The controversial discovery of 68-million-year-old soft tissue from the bones of a Tyrannosaurus rex finally has a physical explanation. According to new research, iron in the dinosaur’s body preserved the tissue before it could decay.

The research, headed by Mary Schweitzer, a molecular paleontologist at North Carolina State University, explains how proteins — and possibly even DNA — can survive millennia. Schweitzer and her colleagues first raised this question in 2005, when they found the seemingly impossible: soft tissue preserved inside the leg of an adolescent T. rex unearthed in Montana.