In the 1800s, many naturalists saw the large dinosaur fossils as similar to modern lizards. Since these early paleontologists did not know what these animals looked like, they made the dinosaurs resemble animals that they did know such as lizards or mammals. The fossils that they found had neither hair nor feathers, therefore the naturalists believed that dinosaurs were sluggish. Moreover, the large size of many fossils also led naturalists to think of dinosaurs as cumbersome and slow. Therefore, people for a long time held the image of a large reptile for a dinosaur. Dinosaurs were the giants of the earth who had to make way for the more nimble humans.
However, since the 1960s, paleontologists have changed their ideas about dinosaurs. As more fossils with feathers were found, more scientists pondered the dinosaur-bird connection. Some of the therapod dinosaurs seemed to have evolved into birds. Since birds are warm-blooded, these particular dinosaurs must be warm-blooded too.
Later, fossils of dinosaurs were discovered at the Poles. This cast further doubt on the cold-blooded theory, as well. Modern reptiles cannot tolerate long periods of continual darkness. Meanwhile, other fossil finds of groups and tracks of dinosaurs indicated that some dinosaurs lived in herds. This meant that they lived in complex social structures, like warm-blooded mammals. No cold-blooded animals today live in herds.
One problem in determining whether dinosaurs were warm-blooded is their large size. An Apatosaurus would be too large to be warm-blooded, and would probably die from heat exhaustion. Also such a large dinosaur would be a gigantotherm, achieving a constant temperature by virtue of its large size.
Also there is no conclusive physical evidence for either warm or cold-bloodedness amongst dinosaurs. These animals share some characteristics of both reptiles and birds. In addition, some mammals have traits of reptiles. For example, the echidna of Australia lays eggs, and stands like a reptile, while the tree sloth of South America requires basking to raise its temperature. Therefore whether dinosaurs were warm or cold-blooded still remains unknown.
What some paleontologists conclude is that dinosaurs could have evolved from cold-blooded species into warm-blooded species. Or dinosaurs could have been a transitional species between warm and cold-bloodedness in animals. The fossil record is incomplete in offering definite proof of any one theory. However for one group of therapods, which evolved into birds, these dinosaurs were probably warm-blooded.
Animals that are ectothermic obtain most of their body heat from their environment. These animals cannot derive internal heat from their own metabolisms. To maintain a constant body temperature, ectotherms take full advantage of the heat outside of themselves. Since they are less dependent on their own metabolism to make heat, ectotherms only need a small amount of food to survive. Therefore, they can exploit environments where food is scarce such as deserts. Examples of ectotherms are the box turtle, marine iguana, naked mole rat, and tuatara.
Animals that are endothermic obtain their body heat from their own internal processes. Because they need to fuel this cellular process, endotherms have to eat constantly. For example, since they have high metabolisms, shrews and hummingbirds require frequent feedings. To conserve their body heat, endotherms are insulated by fat, fur, or feathers. Most mammals and birds are endothermic. Robins, blue jays, squirrels are but a few of examples of endotherms.
Animals such as leatherback sea turtles and saltwater crocodiles are so huge that they exchange little heat with their surroundings. These animals are known as gigantotherms. Because of their large size, gigantotherms, such as the great white shark, have a low heat exchange of surface area to volume ratio. This rate of exchange is small compared to the animal’s total size.