Asked by: Martin Green, Fareham
It’s all to do with the shape of the pelvis. Dinosaurs are divided into two groups: the Saurischia, or ‘lizard-hipped’ dinosaurs (which included predators such as Tyrannosaurus Rex), have a pubis bone that points forward; the Ornithischian or ‘bird-hipped’ dinosaurs (which included plant-eaters including Hadrosaurs and Stegosaurs) have a backwards-pointing pubis. Any fossil with either of these pelvis types is classified as a dinosaur.
Ironically, birds are actually descended from the Saurischia. Their bird hips evolved independently, much later in time.