How to mummify a dinosaur

Some 67 million years ago, in what is now known as Slope County, North Dakota, an adolescent duck-billed dinosaur was killed and chewed up by various predators and scavengers. The Edmontosaurus left behind an impressive fossil nicknamed Dakota the Dinomummy, so called because unlike most dinosaur fossils, which are mostly bones or teeth, this one has exceptionally preserved skin.

In the nearly two decades since Dakota was discovered in the Hell Creek Formation, one of the most studied and fossil-rich regions on Earth, paleontologists have been trying to solve the mystery of how it died and became so well preserved. A recent analysis of the fossil in the journal PLOS ONE not only lends new insight into how it formed, but suggests that dinosaur "mummies" are far more common than previously thought.

Dinosaur mummies aren't like the ancient Egyptian kind. As far as we know, dinosaurs didn't bury their dead. Instead, mummy is a catch-all term for fossilized imprints of soft tissue, especially skin, that doesn't readily form under most conditions. But the examples that survived are stunning.

One such nodosaur mummy was so well preserved. Dubbed the "Mona Lisa" of dinosaurs, scientists could determine that variations in its skin color were a form of camouflage. Another duck-billed mummy nicknamed "Leonardo" even has evidence of parasitic stomach worms.

While dino mummies are already rare, some scientists argue they should be even more scarce. That's because it's hard to make a fossil, but especially a mummy. The conditions necessary are so specific that it only occurs in rare instances.

But life on Earth has been around for so long — about 3.7 billion years — that fossilization has happened enough times for scientists to study these remains, detect patterns and draw conclusions about what life was like in the ancient past. But there are still competing theories as to how these fossils exactly form.

With Dakota the Dinomummy, there are two competing hypotheses, which are believed to be broadly applicable to all dino mummies. The first is that the dinosaur was rapidly buried after death, in just a few minutes or hours, preventing it from decomposing too quickly. The second theory posits that Dakota's carcass desiccated or dried out.

However, neither of these theories satisfactorily explain what happened to Dakota. First of all, analysis of the dirt around the fossil suggests that the dinosaur died in a warm and humid environment that was close to water. There's no evidence that it was buried by a sudden flood, either. So rapid burial has been ruled out. Instead, it seems more likely the ancient beast perished on a sandbar that slowly entombed the corpse.

But before all that, there is ample evidence to suggest predators or scavengers were actively feasting on Dakota. Injuries on the bones and skin seem consistent with the bite marks of crocodyliforms, which are extinct relatives of modern alligators and crocodiles. Unrelated crocodyliform fossils have been discovered at the site, confirming they coexisted with duck-billed dinosaurs like Dakota called hadrosaurids.

"Under the previous explanations of dinosaurian 'mummy' preservation, this fossil should have never formed," Stephanie Drumheller of the University of Tennessee–Knoxville and colleagues, who penned the PLOS ONE study, reported.

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At approximately 35 feet long and 3.5 tons, Dakota was one juicy meal. As carnivores gorged themselves on this this buffet, tears and holes in the dinosaurs skin may have caused it to deflate, pushing out the fluids, gases and microbes that cause decomposition. The paleontologists believe this may have helped in its unique preservation.

"Drying of the overlying skin could have progressed for weeks or months, until sediment from the adjacent river buried the remains," the authors wrote. This may have actually lengthened "the window for preservation" they argue, "in stark contrast to previous explanations of natural mummification."

Because this rationale for how a dinosaur mummy forms is more nuanced — and doesn't require a "spectacularly unlikely convergence of events," as the authors put it — Dakota may indicate why dino mummies are more common than they necessarily should be.

"Not only has Dakota taught us that durable soft tissues like skin can be preserved on partially scavenged carcasses, but these soft tissues can also provide a unique source of information about the other animals that interacted with a carcass after death," Clint Boyd, a senior paleontologist at the North Dakota Geological Survey and one of the study authors, said in a statement.

Recognizing this pathway sheds light on "why fossilized skin, though still uncommon, is not exceptionally rare in the dinosaur fossil record," the authors conclude.