Skeletons

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Skeleton of a dinosaur

From frogs, there evolved reptiles like crocodiles and dinosaurs. Some of these reptiles lived mostly or entirely on land, so they needed stronger bones that could hold them up when they weren't in the water. And most reptiles ate meat, so they needed teeth like fish. Some reptiles evolved the ability to fly. They needed light, hollow bones that wouldn't make them too heavy to fly.

From the smaller kinds of dinosaurs, birds evolved. Birds had hollow bones like the flying dinosaurs. Other reptiles evolved into mammals. Mammals didn't need hollow bones, but they

Different kinds of mammals developed specialized bones for different things. People evolved narrow pelvises so we could walk upright. That let us run faster.