Scientists analyzing fragments of poorly preserved dinosaur bones excavated more than a century ago have discovered what appear to be red blood cells and collagen fibers, soft tissues that thus far have only found rarely, on extremely well-preserved fossils.
The discovery -- which suggests that soft tissue remnants may be more widespread than thought in dinosaur bones -- was something of an accident, said Susannah Maidment, a junior research fellow in the department of earth science and engineering at Imperial College London and co-lead author of a study describing the find published Tuesday in the journal Nature Communications.
Maidment had met fellow lead author Sergio Bertazzo, a biomedical materials scientist also at Imperial College London, at a conference. Maidment studies the skeletons of dinosaurs such as stegosaurus and triceratops; Bertazzo, the microscopic properties of living tissues such as human heart valves. As they discussed their work, Maidment was intrigued. Perhaps using Bertazzo's high-powered microscopes to take a close look at bone crystals in fossils could help her understand the mechanical stresses the ancient beasts' bodies endured.
"I was wondering, has anyone put dinosaur bone under there?" Maidment recalled. "Let's just see what happens."
It took some time for the scientists to convince a museum to allow them to break off pieces of ancient dinosaur bone for the experiment. But eventually Maidment and Bertazzo obtained a claw from a meat-eating dinosaur, estimated to be 75 million years old and discovered at the Dinosaur Park formation in Alberta, Canada, from curators at the National History Museum of London.
Bertazzo stuck a tiny sample tweezed off of the claw onto a scanning electron microscope. He didn't find the bone crystals Maidment hoped to see, but within a couple of hours he found something else that got him excited.
"I received a series of increasingly frantic messages from him," Maidment recalled. "'I need to talk to you,' ... 'I need to talk to you!' ... 'I really need to talk to you!'"
Bertazzo had seen striking oval formations in the sample, which he thought were red blood cells. As a paleontologist, Maidment was skeptical -- thus far, only exceptionally well-preserved fossils had included soft tissue. Scientists believed such protein molecules could not survive for more than 4 million years.
So the team started working to eliminate various possibilities. They compared the oval structures to cells in modern bird blood, contributed by a friend of Maidment's who "happened to have an emu in his freezer," and found similarities in shape.
Using a machine known as a focused ion beam to cut clean nanoscale slices through the possible cells and looking at them again under the electron microscope, the scientists found a dense internal structure that looked like a nucleus. This provided more evidence that the red blood cells weren't a result of contamination. Red blood cells from mammals, including humans, don't have nuclei. Those from other animals, such as reptiles and birds, do.
At one point, Maidment asked a specialist in fungi what the "spores" in the sample were.
"She said, 'I don't know -- they look like blood cells,'" Maidment said.
The collagen was discovered in another sample, a bit of rib from an undetermined dinosaur dating from around the same period of time. Bertazzo was again using the ion beam to slice into the fossil, hoping to examine what had happened to holes in the bones where the collagen should have been when the dinosaur was alive.
Looking at the slice with the electron microscope, he found fibers with distinctive black and white banding -- a signature of collagen that arises from its protein structure. That the banding was still visible, Maidment said, provided evidence that the protein was intact and the collagen had been well-preserved. When the team analyzed the chemical makeup of the purported blood cells and collagen using a mass spectrometer, they found similarities to emu blood and a modern rabbit bone.
In the end, they were reasonably convinced they had found ancient blood and collagen. "Neither Sergio nor I can figure out what else these might be," Maidment said.
Assessing eight samples from the museum in all, Bertazzo and Maidment and their co-authors were able to find blood cells in two and collagen in three, suggesting that there might be a lot of soft tissue lurking in old fossils. Such a potentially large supply of dinosaur blood and collagen could help paleontologists refine their knowledge of the ancient beasts' natural history, Maidment said.
Red blood cell size, for instance, is known to correlate with an animal's warm-bloodedness: the smaller the cell, the higher the animal's metabolic rate. At some point, dinosaurs -- whose ancestors were cold-blooded reptiles and whose descendents are warm-blooded birds -- probably evolved to become warm-blooded. If scientists could find red blood cells in different types of dinosaurs and measure the cells' size, they might be able to pinpoint where and when a transition from cold-bloodedness to warm-bloodedness occurred. Getting a better idea of particular animals' metabolic rates might also help paleontologists understand how they would have behaved when they were alive.
Similarly, scientists might be able to use collagen, which is unique in every animal, as a sort of fingerprint to help them work out the relationships between creatures -- much in the same way researchers use DNA in younger samples to understand relatedness between individuals and species.
Today, scientists use bone structure to try to understand the dinosaur family tree; having the collagen fingerprint information would bring an independent, "fresh set of data" to the discussion, Maidment said.
For now, she added, she and Bertazzo would like to look for blood and collagen in older bones, and in fossils preserved in different types of burial environments.