Researchers say they have found structures resembling red blood cells, along with chemical traces of heme, in a 115-million-year-old dinosaur fossil. The study, published November 2 in the Proceedings of the National Academy of Sciences, could describe the oldest red blood cells identified in a dinosaur fossil if its interpretation is confirmed.
The team examined a hind leg bone from Aratasaurus museunacionali, a small, meat-eating dinosaur discovered in northeastern Brazil in 2008. The fossil was sent to geochemist Kliti Grice and her colleagues in 2024 for analysis. After preparing thin slices of the bone, the researchers used microscopy and several chemical techniques to investigate its contents.
Under a high-resolution microscope, they observed rounded structures with sunken centers inside channels that once carried blood vessels through the bone. The structures measured about 23 micrometers long and 20 micrometers wide. Analyses also detected two chemical signatures of heme, including a degraded form that the researchers say is consistent with an ancient origin. Samples from the surrounding sediment showed different chemical evidence, which the team says argues against contamination from the surrounding material.
The researchers also compared the structures with blood cells from birds and crocodiles, living relatives of dinosaurs. The fossil structures were larger, though the team suggests they may have changed shape during fossilization or that the dinosaur’s cells differed naturally. They also detected evidence of an amino acid associated with collagen.
Grice and her colleagues say the combined evidence—the structures’ location in the bone, their mineralization and the chemical traces of heme—supports their identification as fossilized red blood cells. They propose that rapid burial in water and microorganism-rich sediment slowed decay, while the mineral hausmannite may have helped preserve the structures.
Other researchers remain cautious. Blood and other soft tissues usually decay quickly, and minerals can resemble red blood cells under a microscope. Contamination is another potential concern. Maria McNamara, a paleobiologist who was not involved in the study, described it as a particularly comprehensive effort but said she would still be cautious about identifying the structures as red blood cells.
The team argues that its methods could help researchers look for biological material in other fossils. If the findings hold up, they could offer information about the chemistry of extinct animals’ tissues beyond what bones reveal about anatomy, growth and evolution.
Sources: Smithsonian Magazine.