Once in its laboratory, the team pulverized the bone fragments and subjected them to a battery of chemical tests. And while it found plenty of amino acids—the building blocks of proteins—the profile of these molecules didn’t match collagen. And they weren’t ancient. When living things die, their amino acids gradually flip into a mirror-image state. By measuring the levels of these flipped versions, scientists can deduce how old proteins are. Those in the Centrosaurus were clearly modern. They likely came from microbes, and given the team’s diligence, those microbes likely came from inside the bones.
The team found 50 times more microbial DNA within the bone fragments than in the mudstone immediately surrounding them. It could even see some of the cells under a microscope. “The concentration of bacteria in the bone was huge,” Onstott says. “There’s a very significant community in there.”
This microbial community is dominated by just a few bacterial species that are very different than those in the surrounding sediment. Their exact identity is unclear, but around a third of the DNA is a close match for Euzebya, a bacterium that’s been found in Etruscan tombs and in the skins of Japanese sea cucumbers. “It’s not something I’ve widely encountered, so we’re trying to figure out what they are,” Onstott says. “There seems to be something in the bone that enhances this particular type of bacteria.”
“It’s slightly odd that no one’s ever tried this before,” says Matthew Collins from the University of York, who studies ancient proteins. Despite the collagen controversy, no one had looked directly for microbes inside the bones, and “surprise, surprise! There are microbes present. It doesn’t disprove that researchers can get [original] proteins out of these dinosaur tissues, but it does cast shade on the interpretation.”
Saitta and some of his colleagues take a harder line. They couldn’t find any trace of ancient proteins in the Centrosaurus bones, which contradicts Schweitzer’s discovery, or at least fails to replicate it. And since many bacteria can digest collagen, they say that the presence of microbes in bone makes it even less likely that ancient proteins would have survived for tens of millions of years.
Others feel less strongly. Onstott notes that other dinosaur bones might have no microbes at all, or different communities of them. Some might contain species that help to preserve collagen over time. “I think the discovery of Cretaceous dinosaur proteins is still tentative, and we still don’t have clear, unquestionable evidence,” adds Enrico Cappellini from the University of Copenhagen. Nor is it clear, he says, that chemical signals that have been interpreted as dinosaur proteins came from bacteria.
Schweitzer doesn’t think that the new study contradicts her work, either. “We never claimed that our bones have been preserved without a microbial influence,” she says. That influence is neither surprising nor “mutually exclusive” with the preservation of actual dinosaur proteins—“a claim we support with multiple lines of evidence, rigorous data, and a plethora of controls.”