Scientific American has an interesting article about dinosaur “mummies”. In 1908, Charles Sternberg found the “first dinosaur mummy”. It was an Edmontosaurus dinosaur with what looked like fossilized flesh and skin. It was found in the sandstone rocks of the Lance Formation in eastern Wyoming. New research shows that the “skin” is actually a clay mold, which was molded by bacteria as the animal decayed.

“That’s going to come as a shocker to a lot of people,” says University of Chicago paleontologist Paul Sereno, lead author of the new study, published in Science. This clay molding process was known to preserve the form of soft animals in oxygen-poor areas, such as the muds at the bottom of lagoons and deep-sea trenches, Sereno says. But “no one imagined it could work its magic on a dinosaur buried suddenly by sand in a flooded river,” an environment that is fairly oxygen-rich.

When “mummified” in this way, all of an animal’s outside soft tissues become films of clay less than 1 millimeter thick. Sereno proposes that the creatures’ carcasses were first desiccated in a drought before suddenly being engulfed by sediment—likely brought on by a flood. A layer of bacteria latched onto the wet, porous surface to form a biofilm, which attracted surrounding clay. Then, weeks after they were buried, the soft parts of the carcasses decayed and were washed away by groundwater, leaving the clay mask to forever preserve the forms of what lay just beneath, Sereno says.

“The sort of basic [fossilization] question of ‘How did this thing fossilize in the first place?’ hasn’t received as much attention in the past,” says Stephanie Drumheller, a paleontologist at the University of Tennessee, Knoxville, who was not involved in the study but is mentioned in its acknowledgments section. “I think this was a really great deep dive into this one area where we do see several specimens.”