Warm-blooded vs. Cold-blooded
It was once believed that all dinosaurs were cold-blooded, like present-day
reptiles. But much of their structure and behavior was more like that of
warm-blooded animals than modern reptiles.
Science began thinking of dinosaurs as cold-blooded because of the early
identification of dinosaurs with lizards (witness their name "terrible
lizards), which are cold-blooded. It is now understood that dinosaurs and
lizards are not closely related. In fact, the descendants of dinosaurs
are most likely birds, which are warm-blooded.
Because of the identification of dinosaurs as big lizards, they were depicted
in lizard terms: they were thought to have the sprawling lizard stance
and to be slow moving, as lizards weighing tons would be. Evidence, however,
shows that many dinosaurs -- and not just the small ones -- were extremely
active, moving at great speed. This level of activity argues a metabolism
too high for a cold-blooded animal.
Cold-blooded animals do not function well in cold-climates, but dinosaur
fossils have been found near the polar regions, in what were cold climates
even during the Mesozoic. What were "cold-blooded" animals doing
in such cold climates? There is no mystery if one assumes that these dinosaurs
were warm-blooded. On the other hand, it has been argued that large warm-blooded
dinosaurs would have suffered from heat overloads in the tropical climates
where they are known to have lived. But the largest creatures in tropical
climates today are not cold-blooded reptiles, but warm-blooded mammals,
like the elephant.
Many dinosaurs had dense bone structure that is easily explained -- if
these dinosaurs had the high metabolic rate of warm-blooded animals. Theropods
walked upright, on two feet, like warm-blooded birds, and their bone structure
was more like that of modern birds than reptiles.
There are, in fact, many puzzling characteristics of dinosaurs that are
not mysterious -- if you assume they were warm-blooded.