In 1991, Schweitzer was trying to study thin slices of bones from a 65-million-year-old T. rex. She was having a hard time getting the slices to stick to a glass slide, so she sought help from a molecular biologist at the university. The biologist, Gayle Callis, happened to take the slides to a veterinary conference, where she set up the ancient samples for others to look at. One of the vets went up to Callis and said, “Do you know you have red blood cells in that bone?” Sure enough, under a microscope, it appeared that the bone was filled with red disks. Later, Schweitzer recalls, “I looked at this and I looked at this and I thought, this can’t be. Red blood cells don’t preserve.”
Schweitzer showed the slide to Horner. “When she first found the red-blood-cell-looking structures, I said, Yep, that’s what they look like,” her mentor recalls. He thought it was possible they were red blood cells, but he gave her some advice: “Now see if you can find some evidence to show that that’s not what they are.”
