Asked by: Martin Green, Fareham

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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.


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luis villazon
Luis VillazonQ&A expert

Luis trained as a zoologist, but now works as a science and technology educator. In his spare time he builds 3D-printed robots, in the hope that he will be spared when the revolution inevitably comes.

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