There's still no definite answer whether dinosaurs were warm-blooded or cold-blooded. For many years, scientists thought all dinosaurs were cold-blooded like all reptiles are today. Then, in the 1970s, scientists begin to look at some evidence indicating that dinosaurs may be warm-blooded.

This evidence included several things: the way dinosaurs stood straight-legged like mammals, their big rib cages that could have held mammal-like hearts and lungs, and bones that contained channels for quick blood circulation as found in warm-blooded animals' bones.

More recent studies, however, suggest that dinosaurs were neither warm-blooded nor cold-blooded, but something in between. Big dinosaurs may not have had much control over their body temperature but probably didn't need to - their huge size would have been very effective in insulating them from temperature fluctuations. Whatever the truth, it is clear that dinosaurs were active, dynamic creatures, and not just overgrown lizards.