Researchers say they have found the oldest chemical evidence of blood-related substances in a dinosaur fossil: heme-associated compounds and red-blood-cell-like microstructures in a 115-million-year-old bone from Brazil. The study, published Monday, November 2, in PNAS, examined the small theropod Aratasaurus museunacionali. However, some experts remain unconvinced that the structures are fossilized red blood cells, arguing that their shape and size do not match the cells of reptiles or birds.
Evidence inside the bone
The dinosaur was about 10 feet (3 meters) long. Although previous research has detected blood-related material in dinosaur fossils, the new study is the first to use a chemical process to identify heme-based signatures in dinosaur bones, according to the researchers.
“Traditionally, fossils give us information through their bones, teeth, shells, and morphology. However, exceptionally preserved fossils may contain another archive of information: microstructural and molecular traces of the original organisms,” study co-author Kliti Grice, a professor in the School of Earth and Planetary Sciences at Australia’s Curtin University, told Live Science in an email.
“It opens up the possibility of asking questions about ancient biology that skeletal anatomy alone cannot answer,” Grice said.
Looking deep into the bones
Grice said the team was not initially searching for traces of dinosaur blood. Instead, the research was driven by a broader question: “How much of the original biology of ancient organisms can survive fossilization?”
The researchers studied limb bones from A. museunacionali, an extinct species of small theropod that lived during the Early Cretaceous in Brazil. Theropods were primarily two-legged, carnivorous dinosaurs and included well-known predators such as Tyrannosaurus rex and Velociraptor.
Using microscopy and chemical analyses, the team found numerous microstructures inside the bones’ vascular canals — channels through which blood vessels once ran. The researchers said the structures’ shape, size and location were consistent with red blood cells.
“We combined microscopy, mineralogical analysis and molecular geochemistry,” Grice said. “Importantly, we investigated the chemistry in situ [in the bone itself]. Therefore, chemical signals can be linked to the microstructure in the physical context within the fossil. It’s much more powerful than simply extracting molecules from fossil fragments and inferring their origins.”
The chemical analysis detected conserved heme-related compounds associated with the structures, providing additional evidence that they could be traces of dinosaur blood. Heme is a ring-shaped, iron-containing molecule that forms an important part of hemoglobin in red blood cells.
A fossil time capsule
Grice and her colleagues believe the environment in which the dinosaur was buried may have helped preserve its vascular structures and the red-blood-cell-like material. They suggested that the animal was rapidly covered by sediment in a low-oxygen, or hypoxic, environment. A microbial mat may also have coated the remains.
These conditions could have altered the normal decomposition process, helping preserve both fine structural details and associated chemical features.
“The red blood cell-like structures were petrified with hausmannite, a manganese oxide mineral (Mn3O4),” Grice said. “We believe that early lithification helped stabilize or replicate microscopic biological structures before they were completely destroyed, while aspects of the associated chemistry also survived in altered form.”
The fossil also contained related, conserved amino acids and collagen, a structural protein found in bones.
Not everyone is convinced
Blood-related material has been reported in earlier dinosaur fossils, but not everyone agrees that the latest structures are red blood cells.
“We are extremely grateful to this team for bringing rigor to the study of deep-time fossils,” said Mary Schweitzer, a molecular paleontologist at North Carolina State University who was not involved in the study and has previously investigated dinosaur-era soft tissue.
“I agree that it is endogenous [produced within the body]. Molecules can persist in some form across geological time, meaning they have a lot to teach us. However, I am not convinced that the images they depict are residual red blood cells,” Schweitzer said.
One reason, she said, is that the structures do not convincingly match the expected appearance of red blood cells in nonmammals. Unlike mammalian red blood cells, which lose their nuclei during development, the red blood cells of birds and reptiles retain their nuclei. This usually produces a characteristic nuclear bulge in the cell.
Schweitzer said the structures in the study appear more concave and lack nuclear bulges. She also noted that they are smaller than most nonmammalian red blood cells. “The morphological similarity criterion is not met,” she said.
Schweitzer also questioned whether the study met all the criteria needed to identify fossilized blood cells. She suggested that resonance Raman spectroscopy, a technique used to characterize compounds, could have provided more conclusive evidence. She was part of a team that used the method to detect heme in tissues collected in 2025 from Brachylophosaurus canadensis and Tyrannosaurus.
Although the new study used different methods, Schweitzer welcomed the broader direction of the research.
“Thank you to the authors for bringing innovation to the study of dinosaur fossils,” she said. “I’m really looking forward to seeing this team and others advance molecular approaches to ancient fossil remains.”
Grice, K., Isaji, Y., Wieman, J., Greenwood, P.F., Tripp, M., Diaz-Mateus, M.A., Polopat, S.F., Sun, X., Fox, C.P., Brosnan, L.M., Buntim, RaM., Schwark, L., Visscher, P.T., Ogawa, N.O., Demore, J., Saraiva, A.F., Rickard, W.D.A., Okochi, N., Sayao, J.M., … Kellner, A.W.A. (2026). Heme signature of calcified red blood cells in blood vessels of a 115-million-year-old dinosaur bone. Proceedings of the National Academy of Sciences, 123(0), e2614248123.
Source: www.livescience.com