Dinosaurs have left a wealth of information in their fossils, including how they defended themselves, what they ate, and sometimes even how they died. But when these beasts bit the dust, did they leave any clues about their age, indicating whether they were young, middle-aged or old?
In a word, yes, largely thanks to the “growth rings” of their fossilized bones. These rings, which are arranged similarly to tree rings, were discovered only in recent decades and have revealed that most non-avian dinosaurs It did not live that long, although it sometimes grew to enormous sizes.
For example, the redoubtable Sue at the Chicago Field Museum, one of the most complete Tyrannosaurus Rex specimens never discovered, died at 28 years old, indicate their growth rings. Meanwhile, duck-billed herbivorous dinosaurs appear to have lived only a decade or two, Thomas Holtza vertebrate paleontologist at the University of Maryland told Live Science.
These early ages initially surprised paleontologists.
“I think a lot of people might get the impression that at least some dinosaurs got really big because they lived for a 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 tend to live a long time. African savannah elephants (African Loxodonta), the largest living land animal on Earth, can live up to 70 yearsand bowhead whales (Balaena mysticetus) can live as long as 200 years.
But dinosaurs are another story. Holtz explained that a thin slice of animal bone has a series of parallel lines caused by the outward growth of the bone. A new line is produced each year, so counting these lines gives an accurate estimate of the age of the animal.
These lines are created by annual changes in the way animals grow. In spring and summer, warm weather and an abundance of food mean that animals have abundant nutrition, allowing them to grow more quickly. But in winter, when the weather cools and food becomes scarce, growth slows. These growth slowdowns appear as lines between the bone layers.
But there are some complications with this technique of measuring growth. One is caused by the medullary cavity, the chamber inside a bone that produces bone marrow. As a bone grows, so does the medullary cavity, erasing some of the first growth rings in that expansion. To fix this, researchers can overlay bones from smaller individuals of the same species over missing lines in larger individuals to help estimate the potential total number, Holtz said.
Another complication is the type of bone. Some bones are better than others at detecting growth rings. “Ribs, or bones like the fibula, that don’t carry a lot of stress or weight tend to have a cleaner growth history,” Brusatte said.
While dinosaurs did not live as long as some of today’s largest animals, there are some similarities. For example, larger animals tend to live longer than smaller ones, and the same is generally true for dinosaurs. Sauropods: a group of long-necked leaf-eaters that includes the largest dinosaurs that ever existed – probably had the longest lifespan of any dinosaur, with the oldest known sauropods living about 60 years, according to Holtz.
In contrast, the 110 pounds (50 kilograms) Stenonychosaurus inequalis It grew to its maximum size in only three to five years and probably did not live long afterward.
There are some ideas about why dinosaurs had such a short lifespan. Maybe your metabolisms – or the body’s chemical processes, including the conversion of calories into energy – played a role, Brusatte said. Some dinosaurs were at least partially warm-blooded and would have had incredibly fast metabolisms for reptiles, which could have led to rapid growth and premature deaths, she said.
Or maybe reproduction was a factor. Many dinosaurs produced very large clutches of eggs, which meant they produced a large number of offspring in a short period of time, Holtz said. Long-lived mammals, on the other hand, such as elephants and whales, reproduce more slowly, so natural selection would reward a longer life expectancy. However, this hypothesis is not without flaws. For example, the Galapagos tortoises (Chelonoidis niger) can produce large clutches of eggs but also live to triple-digit ages, so producing a large number of offspring at once does not always indicate a shorter lifespan.
Ultimately, we still don’t know why non-avian dinosaurs died so young.