They discovered them in the remains of a dinosaur. Such evidence has never been found "before".

Researchers from Curtin University have isolated the oldest dinosaur red blood cells, dating back 115 million years. The findings were published in PNAS. The dinosaur remains were discovered in a gypsum mine in Brazil in 2008, specifically the bones of a theropod named Aratasaurus museunacionali. The study identified microscopic structures resembling fossilized red blood cells and traces of mineralized heme. The researchers emphasize that while they found no actual dinosaur blood, the preserved structures provide valuable insights into ancient biology. They caution against assuming that DNA has survived, as larger biomolecules degrade differently than smaller ones. The study highlights the potential for biological information to persist in fossils longer than previously thought.