How Fast Did T.
rex Grow?
A
new discovery led to an answer.
When we think of Tyrannosaurus rex, we think BIG. We imagine a fierce giant stomping through a thick forest. But how big was a baby Tyrannosaurus?
The smallest T. rex skeleton found so far is the size of a golden retriever. Scientists think this dinosaur was two years old. A hatchling T. rex would have been about the size of a cat. Yet it grew into a 42-foot-long adult that weighed about 6 tons and stood over 13 feet tall. That is a lot of growing.
Dr. Gregory Erickson and his fellow scientists wanted
to know how fast T. rex grew. Did
it grow quickly for a short time, like today’s birds and mammals? Or did
it grow slowly for a long time, like today’s reptiles? To find out, the scientists
needed to know how big and how old the dinosaurs were when they died.
Scientists can figure out the weight of an animal from the size of the large bone in the animal’s thigh. Figuring out the age of the dinosaur is harder. In many animals, if you cut through a bone crosswise and look at the cut end under a microscope, you see rings, like the rings in the end of a tree trunk. Because the bones of many modern animals (like crocodiles) form yearly growth rings, scientists think that T. rex bones formed a new ring every year.
But scientists could not use this method in T. rex and some other dinosaurs because the bones became hollow as they grew. The early growth rings were destroyed and cannot be counted. For many years, scientists could not tell the age of any adult T. rex.
In 2002, Dr. Erickson discovered that a full set of growth rings can be found in dense bones from the ribs, shins, and hips. “I was elated,” said Dr. Erickson. “This suggested to me that it was possible to get at the age of T. rex—something I was told was impossible fifteen years ago when I first started in this field.”
The paleontologists compared the body size and age of seven T. rex specimens. They discovered that these dinosaurs grew fastest between the ages of 14 and 18. A teenage T. rex gained almost 5 pounds each day. In a year, a growing T. rex gained about half the weight of a Volkswagen Beetle. These dinosaurs reached adult size by their early twenties. One of the largest and most famous specimens of T. rex (“Sue,” at Chicago’s Field Museum) lived to be 28.
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“This is a tough question and one we don’t truly have an answer for. Large predators tend to show up when large prey are available. But why did the herbivores [plant-eating dinosaurs] get large? It could be that through large size, they gained access to formerly unreachable food. For example, long necks in sauropods [huge plant-eating dinosaurs like Apatosaurus] allowed them to reach the tallest trees. Large size might also have made some dinosaurs too large to be preyed upon.” –Dr. Erickson |
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The
Fastest Growth in the History of the World |
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| Albertosaurus | Apatosaurus |
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al-BUR-tuh-SAW-rus |
uh-PAT-uh-SAW-rus | |
Gaining 5 pounds a day seems like a lot to us, but was it fast for a dinosaur? Dr. Erickson and his co-workers also studied the growth rates of three other large meat-eating dinosaurs: Daspletosaurus, Albertosaurus,and Gorgosaurus. These dinosaurs were big, but only a third to a fifth the size of T. rex, and they grew more slowly, about a pound a day. Some plant-eating dinosaurs grew even faster than T. rex. The record for the fastest growth in dinosaurs goes to Apatosaurus. It could gain 33 pounds each day. In a year, a young Apatosaurus would gain the weight of a school bus. Do any living animals grow this fast? Some baby marine mammals grow very quickly, but only when they are suckling their mother’s high-fat milk. Infant elephant seals can gain 16 pounds a day. Right-whale calves can gain 130 pounds a day, but the fastest growers are blue-whale calves. These whales can gain 200 pounds a day. |
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| Gorgosaurus | Daspletosaurus |
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GOR-goh-SAW-rus |
das-PLEE-tuh-SAW-rus | |
