In 2000, bob harmon discovered the best-preserved tyrannosaurus rex skeleton ever found. the fossilized dinosaur was sticking out of the wall of a canyon in montana. many scientists were eager to study the fossils. dr. mary schweitzer got one of the leg bones to analyze. dr. schweitzer's assistant dissolved the bones in acid and discovered that some soft tissue had survived even after 68 million years. the discovery was a shock to the scientific community. no one had ever found such old tissues. dr. schweitzer found structures identical to modern red blood cells and blood vessels inside the bone. which organ system do blood cells and blood vessels belong to? a. circulatory system b. nervous system c. skeletal system d. respiratory system