115-Million-Year-Old Dinosaur Bone May Hold Oldest Blood Traces Ever Found

The team wasn't hunting for dinosaur blood. Grice says the project started from a broader question: how much of an ancient organism's original biology can survive fossilization? Using limb bones of the Brazilian theropod, they combined microscopy, mineralogical analysis and molecular geochemistry.

"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," Grice said. "That is much stronger than simply extracting molecules from a piece of fossil and assuming where they originated."

How did soft material survive 115 million years?

The researchers credit the burial conditions. They propose the animal was rapidly covered by sediment in a low-oxygen environment containing microbial mats — thin films of microorganisms — which may have disrupted normal decomposition.

The cell-like structures were mineralized with hausmannite, a manganese oxide mineral (Mn₃O₄). "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," Grice said. The fossil also preserved amino acids associated with collagen, the structural protein in bone.

Why are some experts skeptical?

Mary Schweitzer, a molecular paleontologist at North Carolina State University who wasn't involved in the study, offered a mixed verdict. "I very much appreciate this team bringing rigor to the study of deep time fossils," she said. "However, I am not convinced that the images they figure are remnant RBCs [red blood cells] for several reasons."

Her first objection is shape. Bird and reptile red blood cells keep their nuclei — unlike mammalian cells, which shed them — and that typically leaves a characteristic nuclear bulge. Schweitzer says the new microstructures appear concave, with no bulge, and run smaller than most non-mammalian red blood cells. "Morphological similarity is not met," she said.

She also argued the team hadn't fully met the criteria for identifying fossilized blood cells and suggested resonance Raman spectroscopy — a technique that characterizes chemical compounds — would have given more conclusive evidence. Schweitzer was part of a team that used exactly that method in 2025 to detect heme in tissues from Brachylophosaurus canadensis and T. rex.

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

The full paper — 20-plus co-authors led by Grice — appears in Proceedings of the National Academy of Sciences under the title "Heme signatures in mineralized red blood cells within vascular canals of a 115-million-year-old dinosaur bone." Whether the structures are genuine blood cells or mineral mimics, the debate itself shows how much dinosaur paleontology now happens at the molecular scale.