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Researchers studying a dinosaur bone dating back 115 million years say they have uncovered the strongest fossil evidence yet of red blood cells. The discovery could offer rare biological clues about dinosaurs, although some experts remain unconvinced that the microscopic structures are truly fossilized blood cells.

Soft tissues, cells and other biological materials rarely survive fossilization, making claims of such discoveries unusually contentious. Since the late 1990s, several studies have reported structures resembling red blood cells in dinosaur and reptile fossils, but scientists have fiercely debated whether those structures are authentic.

The new study, published Monday in the journal Proceedings of the National Academy of Sciences, combined microscopic imaging with chemical analyses. Researchers identified oval structures resembling red blood cells and detected heme, a molecule found in hemoglobin, within the structures themselves.

According to the study authors, this is the first time heme has been detected in situ inside suspected blood cell structures in a dinosaur fossil. Their case draws on several lines of evidence, including the structures’ location, appearance, mineral composition and associated chemical signals.

Lead author Kliti Grice is an organic geochemist and John Curtin Distinguished Professor at Curtin University in Perth, Australia. She said the findings and the research techniques could help scientists search other fossils for biological information that may have escaped detection.

“The exciting implication is not just what we have found in this dinosaur. It is that we now have approaches that can be applied to other fossils to ask how much original biological information may still be hidden within them,” Grice said in an email.

The fossil is a partial hind limb from Aratasaurus museunacionali, a theropod dinosaur that lived during the Early Cretaceous. Paleontologists excavated the leg bone in 2008 at Mina Pedra Branca in northeastern Brazil, within the Romualdo Formation.

The rocky formation is known for exceptionally preserved fossils. Scientists believe stagnant water at the site restricted oxygen and slowed the decomposition of remains buried there, potentially creating conditions that allowed delicate biological evidence to persist.

Inside the bone’s vascular canals, the researchers found numerous oval microscopic structures that looked like red blood cells. Vascular canals are microscopic tubes containing blood vessels and nerve fibers, placing the structures in a location that the study authors consider significant.

The team also found traces of manganese oxides in the bone. The researchers proposed that these minerals, together with low oxygen burial conditions, may have stabilized the structures and helped preserve the chemical evidence associated with them.

Grice cautioned that a single specimen cannot establish conclusions about the physiology of Aratasaurus. Her team plans to examine fossils from other species, environments and geological periods to determine whether this type of molecular preservation is extraordinarily rare or has simply been overlooked.

If similar evidence appears elsewhere, researchers could investigate how dinosaurs transported oxygen, how their tissues functioned and how their biology compared with animals living today. Museum collections around the world may contain fossils with cellular or molecular information that has never been examined using these approaches.

The finding does not mean scientists are closer to recovering dinosaur DNA, according to Timothy Cleland, a physical scientist at the Smithsonian Institution’s Museum Conservation Institute. Heme and DNA are different molecules that fossilize in bone differently, so evidence that one survived does not demonstrate preservation of the other.

Cleland, who was not involved in the study, said future research could clarify how red blood cells change after death. That work might explain why the structures differ in size and shape from red blood cells in living birds and crocodilians, comparisons the authors also treated cautiously.

Mary Schweitzer, professor emeritus in the department of biological sciences at North Carolina State University, said the findings do not satisfy established standards for identifying fossilized blood cells. She questioned whether the structures adequately match cells in living dinosaur relatives and challenged the significance assigned to hausmannite, the main manganese oxide detected.

Grice responded that the team does not view hausmannite as an original component of the cells. Instead, the researchers believe it formed during fossilization and may have helped preserve both the structures and their associated molecular evidence.

Despite her reservations, Schweitzer called the use of multiple analytical techniques an exciting advance and said the data left her hopeful that dinosaur molecules remain in the bone. “The more studies we can do on very old fossils, the more we can learn about the interaction between the burial environments and the degrading organism.”