It has been noticed that when animals get very large, their methods of temperature control change slightly from that of smaller animals. For example, most warm-blooded creatures have hair (mammals) or feathers (birds) insulating them to keep them warm. Elephants, on the other hand, do not have much hair at all as their large size means they retain heat far more than smaller animals.
(As animals get larger they have less surface area - skin area- relative to their volume, so they loose less heat to their surroundings. Think of a cup of tea and a bath of water at the same temperature. The tea will cool relatively quickly as it has a large surface area:volume ratio but the bath with its smaller surface area:volume ratio will keep warm for longer).
So, the suggestion is that because of the large body size of most dinosaurs, they were able to keep warm just by being large. This is called gigantothermy or mass homeothermy.
Computer models have shown that this would regulate temperatures well in the large dinosaurs but not in juveniles or the much smaller forms and not if winters were very cold. However, there were no cold winters, as the climate when dinosaurs were alive was 15 degrees Celsius warmer than today.
The models also showed that colour was important.Pink elephants would reach a cooler temperature than grey ones and black elephants would reach very high temperatures and be in danger of overheating..
Mass homeothermy can provide a mechanism for temperature regulation (within one degree, if the outside temperature only varies from 22-32 degrees C daily) without the considerable requirements of a high metabolic rate. However, juvenile and the few small dinosaurs would have to have had a different thermoregulatory (gl) mechanism.
Because mass homeotherms cannot cope if the temperature is too low, it has been suggested that dinosaurs became extinct because of climatic change making their biological processes break down.
For more information on the demise of the dinosaurs and what else was effected click here