N 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.

Before laying eggs, female birds store calcium inside their bones that they will need to make eggshells. This layer of bone is called medullary bone. In the T. rex leg bone, Dr. Schweitzer found tissue just like the medullary bone of birds. The dinosaur was a female! If Dr. Schweitzer’s hypothesis is right, she is the first scientist to identify the sex of a T. rex. Which organ system includes the bones?

A. skeletal system
B. muscular system
C. reproductive system
D. integumentary system