In the early 2000s, the fossilized skeletons of two small tyrannosaurs were collected from the famous Hell Creek Formation of Carter County, Montana. Nicknamed Jane and Petey, the individuals would have been slightly taller than a draft horse and twice as long. Settling a debate about whether Jane and Petey represent a separate genus of pygmy tyrannosaurs (Nanotyrannus) or rather just juveniles of Tyrannosaurus rex, an analysis of sliced bones from...