Dinosaur Life Spans – Reasoning By Metabolism
As mentioned on the previous page, bigger animals tend to have longer life spans than smaller animals. By this token, you might expect the average Seismosaurus to live for hundreds of years--especially considering that a full-grown individual would be virtually immune to predation, and could only be killed by disease, hunger, accidents or old age. Estimates like this make good sense if, in fact, sauropods had cold-blooded metabolisms, like today’s giant tortoises.
The metabolism of dinosaurs is still a matter of intense dispute, but lately, some paleontologists have made a convincing argument that the largest herbivores could achieve "homeothermy"--that is, they warmed up slowly and cooled down equally slowly, maintaining a near-constant internal temperature. Since homeothermy is consistent with a cold-blooded metabolism--and since a fully warm-blooded (in the modern sense) Apatosaurus would have cooked itself from the inside like a giant potato--a life span of 300 years seems within the realm of possibility.
What about smaller dinosaurs? Here the arguments are murkier, and complicated by the fact that even small, warm-blooded animals (like parrots) can have long life spans. Most experts believe that the life spans of smaller herbivorours and carnivorous dinosaurs were directly proportional to their sizes--for example, the chicken-sized Compsognathus might live for 5 or 10 years, while a much bigger Allosaurus might live for 50 or 60 years. However, if it can be conclusively proved that dinosaurs were warm-blooded, cold-blooded, or something in between, these estimates will change very quickly!
Next Page: Reasoning by Bone Growth