Illustration of the _\_Aratasaurus museunacionali\__ dinosaur whose fossils contained red blood cells A nondescript 115-million-year-old fragment of leg bone has yielded spectacularly preserved red blood cells, which may be the earliest ever found in dinosaurs. Red blood cells have previously been found in dinosaur remains, including 75-million-year-old bone fragments collected in Canada in 2015 and the femur of a _Tyrannosaurus rex_ that died 68 million years ago, found in Montana in 2005. Now, Kliti Grice at Curtin University in Perth, Australia, and her colleagues say they have isolated the oldest-ever dinosaur red blood cells from the hind-limb of a theropod dinosaur, _Aratasaurus museunacionali_, found in Brazil in 2008.
The _Aratasaurus_ individual studied by the team wasn’t yet fully grown, with an estimated body length of just over 3 metres, and weighed about 34 kilograms. No one has ever seen dinosaur red blood cells in high resolution before.” The cells she found were abundant: in a single slide, just a fifth of a millimetre wide, there were 20 blood cells. All of the cells were within fossilised blood vessels, providing further evidence that they really were red blood cells.
Liquid chromatography–mass spectrometry analysis revealed the presence of haem B, a constituent of haemoglobin that binds to oxygen, adding molecular evidence of the type of cell. The team also had to rule out the possibility of any contamination, especially from the sediments in which the fossil was preserved, which were dominated by algae. Microscopic images of structures resembling red blood cells from the fossilised limb bone of _Aratasaurus_ _museunacionali_ The cells found outside the bone were dominated by chlorophyll-derived compounds, whereas those inside contained haem-derived molecules, which strongly suggests that the latter cells weren’t a result of contamination.
The researchers also compared the blood cells to those of the closest living relatives of the dinosaur fossil, birds and crocodiles. The _Aratasaurus_ red blood cells are significantly larger. This could indicate that the cells swelled during fossilisation, or that the species grew much bigger than the fossilised specimen.
Grice says the discovery may mean that other nondescript dinosaur fossils that have been locked away in museum collections may hold profound secrets.
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