For more than 300 years paleontologists have operated under the assumption that the information contained in fossilized bones lies only in the size and shape of the bones themselves. It was thought that when an animal dies under conditions suitable for fossilization, inert minerals from the surrounding environment eventually replace all the organic molecules—such as those that make up cells, tissues, pigments and proteins—leaving behind what is essentially a “cast” of the once living bones, now composed entirely of mineral.
My first indication that this fundamental tenet of paleontology might not always apply came when I was a relatively new graduate student at Montana State University, studying the microstructure of a Tyrannosaurus rex bone. As I peered through the microscope, what I saw—small, round, red and apparently nucleated structures, restricted to the blood vessel channels coursing through the bone—had not been previously noted, to my knowledge. They looked similar to the nucleated red blood cells of nonmammalian vertebrates. But seeing dinosaur blood cells was impossible, according to the conventional wisdom that shaped my discipline. After I sought opinions on the identity of the red spheres from faculty members and other graduate students, word of the puzzle reached Jack Horner, curator of paleontology at the museum and one of the world's foremost dinosaur authorities. He took a look for himself. Brows furrowed, he gazed through the microscope for what seemed like hours without saying a word. Then, looking up at me with a frown, he asked, “What do you think they are?” When I replied that I did not know but that they were the right size, shape, location and color to be blood cells, he grunted. “So prove to me they aren't.” It was an irresistible challenge, and one that continues to help frame my research.