“After carefully examining thin slices of the fossils, paleontologists located the mineralized red blood cell molecular components, along with a complex manganese oxide called hausmannite. This material helped maintain the cellular structures of heme, a ring-shaped molecule that contains iron and binds many proteins, especially hemoglobin. The hausmannite also helped preserve additional biomolecules found in the blood, such as the amino acids in collagen.”

https://www.popsci.com/science/first-dinosaur-blood-fossil-discovery/