The cupboards of the Natural History Museum in London hold spectacular dinosaur fossils, from 10-centimeter, serrated Tyrannosaurus rex teeth to a 4-meter-long hadrosaur tail. Now, researchers are reporting another spectacular find, buried in eight nondescript fossils from the same collection: what appear to be ancient red blood cells and fibers of ancient protein.

Using new methods to peer deep inside fossils, the study in this week’s issue of Nature Communications backs up previous, controversial reports of such structures in dinosaur bones. It also suggests that soft tissue preservation may be more common than anyone had guessed. “It’s encouraging,” especially because the proteins were found in what appear to be the most unremarkable, ordinary bones, says Matthew Collins, an archaeologist and biochemist at the University of York in the United Kingdom. But he and others caution that the team hasn’t proven beyond doubt that the structures do contain ancient proteins.

As early as the 1970s, researchers captured images of what looked like cellular structures inside dinosaur fossils. But did the structures contain actual tissue? Proteins commonly decay hundreds to thousands of years after an organism dies, but in rare cases they have been known to survive up to 3 million years. In a series of studies beginning a decade ago, a team led by North Carolina State University paleontologist Mary Schweitzer reported that they had extracted what appeared to be collagen, the most abundant protein in bone, from a 68-million-year-old T. rex fossil. They sequenced fragments of the protein and concluded that it closely matched that of birds, dinosaurs’ living descendants (see here and here). But other teams haven't been able to replicate the work, and others suggested that the collagen could be contamination.

The new study, led by materials scientist Sergio Bertazzo and paleontologist Susannah Maidment, both of Imperial College London, has a different strategy for hunting down ancient proteins. Bertazzo, an expert on how living bones incorporate minerals, uses a tool called a focused ion beam to slice through samples, leaving pristine surfaces that are ideal for high-resolution imaging studies. He teamed up with Maidment to apply the technique to eight chunks of dinosaur toe, rib, hip, leg, and claw.

What they found shocked them. Imaging the fresh-cut surfaces with scanning and transmission electron microscopes, “we didn’t see bone crystallites” as expected, Maidment says. “What we saw instead was soft tissue. It was completely unexpected. My initial response was these results are not real.”

The U.K. team tested more fossils and ran microscopic samples from what appear to be collagen fibers through a mass spectrometer to get the weight of the component molecules. The weights came back as identical to those of the three most common amino acids in collagen, the team reports.

But outsiders, including Schweitzer, say that the weights aren’t conclusive proof that the molecules being analyzed are amino acids, or that they came from a dinosaur rather than a contaminant. A different type of mass spectrometer that can provide the sequence of the amino acids in a protein fragment would strongly suggest the existence of collagen and replicate the earlier work, Collins says. Maidment says the team hopes to do such studies soon. If they succeed, the work may spur additional efforts to isolate dinosaur proteins and understand how they differed from those of their modern relatives.