss in the excavation and preparation process. “You’re working in a lot of dust,” he says. “You don’t want to take extra, or you’d just completely miss it. You’d blow it away.”
(Here’s how a group of miners found a nodosaur fossil in Canada.)
Why are there so few dino mummies—and why are most of them hadrosaurs?
Edmontosaurus is far from the only dinosaur to be preserved with remnants of skin and skin impressions. Paleontologists have found skin impressions associated with other hadrosaurs like Saurolophus and Gryposaurus, as well as tyrannosaurs, horned dinosaurs, sauropod dinosaurs, and others. Not all of these specimens contained enough skin fossils to qualify as mummies, and the most extensive skin preservation is often found among hadrosaurs. “You’re very likely to find at least a patch of a skin impression any time you find articulated hadrosaur bones, no need of complete skeletons,” says Royal Tyrrell Museum of Paleontology researcher François Therrien.
Even though dinosaur mummies have been known for a long time, it’s only been relatively recently that paleontologists have understood that such detailed fossils are more common than previously assumed. “Unfortunately, we’ve seen several examples of fossils that seem to have been mummies but were not recognized as such fast enough, so much of the skin was actually prepared away” by experts removing rock from around the bones, says Drumheller-Horton, a paleontologist the University of Tennessee, Knoxville. What paleontologists look for and expect in the fossil record has a role to play in how many dinosaur mummies turn up. For the moment, at least, hadrosaurs have some of the most extensive and best-known examples of skin preservation.
What have we learned from dinosaur mummies?
Dinosaur mummies and other soft tissue fossils help paleontologists and paleoartists envision what the living animals once looked like as they moved around the Mesozoic world. But that’s hardly all. As experts have cataloged various examples of dinosaur mummies and soft tissue fossils, they’ve begun to pick up on some unexpected facets of dinosaur lives.
Along with existing footprints, the hoof-like structures seen in both the Wyoming and Dakota E. annectens mummies help scientists hypothesize how the dinosaurs moved based on the slight differences between its front and back feet. “This is an animal that decides to run on two legs and walk on four,” says Sereno.
Extensive skin impressions from two species of the duckbilled dinosaur Saurolophus have settled debates over whether the two skeletally similar dinosaurs were different species. When University of New England paleontologist Phil Bell compared the skin impressions of the Saurolophus angustirostris found in Mongolia with the other Saurolophus osborni found in Canada in a 2012 study, he found that each had distinctive scale patterns on their bodies. S. angustirostris had vertical bands of scales along the tail and large, bumpy scales along the midline of the back, while S. osborni lacked these ornate specializations. The scale patterns may have helped dinosaurs identify members of their own species.
Scales can reveal dinosaur coloration, as well. First established by comparing the microscopic details of dinosaur feathers to those of modern birds, paleontologists have been able to reconstruct the hues ancient dinosaurs were. The same principles hold for exceptionally-preserved scales.
Take the the small, horned dinosaur Psittacosaurus. A 2016 study of the dinosaur’s skin found that it had darker scales above and lighter scales beneath. This form of camouflage is known as countershading. If spotted from above, the dark colors on the dinosaur’s back would have helped it blend in with the forest habitat. A study of the spiky, armored dinosaur Borealopelta published the following year found a similar pattern. The dinosaur was reddish above and lighter below, which would have helped the dinosaur hide from tyrannosaurs in the ancient forest.
(Read more about how soft tissue finds have given dinosaurs a revamp.)
The future of fossilization research
Even as paleontologists uncover more dinosaur soft tissues in the field and in museum collections, experts are only just beginning to understand how such fossils formed. “There’s just so much amazing research happening now on the different pathways that an organism can take to become a fossil,” Drumheller-Horton says, from researchers looking to how organisms decompose to experts studying the chemical changes dinosaur bodies must have undergone as they became fossilized.
At this point, it’s unclear which mummy-making process is most common. Sereno calls the rock layers in Wyoming that yielded the first dinosaur mummies and the two new E. annectens fossils the “mummy zone.” Two additional tyrannosaur and Triceratops mummies have been found in the same area, and he suspects that more examples of clay-mask mummies could be buried there. Only further investigations will confirm whether clay can preserve skin renderings at other sites, too.
For Boyd, the chemical process that allows skin impressions and soft tissues to fossilize alongside bones is a tantalizing question. “We know why bones fossilizes, since it already has a natural mineral framework that promotes fossilization,” he says, “but these dinosaur mummies typically lack mineralization in their skin and so would require a different pathway to fossilize.” Working out how skin fossilizes would not only solve that mystery, but help paleontologists better look out for fossils that have the potential to preserve soft tissues. Through understanding the making of dinosaur mummies, paleontologists will unveil even more.
