Lizards are cold-blooded. Their temperature rises as the day warms and then falls at night. |
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Were Dinosaurs Warm-Blooded?
Because dinosaurs were reptiles it is easy to imagine that their bodies worked like modern reptiles. But in one key area they may have been distinctly different. At one time scientists used to assume that dinosaurs were cold-blooded, which means that their body temperature depended on their surroundings to be regulated. But in the early 1970s an American biologist, Robert Bakker argued that they might have been warm-blooded, like mammals and birds. His theory radically altered ideas about dinosaur biology.
Some experts also believe that signs of warm-bloodedness can be seen in dinosaur skeletons. Dinosaur bones show signs of sustained rapid growth. This feature is common in warm-blooded animals, but never rarer in cold-blooded ones, except in times when there is a particularly good supply of food. But many paleontologists today find this evidence doubtful, and recent research into dinosaur breathing has prompted different conclusions. Paleontologists have looked at dinosaur noses with x-rays, searching for turbinal bones inside the nasal cavity. In birds and mammals these bones form a complicated collection of paper-thin scrolls that allow warm and moisture to be collected and recycled from outgoing air. But if dinosaurs were cold-blooded, their breath would have also been cold, so there would have been no warmth to recycle and probably not turbinal bones. The results so far show that dinosaurs did not have them.
If dinosaurs were warm blooded, their circulatory systems with have been modified to produce a higher rate of oxygen flow. Dinosaurs would have needed larger hearts than their cold-blooded relatives, and the blood would have almost certainly flowed in a figure 8 circuit. This double circulation system allows oxygen rich blood to become a high pressure and high speed. Unfortunately, soft organs such as dinosaur hearts, hardly ever fossilize, but in 2000 the remains of what looked like a heart found in a fossil of Thescelosaurus, a small herbivore hypsilophodont. Using medical scanning techniques, researchers concluded that it did have a double circulation system, meaning that it could have been warm-blooded.