Traces of 115-million-year-old red blood cells found in dinosaur bone

Researchers have discovered microscopic structures resembling red blood cells inside the leg bone of a dinosaur that lived in Brazil approximately 115 million years ago, offering some of the strongest evidence yet that traces of blood-related biological material can survive in fossils for millions of years.

The structures, found inside tiny channels in the fossilized bone, were oval-shaped and similar in appearance to red blood cells. However, their shape alone was not sufficient to establish their biological origin, as comparable structures can also result from geological processes or microbial activity after an animal dies.

To investigate further, the researchers detected chemical signatures of heme within the structures. Heme is the iron-containing component of haemoglobin, the protein responsible for enabling red blood cells to transport oxygen throughout the body.

According to a study recently published in the Proceedings of the National Academy of Sciences (PNAS), the combination of microscopic appearance, location and chemical composition provides compelling evidence that the structures represent fossilized red blood cell remains.

Kliti Grice, professor of geochemistry at the Western Australian Organic and Isotope Geochemistry Centre at Curtin University and the study’s lead author, said the discovery of original biological molecules in vertebrate fossils has long been controversial. Most biological materials break down relatively quickly and can become mixed with substances from the surrounding environment or microorganisms.

A closer look inside the dinosaur bone

The bone belonged to Aratasaurus, a carnivorous dinosaur from the theropod group whose skeleton was discovered in northeastern Brazil in 2008.

Previous research suggests that the dinosaur was still young when it died, possibly around four years old. It measured approximately 3.1 metres in length and weighed about 34 kilograms.

Inside the channels that once carried blood vessels through its limb bones, researchers identified numerous oval structures measuring an average of approximately 23 micrometres in length and 20 micrometres in width.

Their location was particularly significant: the structures were found within the spaces once occupied by blood vessels, strengthening the possibility that they originated from blood cells rather than from surrounding sediments or later microbial activity.

Grice said the team employed several independent microscopic and chemical techniques to examine the structures and establish whether they contained traces of ancient biological material.

The analyses detected iron, carbon and nitrogen, elements associated with heme. More detailed chemical testing subsequently revealed the presence of heme B, along with another compound believed to have formed as heme underwent chemical changes after burial.

The researchers also found that the chemical signature of heme inside the bone differed from the substances detected in the microbial layer surrounding the fossil.

That surrounding material contained compounds derived from chlorophyll, making it less likely that the heme-related signals originated from the surrounding sediments.

The team also examined alternative explanations for the microscopic structures.

They found no evidence that the structures were fungi or microbial colonies. Minerals such as pyrite and sediment particles, which can sometimes imitate the appearance of biological cells, were also absent.

According to the study, the structures’ confinement to vascular channels, combined with the detection of heme-derived compounds, makes their interpretation as fossilized red blood cell remains the explanation most consistent with the available evidence.

How could traces of blood survive for millions of years?

The researchers believe that the conditions surrounding the dinosaur’s burial played an important role in preserving the biological material.

The bone was surrounded by a microbe-rich layer in an environment with limited oxygen, conditions that may have slowed the breakdown of organic substances and helped preserve some of their chemical components.

One of the study’s most intriguing findings, Grice said, was the presence of hausmannite, a manganese-rich mineral, within the structures resembling red blood cells.

The researchers suggest that this mineral may have helped stabilise the cellular remains and limit damage caused by oxidation.

In effect, it may have acted as a protective mineral coating, helping preserve traces of the original biological material over millions of years.

However, the findings do not mean that intact dinosaur blood or complete red blood cells have survived. The large protein component of haemoglobin was not preserved. Instead, the researchers identified heme-derived molecules and altered organic remains that had undergone chemical changes during fossilisation.

The fossilized structures were also somewhat rounder and slightly larger than red blood cells in modern birds and crocodilians, the closest living relatives of dinosaurs.

These differences may reflect changes that occurred after the dinosaur died or during the fossilisation process.

This uncertainty represents one of the study’s main limitations, as researchers cannot establish precisely what the dinosaur’s red blood cells looked like when it was alive.

The findings also come from a single individual belonging to one dinosaur species, meaning they cannot be generalised to other dinosaurs or establish whether similar preservation processes occurred in other fossils.

The researchers further acknowledge that they cannot completely rule out a contribution from heme-producing microorganisms, although the combined chemical and spatial evidence makes this explanation less likely.

The discovery therefore provides new evidence that traces of biological molecules can survive in fossils for extraordinary periods. It also highlights the importance of combining microscopic and chemical techniques to distinguish ancient biological remains from substances introduced through geological processes or microbial activity after burial.