The bleached skeleton of a hundred-million-year-old Deinonychus can tell us a lot about what this dinosaur 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, estimating the lifespan 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 (preferably) direct analysis of the pertinent fossilized dinosaur bones.

Before anything else, of course, it helps to determine the cause of death of any given dinosaur. Given the locations of certain fossils, paleontologists can often 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 is a good indication 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, or that the dinosaur had recovered from a previously inflicted injury). 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 still 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 lifespans is that modern-day reptiles are some of the longest-lived animals on the earth: giant tortoises can live for 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 the direct descendants of dinosaurs, also have long lifespans. Swans and turkey buzzards can live for over 100 years, and small parrots often outlive their human owners. With the exception of humans, who can live for over 100 years, mammals post relatively undistinguished numbers, about 70 years for an elephant and 40 years for a chimpanzee, and the longest-lived fish and amphibians top out at 50 or 60 years.

One shouldn't rush to conclude that just 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 all dinosaurs were equally cold-blooded. Also, with some important exceptions (such as parrots), smaller animals tend to have shorter lifespans, so the average 25-pound Velociraptor might have been lucky to live beyond a decade or so. Conversely, larger creatures tend to have longer lifespans, 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.

Dinosaur Life Spans: Reasoning by Metabolism

The metabolism of dinosaurs is still a matter of ongoing dispute, but lately, some paleontologists have advanced a convincing argument that the largest herbivores, including sauropods, titanosaurs, and hadrosaurs, achieved "homeothermy," that is, they warmed up slowly in the sun and cooled down equally slowly at night, 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 out like a giant potato, a lifespan of 300 years seems within the realm of possibility for these dinosaurs.

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 herbivorous and carnivorous dinosaurs were directly proportional to their size, for example, the chicken-sized Compsognathus might have lived for five or 10 years, while a much bigger Allosaurus might have topped out at 50 or 60 years. However, if it can be conclusively proved that any given dinosaur was warm-blooded, cold-blooded, or something in between, these estimates would be subject to change.

Dinosaur Life Spans: Reasoning by Bone Growth

You might think that an analysis of actual dinosaur bones would help clear up the issue of how fast dinosaurs grew and how long they lived, but frustratingly, this isn't the case. As the biologist, R.E.H. Reid writes in The Complete Dinosaur, "[bone] growth was often continuous, as in mammals and birds, but sometimes periodic, as in reptiles, with some dinosaurs following both styles in different parts of their skeletons." Also, to establish rates of bone growth, paleontologists need access to multiple specimens of the same dinosaur, at different growth stages, which is often an impossibility given the vagaries of the fossil record.

What it all boils down to is this: some dinosaurs, such as the duck-billed Hypacrosaurus, grew at phenomenal rates, reaching adult sizes of a few tons in a mere dozen or so years (presumably, this accelerated rate of growth reduced the juveniles' window of vulnerability to predators). The trouble is, everything we know about cold-blooded metabolism is inconsistent with this pace of growth, which may well mean that Hypacrosaurus in particular (and large, herbivorous dinosaurs in general) had a type of warm-blooded metabolism, and thus maximum life spans well below the 300 years ventured above.

By the same token, other dinosaurs seem to have grown more like crocodiles and less like mammals, at a slow and steady pace, without the accelerated curve seen during infancy and adolescence. Sarcosuchus, the 15-ton crocodile better known as the "SuperCroc," probably took about 35 or 40 years to reach adult size, and then continued growing slowly for as long as it lived. If sauropods followed this pattern, that would point to a cold-blooded metabolism, and their estimated life spans would once again edge up toward the multiple-century mark.

So what can we conclude? Clearly, until we establish more details about the metabolism and growth rates of various species, any serious estimates of dinosaur lifespans have to be taken with a gigantic grain of prehistoric salt!