Tyrannosaurs had teenage growth spurt
Bone analysis sheds light on dinosaur development.
Tyrannosaurus rex lived fast and died young, research reveals. Examination of fossil ribs has slashed 70 years off the age of Sue, the best-preserved specimen of this enormous Cretaceous carnivore.
It seems that T. rex must have been able to really pile on the pounds. It gained up to two kilograms a day, as much as a modern-day African elephant does, report Gregory Erickson from Florida State University, Tallahassee, and his colleagues in this week's Nature1.
The dinosaur, which roamed the earth some 65 million years ago, was one of the largest terrestrial carnivores ever to live. Adults typically weighed in at around 5,000 kilograms, making them at least 15 times larger than today's largest land-based meat-eater, the polar bear.
This caused palaeontologists to puzzle over how they got so big. Some experts believed they grew slowly throughout their lives, like modern-day reptiles. Others thought they had an initial growth spurt that later subsided, like that in birds and mammals.
But assessing growth rates is tricky, as the creatures are difficult to age. The standard method for using a fossilized skeleton to estimate the age at which a dinosaur died is to count growth rings. These are dense mineral deposits that are laid down in the bones on a yearly basis, as the animal grows. But the technique generally looks at large, weight-bearing bones such as the thigh. In therapods, such as T. rex, these bones are hollow, so the vital rings are missing.
Instead the team looked at bones that do not bear weight, such as ribs and shinbones, which are solid in T. rex. The researchers tested the method in alligators and lizards of known ages and found that they were able to accurately predict what their lifespans had been. The researchers then turned their attention to 7 T. rex fossils of varying sizes.
Using the new method, the team found that the specimens were likely to be between 2 and 28 years old when they died. This made Sue, the oldest and best-preserved T. rex, 70 years younger than was previously thought. "Before this, Sue's age was based on speculation," says Erickson. "Her bones are pretty battered and beat up."
By combining the dinosaurs' ages and sizes, the researchers worked out their growth rates. T. rex, it seems, grew up fast. The animal grew most between 14 and 18 years of age, then retained its large size throughout the rest of its life.
Erickson's team also studied the growth rates of three smaller tyrannosaur species that existed before T. rex. Like T. rex, their growth spurt occurred over a four-year stretch, but their rate of growth was around four times slower. This suggests that T. rex evolved to be so big because of its exceedingly fast growth rate.
However, the beast's enormous dimensions may have led to problems. "As it got bigger, T. rex probably suffered from a progressive decline in its running ability," says dinosaur expert John Hutchinson from the Royal Veterinary College, London.
Younger, smaller animals could have reached speeds of up to 40 kilometres per hour. But as their weight passed 1,000 kilograms, just a fifth of the adult size, this would have become biomechanically impossible, says Hutchinson. For those who believe T. rex was a hunter rather than a scavenger, it is a mystery how the animals managed to eat enough to maintain their growth spurt, given that their ability to chase prey would have been seriously impaired.
- Erickson G.M, et al. Nature, 430. 772 - 775 (2004).