Traces of prehistoric blood could reveal unprecedented clues about dinosaur biology, but they’ve been notoriously difficult to confirm. Now, researchers studying a 115 million-year-old dinosaur bone say they’ve found the strongest evidence yet of red blood cells in a fossil.

Tissues , cells and other soft biological samples rarely ever survive the fossilization process and thus make for controversial discoveries. A few previous studies, dating back to the late 1990s, have reported structures resembling red blood cells in dinosaur and other reptile fossils, but researchers have fiercely debated their legitimacy.

The new study , published Monday in the journal Proceedings of the National Academy of Sciences, used a combination of new microscopic imaging and chemical analyses to find not only microscopic structures resembling red blood cells preserved within the dinosaur bone, but also chemical evidence of heme — a molecule in hemoglobin, the protein in red blood cells that carries oxygen. According to the study’s authors, it is the first time heme has been detected in situ, or within the structures themselves, in a dinosaur fossil.

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The well-preserved Early Cretaceous bone and new research approach bolster the argument that biological structures and molecules can survive in prehistoric fossils, said lead author Kliti Grice, organic geochemist and John Curtin Distinguished Professor at Curtin University in Perth, Australia. “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. However, not all experts are convinced that the microscopic structures are truly red blood cells.

If researchers find similar evidence of red blood cells in more fossil specimens, it “could potentially help us ask much more sophisticated questions about the physiology of extinct animals,” Grice said.

A portion of the fossilized remains of the right hind limb of the theropod dinosaur Aratasaurus, seen at Museu de Paleontologia Plácido Cidade Nuvens, Santana do Cariri, Ceará, Brazil. Curtin University The fossil is a partial hind limb of Aratasaurus museunacionali, a theropod dinosaur that lived during the Early Cretaceous. Paleontologists dug up the leg bone in 2008 at the geological site Mina Pedra Branca in northeastern Brazil within the Romualdo Formation, a rocky area known for its exceptionally preserved fossils. Scientists believe stagnant waters limited oxygen and slowed the decomposition of remains buried in this region.

Within the bone’s vascular canals — microscopic tubes that carry contain blood vessels and nerve fibers — the study authors found an abundance of oval-shaped microstructures that looked similar to red blood cells. The team then used a combination of imaging and chemical analysis to identify whether the structures contained signatures consistent with red blood cells.

Past research may have separately described red blood cell-like structures and possible remnants of blood chemistry in dinosaur fossils, but this study brings together multiple lines of evidence, including the structures’ location, appearance, mineral composition and associated chemical signals, Grice said.

The researchers suggest that low-oxygen burial conditions at the site may have limited the breakdown of organic material, helping the structures survive. Along with heme, the study authors found the bone contained traces of manganese oxides, minerals that may have further helped stabilize and preserve the cells.

Microscopic images of structures resembling red blood cells from the fossilized limb bone of the theropod dinosaur Aratasaurus, taken at a laboratory at Curtin University in Perth, Western Australia. Curtin University “Bones provide extraordinary information about anatomy, growth, locomotion and evolution. But cellular and molecular preservation potentially gives us access to a different layer of biological information: the chemistry of the living animal and its tissues,” Grice said.

However, she emphasized that it is too early to draw conclusions about the physiology of Aratasaurus based on one specimen.

“One of the first questions is whether this is an extraordinarily rare occurrence or whether similar preservation has simply been overlooked in other fossils,” she said. “There may therefore be fossils sitting in museum collections around the world that contain molecular and cellular information that has simply never been investigated.”

Grice and her team plan to expand their research to other fossils from different species, geological time periods and environments to better understand how widespread this type of well-preserved molecular evidence might be.

In the future, more molecular clues could help researchers investigate how dinosaurs transported oxygen through their bodies, how their tissues functioned and how their biology compared with that of animals alive today.

But does finding red blood cells from a dinosaur bring scientists any closer to recreating its DNA, as seen in books and movies like “Jurassic Park”? Not necessarily, said Timothy Cleland, a physical scientist at the Smithsonian Institution’s Museum Conservation Institute. Heme and DNA are different molecules, and they would fossilize in bone in different ways, he explained. Finding evidence of one does not mean the other has survived.

4th March 1938: A Dodo skeleton opposite a reconstructed model of the extinct bird in the National Museum of Wales, Cardiff. Becker/Fox Photos/Getty Images “This study opens new research avenues to explore heme b preservation within bone, especially in exceptional sites like this one,” Cleland said in an email, referring to the most common form of heme. “Additionally, this study begs the question, does this kind of preservation extend beyond this single dinosaur to other species within the Romualdo Formation as well as to other locations with exceptional preservation?”

The structures also differed in size and shape from red blood cells in living birds and crocodilians, the authors noted in the study. But they cautioned that modern species may not be the best comparison for non-avian dinosaurs — and that the cells they found in Aratasaurus museunacionali may not retain their original shape from when the animal was alive.

Cleland, who was not involved in the new study, said future research could help scientists better understand how red blood cells might change after an animal dies, which could possibly explain Aratasaurus’ odd cell shapes. It’s also possible that the structures might not be remnants of red blood cells at all.

The findings do not meet established standards for identifying fossilized blood cells, said Mary Schweitzer, professor emeritus in the department of biological sciences at North Carolina State University. She was not involved in the study.

She questioned whether the structures’ shape and size sufficiently match red blood cells in living relatives of dinosaurs as well as the significance of hausmannite, the type of manganese oxide the study authors predominantly found. Schweitzer noted that this substance is not detected in living red blood cells.

But Grice said the team does not consider hausmannite an original component of the cells. Rather, they believe it formed during fossilization and may have helped preserve the structures and associated molecular evidence. J. Wiemann Still, Schweitzer said in an email that the authors’ use of multiple analytical techniques is an exciting advancement for the field. Based on the data, she said she is hopeful that some molecules from the dinosaur itself remain preserved in the bone.

“The usability of fossil molecules preserved in deep time is only just beginning to be explored, as conventional wisdom states preservation like this is not possible,” Schweitzer said. “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.”

Further research could eventually help scientists answer much bigger questions, including what prehistoric molecules can reveal about disease, biological processes and how molecules have evolved over time, she added.

“If we can get beyond ‘It can’t’ to ‘What if it can?’, there is no telling what may lie ahead!”Schweitzer said. Taylor Nicioli is a freelance journalist based in New York.

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