A 115-million-year-old dinosaur fossil unearthed in Brazil may contain the oldest chemical evidence of blood-related material ever identified in dinosaur remains, according to a new study.
The findings, however, have drawn skepticism from some specialists, who argue that the observed structures do not align with the typical morphology of reptilian red blood cells.
Published Monday, November 2, in the journal PNAS, the research describes microscopic structures resembling red blood cells, along with chemical signatures of heme—the oxygen-carrying component of hemoglobin.
The discovery stems from limb bones of Aratasaurus museunacionali, a roughly 10-foot-long (3-meter) dinosaur. Although earlier studies have reported blood traces in dinosaur fossils, this investigation is the first to apply chemical analysis to detect heme-based signatures within dinosaur bone.
“Traditionally, fossils yield information through bones, teeth, shells, and morphology,” said study co-author Kliti Grice, a professor in the School of Earth and Planetary Sciences at Curtin University in Australia, in an email to Live Science. “Exceptionally preserved fossils, however, may hold another archive: microscopic structures and molecular traces of the original organism. That opens the door to questions about ancient biology that skeletal anatomy alone cannot answer.”
Deep in the Bones
Grice explained that the team did not set out to find dinosaur blood. Instead, the research was motivated by a broader question: how much of an ancient organism’s original biology can survive the fossilization process?
To investigate, Grice and her colleagues examined the limb bones of A. museunacionali, a small theropod from the Early Cretaceous period of Brazil. Theropods were primarily carnivorous, bipedal dinosaurs that included iconic predators such as Tyrannosaurus rex and Velociraptor.
Through microscopic and chemical analysis of the theropod’s limb bones, the researchers identified numerous microscopic structures inside vascular canals—the channels that once transported blood vessels through bone. The structures’ shape, size, and position were consistent with red blood cells, according to the team.
“We combined microscopy, mineralogical analysis, and molecular geochemistry,” Grice said. “Critically, we investigated the chemistry in situ—within the bone itself—allowing us to link chemical signals to microscopic structures in their physical context. This is far stronger than extracting molecules from a fossil fragment and assuming their origin.”
Chemical analyses also detected preserved heme-related compounds associated with the structures, adding further chemical support to the possibility that the finds represent traces of dinosaur blood. Heme is the iron-containing, ring-shaped molecule that forms a central component of hemoglobin in red blood cells.
A Time Capsule
Grice and her team propose that the burial conditions of the theropod helped preserve the vascular structures and the cells within them. They suggest the animal was rapidly buried by sediment in a low-oxygen environment containing microbial mats—thin biofilms of microorganisms. These conditions may have altered normal decomposition, helping preserve both the microscopic structures and their associated chemical signatures.
“The red-blood-cell-like structures were mineralized with hausmannite, a manganese oxide mineral (Mn₃O₄),” Grice said. “We believe early mineralization helped stabilize or replicate microscopic biological structures before they were destroyed, while elements of the associated chemistry survived in altered form.”
The fossil also preserved amino acids linked to collagen, the structural protein found in bone.
Not Everyone Is Convinced
Blood has been reported in dinosaur fossils previously, but not all experts are convinced that these latest findings represent genuine red blood cells.
“I greatly 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 research but has previously studied dinosaur-age soft tissues. “I agree that endogenous molecules—those produced within the body—can persist over geological time in some form, and that they can teach us a great deal. However, I am not convinced that the structures shown are remnant red blood cells for several reasons.”
Schweitzer noted that the structures do not convincingly match the expected appearance of red blood cells in nonmammalian animals. Unlike mammalian red blood cells, which lose their nuclei during development, those of birds and reptiles retain them, typically producing a characteristic nuclear bulge in cell shape.
According to Schweitzer, the microstructures appear more concave and lack a nuclear bulge. They are also smaller than most nonmammalian red blood cells, “so morphological similarity is not met,” she said.
“I appreciate these authors bringing technological innovation to the study of dinosaur fossils,” she said. “I am excited to see this team and others advancing a molecular approach to ancient fossil remains.”