Red blood cells found in a 115 million-year-old dinosaur fossil from Brazil are the oldest chemical evidence of blood-related material in a dinosaur fossil, a new study claims.

However, not everyone agrees with the research team's conclusions, with some experts saying the shape doesn't match those of reptilian red blood cells.

In the new research, published Monday (Nov. 2) in the journal PNAS, the scientists said they identified microscopic structures resembling red blood cells alongside chemical signatures of heme, the oxygen-carrying component of hemoglobin.

The new findings come from limb bones of a roughly 10-foot-long (3 meters) dinosaur called Aratasaurus museunacionali. While previous research has detected blood in dinosaur fossils, this is the first study to use chemical processes to identify heme-based signatures in dinosaur bones.

"Traditionally, fossils give us information through bones, teeth, shells and morphology. But exceptionally preserved fossils may contain another archive of information: microscopic structures and molecular traces of the original organism," study co-author Kliti Grice, a professor in the School of Earth and Planetary Sciences at Curtin University in Australia, told Live Science via email. "That opens the possibility of asking questions about ancient biology that skeletal anatomy alone cannot answer."

Deep in the bones

The team was not originally looking for traces of dinosaur blood, Grice said. Her research was driven by a broader question: How much of an ancient organism's original biology can survive fossilization?

To investigate, Grice and her team studied the limb bones of A. museunacionali, an extinct species of small theropod that lived during the Early Cretaceous period in Brazil. Theropods were a group of mostly meat-eating dinosaurs that walked on two legs. They included famous predators such as Tyrannosaurus rex and Velociraptor.

Using microscopic and chemical analysis on the theropod's limb bones, the researchers found numerous microscopic structures within vascular canals, channels that once carried blood vessels through the bone. Their shape, size and location were consistent with red blood cells, the team said.

"We combined microscopy, mineralogical analyses and molecular geochemistry," Grice said. "Critically, we investigated the chemistry in situ [in the bone itself], so we could relate the chemical signals to the microscopic structures in their physical context within the fossil. That is much stronger than simply extracting molecules from a piece of fossil and assuming where they originated."

The team's chemical analyses detected preserved heme-related compounds associated with these structures, providing more chemical evidence to support the idea that the finding could be traces of dinosaur blood, as heme is the iron-containing, ring-shaped molecule that forms a key part of the molecule hemoglobin in red blood cells.

A time capsule

Grice and the team think the conditions the theropod dinosaur was buried in could have helped preserve the vascular structures and the red blood cells within them. They suggested that the animal was rapidly covered by sediment in a low-oxygen environment containing microbial mats ‪—‬ thin biofilms made of microorganisms. These conditions may have altered the normal process of decomposition, helping to preserve the microscopic structures and associated chemical signatures.

"The red-blood-cell-like structures were mineralised with hausmannite, a manganese oxide mineral (Mn₃O₄)," Grice said. "We think early mineralisation helped stabilise 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 preserved amino acids associated with collagen, the structural protein found in bone.

Not everyone is convinced

While blood has been found in dinosaur fossils before, not everyone agrees that these latest findings are truly red blood cells.

"I very much appreciate this team 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 but has previously studied soft tissues from the dinosaur age. "I am in agreement that endogenous [produced within the body] molecules can persist over geological time in some form, and that there is much these can teach us. However, I am not convinced that the images they figure are remnant RBCs [red blood cells] for several reasons."

For one, the structures do not convincingly match the expected appearance of red blood cells in nonmammalian animals, Schweitzer said. Unlike mammalian red blood cells, which lose their nuclei during development, those of birds and reptiles retain their nuclei. This typically produces a characteristic nuclear bulge in the shape of the cell.

According to Schweitzer, the latest findings show the microstructures having more of a concave shape with no nuclear bulge. The structures also were smaller than most non-mammalian red blood cells, "so morphological similarity is not met," Schweitzer said.

Schweitzer also questioned whether the researchers had adequately met the criteria for identifying fossilized blood cells. She suggested that resonance Raman spectroscopy, an analytical technique used to characterize chemical compounds, would have provided more conclusive evidence. Schweitzer was part of a team that used this method in 2025 to detect heme in tissues recovered from Brachylophosaurus canadensis and T. rex.

Although this study did not use the same methods, Schweitzer welcomed the broad direction of the research.

"I appreciate these authors bringing technological innovations to the study of dinosaur fossils," she said. "I am excited to see that this team and others are moving forward with a molecular approach to ancient fossil remains."