What was a Dinosaur's Lifespan?

After recently publishing some questions and answers about dinosaurs, I've been getting some new challenges for dinosaur information, including a question I had never been asked - or thought about - before: What was the lifespan of a dinosaur? Before I go any further, I need to remind you that I am no paleontologist (scientist who studies fossils and other remains of prehistoric animals), just a dinosaur fan who never outgrew his love for the ancient beasts.

This is a particularly tricky dinosaur question. Paleontologists used to associate the dinosaur's size with growth rates for living animals to determine the lifespan. That posed a problem for the lifespan of a really enormous dinosaur, because we have no way of determining how long it would take to grow that large.

Paleontologists have recently discovered that fossils of dinosaur bones show growth lines, like the growth rings of trees, and they are using this new evidence to determine the lifespan of dinosaurs.

Paleontologists have found two different kinds of growth lines in dinosaur fossils. One was laid down in the regular growth of the bone, while the other, thinner kind represents times of no growth. Similar lines are found in the bones of living animals, indicating periods of rapid bone development its disruptions.

Captive vertebrates, maintained with a steady diet and temperature throughout the year nonetheless exhibit growth lines as those of similar animals in the wild. There is a regular pattern of the development of bones in these animals. Since paleontologists assume a similar yearly cycle in the bone growth of dinosaurs throughout their lifespan, they can use bone growth shown in the fossils to determine an individual dinosaur's lifespan.

We still have some problems. In the bones of many modern animals, bone tissue is absorbed, perhaps recycled, as new bone forms. So, even with a living animal, there would be a fair amount of guesswork to determine the animal's age, not to mention lifespan, considering that growth lines from its earliest years may no longer be visible.