Duck-billed dinosaurs, 30-foot-long plant-eaters that once roamed much of North America, were hardly bigger than toy poodles when they hatched. "There is almost nothing you can think of as vulnerable as a 16-inch-tall duck-billed dinosaur," says paleontologist Jack Horner of the Museum of the Rockies in Bozeman, Montana. So what's a tiny baby dino to do? Grow fast like a bird, not slowly like a lizard.
Horner, paleontologist Kevin Padian of the University of California at Berkeley, and their colleagues examined fossil embryos and hatchlings from three types of duck-bills to figure out how they matured. The researchers counted the number of microscopic blood vessels in the part of the bone surrounding the marrow--a large blood supply denotes fast growth. They also examined embryonic bone from modern alligators, ostriches, and emus.
"The dinosaurs had even more vascular space than the birds. It looks like they grew up to somewhere between nine and 12 feet long in just one year," Horner says. They may have looked like lizards, but Horner finds that dinosaurs had evolved a birdlike, fast-paced development style to survive the cretaceous competition. "In other words, birds didn't invent this method of growth. It was invented by their dinosaur ancestors."