The life and times of a dinosaur may be written in its blood — if blood can be preserved in bone.

Tiny microstructures found in the limb bones of a fossilized 115-million-year-old specimen are the remnants of red blood cells, researchers report November 2 in the Proceedings of the National Academy of Sciences . The team says these are both the oldest red blood cell structures yet identified and the first to be chemically proven. But not everyone is as sanguine about that conclusion.

The ability to identify such structures could offer a powerful molecular window into the physiology of long-extinct animals, including their metabolism and even activity levels, says paleontologist Kliti Grice of Curtin University in Bentley, Australia.

The bone, from a fossil-rich site in northeastern Brazil called Mina Pedra Branca, belonged to the dinosaur Aratasaurus museunacionali .…