Tracking the growth of dinosaurs and how they changed as they grew is difficult since all the evidence there is consists of fossils. Using a combination of biomechanical analysis and bone histology, palaeontologists from Beijing, Bristol, and Bonn have shown how one of the best-known dinosaurs switched from four feet to two as it grew. Psittacosaurus, the ‘parrot dinosaur’ is known from more than 1000 specimens from the Cretaceous, 100 million years ago, of China and other parts of east Asia. As part of his PhD thesis at the University of Bristol, Qi Zhao, now on the staff of the Institute for Vertebrate Paleontology in Beijing, carried out the intricate study on the bones of babies, juveniles and adults.