The study, published Monday in the journal Proceedings of the National Academy of Sciences, examined limb bones of Aratasaurus museunacionali, a small theropod dinosaur that lived during the Early Cretaceous period in what is now Brazil. Using microscopic and chemical analysis, the team found ellipsoidal microstructures within vascular canals, the channels that once carried blood vessels through the bone.
“Something that looks like a blood cell under a microscope is interesting, but morphology by itself isn't enough to establish its biological origin,” said Kliti Grice, an organic geochemist at Curtin University in Australia. “Our approach brings together microscopic observations with chemical evidence associated with the structures. The convergence of independent lines of evidence is much more powerful than saying, 'this looks like a red blood cell'.”
The team conducted high-performance liquid chromatography–mass spectrometry on the bone extract and found it was dominated by heme B, a key component of hemoglobin. Elemental mapping showed the cell-like structures were correlated with iron, carbon, and nitrogen, while the rest of the bone contained phosphorus, calcium, fluorine, and carbon. Grice said the microstructures had been mineralised with hausmannite, a manganese oxide mineral, which may have helped preserve them.
The researchers suggest the fossil was rapidly buried in a low-oxygen environment containing microbial mats, conditions that may have altered normal decomposition and helped preserve the microscopic structures and associated chemical signatures.
But Mary Schweitzer, a molecular paleontologist at North Carolina State University who was not involved in the study but has previously studied soft tissues in dinosaur fossils, expressed scepticism. “I am in agreement that endogenous 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 for several reasons,” Schweitzer told Live Science.
She noted that the structures did not show a characteristic nuclear bulge typical of non-mammalian red blood cells, which retain their nuclei, and that they were smaller than most non-mammalian red blood cells. Schweitzer also questioned whether the researchers had adequately met the criteria for identifying fossilised blood cells and suggested that resonance Raman spectroscopy would have provided more conclusive evidence.
Grice said further studies were needed. “The immediate priority is replication,” she said. “We need to examine additional fossils, different geological settings and other parts of the dinosaur family tree using complementary microscopic, chemical and molecular techniques. If similar signals can be demonstrated independently, we can begin asking much bigger questions about preservation and, cautiously, about ancient physiology.”
If confirmed, the finding would be the oldest known example of possible dinosaur blood cells, surpassing previous finds dating back about 80 million years.