Dinosaurs have left behind a plethora of evidence in their fossils, including their defense mechanisms, dietary preferences, and sometimes even their cause of death. However, did these creatures leave any indications of their age at the time of death? Were they young, middle-aged, or elderly?
The answer is yes. This is primarily due to the discovery of “growth rings” in their fossilized bones, which resemble tree rings. These rings were only discovered in the past few decades and have shown that despite their massive size, most non-avian dinosaurs did not live that long.
For example, Sue, one of the most complete T-Rex specimens ever discovered at the Chicago Field Museum, died at the age of 28, according to her growth rings. Duck-billed herbivorous dinosaurs, on the other hand, appear to have only lived for one or two decades, according to Thomas Holtz, a vertebrate paleontologist at the University of Maryland, as reported by Live Science. Initially, paleontologists were surprised by these young ages.
“I think a lot of people might have the impression that at least some dinosaurs got to be really big because they lived for a really long time,” Steve Brusatte, a vertebrate paleontologist at the University of Edinburgh in Scotland, told Live Science. “Certainly, scientists used to think that.”
Large modern animals have long lifespans. For instance, African bush elephants, the largest land animal on Earth, can live up to 70 years, whereas bowhead whales can live up to 200 years.
Dinosaurs, however, have a different story. As per Holtz’s explanation, a thin slice of animal bone has parallel lines caused by the outward growth of the bone, and each line represents a year of growth. These lines are caused by annual changes in the way animals grow. During spring and summer, when the weather is warm and food is plentiful, animals grow more quickly. But in winter, when the weather cools and food becomes scarce, growth slows down. These slowdowns in growth show up as the lines between bone layers.
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However, there are some complications with this technique of measuring growth. The medullary cavity, the chamber inside a bone that produces bone marrow, can erase some of the early growth rings in that expansion as a bone grows. To counter this, researchers can superimpose the bones from smaller individuals of the same species over those missing lines in larger individuals to help estimate the potential total number. Moreover, some bones are better for spotting growth rings than others. Bones like ribs or those that don’t bear a lot of weight tend to have a cleaner record of growth.
Although dinosaurs didn’t live as long as some of today’s large animals, many similarities still exist. For example, bigger animals tend to live longer than smaller ones, and the same generally holds true with dinosaurs. Sauropods, the largest dinosaurs that ever lived, were a group of long-necked leaf eaters that likely had the longest lives of all dinosaurs, with the oldest-known sauropods living to around 60 years old.
On the other hand, Stenonychosaurus inequalis, which weighed 110 pounds (50 kg), grew to its maximum size in just three to five years and didn’t likely live long afterward. As to why dinosaurs had such short life spans, there are some theories. One theory suggests that their metabolisms, or the body’s chemical processes, including converting calories into energy, played a role. Some dinosaurs were at least partially warm-blooded, and they would have had incredibly fast metabolisms for reptiles, which could have led to rapid growth and early deaths.
Another theory suggests that reproduction was a factor. Many dinosaurs produced very large clutches of eggs, which meant they produced a lot of offspring in a short period of time. Whereas long-lived mammals, such as elephants and whales, reproduce more slowly, natural selection would reward longer lifespans. However, this hypothesis is not without its flaws. For example, Galapagos tortoises can produce large clutches of eggs but also live well into triple-digit ages, so producing a large number of offspring in one go does not always indicate a shorter life.
Ultimately, the reason why non-avian dinosaurs died young is still unknown.