Biologists follow ‘fossilizable’ clues to pinpoint when mammal, bird and dinosaur ancestors became athletes

A new study is the first to draw a link between RBC size and microscopic traces of blood vessels and bone cells inside bones. They found that extinct mammal and bird relatives had smaller RBCs and were likely better athletes than earlier terrestrial vertebrates. The timing of RBC-size reduction coincided with Earth’s greatest mass extinction 252 mya.
Paleontology News — ScienceDaily