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medullary bone. It’s a reproductive tissue that’s only found in birds. Birds are constrained by the fact that they have very thin bones, which are an adaptation for flight, and they make calcified eggshells,” she said. There is not a whole lot of calcium available from the skeletal bones because they are lightweight, but birds need calcium for eggshells. “So,” she said, “they developed a reproductive tissue that is laid down with the first spike of estrogen that triggers ovulation.”
    It was easy to spot, since it looked very different from other types of bone. Medullary bone is produced rapidly, has lots of blood vessels, and has a kind of spongy, porous look and feel to it. Since birds are dinosaurs, and T. rex is in the family of nondinosaurs from which birds claim descent, the presence of medullary bone made sense. Paleontologists had hoped to find medullary bone in dinosaur fossils, but they had not yet. If she was right in her snap judgment, this was not only scientifically important but a treat for all of us who love dinosaurs—a girl tyrannosaur.

THE SECOND EXCAVATION
    And that is how the second excavation of B. rex began. The first, the old-fashioned kind, was to dig into the rock to free the fossil bone. The second excavation, of a sort that will mark a sea change in paleontology as it becomes more common, was to dig into the fossil itself, not with dental pick and toothbrush, but with the tools of chemical and physical analysis. Most of our current knowledge of dinosaurs and other extinct animals consists of the fruits of first excavations. I am not undervaluing this knowledge. In fact, it is almost impossible to overstate its value.
    The work of traditional paleontology has produced a record of evolution on earth. The great skeletons that tower over museum exhibition halls are flashy, but they are mere points of data in the grand accumulation of knowledge. Fossils that show how jaws evolved or when a toe moved, or an opening in a skull appeared, are equally as important in mapping not just the existence of the past, but the process of evolution, and eventually the laws that govern its progress.
    But there are now new means of tracing the past and some paleontologists are using them, although they don’t seem to spread as fast as they might. As long ago as 1956 Philip Abelson reported amino acids in fossils more than a million years old. In the 1960s and 1970s other scientists pushed for the importance of molecular biology for scientists who study the past. Bruce Runnegar of UCLA summed up a new view at a 1985 conference when he said, “I like to take the catholic view that paleontology deals with the history of biosphere and that paleontologists should use all available sources of information to understand the evolution of life and its effect on the planet. Viewed in this way the current advances being made in the field of molecular biology are as important to present-day paleontology as studies of comparative anatomy were to Owen and Cuvier.”
    Change does not come easy, however. Scientific disciplines are more like barges than speedboats, slow to turn in a new direction. This is as true for scientists who study dinosaurs as for any others. And there are significant obstacles to moving in a new direction. For one thing, dinosaur fossils are so old that recovering biological materials from them has been a major challenge.
    Of course we still excavate bones, and we need to. But we also need to look deep into the bones, into their chemistry. A first step is to narrow and deepen our vision, looking at microscopic evidence like the internal structure of bone, and moving even deeper to seek fossil molecules. Mary is a pioneer in this research, and as an inveterate digger myself, I like to think of her work in a similar framework. She is digging, too, but for her the fossil bone is the equivalent of the siltstone of the Hell Creek Formation, and the fossils she is trying to extract are not femurs and