Duck-Billed Dinosaurs "Outgrew" Their Predators

Talk about being a big baby. The duck-billed dinosaur Hypacrosaurus grew three to five times faster than the fearsome predators that hunted it, reaching its full size by age ten, according to a new study.

Unlike other plant-eating dinosaurs, duckbills such as Hypacrosaurus didn't have piercing horns, dagger-like teeth, or hulking body armor.

So the ability to grow bigger faster provided the animals with a size advantage that likely served them well in their early years.

For example, baby duckbills were probably about the same size as Tyrannosaurus rex hatchlings, said study co-author Drew Lee of Ohio University's College of Osteopathic Medicine.

But by five years old the duckbill would be the size of a grown cow, while the T. rex would be only as big as a large dog.

"It's harder to hunt a larger animal, because it's physically dangerous," said study co-author Lisa Noelle Cooper of Kent State University and the Northeastern Ohio Universities College of Medicine.

"Growing Like Crazy"

Duck-billed dinosaurs, or hadrosaurs, were diverse and abundant in the late Cretaceous period, about 70 million years ago.

The soft-bodied animals flourished in what is now North America alongside many different meat-eating, two-legged dinosaurs known as theropods. (See a picture of the diversity of life in the Cretaceous.)

By counting and measuring rings in fossil leg bones, the researchers compared growth rates for Hypacrosaurus and three common theropods: the tyrannosaurs Albertosaurus and T. rex and the small velociraptor-like Troodon.

As with trees, each ring in the fossil bones represents a year of life. The wider the ring, the more growth occurred that year.