Duck-Billed Dinosaurs "Outgrew" Their Predators

Ker Than
for National Geographic News
August 6, 2008
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.

The results suggest that Hypacrosaurus grew to its adult size of more than 30 feet (9 meters) long within 10 to 12 years.

By comparison, the T. rex grew to as much as 40 feet (12 meters)—but took more than twice as long to reach adulthood. (Related: "For Tyrannosaurs, Teen Years Were Murder" [July 13, 2006].)

"We were shocked at how fast [the hadrosaur] grew," Cooper said.

"If you look at a cross section of the bone of a nestling or even from within the egg, there are huge spaces in which blood supply was going through the bone, which means they were growing like crazy."

The research appears online this week in the journal Proceedings of the Royal Society B.

Based on its growth rate, the team also suspects Hypacrosaurus reached sexual maturity early, at only two or three years of age, allowing the animals to get a head start on reproduction.

The combination of large size and early reproduction would have provided duck-billed dinosaurs with a huge advantage over their predators, scientists say.

"These things together are like a double punch," said Kristina Curry-Rogers, a paleontologist at Macalester College in St. Paul, Minnesota, who was not involved in the study.

"It is clear that these duck-billed dinosaurs weren't helpless prey and that their growth rates significantly contributed to their long-term success."

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