Dinosaur ‘mummies’ may not be rare flukes after all

It might be easier for dinosaurs to “mummify” than scientists thought.

Unhealed bite marks on fossilized dinosaur skin suggest that the animal’s carcass was scavenged before being covered in sediment, researchers report October 12 in PLOS ONE. The finding challenges the traditional view that burial very soon after death is required for dinosaur “mummies” to naturally form. 

The new research centers on Dakota, an Edmontosaurus fossil unearthed in North Dakota in 1999. About 67 million years ago, Dakota was a roughly 12-meter-long, duck-billed dinosaur that ate plants. Today, Dakota’s fossilized limbs and tail still contain large areas of well-preserved, fossilized scaly skin, a striking example of dinosaur “mummification.”

The creature isn’t a true mummy because its skin has turned into rock, rather than being preserved as actual skin. Researchers have come to refer to such fossils with exquisitely preserved skin and other soft tissues as mummies.

In 2018, paleontologist Clint Boyd of the North Dakota Geological Survey in Bismarck and colleagues began a new phase of cleaning up and examining the dinosaur fossil. The team had found what looked like tears in the tail skin and puncture holes on the animal’s right front foot. To investigate what may have caused the skin marks, the researchers teamed up with Stephanie Drumheller, a paleontologist at the University of Tennessee in Knoxville, to remove extra rocky material around the marks. 

The holes in the skin — particularly those on the front limb — are a close match for bite wounds from prehistoric relatives of modern-day crocodiles, the researchers say. “This is the first time that’s been seen in dinosaurian soft tissues,” Drumheller says. 

Because the marks on the tail are larger than those on the front limb, the team thinks that at least two carnivores munched on the Edmontosaurus carcass, probably as scavengers because the wounds didn’t heal. But scavenging doesn’t fit into the traditional view of mummification.

“This assumption of rapid burial has been baked into the explanation for mummies for a while,” Drumheller says. That clearly wasn’t the case for Dakota. If scavengers had enough time to snack on its body, then the deceased dino had been out in the open for a while.

Observing Dakota’s deflated skin envelope, shrink-wrapped to the underlying bone with no muscle or other organs, Drumheller had an unexpected “eureka moment,” she says. “I had seen something like this before. It just wasn’t in the paleontological literature. It was in the forensics literature.”

When some smaller modern scavengers like raccoons feed on the internal organs of a larger carcass, the scavengers rip open the carcass’s body. The forensics research showed that this hole gives any gasses and fluids from further decomposition an escape route, allowing the remaining skin to dry out. Burial could happen afterward.

The researchers “make a very good point,” says Raymond Rogers, a researcher at Macalester College in Saint Paul, Minn., who studies how organisms decay and fossilize and wasn’t involved in the research. “It would be very unlikely for a carcass to achieve advanced stages of desiccation and also experience rapid burial. These two generally presumed prerequisites for mummification seem to be somewhat incompatible.”

Fossilization of soft tissues — like skin or brains or fleshy head combs — is uncommon, but not unheard of (SN: 8/20/21; SN: 12/12/13). “If [soft tissue] requires some spectacular confluence of weird events to get it fossilized at all, it’s far more common than then you would expect if that was the case,” Drumheller says. Perhaps, then, mummies originating from common carcass fates could explain this.

But while dry, “jerkylike” skin could survive long enough to be buried, the conditions involved aren’t necessarily common, says Evan Thomas Saitta, a paleontologist at the University of Chicago who was not involved with the study.

“I still suspect that this actual process is a very precise sequence of events, where if you get the timing wrong, you end up without a mummy dinosaur,” he says.

Understanding that sequence of events, and just how common it is, requires figuring out how fossilization proceeds after a mummy’s burial. This is an area of research that Boyd says he’s interested in looking into next.

“Is it just the same fossilization process as for the bones?” he asks. “Or do we also need a different set of geochemical conditions to then fossilize the skin?”