The bleached skeleton of a hundred-million-year-old Deinonychus can tell us a lot about what it ate, how it ran, and even how it interacted with others of its kind--but not much about how long it lived before dropping dead of old age. The fact is, guessing the life span of the average sauropod or tyrannosaur involves drawing upon numerous strands of evidence, including analogies with modern reptiles, birds and mammals, theories about dinosaur growth and metabolism, and (sometimes) direct analysis of dinosaur bones.
Before anything else, of course, it helps to determine the cause of death of a particular dinosaur specimen. Based on the locations of some skeletons, paleontologists can figure out if the unlucky individuals were buried by avalanches, drowned in floods, or smothered by sandstorms; also, the presence of bite marks in solid bone may indicate that the dinosaur was killed by predators (though it’s also possible that the corpse was scavenged after the dinosaur had died of natural causes). If a specimen can be conclusively identified as a juvenile, then death by old age is ruled out, though not death by disease (and we know very little about the diseases that afflicted dinosaurs).
Dinosaur Life Spans - Reasoning By Analogy
Part of the reason researchers are so interested in dinosaur life spans is that modern-day reptiles are some of the longest-lived animals on the planet: giant tortoises can live over 150 years, and even crocodiles and alligators can survive well into their sixties and seventies. Even more tantalizingly, some species of birds--which are directly descended from dinosaurs--also have long life spans. Swans and turkey buzzards can live for over 100 years, and small parrots often outlive their human owners. Mammals (except humans) post relatively undistinguished numbers--about 70 years for an elephant, 40 years for a chimpanzee--and the longest-lived fish and amphibians top out at 50 or 60 years. (The exception among mammals is the bowhead whale, which may live for over two centuries!)
However, one shouldn't rush to conclude that because some of the relatives and descendants of dinosaurs regularly hit the century mark, dinosaurs must have had long life spans as well. Part of the reason a giant tortoise can live so long is that it has an extremely slow metabolism; it's a matter of debate whether dinosaurs were equally cold-blooded (see the next page). Also, with some important exceptions (such as parrots), smaller creatures tend to have shorter life spans, so the average Velociraptor might have been lucky to hit the tender age of 20. Conversely, larger creatures (witness that bowhead whale) tend to have longer life spans--but just because a Diplodocus was 10 times bigger than an elephant doesn’t necessarily mean it lived ten times (or even twice) as long.
Next Page: Reasoning by Metabolism