Researchers identified red blood cells and haem B in a 115-million-year-old Aratasaurus museunacionali fossil.

Soft tissues and blood cells rarely survive fossilization because organic material typically decays within a few years. For decades, paleontologists have debated whether microscopic structures inside dinosaur bones preserve original biology.

Earlier reports, including flexible blood vessels in Tyrannosaurus rex fossils and cell-like shapes in 75-million-year-old dinosaur bones from Canada in 2015, drew skepticism from scientists who argued the features were bacterial biofilms or mineral deposits. Because prior studies lacked definitive chemical biomarkers, proving that fossilized structures are genuine blood remnants has remained a major hurdle in paleontology.

The fossil used in the latest research belongs to Aratasaurus museunacionali, a juvenile carnivorous theropod unearthed in a Brazilian gypsum mine in 2008 that survived the 2018 National Museum of Brazil fire before being named a new species in 2020.

1. A research team led by geochemist Kliti Grice of Curtin University used hydrochloric acid to dissolve surrounding rock from the Aratasaurus museunacionali leg bone, exposing vascular canals that once carried blood vessels. Electron microscopy revealed numerous ellipsoidal structures averaging 23 micrometers long and 20 micrometers wide that match the spatial distribution of red blood cells.

2. Liquid chromatography-mass spectrometry and spectroscopic analyses detected intact heme B, the iron-containing molecule in hemoglobin that transports oxygen, alongside its chemical breakdown product 3,8-diethyl heme B directly within the cell-like structures. Chemical tests also identified the amino acid building blocks of collagen, a key structural protein in connective tissue.

3. To rule out external contamination from the burial environment, researchers ran identical chemical analyses on the surrounding rock matrix. The sediment contained no heme B and instead showed chemical signatures of chlorophyll and bacteriochlorophyll, indicating that the heme-derived molecules originated inside the dinosaur bone.

4. Researchers identified a complex manganese oxide mineral called hausmannite inside and around the cellular structures. The team concluded that rapid burial in a low-oxygen, microbially active environment allowed hausmannite mineralization to encapsulate and stabilize the delicate red blood cells and heme molecules before they could degrade.

1. Paleontologist Mary Schweitzer of North Carolina State University questioned the identification of the structures as red blood cells based on their morphology. Because birds and reptiles possess nucleated red blood cells, dinosaur cells should display a dome-like nuclear bulge on both sides, whereas the structures found in the Aratasaurus bone are concave.

2. The fossilized structures are rounder, wider, and significantly larger than the red blood cells of living archosaurs, which include modern birds and crocodilians. Some independent researchers noted that the microscopic shapes could represent secondary mineral formations, such as manganese oxide deposits, rather than original cell membranes.

3. Although chemical comparisons with the surrounding rock reduced the likelihood of external sediment contamination, the study authors explicitly acknowledged in their paper that heme-producing microbes living within the bone during fossilization cannot be completely ruled out as an alternative source of the heme signatures.

4. The findings rely on a single bone fragment from one immature dinosaur specimen. Lead author Kliti Grice cautioned that the mineralization of small, stable ring molecules like heme does not mean intact hemoglobin proteins or large fragile molecules such as DNA can survive across 115 million years.