But that view is changing — and it's possible that many ancient fossils may preserve more detail than meets the eye. The sort of biological tissue now being found in some fossils could tell us about dinosaur anatomy, behavior, and evolution in ways that weren't possible just a few years ago.
The photo above, from a new study published today in Nature Communications and led by Sergio Bertazzo of Imperial College London, shows an extremely zoomed-in view of a 75-million-year-old theropod claw, taken from the London Natural History Museum's collection. When researchers scraped tiny pieces off the fossil and looked at them under an electron microscope, they found tiny structures that look a lot like collagen fibers present in our own ligaments, tendons, and bones.
In other dinosaur fossils, the researchers found features that resemble red blood cells. Tests showed that they have a similar chemical composition to the blood of an emu (a bird thought to be a relatively close relative to dinosaurs).
The idea that dinosaur fossils might harbor soft tissue first surfaced about a decade ago, when paleontologist Mary Schweitzer found evidence of blood cells preserved inside T. rex fossils.
But what's so exciting about this new study is that the fossils used, unlike Schweitzer's, aren't particularly well-preserved. Susannah Maidment, one of the paleontologists who worked on the paper, called them "crap" specimens. If they have preserved soft tissue inside them, it could be a sign that thousands of other fossils in museum collections do too.
How paleontologists found blood inside dinosaur fossilsFor hundreds of years, most paleontologists never considered that their fossils might preserve these sorts of microscopic soft-tissue features. It was assumed that the proteins and other molecules they're made of would deteriorate in just a few million years.
What's more, looking inside them to confirm this would require that people damage the fossil, either by breaking it open or by dissolving the hard, mineralized outside, as Schweitzer did with her T. rex. "No right-thinking paleontologist would do what Mary did with her specimens," paleontologist Thomas Holtz told Smithsonian for a 2006 story on Schweitzer's discovery. "We don’t go to all this effort to dig this stuff out of the ground to then destroy it in acid."
Schweitzer did so after a veterinarian at a conference happened to see microscope slides of T. rex bone slices and observed that there were red blood cells inside it. But her claim remained controversial among paleontologists — even after her 2006 paper, which presented more thorough testing.
More recent chemical analysis has provided further evidence that the
T. rex bones do indeed contain blood cells, and Schweitzer has since found soft tissue preserved inside an 80-million-year-old hadrosaur. It's still unclear exactly how this soft tissue is able to survive, but some hypothesize that iron molecules might bind to proteins in the tissue, making it more stable.
This newest paper, conducted with weathered, run-of-the-mill fossils rather than pristine ones, suggests that this process might be the rule, not the exception. If so, these findings could be the first of many to come.
Dinosaur blood and proteins could tell us about their behavior and evolutionYou can only learn so much about an organism from its bones. As much as we've discovered from the hundreds of thousands of dinosaur fossils excavated around the world, we're still debating whether dinosaurs were warm- or cold-blooded and how many of them had feathers.
Peering inside these dinosaurs' bones — to look at their blood cells, connective tissue, and other microscopic features — could dramatically improve our understanding of their biology as a whole. The structure of their blood cells, for instance, could hint at their behavior and physiology in ways that their bones simply can't.
The tissue might help scientists better understand evolutionary relationships between species
The new information might also help scientists better understand evolutionary relationships between species. In the study, researchers found that the proteins inside the collagen-like fibers are well-preserved, with the specific sequence of amino acids that they're built from largely intact. Amino acid sequences in proteins gradually evolve over time and vary from species to species, somewhat like DNA — so analyzing them in dinosaurs could lead to better knowledge about the evolutionary relationships between them and other species, like birds.
But there's one thing we can't do with this soft tissue: extract dinosaur DNA and make Jurassic Park a reality. Compared with collagen fibers and red blood cells, DNA is much, much smaller and more fragile.
Perhaps DNA could also be more readily preserved than thought. But scientists currently estimate that it has a half-life of just 521 years, and dinosaurs largely died off 65 million years ago.