Sometimes what seems a loss is actually a gain. When Museum of the Rockies paleontologist Jack Horner hired a helicopter to transport his newly found Tyrannosaurus rex fossils from a remote corner of Montana in May 2002, the dinosaur’s two-ton femur proved too heavy. Reluctantly, Horner broke the rare bone in half, making fragments that he later shipped to his colleague Mary Schweitzer at North Carolina State University. As soon as Schweitzer unpacked the samples, she knew she had something surprising. “I pulled out the bone, and I looked at my technician and said, ‘We have a girl—and it’s pregnant,’ ” Schweitzer recalls.
Schweitzer had noticed that the femur had a special inner layer, known as medullary bone, which had previously been found only in ovulating female birds. Birds, and evidently some dinosaurs, grow medullary bone to store up calcium so egg production doesn’t weaken the load-bearing outer layer of their bones. Schweitzer hypothesizes that only members of the theropod dinosaur family, which have thin, hollow bones like birds, grew this special layer. Her discovery gives paleontologists a new way of distinguishing dinosaur gender and solidifies the link between dinosaurs and birds. “What we’ve found is that the similarity goes beyond morphology to reproductive physiology,” Schweitzer says.
The broken bone was a gift that kept on giving. Because fossils occur when ancient bone is replaced by minerals, “people had just assumed that if you decalcify [it], there wouldn’t be anything left,” Horner says. But when a technician in Schweitzer’s lab tried just that—68 million years after the T. rex’s death—she found flexible, soft tissue that looked like bone cells and blood vessels with red blood cells inside.
Schweitzer’s next goal is to understand the tissues’ chemical changes, which will also determine whether any DNA was preserved. “We don’t have any chemistry that explains how these soft tissues still exist,” she says. Schweitzer believes understanding preservation at a molecular level could help paleontologists find traces of ancient life on Earth, as well as on planets like Mars. “This is just the start of something that hopefully will go in a million directions and give us information that we’re not even looking for.”