Until recently, paleontologists had no way to measure the age of dinosaurs or to figure out how they grew. So, we assumed dinosaurs had a physiology similar to modern reptiles. But it turns out that the clues we needed were locked in the animals’ bones all along—in growth lines similar to the annual growth rings in trees. John R. Horner, Kevin Padian, and Armand de Ricqlès, who have studied dinosaur bones together for more than 20 years, break down how they helped to determine the growth rates of many dinosaur species.