For more than a hundred years after the first dinosaur fossils were dug up, paleontologists just assumed that dinosaurs were cold-blooded animals, like modern lizards and snakes are. Dinosaurs were big, and they were slow, and they were stupid——and none of these characteristics are consistent with small, active, intelligent, warm-blooded animals like birds and mammals.

But in the late 1960s, two young Yale scientists, Robert Bakker and John Ostrum, came out with the hypothesis that dinosaurs were in fact faster, smarter and more energetic organisms than science previously thought. This radical idea caused an immediate stir, and the controversy about dinosaur physiology is still continuing today.

First, here are some of the arguments put forth for warm-bloodness in dinosaurs. Two: the dinosaurs evolved alongside mammals and competed with them for 170 million years, so they must have been competitive with other warm-blooded species. Three: some dinosaurs had very large bodies and very long necks, so they would need a four-chambered heart and a lot of energy to move their blood those distances. And four: the structure of dinosaur bone is more similar to bird and mammal bone than it is to typical reptile bone. Those are some of the arguments that are used to support the idea that dinosaurs were warm-blooded.