That came as a huge surprise to Tullis Onstott, a microbiologist from Princeton who worked with Saitta, and who always thought of fossils as inert and inanimate. “I thought that dinosaur bone must be some kind of sealed sarcophagus,” he says. “It’s not, by any means. It’s basically a condo for bacteria. Now the question becomes: Is this true for all dinosaur bones?”

The team’s study, which has been uploaded as a preprint and has yet to be reviewed and published, complicates a heated debate that has rocked the world of paleontology for more than a decade. In the mid-2000s, Mary Schweitzer from North Carolina State University reportedly found blood vessels, cells, and traces of collagen protein from the thigh bones of two dinosaurs: a 68-million-year-old Tyrannosaurus and an 80-million-year-old Brachylophosaurus. Though a far cry from Jurassic Park, since no DNA had been discovered, the discovery was still an extraordinary one. If proteins really could survive that long, they would allow scientists to study dinosaurs at a molecular level, just as they do modern animals.

Others were skeptical. Most ancient proteins are hundreds of thousands of years old at most. A few exceptional molecules have lasted for 3 to 4 million years, protected either by exceptional cold or unique minerals. The supposed collagen from Schweitzer’s dinosaurs enjoyed no such protection, and would have been 20 to 30 times more ancient. Collagen is tough, but after such a long time, the chemical bonds that hold it together would likely have ruptured. Some critics argued that the proteins Schweitzer had detected weren’t actually there, or that the cells she had seen were actually bacterial colonies. Others suggested that the molecules and tissues must have come from modern organisms that contaminated the samples.

Schweitzer’s team addresses that last critique by taking extra care to stop external microbes from getting into its fossils. When it recently reanalyzed fragments from its Brachylophosaurus, the team even disassembled one of its analytical instruments and soaked its pieces in alcohol to kill any contaminating microbes.

But based on his new findings, Saitta now argues that such measures wouldn’t have done anything to remove microbes living inside the bones themselves. And if such microbes prove to be common, researchers must take extra steps to prove that proteins in a dinosaur’s bone actually belonged to the animal itself, and not to modern microbes that have infiltrated its remains.

His team tried to do that with their Centrosaurus specimen. Like Schweitzer’s group, it took care to stop foreign microbes from landing in the samples. The scientists washed their gloves and face masks in alcohol. They sterilized their tools with bleach and a blowtorch. They extracted bones that were still encased in mudstone and wrapped them in sterilized foil. “Collecting bones aseptically is basically an impossible task because the techniques of paleontology haven’t changed in the last 100 years, but I think we did a good job,” Saitta says.